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Characteristics of hydrothermal eruptions, with examples from New Zealand and elsewhere
Earth-Science Reviews 52 Ž2001. 299–331 www.elsevier.comrlocaterearscirev
Characteristics of hydrothermal eruptions, with examples from New Zealand and elsewhere P.R.L. Browne a,) , J.V. Lawless b a
Geothermal Institute and Geology Department, UniÕersity of Auckland, PriÕate Bag 92019, 23 Symonds Street, LeÕel 2, Auckland, New Zealand b Sinclair Knight Merz Limited, PO Box 9806, Auckland, New Zealand Received 6 November 1998; accepted 17 August 2000
Abstract Hydrothermal eruptions have occurred in many hot water geothermal fields. This paper concentrates on examples from New Zealand but also mentions others elsewhere, which demonstrate points of particular interest. Numerous small eruptions Žmaximum focal depths of about 90 m. have occurred in historic times Žpast 150 years. at WairakeirTauhara, Rotorua, Tikitere, Ngatamariki, Mokai and Waimangu. The presence of breccia deposits shows that much larger Žwith estimated maximum focal depths of about 450 m., prehistoric hydrothermal eruptions have also occurred at Kawerau, Wairakei, Tikitere, Orakeikorako, Te Kopia, Rotokawa and Waiotapu. One of the largest known hydrothermal eruptions in New Zealand took place at Rotokawa 6060 " 60 years ago; this produced a deposit that extended over an area with a diameter of 4 km, and has a maximum thickness of 11 m. Deposits from hydrothermal eruptions are typically very poorly sorted, matrix-supported, and may contain hydrothermally altered clasts that derive from within the geothermal reservoir. Their lithologies and alteration mineralogies are useful guides to subsurface conditions. Hydrothermal eruptions do not require any direct input of either mass or energy derived directly from a magma and, thus, differ from both phreatic and phreatomagmatic eruptions. Many hydrothermal eruptions in a hot water field start very close to the ground surface and result from the rapid formation of steam due to a sudden pressure reduction. This steam provides the energy necessary to brecciate, lift and eject fragments of the host rocks as a flashing front descends and water nearby in the reservoir boils. A rock brecciation zone accompanies this front, and both precede the descent of the eruption surface. A hydrothermal eruption continues until the steam is produced too slowly to lift the brecciated rocks. There is no genetic difference between the small eruptions induced by exploitation and those which occur as a geothermal system evolves naturally and whose effects may penetrate to much greater depths. Hydrothermal eruptions do not need the presence of either field-wide cap rocks or pressures within a reservoir that exceed that provided by a hydrostatic column of water very close to its boiling temperature. q 2001 Elsevier Science B.V. All rights reserved. Keywords: geothermal; hydrothermal eruption; phreatic; epithermal; geohazard
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Corresponding author. Tel.: q64-9-373-7599; fax: q64-9-373-7436. E-mail address: [email protected] ŽP.R.L. Browne..
0012-8252r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 5 2 Ž 0 0 . 0 0 0 3 0 - 1
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P.R.L. Browne, J.V. Lawlessr Earth-Science ReÕiews 52 (2001) 299–331
1. Introduction Hydrothermal eruptions have been common events in many geothermal systems in New Zealand and elsewhere ŽTable 1.. There have been numerous eruptions in historic times. Some of these have been induced by exploitation, such as at WairakeirTauhara in New Zealand and at Tiwi in the Philippines, but others have occurred during the systems’ natural evolution. Indeed, hydrothermal eruptions are typical manifestations of active geothermal fields. Prehistoric hydrothermal eruptions are largely recognised from the presence of eruption breccias. However, evidence for hydrothermal eruptions is often overlooked because their deposits are not recognised, are poorly preserved or else buried under younger materials. Nevertheless, hydrothermal eruptions and their products deserve to be studied for the following reasons: Ža. They are destructive events that can cause loss of life and damage or destroy structures, such as geothermal plant and pipelines. Examples are given below. Mapping and dating of hydrothermal eruption deposits will obviously assist in locating the sites of prehistoric events and in determining eruption frequency. This information will assist in avoiding potentially dangerous sites when locating pipelines and power plants and also in planning reinjection strategies. The same applies to mining epithermal gold deposits that occur in active geothermal areas, such as the Ladolam deposit on Lihir Island, Papua New Guinea ŽMoyle et al., 1990.. Žb. The material ejected by a hydrothermal eruption provides clues about the lithology of the geothermal reservoir from which it derives. Žc. The presence of specific hydrothermal minerals in ejecta may reveal information about the subsurface temperatures and fluid types present, while fluid inclusion geothermometry may be used to estimate these temperatures more precisely as well as to determine fluid salinities. Žd. Hydrothermal eruptions may cause major changes to the hydrology of a field. Že. Hydrothermal eruption events can be intimately related to mineralisation Že.g. Sillitoe, 1985; Sillitoe et al., 1984; Hedenquist and Henley, 1985; Nelson and Giles, 1985; Bogie and Lawless, 1987; Robson and Stevens, 1991; Rutherford and Fransen,
1991.. The resulting breccias may also later become mineralised because of their high permeability ŽEbert and Rye, 1997., offering attractive targets for epithermal mineral exploration.
2. Nomenclature Terminology has not always been consistently used in the literature. The terms used in this paper are defined as follows: Geyser: An eruption of hot water, steam and nonaqueous gas from a hydrothermal system, usually but not always of cyclic occurrence, which ejects at most only trivial amounts of solid material. The ejection mechanism is volume change due to boiling with or without significant effervescence of gas, as opposed to ejection of water because of artesian pressure alone. Hydrothermal eruption: An eruption ejecting at least some solid material and whose energy derives solely from heat loss and phase changes in a convecting hot water or steam-dominated hydrothermal system. Phreatic eruption: An eruption which is caused by heating and flashing of water produced when magma comes into contact with water but only country rock or overburden is ejected Ži.e. no juvenile magmatic material.. If there is unequivocal evidence for the ejection of any juvenile igneous material, it should be classed as a phreatomagmatic eruption. Eruptions of these classes include those where lava contacts water, as in littoral explosions when lava flows into the sea, subglacial eruptions, or when water is introduced after an eruption, as in some Arootless fumarolesB when rainwater percolates into hot pyroclastic deposits Žsee Mastin, 1995 for a compilation.. One of us ŽJVL. witnessed eruptions of this type following the 1984 volcanic eruptions of Mt. Mayon, Philippines, whereby heavy rainfall gullying into and contacting andesitic pyroclastics some 6 weeks after their deposition, caused eruptions, which ejected clasts, up to 3 m in diameter, to heights of several hundred metres. An interesting example of distinguishing phreatic from phreatomagmatic and hydrothermal Žsensu stricto. eruptions is provided by Sigvaldason Ž1992.. To distinguish these from the following class of eruption, we limit phreatic
P.R.L. Browne, J.V. Lawlessr Earth-Science ReÕiews 52 (2001) 299–331
eruptions to those where the water phase is essentially cold, which requires all of the eruptive energy to come from the magma. Magmatic-hydrothermal eruption: An eruption that occurs when injection of magmatic material into a pre-existing convecting hydrothermal system causes a heat pulse that triggers an eruption ŽLawless et al., 1997.. In this case, the bulk of the energy responsible for the eruption is derived from the hydrothermal system itself, but the magmatic input has an essential triggering role. The resulting eruption may be larger than would be possible for a purely hydrothermal eruption or for a phreatic eruption involving the same amount of magma. Juvenile magmatic material may or may not be identifiable. One of the clearest examples of an historic magmatic-hydrothermal eruption was the 1886 eruption at Rotomahana ŽSimmons et al., 1993; Keam, 1988. as were probably the 1967–1977 eruptions of Soufriere de Guadaloupe ŽWohletz and Heiken, 1992, p. 278.. Mastin Ž1991. also considered this mechanism might have caused the 550,000 BP eruption at Inyo Craters, Long Valley Caldera. Note that this terminology has not been previously formally defined in this way in the literature, as this distinction has rarely been made, although the concept was mentioned by White Ž1955. and Nairn Ž1979.. It is, however, significant, since it is crucial to understanding the formation of some mineral deposits Že.g. Kelian, Indonesia: Van Leeuwen et al., 1989; Sillitoe, 1997; Davies et al., 1999. and assessing hydrothermal eruption risks. Hydrothermal eruption crater: A crater formed by a hydrothermal eruption. Maar: A crater formed by a phreatic or phreatomagmatic eruption. These craters are characteristically wide wup to several kilometres according to Wohletz and Heiken Ž1992.x but shallow, with floors below the level of their surrounds ŽLorenz, 1973.. Hydrothermal eruption breccia: A clastic surface deposit produced from a hydrothermal eruption. Hydrothermal breccia: A general term, which can include both hydrothermal eruption breccias Žexogenous. and breccias formed in the subsurface Žendogenous. by forceful disruption by fluid within a hydrothermal system. The terms Ahydrofractured brecciaB or Ahydraulically fractured brecciaB apply to breccias formed subsurface where there is clear evidence of brittle opening of the host rocks. How-
301
ever, where there has been vigorous clast transport and milling, this may not be apparent ŽFig. 1. and these terms should be avoided. Diatreme: A body of brecciated rock, usually with a subvertical, upwards-flaring form. It may infill an eruption channel, a phreatic or phreatomagmatic eruption vent, or be formed by other explosive magmatic events, such as a sudden loss of CO 2 when kimberlite forms ŽBrey and Ryabchokov, 1994.. It underlies a maar Žif preserved..
3. Characteristics of hydrothermal eruptions and their deposits A review of the literature and our own observations lead to the following generalisations about the characteristics of hydrothermal eruptions and conditions in a geothermal reservoir prior to an eruption. Some examples of individual hydrothermal eruptions are presented in Appendix A. 3.1. Craters and the focal depths of eruptions Some large prehistoric hydrothermal eruptions apparently had deep focal depths. At Waiotapu, eruptions ejected clasts derived from depths of at least 350 m ŽHedenquist and Henley, 1985., and some at Kawerau came from at least 200 m ŽNairn and Wiradiradja, 1980.. A comparison of the lithology of ejected clasts with the lithology of cores recovered from drillholes at Rotokawa shows that at least some material could have derived from as deep as 450 m ŽCollar and Browne, 1985.. However, this is not unequivocal, as it is possible that the drillholes did not penetrate some of the units represented among the clasts. Clasts from Inyo Craters were considered by Mastin Ž1991., on the basis of litho-stratigraphy, to have come from below 450 m depth. Smaller eruptions presumably had shallower focal depths, though it is difficult to find direct evidence of this. Fytikas and Marinelli Ž1976. suggested that the diameter of hydrothermal eruption craters could be correlated with the depth of the eruption focus and that this, in turn, could be used to predict the depth of exploitable hydrothermal reservoirs, but there are so many possible combinations of fluid state and
P.R.L. Browne, J.V. Lawlessr Earth-Science ReÕiews 52 (2001) 299–331
302
Table 1 Summarised examples of hydrothermal eruptions Location
Date
Crater Diameter Žm.
A. Historic eruptions Waimang 1900–1904 170 = 77 AGeyserB, NZ Žepisodic with average 36-h repeat. Waimangu ŽTrinity Terrace.
1973
Within lake
Waimangu ŽFrying Pan Flat. several events over 2 months
1917
120?
Waimangu ŽMud Rift.
1978–1979 2:18, 30 Žseveral. 1981
Tauhara, NZ
1981 Žalso 1974.
Karapiti, NZ ŽCrater G.
1978 1980 1981 1983
ŽCrater C. ŽCrater D. ŽCrater B.
Ngatamariki, NZ Rotorua, NZ
Ejecta Depth Žm.
Max. radius Žm.
15
Max. thickness Žm.
Area Žm2 .1
Vol. Žm 3 . 2
Max. clast Žm.
2.5
70
970
0.3
) 40
0.7
800?
3000
3200?
30
1.6 = 10 7 ? 5 = 10 6
4
10–23
0.22
1200
280
90
4.5
) 800
2.0
17,000
6800
1.2
15
) 10
) 100
60
15
1000– 2000
0.5
100
) 22,000
2.0
30,000
30
1961 ) 22,000 1964 1967 1997 1948 1894 1895 Ž2. 1897 1901 1902 1906 1931 1932 1945 1998–2000 Ž6.
) 100
20
5
0.5
0.15 100 30–60 15 30 30 30
15 4 Žfive small vents.
Shallow
5
0.5
Min. ejection velocity Žmrs.
460
0.45
120
Height Žm.
55
130
0.5
60–120 60 20
P.R.L. Browne, J.V. Lawlessr Earth-Science ReÕiews 52 (2001) 299–331
Min. focal depth Žm.
90
Basis
Stratigraphy
275?
Estimated energy ŽkJ. 3
Initiating mechanism
Reservoir characteristics
Commentsr loss of life
Reference
Magmatic?
Rejuvenated following 1886 Tarawera eruption. L iquiddominated water level at surface
Four deaths, several injured on August 31, 1903
Warbrick Ž1934., Lloyd and Keam Ž1965., Scott and Lloyd Ž1982.
10 8 (Õ)
Chloride water
Possible precursors; damage to vegetation
Lloyd and Keam Ž1974.
10 11 (Õ)
Magmatic?
Two deaths, one injured, building destroyed. Lateral blast
Morgan Ž1917., Warbrick Ž1934., Rae Ž1991.
No damage or loss of life
Scott and Lloyd Ž1982.
Near urban area, but no damage or loss of life
Scott and Cody Ž1982.
No damage or loss of life in any eruptions at Karapiti
Allis Ž1984.
10 6 (Õ) 10–27
Stratigraphy
303
10 8 (Õ)
Shallow
Drought
Drought, then rain
Shallow steam cap over deep steam and water, drawn-down at Wairakei Shallow steam zone connected to drawdown Wairakei reservoir
10 7 (Õ) Allis Ž1979.
AshallowB
Fracturing of cap? Draw-down
10 7 (Õ)
Liquid-dominated
Not seen, remote
Liquid-dominated, later drawn-down by exploitation, then partially recovered following reduction in draw-off
In urban area, damage to houses but no loss of life
Brotheridge et al. Ž1995. Donaldson Ž1985., SKM unpublished notes
Burial and man-made modification of vent may have contributed to eruptions in 1998– 2000 (continued on next page)
P.R.L. Browne, J.V. Lawlessr Earth-Science ReÕiews 52 (2001) 299–331
304
Table 1 Ž continued . Location
Date
Mokai, NZ
1996
Tikitere, NZ Lake City California, USA ŽMud Volcano. reference also made to eruptions at Gerlach Hot Springs, NV Tiwi, RP ŽNaglagbong.
1966 1951
1980–1981 Žthree episodes.
Crater
Ejecta
Diameter Žm.
Depth Žm.
20
5
60, several ) 3 vents within 210-m diameter
Max. radius Žm.
Max. thickness Žm.
0.3 100, plus ) 4 minor airfall to 7200
Area Žm2 .1
Vol. Žm 3 . 2
1.3 = 10 5
1.7 = 10 5
Max. clast Žm.
10
50
2
5500
1300
Tongonan, RP 1983 ŽBao.
10
8
20
0.5
900
AsmallB
0.1
Yellowstone, 1963 USA ŽSeismic Geyser.. Eruptions of Sapphire Pool Ž1959–1961., Excelsior Geyser Ž1881., Link Geyser Ž1957–1958., Steamboat Geyser Žseveral. also referred to ŽPorkchop 1989 geyser.
) 12
)6
10?
220
AsmallB
1.0
66
5400
La Soufriere ` de Guadaloupe, Lesser Antilles Iwo Jima
1797, 1809– 1810, 1836– 1837, 1956, 1976–1977 1957 Žalso 1922.
33
Dieng, Indonesia
1979
3: largest ; 300
Min. ejection velocity Žmrs.
0.3 0.3
25
14
Height Žm.
1.88
30
30–150
14
100
150 plus lahars to 4000
7
7600
8 = 10 5
1
0.4
40
P.R.L. Browne, J.V. Lawlessr Earth-Science ReÕiews 52 (2001) 299–331
Min. focal depth Žm.
- 90 30?
Basis
Stratigraphy geometry
AshallowB
Estimated energy ŽkJ. 3
1.6 = 10 8 Ž k ., 2.7 = 10 9 Ž t .
10 7 (Õ)
AshallowB
AshallowB
Initiating mechanism
Alteration, morphology
10 8 (Õ)
305
Reservoir characteristics
Commentsr loss of life
Reference
Local steam-heated zone over outflow Steam-heated Steam-heated
No damage or loss of life
KML Ž1998.
Steam cap over water draw-down following exploitation
No damage or loss of life
Espanola Ž1974. White Ž1955.
Grindley Ž1982., personal observation ŽJVL., Yuhara Ž1995. Personal observation ŽJVL., PNOC Ž1982. Marler and White Ž1975., White et al. Ž1988.
Drought
Local steam heated zone over outflow
Seismic fracturing of silica cap
Over pressured liquid-dominated
Seasonal hydrological changes Tectonic fracturing
Liquid-dominated, overpressured
Fournier et al Ž1991.
Steam over liquid, close magmatic connection
Zlotnicki et al. Ž1992.
Artificial burial of natural vents? Earthquake
Steam zone, probably magmatic component
Satellite crater formed by collapse
Corwin and Foster Ž1959.
High pressure, vapour cap
142 killed but by gas
Le Guern et al. Ž1982.
(continued on next page)
P.R.L. Browne, J.V. Lawlessr Earth-Science ReÕiews 52 (2001) 299–331
306
Table 1 Ž continued . Location
Date
Crater
Ejecta
Diameter Žm.
Depth Žm.
Max. radius Žm.
200? 40
5
130
Agua Shuca, Ahuachapan, El Salvador Nakano-yu, Japan
1868? 1990
Nisyros, Greece
Prehistoric, At least up to 30 1871, 1873 23, largest Žseveral. 1887 350
Max. thickness Žm.
1995
5.9 = 10 6 Žsingle unit.
3.7 = 10 6 Žsingle unit. 10 6 2 = 10 7
1
10 7
3
1.6 = 10 5
1
10 6
3
) 1300
4 8
Whakarewa42,000 rewa, Rotorua ŽNgawha Crater. Yellowstone 23–25,000 Žmany.
100
4
100?
250– 300
11 Žlargest single unit., 40 Žtotal.
200
1.0
1.3 = 10 5
Kawerau Ž2.
- 1800– 16,000
4000– 6000 3 = 10 5
13
Rotokawa Ž8?. 3700–20,000
Height Žm.
9 Žthickest unit.
2500
300–500
Max. clast Žm.
500 including debris flows
) 20
9000–14,500
Vol. Žm 3 . 2
550
130
5
1.3 = 10 7 Žsingle unit., 1.5 = 10 7 Žtotal. 7 = 10 5 Žlargest single unit.
50–150
150–200 2
30
350–1600 ) 35
1200
20
10 6
1.4 = 10 7
ŽMary Bay.
- 13,650
) 2500
100
2000
Inyo Craters, USA Ž12. Te Kopia area, NZ Žseveral. Ngawha, NZ Žat least 6.
650,000– 550,000 ) 2000
200
52
1000
13
10 6
1.2 = 10 9
) 0.7
200
2
10 5
10 4
0.5
?
100–200
Whangairorohea, NZ Tikitere, NZ
?
40–75
14,000
Min. ejection velocity Žmrs.
1600
B. Prehistoric eruptions (age BP) Waiotapu 900–15,000 60–250 Žseveral.
Orakei Korako Ž11.
Area Žm2 .1
300 ) 10
100
P.R.L. Browne, J.V. Lawlessr Earth-Science ReÕiews 52 (2001) 299–331
Min. focal depth Žm.
Basis
Estimated energy ŽkJ. 3
307
Initiating mechanism
Reservoir characteristics
Commentsr loss of life
Reference
26 deaths
4.6 = 10 10 Ž k . , 6.3 = 10 12 Ž t . 10 9 (Õ)
Excavation
Liquid-dominated vapour cap, drawn down by exploitation Magmatic?
Bruno et al Ž1992., Goff and Goff Ž1997. Yuhara Ž1997.
Seismic
Liquid-dominated?
Chiodini et al. Ž1992., Marini et al Ž1993.
Silica sealing
Now mainly liquiddominated
Cross Ž1963., Hedenquist and Henley Ž1985. Nairn and Wiradiradja Ž1980.
Damage to houses and road, four killed
100–350
Alteration, stratigraphy
2.3 = 10 9 to 1.4 = 10 11 Ž k .
190
Stratigraphy
5 = 10 12 Ž t .
450?
Stratigraphy
10 11 (Õ)
Some may be magmatic?
) 120, possibly - 20
Stratigraphy
10 9 (Õ)
Faulting?
Now mainly liquiddom inated, som e steam heated features
Lloyd Ž1972.
Change in lake level?
Mainly liquiddominated, now drawn-down
Lloyd Ž1975.
Glacial fluctuations in groundwater
Now overpressured, mainly liquiddominated
Faulting
Maybe phreatomagmatic Steam over water
Muffler et al. Ž1971.,Wold et al. Ž1977., White et al. Ž1988. Mastin Ž1991.
Draining lake?
Highly overpressured
Faulting?
Liquid-dominated
42
Stratigraphy
450
Stratigraphy
4 = 10 9 – 4 = 10 11 Ž k . 10 10 –8 = 10 11 Žt. 10 10 Ž k ., 10 12 Žt. 10 8 (Õ)
Now mainly liquiddominated, some excess enthalpy Now mainly liquiddominated, some excess enthalpy
Collar Ž1985., Collar and Browne Ž1985.
Browne et al. Ž1994., Clark Ž1999. Browne et al. Ž1981., Lawless Ž1988. Youngman Ž1996. Espanola Ž1974. (continued on next page)
P.R.L. Browne, J.V. Lawlessr Earth-Science ReÕiews 52 (2001) 299–331
308
Table 1 Ž continued . Location
Date
Crater Diameter Žm.
Atiamuri, NZ
Ejecta Depth Žm.
Max. radius Žm.
13,000
Wairakei, NZ ? ŽRautehuia Breccia.
Max. thickness Žm.
Area Žm2 .1
Vol. Žm 3 . 2
3 ?
Max. clast Žm.
Height Žm.
Min. ejection velocity Žmrs.
0.5
229 m? Žseveral units ?.
3 = 10 6 ?
Various data and estimates from the original sources are presented in this table in normal font, verbatim, with no modification except for conversion of units, where we have estimated derived parameters on the basis of the data given, these are in italics, using the following assumptions: Ž1. Where no more detailed data are available, areas of deposits are estimated from the maximum radius of deposition and assuming an elliptical distribution with an obliquity of 0.7. Ž2. Where no more detailed data are available, volumes of deposits are estimated from the area and maximum thickness by Eqs. 2–7 of Wohletz and Heiken Ž1992., assuming r h s one-fifth of the maximum ejection radius. Ž3. For previous estimates of energy involved in eruptions, k s mechanical energy required to transport ejecta on a kinetic or ballistic basis, t s total available energy. Estimates of t are usually much larger than k. Where no more detailed data are available, we have estimated the energy of eruption from the volume of ejecta Ž Õ ., by assuming 10 Jrcm3 is required.
eruption mechanism as to make this method of doubtful utility. Collar and Browne Ž1985. depicted vents at Rotokawa as overlapping and funnel-shaped rather than cone-shaped. Vents commonly ejected clasts, which were brecciated during earlier eruptions but then fell back into their vent. Many vents are not recognisable because: Ž1. much of the ejected material fell back into the crater and remains there; Ž2. many became sites of intense geothermal activity that masks their morphology; andror Ž3. inward slumping of the crater walls disguises the crater margins. However, the locations of some craters can be recognised from the raised apron of breccia surrounding them, by the distribution and thickness of the breccia and especially by the maximum size of its clasts. This also helps to distinguish them from the many crater-like features in geothermal areas that are not produced by hydrothermal eruptions but by other processes, such as ground collapse following the dissolution of near-surface rocks by acid waters. A good example of a discussion of whether certain craters are of hydrothermal origin is provided by White et al. Ž1988.. Pike Ž1974. provides criteria for using systematic numerical taxonomic methods to help reveal their origin.
The width of vent craters differs greatly. Examples are listed in Table 1, with the larger craters, such as those at Kawerau, Rotokawa and Ngawha, being 200–500 m across, whereas smaller ones, such as those at Ngatamariki, and Tongonan in the Philippines, are 10–20 m in diameter. The largest one reported if, indeed, it is of hydrothermal origin, is at Mary Bay, Yellowstone and is more than 2500 m across ŽWold et al., 1977.. 3.2. Breccia deposits Nelson and Giles Ž1985. and Sillitoe Ž1985. have discussed the distinguishing features of hydrothermal eruption breccias. Drawing on the above examples, we make the following comments: Ž1. Extent: The distribution of the products from a hydrothermal eruption obviously depends upon the magnitude of the eruption, the nearby topography, the geometry of the vent, the nature of the ejected material, whether or not the eruption occurred through water and the post depositional erosion history of the breccia. However, even the largest hydrothermal eruptions produce deposits that are several orders of magnitude smaller in volume than those deposited from a typical silicic pyroclastic
P.R.L. Browne, J.V. Lawlessr Earth-Science ReÕiews 52 (2001) 299–331
Min. focal depth Žm.
Basis
Estimated energy ŽkJ. 3
Initiating mechanism
Reservoir characteristics
Activity extinct now Magmatic?
eruption. Lahar deposits are more likely to occur within valley floors. Breccias deposited from the largest hydrothermal eruption at Rotokawa ŽFig. 2. covered an area of about 12 km2 , with a maximum thickness of 11 m ŽCollar and Browne, 1985. and had a volume possibly as great as 10 7 m3 Žit has been eroded considerably.. The Lower Eruption Breccia at Kawerau is estimated to have a volume of 2 = 10 7 m3 ŽNairn and Wiradiradja, 1980.. Five of the hydrothermal eruptions younger than 1800 years old at Waiotapu ŽLloyd, 1959; Hedenquist and Henley, 1985. produced ejecta that covered areas of between 2 and 6 km2 ; one deposit has a maximum thickness close to its vent of 13 m and a volume of 3.7 = 10 6 m3. The thickness of this, and most other eruption deposits, decreases exponentially away from their craters. These very large deposits are, however, exceptional and the great majority of hydrothermal eruptions probably produce deposits with volumes less than 10 5 m3. The distribution of ejecta around a vent can vary from uniform to quite asymmetrical, depending upon vent geometry and wind conditions. The most extreme historical example of the latter is probably the directional eruption blast at Waimangu in 1917 ŽMorgan, 1917., when some ejecta, described as
Now a shallow steam zone over a liquiddominated resource
Commentsr loss of life
309
Reference
Browne and Lloyd Ž1986. ECNZ Ž1992., Bogie et al. Ž1995.
AmudB, reputedly travelled 3200 m from the vent, and debris from a house destroyed by the blast was transported 1600 m. Fine air-fall material from the eruption cloud at the Lake City eruption ŽWhite, 1955. travelled 7200 m from the vent. Ž2. Textural Characteristics: Typical hydrothermal eruption breccias are very poorly sorted and invariably matrix-supported Žimplying rapid settling velocities with little opportunity for sorting.. Ejected material ranges from clay size to clasts several meters in diameter. Vertical and lateral grading of clasts is usually poorly developed although the largest clasts usually lie closest to their craters. Bedding forms are rare, unlike in base surge deposits. Probably because many clasts were milled within their vents during the course of an eruption before being ejected, they are typically subround. Larger blocks sometimes impact into underlying material and this is one way to distinguish hydrothermal eruptions from mudflow deposits, which they may superficially resemble. Some hydrothermal eruption breccias show evidence that they were deposited wet, such as the presence in them of accretionary lapilli or the vesicular texture of their matrix Že.g. Mastin, 1991.; some may even be wet enough to generate lahars Že.g. Marini et al., 1993.. Similar textures present in subsurface brec-
310
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clasts present in hydrothermal eruption breccias may, thus, differ from those in debris flows and lahars, both of which have clasts that are more likely to be monomict and altered differently Žif at all.. For example, it is unlikely that clasts in a lahar of volcanic origin would contain alteration minerals, such as epidote, adularia or perhaps zeolites, produced at moderate temperatures by near-neutral pH waters. Nor would products, such as vein quartz and platy calcite Žwhich have been recognised in hydrothermal eruption breccias., indicative of shallow two phase conditions in a convective reservoir, usually occur in a volcanic lahar whose alteration is more likely to indicate acid andror vapour-dominated conditions, and comprise kandites, sulphur and alunite. If an eruption originated within a secondary steam-heated aquifer, the alteration of the ejected clasts may resemble those of volcanic origin, but probably without very abundant sulphur and the
Fig. 1. Idealised development of hydrothermal veins and breccias.
cias, which are arguably of hydrothermal origin, were noted by Hulen et al. Ž1999.. In other eruptions the deposits may be quite dry, however, such as the historic eruptions at Karapiti, New Zealand, where the mobile phase was steam derived from a vapourdominated zone in the top of the reservoir. Thus, in terms of the eruption types discussed by Mastin Ž1995., it is important to appreciate that hydrothermal eruptions derived from a steam zone can fall into his category of Agas eruptionsB even at well below magmatic temperatures. Similarly, consideration of comments, such as those of Sheridan and Wohletz Ž1981. regarding waterrmagma ratios in hydrovolcanic eruptions could lead one to conclude that hydrothermal eruptions should always be wet, but this is clearly not the case. Ž3. Lithology and alteration of clasts: Much of the material ejected derives from the geothermal reservoir and, thus, reveals at the surface the types of rocks that comprise it. Many clasts are hydrothermally altered, so that they can be studied to interpret hydrological conditions in the reservoir itself using standard mineralogical techniques. The lithology of
Fig. 2. Distribution of 6060-year-old hydrothermal eruption breccia at the Rotokawa geothermal field. Isopachs are in metres. Also shown are deduced vent locations Žfrom Collar, 1985; Collar and Browne, 1985..
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phosphorous-bearing phases, such as woodhouseite, that appear to be diagnostic of primary magmatic volatiles ŽBogie and Lawless, 2000.. Possibly breccias deposited from an eruption originating within a steam-dominated reservoir could be distinguished by the occurrence of clasts with phases such as pyrrhotite, and the absence of phases indicating liquid conditions, such as abundant vein quartz. However, in practice, such observations tend to be equivocal. For reasons discussed below, large, deepfocussed eruptions from steam reservoirs are believed to be uncommon. 3.3. Amount of energy released Estimates of energy involved in eruptions can be based on a number of principles, but fundamentally they fall into two categories: those that assess mechanical energy required to produce the effects, in terms of cratering and ejecta, and those that look at energy supply and the thermodynamics of eruptive processes. In the first category, some methods are empirical, based on measurement made with large artificial explosions Že.g. Murphy and Vortman, 1961.. Others are ballistic, based on the distance that large clasts are ejected Že.g. Sherwood, 1967; Wilson 1972, 1980; Steinberg, 1977.. An alternative is to look at the amount of energy required to transport the mass of ejecta Že.g. Muffler et al., 1971.. In the second category are analyses of thermal energy available in hydrothermal systems and the thermodynamics of the resulting eruptions, Že.g. Muffler et al., 1971; Nelson and Giles, 1985; Mastin, 1995., or various volcanic processes Že.g. Self, Wilson and Nairn, 1970.. All of these methods have been applied to assessing the energy involved in hydrothermal eruptions, with varying degrees of sophistication. Some of the resulting estimates for individual eruptions are quoted in Table 1 and discussed below. Caution must be used in comparing these estimates, since the different methods will yield very different results. Some of the difficulties are as follows: Methods based on ballistics of large ejecta are not applicable where they are mainly fine-grained material or mud, as in some hydrothermal eruptions. Also, while maximum distances of ejecta are often given, and maximum clast size, what is needed for v
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an analysis of this type is the ejection distance of a particular clast of known size. The differences in scale, temperature, mechanisms and fluid properties between steam-driven eruptions and volcanic, nuclear or high-explosive blasts mean that analyses based on the thermodynamics of volcanic or artificial explosions are of doubtful applicability. Some estimates based on total ejecta mass or available energy assume isentropic decompression, whereas others are isenthalpic. This is discussed by Mastin Ž1995.. Estimates based on energy required to transport ejecta will usually be much smaller than estimates based on available thermal energy, because not all of the energy in the clasts is transferred to the fluid medium. Estimates of the efficiency of such heat transfer vary widely Že.g. Nairn and Wiradiradja, 1980; Sheridan and Wohletz, 1981.. Some estimates of the difference between kinetic and total energy are as great as 10 3 times ŽMastin, 1991.. Some of the more significant estimates that have been made for hydrothermal eruptions include the following. A common rule of thumb is that the energy required to form a crater varies according to the third power of the crater’s dimensions ŽWohletz and Heiken, 1992., although this breaks down for very large explosions, and the depth of focus of the explosion is also important in determining the radius of the crater ŽMurphy and Vortmann, 1961.. Muffler et al. Ž1971. calculated that the energy required Žisenthalpically, but including a component for comminution; Charles, 1957. for a hydrothermal eruption at Yellowstone, which ejected material from a depth of 70 meters was about 8.4 Jrcm3 and provided curves for estimating eruptive energy based on crater radius. Their method probably seriously under-estimates the energy required to form larger craters, since they assumed a constant depth and height of eruption. This compares with a range of 1.3 to 210 Jrcm3 for large crater-producing artificial explosions ŽMurphy and Vortmann, 1961.. Thermodynamic analysis by Mastin Ž1995. showed that the maximum energy available for hydrothermal eruptions is about 250 kJrkg of fluid–rock mixture Žabout 500 Jrcm3 ., and that maximum clast velocities are unlikely to exceed 400 mrs, which is consistent with ballistic observations. For the larger erupv
v
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tions, such as those at Rotokawa, with a volume of 10 7 m3, an energy requirement of 10 Jrcm3 corresponds to 10 11 kJ. For comparison, Hedenquist and Henley Ž1985. compared fall velocities of large clasts at Waiotapu to their probable trajectories following Keiffer Ž1977.. They then estimated that the kinetic energy required was between 2.3 = 10 9 hydrothermal eruption at the Kawerau field released Žnot requ ) and 1.4 = 10 11 kJ. Similar analyses by Nthey areairn and Wiradiradja Ž1980. estimated that one prehistoric ired. 5 = 10 12 kJ of thermal energy. These are of similar magnitude to son and Gthe estimates made for Yellowstone. Nelson and Giles Ž1985. approached the question of eruptive energy from the point of view of energy storage in a hydrothermal system and reached the conclusions that there was more than enough energy available in them to generate hydrothermal eruptions. White Ž1955. and Mastin Ž1991. estimated the kinetic energy of the South Inyo Crater eruption to be of the order of 10 10 kJ, and its total energy to be 10 11 –10 12 kJ, although there is some doubt whether that eruption was hydrothermal or phreato-magmatic. Some large hydrothermal systems in the Taupo Volcanic Zone are reported to have natural heat fluxes in the range 100–300 MW Žthermal: hereinafter designated MWth . ŽHochstein et al., 1993.. While noting that estimates based on surface heat flow measurements can be too low by a factor of three because they neglect the importance of subsurface boiling Že.g. Grant, 1985: cf. KRTA, 1986., a value of 200 MWth can be regarded as typical. The larger eruptions mentioned above, therefore, required energies equivalent to between hours and months of the natural heat flux of these systems. A similar observation was made by Hedenquist and Henley Ž1985., who stated that the eruptions at Waiotapu required energy equivalent to a few days of the natural heat flux of the system. Thus, even allowing that eruption processes are poor means to transfer energy ŽSheridan and Wohletz, 1981., it is evident that hydrothermal eruptions need not be rare events within hydrothermal systems lasting several hundred thousands of years Že.g. Browne, 1979.. The energy depletion, by itself, would leave no identifiable trace within the reservoir rocks in terms of hydrothermal alteration overprinting. These estimates clearly show
that hydrothermal eruptions can occur without additional heat being added to a geothermal system whose reservoir fluid is near boiling. 3.4. Fluid state in hydrothermal systems Although many geothermal systems have been drilled and tested, there are few reliable measurements of the pressures and temperatures prevailing in the upper few hundred meters Ži.e. the region where hydrothermal eruptions occur.. Conditions here usually need to be inferred, with a few exceptions, as noted below. The Lihir gold mine in Papua New Guinea Žcommissioned in 1996., where progressive dewatering and associated monitoring of an active hydrothermal system to permit mining is required, will offer a unique opportunity to collect such data ŽMoyle et al., 1990; Clayton, 1999.. A liquid-only hydrothermal system, without any dissolved gases, and discharging to the surface represents the simplest case. Here, the temperatures of the heat source and the boiling temperature of water at the ground surface Žassuming conditions are everywhere subcritical. limit the two ends of its temperature gradient. In the simplest situation, water in this system is just slightly below boiling temperature throughout. Many of the New Zealand systems, in their undisturbed state, have down-well temperature profiles that are close to the boiling-point-for-depth gradient with a piezometric surface close to the ground surface. Assuming that the pressure is determined by the weight of the overlying water column, pressures in such a system will everywhere be below that of cold hydrostatic and certainly far below lithostatic pressure. In the undisturbed state, hydrothermal eruptions will not occur; but even a very slight disturbance that causes a drop in confining pressure at any depth can initiate subsurface boiling, which may then trigger a hydrothermal eruption. As pointed out by Donaldson et al. Ž1981., where convection occurs, the resulting pressure gradient will be hydrodynamic due to frictional pressure drop. Parts of such geothermal systems may, therefore, be AoverpressuredB and artesian ŽCathles, 1977.. The overpressure is derived from the balance between the descending column of cold water providing recharge to the system, and the upflowing, less dense hot
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water — the so-called thermo-artesian effect ŽElder, 1968.. Similarly Hayba and Ingebritsen Ž1997. concluded, from numerical modelling of convective hydrothermal fluid flow, that overpressures of 50 % above hydrostatic could occur. Careful and crucial measurements made in some research drillholes at Yellowstone National Park by White et al. Ž1975. showed temperatures in this hot water reservoir tended to follow the boiling curve at depth but conductive gradients existed in the upper few tens of metres. Pressure gradients were as much as 47% above that of cold hydrostatic. This condition was ascribed to self-sealing ŽWhite et al., 1988.. Smith Ž1970. reported pressures 0.9 MPa in excess of cold hydrostatic at shallow depths at Kawerau. At Rotokawa the measured pressure exceeds the expected hydrostatic pressures by about 1 bar per 100 m depth Ži.e. by about 10%, Grant, 1985.. Well AkicksB Žback-flow from overpressured zones. are also common during drilling in geothermal systems, showing that, locally, pressure is above that of cold hydrostatic. In this situation, hydrothermal eruptions can more easily be initiated. The situation is complicated where increasing amounts of dissolved gases cause boiling to occur at progressively lower temperatures and greater depths within a reservoir, e.g. Ohaaki ŽSutton and McNabb, 1977; Mahon et al., 1980., and recharge is from a similar, or higher, elevation to the ground surface over the upflow zone. Systems with high gas contents, such as Ngawha in New Zealand and Lardarello in Italy, have thick vapour-rich zones above a liquid only phase. Unlike steam, gas that separates from deeper boiling water will not condense Žalthough some may redissolve in perched waters. as it moves to cooler regions. Condensation of steam may allow AnoncondensibleB gases to accumulate beneath rocks of low permeability creating a gas pocket that tends to spread laterally and vertically until it escapes. It can be an effective transmitter of high pressures from below. Pressures could become sufficiently high for a hydrothermal eruption to occur. Extrapolation of deep pressures to near-surface at Ngawha show that the pressure is potentially sufficient today to permit a hydrothermal eruption by lifting off a lithostatic load of about 130 m of rock ŽFig. 3.. That this does not happen is probably due to the capping of about 500 m of low-permeability
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sedimentary rocks, which do not permit the gas pressures to be transmitted close to the surface. By contrast, geothermal systems in steep terrain, such as most in Indonesia and the Philippines, have deep zones where the temperatures are close to boiling but their piezometric surfaces are as much as 1000 m below the ground surface; this is because of the lower elevations of the recharge zones. Where vertically extensive steam zones exist their pressures usually correspond to the saturation value at the maximum enthalpy point of steam, about 34 bars ŽRogers and Mayhew, 1980.. They could, therefore, theoretically, lift off a lithostatic load equivalent to about 180 m of rock. However, the steam zones do not extend closely enough to the surface to permit this to happen, presumably because they take a long time to develop. The upper boundary of steam zones in this situation may be a thick layer of low-permeability alteration products produced by moderate temperature, acid–sulphate–bicarbonate waters and characterised by clays and, in some cases, anhydrite ŽBogie et al., 1987.. The fact that vertically extensive steam zones are isothermal and, therefore, isobaric means that hydrothermal eruptions caused by fluid overpressuring cannot originate very deep within such systems. Note
Fig. 3. Pressure–depth profiles in explored New Zealand geothermal fields Žmodified after Browne et al. 1981..
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also that at temperatures below critical point, a given volume of reservoir containing a single liquid phase will have more energy to provide to a hydrothermal eruption than will a steam-dominated reservoir of the same volume. For a reservoir with a porosity of 30% and water saturation of 30% by volume, this difference is about 20% ŽFig. 4.. The effect increases as the temperature increases. Although the enthalpy of steam per unit mass is greater than that of water, it is almost invariant with temperature in the subcritical range and the enthalpy per unit Õolume is much less than that of water. In fact, in terms of stored heat per reservoir volume, more energy is stored in the residual liquid water in a steam zone Žat say 30% by volume water saturation. than in the accompanying steam. The contribution of energy from the reservoir rocks will be the same in each case Žthough perhaps the rate of energy transfer will not.. In terms of fluid flow through a porous reservoir to a fracture over a short time, it is the distance that the fluid has to flow and, hence, the reservoir Õolume supplying the fluid that matters, not the mass.
4. Physical mechanisms of hydrothermal eruptions When attempting to understand the physical mechanisms of hydrothermal eruptions, and hope-
Fig. 4. Amount of energy in fluid per unit reservoir volume in a geothermal reservoir at varying degrees of water saturation by volume. Curves are shown for a typical near-surface steam zone at 1508C, a typical deeper steam zone at 2408C, characteristic of those exploited for geothermal energy, and a zone at 2708C more typical of a deep, boiling two-phase source zone. In all cases, porosity s 30%.
fully to arrive at a model which has some predictive value, it is necessary to recognise that not all hydrothermal eruptions are the same. There are several different types, but some require special circumstances and so probably occur infrequently. Some mechanisms may be common to more than one type. The following types can be distinguished and are presented here in the order of requiring progressively lower subsurface fluid pressures: 4.1. Pressures exceeding lithostatic The simplest model of a hydrothermal eruption is one whereby a field wide ‘cap’ or cover rock allows pressures within the reservoir to increase until they exceed lithostatic pressure, plus the tensile strength Žif any. of the rock, at some place. When this happens there is a single, short eruption Žthough the eruption may then continue driven, through other mechanisms., originating within the reservoir that brecciates the host rocks, then ejects the resulting clasts. This mechanism was demonstrated to be a factor for the 1976 phreatic eruption of La Soufriere ` de Guadaloupe, which was preceded by ground tilt, indicating subhorizontal volcanic hydrofracturing occurred Žsummarised by Wohletz and Heiken, 1992, p. 97.. This mechanism is probably most common in the case of very small, near-surface eruptions, but it does not account for the following observations and features: Ž1. Hydrothermal eruptions occur in some geothermal fields where measured pressures may only slightly exceed hydrostatic. Hydrothermal eruptions have occurred at all of the fields shown in Fig. 3, except Ohaaki, despite the fact that only one of the systems ŽNgawha. is significantly overpressured with respect to lithostatic pressure. Eruptions have occurred within both the liquid and vapour-dominated portions of such systems. Ž2. Very few active hydrothermal fields have field-wide lithological cap rocks, otherwise they could not discharge any steam or water derived from the reservoir. Condensation of steam would usually make such a situation short-lived. An absence of surface activity above a hydrothermal system is usually due to hydrological factors Že.g. steep terrain., not because it has a lithological AlidB. The hydrological importance of near-surface low permeability
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caps, where they do occur, is mainly to keep cold water out of a reservoir rather than hot water in. Ž3. Since hydrothermal eruptions are typical features of so many geothermal systems, it should not be necessary to invoke complex or unusual mechanisms to explain their occurrence. They are probably most frequent when a geothermal system is young, as is the case at Waimangu, or perhaps under stress, as may have been the condition at Waiotapu following injection of magmatic gases into the reservoir fluids ŽHedenquist and Henley, 1985.. The latter were magmatic-hydrothermal eruptions following, perhaps, the injection of dykes ŽLawless, 1988.. However, even long-established systems, such as those at Orakeikorako and Kawerau, have been the sites of large hydrothermal eruptions; the latter system, for example, has been active, in some form or other, for the past 200,000 years ŽBrowne, 1979.. Any model of hydrothermal eruption mechanisms must also account for the frequent occurrence of eruptions in exploited systems, where there has very rarely been observed any chemical changes consistent with a sudden input of magmatic volatiles. The question which then arises is whether or not there are any genetic differences between the small hydrothermal eruptions, which occur in both the exploited and nonexploited fields and those of much greater magnitude, whose effects penetrate to several hundred meters depth. There is no evidence that hydrothermal eruptions, which occur in exploited geothermal areas are any different, except in their magnitudes, from those that take place during a geothermal system’s natural evolution. Ž4. Observations made by Allis Ž1986. of hydrothermal eruptions at the Craters of the Moon area ŽWairakei. show that these events lasted from 15 minutes to several hours. During this time, material was ejected Žand on falling back reejected., i.e. these events were not over instantly, which would probably have been the case were they single eviscerating eruptions that originated deep within a reservoir with almost all the energy being released at once. According to Warbrick Ž1934., the 1917 hydrothermal eruption at Waimangu lasted for almost 3 days, although activity declined during this time. Ž5. Hydrothermal eruptions reoccur repeatedly at the same sites, e.g. at the Puarenga Stream, Rotorua, in some instances after less than a year. This is true
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of both small and larger eruptions and requires that the hydrology of the field be restored nearly to its previous condition between eruptions. The extreme case was the Waimangu AGeyserB, which was a cyclical hydrothermal eruption with a periodicity averaging 36 h, but on occasion as short as 8 minutes Žquoted by Keam, 1962.. It is hard to see that super-lithostatic pressures could be built up this quickly and repeatedly. 4.2. Accumulation of steam and r or gas Eruptions can occur when ascending steam reaches a depth where its pressure exceeds that of lithostatic. The shallower the steam zone extends, the lower the initiation pressure, that is to say that cooler steam can be a problem in terms of eruption risk if it reaches shallow levels ŽFig. 5.. The only precursory condition is that there is subsurface boiling occurring at any depth, and there is sufficient permeability for the steam to ascend. This is the mechanism that is thought to cause hydrothermal eruptions in exploited fields. Here, declining water levels permit higher steam pressures to develop close to the ground surface as boiling conditions descend deeper into the reservoir ŽFig. 6.. Also, water draining from the formations generates a higher steam flux as it boils, deriving its energy to do so from the rocks. Most commonly, such an accumulation of steam occurs only at shallow depths Ždown to a few tens of metres., due to near-surface dewatering. It would usually be preceded by some surface emissions of steam. Less commonly, higher-pressure accumulations of steam could occur at depth, due to deeper changes within the reservoir. These may lead to larger hydrothermal eruptions, with no warning at the surface. The collection of carbon dioxide gas, near the surface but below a seal, may accentuate the effect, as unlike steam, it will not condense. 4.3. Subsurface pressure release If pressures are released suddenly at depth in a geothermal system, steam will form and gas separated from the liquid phase will be released, whereupon the eruption follows the same course as that described above. A pressure release could be due to
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some minor hydrothermal eruptions, were due to the effects of subsurface hydrofracturing. Marini et al. Ž1993. identified the same mechanism as the cause of the 1873 hydrothermal eruption at Nisyros, Greece. Zlotnicki et al. Ž1992. tracked progressive migration of hydrothermal fluid using a variety of techniques prior to the 1976–1977 eruptions of La Soufriere ` de Guadeloupe. They considered that opening of permeable channels through argillised aquicludes, with associated seismicity, was a factor leading to these phreatic eruptions. The necessary condition for an eruption of this type to occur is that water at the local boiling temperature exists at some depth. This water does not have to reach the surface initially, but simply be close enough to connect with it when a fracture opens. Fig. 5. Depth of possible eruption focus due to lithostatic unroofing based on depth of steam generation.
subsurface hydraulic fracturing ŽGrindley and Browne, 1976., or result from local tectonic dilatancy ŽSibson, 1989., or be caused by removal of overburden through erosion, landsliding, draining of a lake ŽMuffler et al., 1971., or lowering of the water table due to dry weather. If subsurface fracturing connects permeable channels between zones of higher and lower pressures, permitting fluid to flow rather than flash, then this may trigger an eruption immediately, or it may delay it until enough fluid accumulates. The best known example of an eruption of this type, with some delay, was in 1959 in Yellowstone, USA. A large earthquake Žthe Hegben Lake earthquake. was followed, within 24 hours, by eruptions of 289 springs as geysers, some of which ejected solid material and so can be classed as hydrothermal eruptions. One hundred and sixty of the springs had no previous record of eruption ŽMarler and White, 1975.. By contrast, one new feature created in that earthquake became a fumarole and then a geyser, which later ejected solid material. This evolution took place over more than 2 years. White et al. Ž1988. also concluded that the episodic and frequently seasonal disturbances of surface thermal activity in Norris Geyser Basin, Yellowstone, including
4.4. Addition of magmatic heat or gas Addition of heat to an active geothermal system could trigger a large eruption, but as discussed above, this should strictly be classed as a magmatic-hydrothermal eruption, not a hydrothermal eruption per se. A more subtle effect might be for an input of magmatic gas to cause boiling and, hence, an eruption. In
Fig. 6. Pressure–depth relations in a hydrothermal system, showing that a decline in water level and a drop in deep pressure can cause shallow pressures to rise due to the formation of a steam zone.
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practice, unless there was some distinctive hydrothermal mineralogy formed in the process it would be very hard to distinguish this event from a AnormalB hydrothermal eruption. If such an event occurred within an exploited system it could be expected to cause observable chemical changes; that such changes have rarely been observed in fluid composition suggests that this mechanism is not the most common. 4.5. ProgressiÕe flashing: the A top-downB model This is the mechanism that we consider is the most common cause of hydrothermal eruptions, especially in fields with low subsurface pressures. All the causes suggested earlier require a fluid near boiling temperature, a sudden lowering of pressure and the formation of steam. McKibbin Ž1989. pointed out that the material is ejected by the lifting power of the steam formed, and he modelled mathematically the resultant changes within a reservoir once an eruption started, and showed that a fluid flashing front descends ahead of the eruption surface. This work was extended by Smith and McKibbin Ž1999., who demonstrated analytically that eruptions could be initiated with a fluid pressure as little as 10% in excess of hydrostatic. They also demonstrated a physical model that confirmed the effectiveness of the process. The same general concepts were advanced by Civetta et al. Ž1974. in discussing the role of fluidisation in forming maars. The gas content of the fluid is not very critical to the mechanism, except for its control on the temperature and thus depth, of initial flashing ŽMcKibbin, 1996., although as Nelson and Giles Ž1985. pointed out, dissolved gas adds to the energy available in a geothermal fluid. We propose here that most natural hydrothermal eruptions start at the ground surface, or very close to it, and penetrate downwards into the reservoir. With this mechanism there is no need for any confining pressure and, indeed, it is possible that an eruption of this type could start from a free water surface, as at the Waimangu AGeyserB. The conditions necessary for an eruption of this type to occur are that boiling water exists at, or close to, the surface and this is underlain by water at a boiling point for depth temperature gradient.
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We think it unlikely that large eruptions begin at great depth within a geothermal reservoir since brecciating and lifting, almost instantaneously, a coneshaped mass of rock above a single focal point deep within a reservoir requires a large amount of energy. This energy would be needed to overcome a combination of the tensile strength Žif any. of the overlying rocks and the high lithostatic pressures they impose. It is energetically much easier for a hydrothermal eruption to begin within a meter or so of the ground surface below a very thin cap ŽFig. 7.. The initial steam burst ejects the covering materials together with entrained water and mud. Because the initial eruptive phase reduces pressures still further within the reservoir, more steam then forms from any residual meteoric or thermal water present. This steam then provides the lift required to brecciate and disperse more rocks. The result is the flashing front and brecciation surface move progressively downward within the reservoir followed by the eruption front. Water present in joints or cracks adjacent to the developing crater also flashes to steam as pressures reduce suddenly, causing the sides of the enlarging vent to brecciate and implode. This may occur more readily where the host rocks are brittle, perhaps through their being silicified. Ductile rocks or sediments are less likely to shatter and brecciate, rather they absorb much of the energy released by the formation of the steam. Most of the erupted material Žrock, mud and water. probably falls back into the crater, to be reerupted more than once. This results in breccia deposits outside and within the crater that are mixed with respect to clast rock type, i.e. the resultant deposits do not show a vertical sequence of clast lithology that is the inverse of the stratigraphic sequence of the reservoir rocks. The very earliest-deposited material, however, must be from nearest the ground surface. The hydrothermal eruption continues until the steam forms too slowly to provide sufficient lifting power to eject rocks from the crater, although some steam may continue to discharge for several years or longer. The maximum depth of disruption within the reservoir, or the apparent focal depth of the eruption, depends on several factors. These include the physical properties of the host rocks, especially the presence of near-vertical permeable pathways, such as
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fractures, the depth of the piezometric surface, the availability of meteoric water, and the amount of energy available from the host rocks, which must also be at temperatures close to boiling. It has long been recognised that thermal equilibrium may not be rapidly achieved in the erupted material Že.g. Mastin, 1995., but the significance of heat transferred from wall rock to fluid in fractures may have been underestimated in the past ŽScott and Watanabe, 1998; Henley and Hughes, 2000.. The hydrothermal eruptions that penetrate to the greatest depths are those which have the most permeable reservoir rocks. This is because they can
provide large amounts of water that can flash to steam to sustain and provide lift for the ejecta. The size of a hydrothermal eruption, therefore, largely depends upon the volume and supply rate of nearboiling water. Where the consequences of a hydrothermal eruption, such as rapid cooling, hydraulic fracturing and brecciation, penetrate to considerable depths, highly permeable conditions extend to the surface and sudden, severe changes to the hydrology of a field may result in, for example, mixing between condensate, meteoric and thermal fluids. Because of the enhanced permeability and steep thermal gradients,
Fig. 7. Ža–f. The development and course of a hydrothermal eruption and postulated change in position of its piezometric surface in a hot water field with a steam cover. Note that brecciation of the host rocks occurs where water changes to steam. This steam provides the energy needed to lift rocks out of the vent. Final sketch is of a situation when the piezometric surface has restored and hydrothermal alteration causes sealing. Scale is not given but may be from 5 to 300 m over the vertical distance shown.
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Fig. 7 Ž continued ..
hydrothermal mineral deposition in the brecciated zone is fast, and sealing of cracks and cementing of
clasts proceeds rapidly. This eventually produces horizons of brittle, coherent rocks that may partici-
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pate in hydrothermal eruptions that occur after the hydrology of the field has been more or less restored ŽFacca and Tonani, 1967.. This has happened at Kawerau, for instance, where a period of 5500 years separated large hydrothermal eruptions from the same site ŽNairn and Wiradiradja, 1980.. When an eruption ceases, the sides of the craters commonly collapse as a result of slumping, or later from acid dissolution of surrounding rocks. Some craters develop lakes Že.g. at Waiotapu., but the craters of hydrothermal eruptions are seldom spectacular in appearance or long lasting. Those older than about 1800 years in the Taupo Volcanic Zone, for example, have little expression and have been recognised almost entirely from the distribution and nature of the deposits they ejected. 4.5.1. Initiating mechanisms The trigger, which initiates a hydrothermal eruption of the Atop-downB type, is uncertain except in cases where overburden is removed. Seismic activity is a likely trigger ŽMarler and White, 1975. but does not always occur. The occurrence of eruption vents aligned in a northeast direction at Rotokawa ŽCollar, 1985. indicates a strong structural control that here parallels the regional fault pattern. Many hydrothermal eruptions are probably initiated by small and subtle events, such as a reduction in atmospheric pressure ŽAllis, 1986. that affect a reservoir filled with water very close to boiling temperatures. 4.5.2. Terminating mechanisms Hydrothermal eruptions that result from local and shallow overpressuring will cease within a few seconds. By contrast, hydrothermal eruptions that occur in the manner described in our model may last for hours or more and their level of intensity declines slowly. In this case, they are analogous to well blow outs whose effects continue for some time and also gradually diminish in their activity. The case of well WK 204 at Wairakei is instructive in this regard ŽThompson, 1976.: following a blow-out, it discharged episodically for 13 years, ejecting much solid material and forming a crater about 15 m across, before the discharge gradually became wetter and spontaneously ceased. The most obvious cause for a cessation of activity is for the supply of hot water to run out or to become
insufficient to generate enough steam to brecciate and lift any more rocks clear of the vent. It is notable that vents at the Craters of the Moon continued to discharge dry steam, but not rocks, after observed eruptions there ended Žour observations.; this is consistent with our model. Eruptions will also end when the vents become flooded with ground water, effectively quenching them as in the case of WK 204 described above. Hydrothermal eruptions originating below a lake, for example, will cease when their vents are flooded with cold lake water. Vents may become blocked as their sides collapse. A slower, but effective, mechanism is one whereby deposition of hydrothermal minerals blocks channels supplying fluid to a vent, in a way analogous to the manner in which geothermal wells can block by mineral deposition, sometimes within a matter of days.
5. Predicting and preventing hydrothermal eruptions 5.1. Predicting eruptions Both shallow and deep focus hydrothermal eruptions are potentially damaging to life and property ŽBixley and Browne, 1988; Bromley and Mongillo, 1994.. They are difficult to predict since there is no limiting pressure or depth below which they will not penetrate. Rather, it is a matter of identifying danger signs and comparing the nature of a reservoir with others where hydrothermal eruptions have occurred. Danger signs could include the following: Evidence of previous hydrothermal eruptions, such as craters and eruption breccias. Where possible these should be dated. Identifying past eruptions as having been phreatomagmatic or magmatic-hydrothermal, rather than strictly hydrothermal, is particularly important in this regard, since hydrothermal eruptions Žsensu stricto. may take place on a more or less regular Žand repeatable. time scale Ždepending on the rate of energy throughput of the convective hydrothermal system and possibly shallow self-sealing.. By contrast, the occurrence of the other types of eruptions is more stochastic within the time scale of human activities ŽLawless, 1988. and these may be triggered by external events, such as an earthquake or sudden unloading, but such occurrence can v
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be expected to be less frequent. For example, dating of the large eruption breccia at Awibengkok as being older than 8400 years ŽStimac and Sugiaman, 2000., together with recognition that it contained clasts with alteration atypical of current shallow reservoir conditions, were factors in deciding that it represented an unusual event, which was unlikely to be repeated within the lifetime of the power project ŽJVL, personal observation.. Evidence for extensive self-sealing, such as the occurrence of silica aprons, or other near-surface impermeable formations, such as clay-rich lacustrine sediments. Liquid-dominated systems with pressure profiles that equal, or exceed, boiling temperatures for depth from the surface downwards. Superheated steam emissions, or shallow steam and gas accumulations. However, to monitor these comprehensively would require very many wells with a range of depths at each location. In practice, a compromise would probably be reached by specifying a finite number of monitoring places, having regard to what is known of the near-surface hydrology. Shallow well AkicksB during drilling. Reservoir fluids with high gas contents, say over 1 wt.%. Changes in thermal activity following exploitation, especially falling water levels or drying up of previously boiling springs, higher chemical geothermometer temperatures Že.g. Fournier et al., 1991., or a change in water composition from neutral chloride to acid sulphate. Evidence of fluctuating groundwater levels. Signs of slope instability in thermal areas that could lead to sudden removal of overburden. The general nature of reservoir response to exploitation can be predicted by numerical simulation modelling, but this will not make unambiguous predictions of the occurrence of individual eruptions or their locations. v
v
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v
v
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have been from sites of previous thermal activity, albeit apparently extinct. In the sole case that we know of where this was not so, namely the third recorded eruption at Tiwi Ž1982., its site lay along the strike of a fault known from previous thermal activity and eruptions. A simple precaution for avoiding effects from future eruptions, therefore, is to not site any facilities on, or close to, areas of present or past thermal activity. In view of the small size of eruptions observed in exploited fields, a safety zone of 200 m should be adequate. If disturbing sites of present or past thermal activity is unavoidable, then it is better not to excavate the ground surface, so as not to lower overburden pressure and thereby initiate an eruption, as happened at Nakano-yu ŽYuhara, 1997.. If ground disturbance is essential, then using gravel pack fill will allow easy escape for steam. This has been done successfully at geothermal projects in Indonesia and the Philippines. Installing large areas of asphalt or concrete slabs should not be done. It is noteworthy that there are three high-temperature, liquid-dominated geothermal fields in the world, which have been exploited on a large scale with only partial, or no, reinjection of spent thermal fluid, namely Wairakei, Tiwi and Ahuachapan. All have suffered large pressure declines within the liquid zone of their reservoirs Ž) 10 bars.. It is probably not coincidental that post-exploitation hydrothermal eruptions have occurred at all three. Reinjection of spent thermal fluid is now being practised at all of them.
v
v
5.2. AÕoiding eruptions It is noteworthy that almost all known historic hydrothermal eruptions, in both undisturbed and exploited fields Žexcept those with a clear magmatic character, such as the activity at Waimangu that followed the Tarawera volcanic eruption in 1886.,
5.3. PreÕenting eruptions If shallow monitor wells start to record dangerously high steam pressures, then it may be possible to vent the steam. To the best of our knowledge, however, this has not yet been done anywhere on a regular or systematic basis. An alternative approach is to inject cool or cold water to prevent steam from accumulating. A scheme for selective reinjection of separated water and condensate in this way was recently proposed for a geothermal development at Tauhara, New Zealand ŽMercury Geotherm, 1996; Menzies and Lawless, 2000..
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It may be possible to prevent eruptions by flooding the site to raise the hydrostatic pressures. This method was used in an unsuccessful attempt to stop the eruption of well WK 204 at Wairakei ŽThompson, 1976.. It was also done at Tokaanu, New Zealand, where the tailrace from a hydroelectric power station comprises an artificial pond located and designed to suppress eruptions from a thermal area there. 6. Discussion and concluding remarks Hydrothermal eruptions do not all require a fieldwide cap or cover rock, which allows reservoir pressures to exceed those of lithostatic or the tensile strength of this rock Žthough they can occur this way.. Rather, they most commonly occur in waterdominated reservoirs with water nearly at its boiling temperature, so that any local depressurisation permits it to boil. In both cases, water flashes to steam and a flashing front descends into the reservoir. Energy for boiling comes principally from the fluid, although heating by wallrocks may also contribute. The large specific volume change of water when it boils provides the mechanism for the steam, thus, generated to brecciate and eject the reservoir rocks so that a zone of rock brecciation accompanies the descending flashing front. Both precede penetration of the eruption surface itself. Eruptions may continue for as long as several hours to days, ceasing only when steam is produced at a rate insufficient to lift any more rock fragments. A large proportion of the erupted material falls back into the vent, where it is commonly later cemented and altered by steam andror hot water. Where an eruption crater erodes its former presence may be recognised by the occurrence of altered clasts within a usually circular zone of breccia deposits. Acknowledgements We thank Arnold Watson, Ian Bogie, Phil White, Brian Barnett, Tony Cartwright, Colin Harvey and Stuart Simmons for their helpful suggestions and comments. Material on the 1999 eruptions at Rotorua was kindly provided by Peter Barnett. Trevor Hunt provided much useful information including a partial
translation of the paper by Yuhara Ž1997., made by Toshi Tosha of the Geological Survey of Japan. The paper benefited substantially from reviews by two anonymous referees for AEarth Science ReviewsB. Appendix A. Descriptions of selected hydrothermal eruptions A.1. Selected examples of historic hydrothermal eruptions For convenience, we consider separately hydrothermal eruptions that have occurred as part of the ‘natural’ evolution of a geothermal system and those that clearly result from its exploitation. However, there is no evidence that there is any difference between these except in their magnitudes, as discussed below. Excluded from discussion are phreatomagmatic eruptions, such as those that occurred at Crater Lake, Ruapehu ŽNairn et al., 1979., and at Rotomahana– Waimangu in 1886 ŽLloyd and Keam, 1965; Nairn, 1979.. Similarly, the eruption at Suoh, South Sumatra in 1933 ŽStehn, 1934., has previously been quoted as an example of a very large hydrothermal eruption, but recent petrology has shown this to be of magmatic-hydrothermal origin ŽKingston Morrison, unpublished data 1994.. We select only eruptions that have been comparatively well documented, which are known to us from personal observations and have not been reported previously, or which have particularly informative characteristics. Other examples are listed in Table 1. The data in Table 1 are as presented in the original sources: no attempt has been made to assess the accuracy of parameters estimated by others. Units have, however, been converted for consistency. In addition, where the data permit, we have provided estimates of ejecta area, volume and eruptive energy. Our estimates are given in italics. Where no better data are available, to estimate areas we have assumed that the ejecta has an elliptical distribution with an obliquity of 0.7. Ejecta volumes have been calculated by assuming that the thickness drops off exponentially from the vent ŽWohletz and Heiken, 1992, Eqs. 2–7., and that the half-thickness occurs at one-fifth of the maximum radius. Analysis of the 11 examples where sufficient isopach data are available
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to determine the ratio of half-thickness radius to total radius gives a range from 2.1 to 26 Žthough with only one estimate of over 10, namely Tauhara 1981 ŽScott and Cody, 1982., which was a lateral blast and also may be unduly influenced by minor air-fall material. with a mode of 3.5 and a mean of 6, but the better-documented examples tend to support the larger ratios. Where the crater size is large compared to the radius of the ejecta, this method may tend to overestimate the volume of the ejecta. Where there are multiple events at the same location, it is the area and volume for the largest single identifiable unit that is given, not the total, since it is the size of individual eruptions that is of the most interest in this context. For estimates of eruptive energy required, we have simply used a factor of 10 Jrcm3 of ejecta. These estimates are admittedly crude, but they do give a consistent basis of comparison and the completeness of the data in most cases does not justify any more detailed analysis. A.1.1. Historical ‘natural’ hydrothermal eruptions in undisturbed fields In areas that are frequently visited by tourists, numerous small hydrothermal eruptions have been observed. Eruptions of the same magnitude in more remote areas would scarcely be noticed, as their products are quickly eroded or else covered by vegetation. For example, a small hydrothermal eruption occurred a few hundred meters away from the main tourist route at Waimangu in 1981, but this event went unrecognised until several months afterwards ŽR F. Keam, personal communication.. Very small hydrothermal eruptions have occurred at Orakeikorako, where a silica sinter terrace ŽAArtist’s PaletteB . has been locally cracked or broken from below to form broken blister forms or linear features, resembling pressure ridges, that extend laterally for only a meter or so. Larger hydrothermal eruptions occurred at Waimangu, New Zealand ŽFig. 8. in 1915, 1917, August 1924, February 22 1973, several in the period June 1978– November 1979, 2–11 May and possibly late in 1981. Other AdisturbancesB in 1975 and 1978, which disrupted the usual activity, were not large enough to be unequivocally described as eruptions. The largest eruption, in 1917, caused two fatalities as a result of a laterally directed blast. The
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Waimangu ‘Geyser’, which erupted between 1901 and 1904, also showed many features characteristic of hydrothermal eruptions ŽWarbrick, 1934, Lloyd and Keam, 1974.. It is the only known historical example of a hydrothermal eruption with a repetitive, short-period cycle of about 36 h, although there are other examples of repeat eruptions at much longer and irregular intervals from the same site, e.g. Karapiti, Rotorua, and repeated eruptions interspersed with geysering at Steamboat Geyser, Yellowstone ŽWhite et al., 1988.. The hydrothermal eruption of February 22, 1973 is the best studied one at Waimangu, although not the largest. It occurred through a hot Žabout 608C. lake in early morning darkness and lasted for at least 15 min. People sleeping 850 m away were not awakened, so it must have been a quiet event ŽLloyd and Keam, 1974.. Ejecta fell over an area of approximately 3000 m2 to a maximum thickness of 0.45 m. Undoubtedly most of the ejecta fell back into the lake but much vegetation was scalded, stripped of foliage, or killed by hot
Fig. 8. Location map of Taupo Volcanic Zone showing geothermal systems where hydrothermal eruptions have occurred and other places mentioned in the text.
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water, steam, mud, stones and blocks. Lloyd and Keam Ž1974. calculated the erupted volume as being 970 " 150 m3. The ejected material consisted of a mixture of hydrothermally altered angular blocks, up to 0.3 m diameter, some hotter than 1008C when they landed; other ejecta fell as wet mud, probably at the lake temperature. Some material was ejected to heights of at least 40 m. Lloyd and Keam Ž1974. concluded that the depth of rock disruption extended to some 90 m below the lake bottom, which itself is not more than 30 m deep. The lake increased in area by 285 " 15 m2 as a result of the eruption and subsequent slumping. Precursors to the eruption were few and feeble. About 5 weeks before it occurred, two small springs appeared at the then lake edge, and boiling muddy water splashed up to 0.5 m from them; nearby, 50-mm-diameter stones were dislodged from enlarging vents. Other new springs appeared, some hot Ž99.58C. and turbulent, but some thermal activity on-land ceased, being replaced by that offshore, where gas discharges increased. Changes in thermal activity in this young, active geothermal field are common, and it is only with hindsight that these events were recognised as being possible precursors to the February 22nd eruption. Lloyd and Keam Ž1974. believe that the eruption occurred after a fluid flow path with higher permeability was created in rocks below the lake. A small hydrothermal eruption occurred at the Ohineraki thermal area in the northern part of the Mokai ŽNew Zealand. field in 1996 ŽSam Andrews, personal communication; Kingston Morrison, 1998.. It changed the shape of the main crater but ejecta were no longer recognisable in mid-1997. A hydrothermal eruption at Ngatamariki ŽNew Zealand. occurred in 1948 and produced a crater about 10 m in diameter ŽBrotheridge et al., 1995.. This eruption was not seen but its deposits partly surround the crater. Large eruptions occurred from three adjacent craters at the Dieng field, Java, in 1979 ŽLe Guern et al., 1982.. These occurred prior to power production from the field and resulted in large loss of life Ž142 persons., which was, however, caused by the accompanying gas release rather than the impact of ejecta or thermal effects. Most people died more than a kilometre from the craters. The eruption continued
for slightly less than one day and it is likely that the gas flowed downslope from the craters. The largest crater also emitted a lahar that flowed further than 3 km. Several earthquakes occurred between 1 and 3 h before the eruption. A.1.2. Hydrothermal eruptions that haÕe occurred in exploited fields Hydrothermal eruptions are very frequent in geothermal fields undergoing exploitation. These have been mainly very small events, lasting up to a few hours and produce craters up to 50 m across ŽTable 1.. A key question in this case is whether exploitation has in some way caused the eruptions Žsince that offers clues to understanding their mechanisms and whether mitigation is feasible., or whether they are AnaturalB events, which were reported because they occur in much visited areas. Some of the observed eruptions described below, such as those at Rotorua and Tongonan, cannot be directly linked to exploitation and their occurrence during a period of exploitation appears to be coincidental. In other cases, such as at Wairakei, there is a clear link between the eruptions and exploitation-induced pressure changes in the reservoir, or modification of the overburden through landslides or excavation. Hydrothermal eruptions have occurred at both Wairakei and Tauhara ŽFig. 8. ŽAllis, 1981, 1983. and these events were undoubtedly a consequence of the exploitation of these hydrologically linked systems. Two eruptions occurred in the Tauhara field at the Taupo Pony Club, in 1974 and 1981, 7 km from the Wairakei Power Station, and products ejected by the latter have been described in detail by Scott and Cody Ž1982.. Allis Ž1979, 1984, 1986. gives detailed accounts of the Craters of the Moon ŽKarapiti. area, within the Wairakei field, where at least 20 hydrothermal eruptions have taken place in the past 20 years. The most recent events were in March 2000 ŽC.J. Bromley, personal communication.. The craters produced are only about 2–20 m in diameter and depth, but ejected mud and pumice rose to heights of about 30 m. Eruptions in exploited fields are of short duration Žhours at longest. and of shallow focus Ža few meters.. Ejecta travels only 100 m or so from their vents, although a blast may be directed laterally; for example, the eruption in 1981 at the Taupo Pony
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Club ejected material which has an asymmetric distribution around its vent. The deposit consisted of silts, muds, and pumice blocks up to 40 cm in diameter ŽScott and Cody, 1982.. Numerous small hydrothermal eruptions have occurred at the Rotorua geothermal field. The main events are documented by Donaldson Ž1985. as occurring in 1894, 1895Ž2., 1897, 1901, 1902, 1906, 1931, 1932, and 1945. Others have occurred in 1998–2000. They are included in this section as the Rotorua field has been exploited by numerous shallow wells, but many of the eruptions took place before significant exploitation began, implying exploitation was not the cause of these eruptions. Some interesting features about the eruptions at Rotorua are as follows: Ž1. Numerous eruptions have occurred from the same sites Žespecially at the mouth of the Puarenga Stream.. Ž2. Several eruptions Žalthough not well-documented. occurred as part of a general upsurge in thermal activity in the few weeks after the brief 1886 volcanic eruption at Mt. Tarawera ŽKeam, 1988.. The volcanic activity at Mt Tarawera was clearly related to emplacement of a basaltic dyke along the NE–SW structural trend of the Taupo Volcanic Zone ŽNairn, 1979 .. Concurrently, large phreatic, phreatomagmatic and magmatic-hydrothermal eruptions occurred along the strike to the SW at Rotomahana and Waimangu. However, Rotorua is too far away Ž25 km., and in a direction normal to the structural trend for this same dyke to affect thermal activity at Rotorua. Possibly a discrete en-echelon dyke was injected at Rotorua or, more probably, there was simultaneous structural dilatancy at both sites, which permitted dyke injection at Tarawera and a hydrological change permitting increased fluid flow at Rotorua. R.F. Keam Ž1998, personal communication. suggested an interesting alternative explanation, namely that slumping of lake-shore sediments at Lake Rotorua, caused by several small felt earthquakes preceding the eruption at Tarawera, either reduced the overburden or increased near-surface permeability within the hydrothermal system, thus stimulating the activity. Ž3. The series of eruptions in 1998–2000 were small and did not cause any loss of human life, but they did render some houses uninhabitable. They are
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the only examples known of eruptions occurring as a results of cessation of exploitation. The water level in the Rotorua geothermal system was drawn-down by some 7 m due to exploitation by 1987, causing a drastic reduction in natural thermal activity, including the geysers, which are a major tourist attraction. In that year, a large number of geothermal bores were compulsorily closed, and fluid withdrawal since then has been strictly regulated. This has had the effect of allowing reservoir pressures to recover by about half of the previous draw-down, and the geysers are now very active again, but it also led to hydrothermal eruptions near Kuirau Park, damaging houses, which had been built close to the thermal areas during the period of inactivity. Three hydrothermal eruptions occurred at Tiwi ŽPhilippines. in 1980–1981 ŽGrindley, 1982. following its large-scale exploitation, which began in 1977, without reinjection, caused a large drop in liquid-zone pressures. This situation was closely analogous to the Karapiti eruptions at Wairakei. One difference, however, was that at Tiwi the first eruption took place at an area ŽNaglagbong. of geysering alkali-chloride springs, which ceased to discharge and became fumarolic some weeks before the eruption. At Karapiti, the site of the eruption was always fumarolic. The significance of the Tiwi events is that the presence of alkali-chloride springs at the site of the eruptions shows that there was very little hydraulic resistance to the upflow of primary thermal fluids there prior to exploitation. Hence, there was very little physical containment of steam prior to the eruption. Three small hydrothermal eruptions occurred over a period of 6 weeks at the same site in the Bao Valley, Tongonan geothermal field, Leyte, Philippines, in August 1981 ŽJVL, personal observation; PNOC, 1982.. They left a crater about 10 m across blocking a project road, and ejecting mud to a height of 10–20 m and a radius of 20–30 m. This occurred shortly before the Tongonan power station was commissioned and there had not been any drilling or well testing within 2 km of the eruption site for several years. Hence, it is unlikely that exploitation had any relation to these eruptions. The eruptions occurred after unusually dry weather, a factor that Allis Ž1984. considered also provoked the eruptions at Karapiti. An eruption occurred near the Nakano-yu hot springs, Japan, in 1995 ŽYuhara, 1997.. There were
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four fatalities. It is not clear if the victims were buried by eruption debris per se, or by an accompanying landslide. The eruption occurred during excavation for a road, at the bottom of a 4-m-deep trench. This must be classed as an eruption due to human disturbance, presumably due to overburden removal, although not from geothermal energy exploitation. Sulphur dioxide gas Žalong with H 2 S. was detected at the site three days after the explosion, suggesting a direct magmatic volatile component was involved in the event ŽMiyake and Ossaka, 1998.. Yuhara Ž1995. refers to other historic eruptions at Hakone, Noboribetsu, Unzen, Kirishima and Aso in Japan, but insufficient information is available to justify including these in Table 1. A landslide causing damage and 23 fatalities occurred at the Zunil geothermal field in Guatemala in 1991 ŽGoff and Goff, 1997.. This caused a steam eruption due to damage of a geothermal wellhead, in an area of thermal activity. Probably, however, the cause of the landslide was slope failure of hydrothermally altered rocks rather than a hydrothermal eruption, although it has often been cited as an exploitation induced eruption. A hydrothermal eruption from the Agua Shuca thermal area at the Ahuachapan geothermal field, El Salvador in 1990 ŽBruno et al., 1992, Goff and Goff, 1997., caused 25 fatalities. Historical accounts show that there had been considerable variability in the thermal activity there and hydrothermal eruptions occurred at the same place in about 1868 and at another thermal area at Ahuachapan at an earlier but unknown date. It is possible, therefore, that the Agua Shuca eruption was a purely natural phenomenon. However, the Ahuachapan geothermal field had been under production, with only minor reinjection, since 1975, which had caused a pressure drop within the reservoir of 12 bars by 1984 ŽVides-Ramos, 1985., and this probably contributed to the cause. A.2. Examples of some prehistoric hydrothermal eruptions Inevitably only larger prehistoric hydrothermal eruption can be recognised. There have been many large, prehistoric eruptions at several New Zealand fields ŽFig. 8.. At Kawerau, 14,500 and 9000 years ago ŽNairn and Wiradiradja, 1980.; at Whakare-
warewa ŽRotorua., about 42,000 years ago ŽLloyd, 1975.; at Te Kopia where there have been at least 2 ŽBrowne et al., 1994.; on at least four occasions at Orakeikorako during an 8000-year period ŽLloyd, 1972.. Eruptions also occurred at Waiotapu, more than 13,450 years ago, and at least five, probably more violent, ones in the past 1800 years ŽLloyd, 1959; Hedenquist and Henley, 1985.. At Upper Atiamuri there were eruptions about 14,000 years ago ŽBrowne and Lloyd, 1986.; at Tikitere ŽEspanola, 1974.; and at Wairakei ŽBogie et al., 1995.. Some of these may have been phreatic or magmatic-hydrothermal rather than strictly hydrothermal, however. The prehistoric eruptions are recognised mainly from the deposits they ejected, and there have undoubtedly been many eruptions of which no geological record survives, or whose deposits have yet to be recognised. In many places Že.g. Rotokawa., the craters have been completely filled, and can only be located by mapping the distribution of ejecta. Others are of very subdued morphology, for example, a large crater at the Te Kopia field is best seen on air photos. It is also likely that there have been hydrothermal eruptions at the Ngawha field in Northland ŽSkinner, 1981; Sinclair Knight Merz, 2000.. There are six large craters there, up to 300 m in diameter, plus several smaller ones. Mining for mercury and kauri gum, and slumping of the walls have modified some. A notable feature of the Ngawha craters is that all of the large ones are about the same size, are in a generally similar state of preservation Žhaving regard to local differences in erosion rate due to hydrology, or human modification., and they do not overlap. It is probable, therefore, that all of these craters are about the same age and may have formed in a single episode in response to pressure release following the rapid draining of an extensive lake Ž2 = 5 km., possibly as recently as the Holocene. This once covered the area and deposited lacustrine sediments up to 10 m thick ŽDavey and van Moort, 1974; Fleming, 1945.. At least eight, and probably as many as thirteen, large hydrothermal eruptions have occurred in the Rotokawa area ŽFig. 8.. Probably seven took place between 11,000 and 22,700 years ago; three between 9000 and 9700 years ago; at least one took place 6060 " 60 years ago; one between 4000 and 4500 years ago, and one about 3700 years ago ŽCollar,
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1985; Collar and Browne, 1985.. It is also possible, but not proven, that there were some hydrothermal eruptions prior to 22,600 years ago. There is no evidence, however, that any large hydrothermal eruptions have occurred there in the last 1,800, and possibly the last 3400s years. The eruption events at Rotokawa were dated because their deposits are interbedded with rhyolitic tephras of regional extent ŽFig. 9. whose ages are known from 14 C dating ŽWilson and Walker, 1985.. In addition a tree buried by a hydrothermal eruption at Rotokawa was 14 C
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dated as being 6060 q 60 years old. Collar Ž1985. located the vents for the hydrothermal eruptions by measuring the sizes of clasts present in their deposits; the largest clasts occur closest to vents. This is a better guide to deducing vent positions than isopach mapping since eruption breccias are readily, but unevenly, eroded and their present day thickness is not the same as their initial thickness. Breccias are invariably poorly sorted, matrix-supported, and consist of angular to subangular fragments of various types of tuffs, rhyolite lava, and mudstone present in
Fig. 9. Composite stratigraphic section at Rotokawa Žfrom Collar, 1985; Collar and Browne, 1985.. Tephra, ignimbrite and lapilli units are of regional distribution.
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a fine-grained matrix. The 6060-year-old breccia at Rotokawa also contains logs of wood and occasional lumps of sulphur. Breccia deposits are typically massive; some contain clasts that have their longest axes vertical, showing that they impacted into soft mud; some large blocks following ballistic trajectories deformed underlying tephra on impact ŽCollar, 1985.. Many ejected clasts had been hydrothermally altered and hydraulically brecciated prior to eruption, showing that they had once been AstewingB within the reservoir. No juvenile magmatic material has been recognised in the Rotokawa deposits, indicating they are not of phreatomagmatic origin; but since the country rocks are themselves young pyroclastics, it is not certain that such material would be easily recognised. The very large volume and deep focus of these eruptions suggests they could be of phreatic or magmatic-hydrothermal origin. There is no direct evidence for this, but this would be consistent with certain features of the hydrothermal system suggesting Rotokawa has a closer magmatic affinity than similar systems in the Taupo Volcanic Zone. These features are: Ž1. the youthful nature of the system, as suggested by the persistence of low temperature Ždisequilibrium. mineral assemblages within its hotter parts; Ž2. spring waters with the highest Cl contents of any hydrothermal system in the TVZ apart from Tokaanu; and Ž3. a near-surface sulphur deposit, which is larger than the current flux of H 2 S through the system could possibly deposit in the time available ŽKrupp and Seward, 1987; KRTA, 1986.. Deposits from the eruptions cover about 12 km2 , and together they locally total 40 m in thickness. The 6060-year-old eruption ŽFig. 2. vented from the lake and ejected a mass of country rock, mud, sulphur, water and steam, which slurped out of the Rotokawa Basin into the Waikato River. This must have been a very destructive event, causing major flooding for a long distance down the river. The other hydrothermal eruptions produced deposits of more local distribution, but these also undoubtedly affected the Waikato River. The very much smaller Ngatamariki eruption in 1948, for example, reportedly discoloured the Waikato River for several days ŽR.F. Keam, personal communication.. The craters formed by the hydrothermal eruptions at Rotokawa are deduced to have been located along
a northeast-oriented structural feature extending from within the lake itself to a position about 1.5 km distant ŽFig. 2.. However, these craters, which may have been up to 250 m in diameter ŽCollar, 1985., are now completely filled with young tephra, notably Taupo Ignimbrite, which deposited in the second century AD ŽWilson and Walker, 1985.. The existence, magnitude and nature of these hydrothermal eruptions are now only evident from study of the deposits that they produced. It is, therefore, possible that there could have been several very much smaller hydrothermal eruptions of which no record is now visible. References Allis, R.G., 1979. Thermal history of the Karapiti area, Wairakei. N. Z. Dep. Sci. Ind. Res., Geophys. Div. Rep. 137. Allis, R.G., 1981. Changes in heat flow associated with exploitation of Wairakei geothermal field, New Zealand. N. Z. J. Geol. Geophys. 24, 1–19. Allis, R.G., 1983. Hydrologic changes at Tauhara geothermal field, New Zealand. N. Z. Dep. Sci. Ind. Res., Geophys. Div. Rep. 193. Allis, R.G., 1984. The 9 April 1983 steam eruption of Craters of the Moon thermal area, Wairakei. N. Z. Dep. Sci. Ind. Res., Geophys. Div. Rep. 196. Allis, R.G., 1986. Physical effects of exploitation of Wairakei Geothermal Field. Tour Guide, TVZ. N. Z. Geol. Surv. Rec. 11, 166–169. Bixley, P.R., Browne, P.R.L., 1988. Hydrothermal eruption potential in geothermal development. Proc. 10th N. Z. Geotherm. Workshop, pp. 195–198. Bogie, I., Lawless, J.V., 1987. Controls on the hydrology of large volcanically hosted geothermal systems: implications for exploration for epithermal mineral deposits. Proc. 1st PACRIM Congr., pp. 57–60. Bogie, I., Lawless, J.V., Pornuevo, J.B., 1987. Kaipohan: an apparently non-thermal manifestation of hydrothermal systems in the Philippines. J. Volcanol. Geotherm. Res. 31, 281–292. Bogie, I., Lawless, J.V., MacKenzie, K.M., 1995. Geological results from drilling in the Poihipi Žwestern. sector of the Wairakei geothermal field, New Zealand. Proc. 17th N. Z. Geotherm. Workshop, pp. 55–60. Bogie, I., Lawless, J.V., 2000. Application of mineral deposit concepts to geothermal exploration. Proc. World Geotherm. Congr. Žin press.. Bromley, C.J., Mongillo, M.A., 1994. Hydrothermal eruptions — a hazard assessment. Proc. 16th N. Z. Geotherm. Workshop, pp. 45–50. Brotheridge, J.M., Browne, P.R.L., Hochstein, M.P., 1995. The Ngatamariki geothermal field, New Zealand: surface manifestations past and present. Proc. 17th N. Z. Geotherm. Workshop, pp. 61–66.
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Mastin, L.G., 1991. The roles of magma and groundwater in the phreatic eruptions at Inyo Craters, Long Valley Caldera, California. Bull. Volcanol. 57 Ž2., 85–98. Mastin, L.G., 1995. Thermodynamics of gas and steam-blast eruptions. Bull. Volcanol. 57, 85–98. Menzies, A.J., Lawless, J.V., 2000. Two-dimensional modelling of the Tauhara geothermal field. GRC 24, 611–616. Mercury Geotherm, 1996. Assessment of Environmental Effects, Tauhara Geothermal Project. Unpublished Report to Environment Waikato in support of Resource Consent Application. Miyake, Y., Ossaka, J., 1998. Steam Explosion of February 11th, 1995 at Nakanoyu hot spring, Nagano Prefecture, central Japan. Bull. Volcanol. Soc. Jpn. 43 Ž3., 113–122 Žin Japanese.. Morgan, P.G., 1917. Eruption of Frying Pan Flat, near Waimangu, Rotorua District. N. Z. Geol. Surv. 11th Annu. Rep. C-2B, 11–12. Moyle, A.J., Doyle, B.J., Hoogvliet, H., Ware, A.R., 1990. Ladolam gold deposit, Lihir Island. AusIMM Monogr. 14, 1793–1806. Muffler, L.J.P., White, D.E., Truesdell, A.H., 1971. Hydrothermal explosion craters in Yellowstone National Park. Geol. Soc. Am. Bull. 82, 723–740. Murphy, B.F., Vortman, L.J., 1961. High-explosive craters in desert alluvium, tuff and basalt. J. Geophys. Res. 66, 3389– 3404. Nairn, I.A., 1979. Rotomahana–Waimangu, 1886: base surge and basalt magma. N. Z. J. Geol. Geophys. 22, 363–378. Nairn, I.A., Wiradiradja, S., 1980. Late Quaternary hydrothermal explosion breccias of Kawerau geothermal field, New Zealand. Bull. Volcanol. 43, 1–13. Nairn, I.A., Wood, C.P., Hewson, C.A.Y., 1979. Phreatic eruptions of Ruapehu, April 1975. N. Z. J. Geol. Geophys. 22, 155–173. Nelson, C.E., Giles, D.L., 1985. Hydrothermal eruption mechanisms and hot spring gold deposits. Econ. Geol. 89, 1633– 1639. Pike, R.J., 1974. Crater on Earth, Moon and Mars: multivariate classification and mode of origin. Earth Planet Sci. Lett. 22, 245–255. PNOC, 1982. Bao activity intensifies. Geosci. Newslett. Philippines National Oil Company — Energy Development Corporation II Ž2., 1–2. Rae, A.J.,1991. The petrology and mineralogy of the 1917 hydrothermal eruption breccia, Waimangu. Geothermal Institute Diploma Report, 91.15, 53 pp. Robson, R.N., Stevens, M.R., 1991. An occurrence of hydrothermal eruption breccia, Onemana, Coromandel Peninsula, New Zealand. Proc. 25th Annu. Conf. N. Z. Branch AusIMM, 9 pp. Rogers, G.F.C., Mayhew, Y.R., 1980. Thermodyanamic and Transport Properties of Fluids. 3rd edn. Blackwell, UK, 24 pp. Rutherford, P.G., Fransen, P.J.B., 1991. Timing of epithermal activity in the Horohoro–Matahana basin area, Taupo Volcanic Zone. Proc. 25th Annu. Conf., N. Z. Branch AusIMM, 270–278. Scott, B.J., Cody, A.D., 1982. The 20 June 1981 hydrothermal explosion at Tauhara geothermal field, Taupo. N. Z. Geol. Surv. Rep. 103.
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Characteristics of hydrothermal eruptions, with examples from New Zealand and elsewhere P.R.L. Browne a,) , J.V. Lawless b a
Geothermal Institute and Geology Department, UniÕersity of Auckland, PriÕate Bag 92019, 23 Symonds Street, LeÕel 2, Auckland, New Zealand b Sinclair Knight Merz Limited, PO Box 9806, Auckland, New Zealand Received 6 November 1998; accepted 17 August 2000
Abstract Hydrothermal eruptions have occurred in many hot water geothermal fields. This paper concentrates on examples from New Zealand but also mentions others elsewhere, which demonstrate points of particular interest. Numerous small eruptions Žmaximum focal depths of about 90 m. have occurred in historic times Žpast 150 years. at WairakeirTauhara, Rotorua, Tikitere, Ngatamariki, Mokai and Waimangu. The presence of breccia deposits shows that much larger Žwith estimated maximum focal depths of about 450 m., prehistoric hydrothermal eruptions have also occurred at Kawerau, Wairakei, Tikitere, Orakeikorako, Te Kopia, Rotokawa and Waiotapu. One of the largest known hydrothermal eruptions in New Zealand took place at Rotokawa 6060 " 60 years ago; this produced a deposit that extended over an area with a diameter of 4 km, and has a maximum thickness of 11 m. Deposits from hydrothermal eruptions are typically very poorly sorted, matrix-supported, and may contain hydrothermally altered clasts that derive from within the geothermal reservoir. Their lithologies and alteration mineralogies are useful guides to subsurface conditions. Hydrothermal eruptions do not require any direct input of either mass or energy derived directly from a magma and, thus, differ from both phreatic and phreatomagmatic eruptions. Many hydrothermal eruptions in a hot water field start very close to the ground surface and result from the rapid formation of steam due to a sudden pressure reduction. This steam provides the energy necessary to brecciate, lift and eject fragments of the host rocks as a flashing front descends and water nearby in the reservoir boils. A rock brecciation zone accompanies this front, and both precede the descent of the eruption surface. A hydrothermal eruption continues until the steam is produced too slowly to lift the brecciated rocks. There is no genetic difference between the small eruptions induced by exploitation and those which occur as a geothermal system evolves naturally and whose effects may penetrate to much greater depths. Hydrothermal eruptions do not need the presence of either field-wide cap rocks or pressures within a reservoir that exceed that provided by a hydrostatic column of water very close to its boiling temperature. q 2001 Elsevier Science B.V. All rights reserved. Keywords: geothermal; hydrothermal eruption; phreatic; epithermal; geohazard
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Corresponding author. Tel.: q64-9-373-7599; fax: q64-9-373-7436. E-mail address: [email protected] ŽP.R.L. Browne..
0012-8252r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 5 2 Ž 0 0 . 0 0 0 3 0 - 1
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1. Introduction Hydrothermal eruptions have been common events in many geothermal systems in New Zealand and elsewhere ŽTable 1.. There have been numerous eruptions in historic times. Some of these have been induced by exploitation, such as at WairakeirTauhara in New Zealand and at Tiwi in the Philippines, but others have occurred during the systems’ natural evolution. Indeed, hydrothermal eruptions are typical manifestations of active geothermal fields. Prehistoric hydrothermal eruptions are largely recognised from the presence of eruption breccias. However, evidence for hydrothermal eruptions is often overlooked because their deposits are not recognised, are poorly preserved or else buried under younger materials. Nevertheless, hydrothermal eruptions and their products deserve to be studied for the following reasons: Ža. They are destructive events that can cause loss of life and damage or destroy structures, such as geothermal plant and pipelines. Examples are given below. Mapping and dating of hydrothermal eruption deposits will obviously assist in locating the sites of prehistoric events and in determining eruption frequency. This information will assist in avoiding potentially dangerous sites when locating pipelines and power plants and also in planning reinjection strategies. The same applies to mining epithermal gold deposits that occur in active geothermal areas, such as the Ladolam deposit on Lihir Island, Papua New Guinea ŽMoyle et al., 1990.. Žb. The material ejected by a hydrothermal eruption provides clues about the lithology of the geothermal reservoir from which it derives. Žc. The presence of specific hydrothermal minerals in ejecta may reveal information about the subsurface temperatures and fluid types present, while fluid inclusion geothermometry may be used to estimate these temperatures more precisely as well as to determine fluid salinities. Žd. Hydrothermal eruptions may cause major changes to the hydrology of a field. Že. Hydrothermal eruption events can be intimately related to mineralisation Že.g. Sillitoe, 1985; Sillitoe et al., 1984; Hedenquist and Henley, 1985; Nelson and Giles, 1985; Bogie and Lawless, 1987; Robson and Stevens, 1991; Rutherford and Fransen,
1991.. The resulting breccias may also later become mineralised because of their high permeability ŽEbert and Rye, 1997., offering attractive targets for epithermal mineral exploration.
2. Nomenclature Terminology has not always been consistently used in the literature. The terms used in this paper are defined as follows: Geyser: An eruption of hot water, steam and nonaqueous gas from a hydrothermal system, usually but not always of cyclic occurrence, which ejects at most only trivial amounts of solid material. The ejection mechanism is volume change due to boiling with or without significant effervescence of gas, as opposed to ejection of water because of artesian pressure alone. Hydrothermal eruption: An eruption ejecting at least some solid material and whose energy derives solely from heat loss and phase changes in a convecting hot water or steam-dominated hydrothermal system. Phreatic eruption: An eruption which is caused by heating and flashing of water produced when magma comes into contact with water but only country rock or overburden is ejected Ži.e. no juvenile magmatic material.. If there is unequivocal evidence for the ejection of any juvenile igneous material, it should be classed as a phreatomagmatic eruption. Eruptions of these classes include those where lava contacts water, as in littoral explosions when lava flows into the sea, subglacial eruptions, or when water is introduced after an eruption, as in some Arootless fumarolesB when rainwater percolates into hot pyroclastic deposits Žsee Mastin, 1995 for a compilation.. One of us ŽJVL. witnessed eruptions of this type following the 1984 volcanic eruptions of Mt. Mayon, Philippines, whereby heavy rainfall gullying into and contacting andesitic pyroclastics some 6 weeks after their deposition, caused eruptions, which ejected clasts, up to 3 m in diameter, to heights of several hundred metres. An interesting example of distinguishing phreatic from phreatomagmatic and hydrothermal Žsensu stricto. eruptions is provided by Sigvaldason Ž1992.. To distinguish these from the following class of eruption, we limit phreatic
P.R.L. Browne, J.V. Lawlessr Earth-Science ReÕiews 52 (2001) 299–331
eruptions to those where the water phase is essentially cold, which requires all of the eruptive energy to come from the magma. Magmatic-hydrothermal eruption: An eruption that occurs when injection of magmatic material into a pre-existing convecting hydrothermal system causes a heat pulse that triggers an eruption ŽLawless et al., 1997.. In this case, the bulk of the energy responsible for the eruption is derived from the hydrothermal system itself, but the magmatic input has an essential triggering role. The resulting eruption may be larger than would be possible for a purely hydrothermal eruption or for a phreatic eruption involving the same amount of magma. Juvenile magmatic material may or may not be identifiable. One of the clearest examples of an historic magmatic-hydrothermal eruption was the 1886 eruption at Rotomahana ŽSimmons et al., 1993; Keam, 1988. as were probably the 1967–1977 eruptions of Soufriere de Guadaloupe ŽWohletz and Heiken, 1992, p. 278.. Mastin Ž1991. also considered this mechanism might have caused the 550,000 BP eruption at Inyo Craters, Long Valley Caldera. Note that this terminology has not been previously formally defined in this way in the literature, as this distinction has rarely been made, although the concept was mentioned by White Ž1955. and Nairn Ž1979.. It is, however, significant, since it is crucial to understanding the formation of some mineral deposits Že.g. Kelian, Indonesia: Van Leeuwen et al., 1989; Sillitoe, 1997; Davies et al., 1999. and assessing hydrothermal eruption risks. Hydrothermal eruption crater: A crater formed by a hydrothermal eruption. Maar: A crater formed by a phreatic or phreatomagmatic eruption. These craters are characteristically wide wup to several kilometres according to Wohletz and Heiken Ž1992.x but shallow, with floors below the level of their surrounds ŽLorenz, 1973.. Hydrothermal eruption breccia: A clastic surface deposit produced from a hydrothermal eruption. Hydrothermal breccia: A general term, which can include both hydrothermal eruption breccias Žexogenous. and breccias formed in the subsurface Žendogenous. by forceful disruption by fluid within a hydrothermal system. The terms Ahydrofractured brecciaB or Ahydraulically fractured brecciaB apply to breccias formed subsurface where there is clear evidence of brittle opening of the host rocks. How-
301
ever, where there has been vigorous clast transport and milling, this may not be apparent ŽFig. 1. and these terms should be avoided. Diatreme: A body of brecciated rock, usually with a subvertical, upwards-flaring form. It may infill an eruption channel, a phreatic or phreatomagmatic eruption vent, or be formed by other explosive magmatic events, such as a sudden loss of CO 2 when kimberlite forms ŽBrey and Ryabchokov, 1994.. It underlies a maar Žif preserved..
3. Characteristics of hydrothermal eruptions and their deposits A review of the literature and our own observations lead to the following generalisations about the characteristics of hydrothermal eruptions and conditions in a geothermal reservoir prior to an eruption. Some examples of individual hydrothermal eruptions are presented in Appendix A. 3.1. Craters and the focal depths of eruptions Some large prehistoric hydrothermal eruptions apparently had deep focal depths. At Waiotapu, eruptions ejected clasts derived from depths of at least 350 m ŽHedenquist and Henley, 1985., and some at Kawerau came from at least 200 m ŽNairn and Wiradiradja, 1980.. A comparison of the lithology of ejected clasts with the lithology of cores recovered from drillholes at Rotokawa shows that at least some material could have derived from as deep as 450 m ŽCollar and Browne, 1985.. However, this is not unequivocal, as it is possible that the drillholes did not penetrate some of the units represented among the clasts. Clasts from Inyo Craters were considered by Mastin Ž1991., on the basis of litho-stratigraphy, to have come from below 450 m depth. Smaller eruptions presumably had shallower focal depths, though it is difficult to find direct evidence of this. Fytikas and Marinelli Ž1976. suggested that the diameter of hydrothermal eruption craters could be correlated with the depth of the eruption focus and that this, in turn, could be used to predict the depth of exploitable hydrothermal reservoirs, but there are so many possible combinations of fluid state and
P.R.L. Browne, J.V. Lawlessr Earth-Science ReÕiews 52 (2001) 299–331
302
Table 1 Summarised examples of hydrothermal eruptions Location
Date
Crater Diameter Žm.
A. Historic eruptions Waimang 1900–1904 170 = 77 AGeyserB, NZ Žepisodic with average 36-h repeat. Waimangu ŽTrinity Terrace.
1973
Within lake
Waimangu ŽFrying Pan Flat. several events over 2 months
1917
120?
Waimangu ŽMud Rift.
1978–1979 2:18, 30 Žseveral. 1981
Tauhara, NZ
1981 Žalso 1974.
Karapiti, NZ ŽCrater G.
1978 1980 1981 1983
ŽCrater C. ŽCrater D. ŽCrater B.
Ngatamariki, NZ Rotorua, NZ
Ejecta Depth Žm.
Max. radius Žm.
15
Max. thickness Žm.
Area Žm2 .1
Vol. Žm 3 . 2
Max. clast Žm.
2.5
70
970
0.3
) 40
0.7
800?
3000
3200?
30
1.6 = 10 7 ? 5 = 10 6
4
10–23
0.22
1200
280
90
4.5
) 800
2.0
17,000
6800
1.2
15
) 10
) 100
60
15
1000– 2000
0.5
100
) 22,000
2.0
30,000
30
1961 ) 22,000 1964 1967 1997 1948 1894 1895 Ž2. 1897 1901 1902 1906 1931 1932 1945 1998–2000 Ž6.
) 100
20
5
0.5
0.15 100 30–60 15 30 30 30
15 4 Žfive small vents.
Shallow
5
0.5
Min. ejection velocity Žmrs.
460
0.45
120
Height Žm.
55
130
0.5
60–120 60 20
P.R.L. Browne, J.V. Lawlessr Earth-Science ReÕiews 52 (2001) 299–331
Min. focal depth Žm.
90
Basis
Stratigraphy
275?
Estimated energy ŽkJ. 3
Initiating mechanism
Reservoir characteristics
Commentsr loss of life
Reference
Magmatic?
Rejuvenated following 1886 Tarawera eruption. L iquiddominated water level at surface
Four deaths, several injured on August 31, 1903
Warbrick Ž1934., Lloyd and Keam Ž1965., Scott and Lloyd Ž1982.
10 8 (Õ)
Chloride water
Possible precursors; damage to vegetation
Lloyd and Keam Ž1974.
10 11 (Õ)
Magmatic?
Two deaths, one injured, building destroyed. Lateral blast
Morgan Ž1917., Warbrick Ž1934., Rae Ž1991.
No damage or loss of life
Scott and Lloyd Ž1982.
Near urban area, but no damage or loss of life
Scott and Cody Ž1982.
No damage or loss of life in any eruptions at Karapiti
Allis Ž1984.
10 6 (Õ) 10–27
Stratigraphy
303
10 8 (Õ)
Shallow
Drought
Drought, then rain
Shallow steam cap over deep steam and water, drawn-down at Wairakei Shallow steam zone connected to drawdown Wairakei reservoir
10 7 (Õ) Allis Ž1979.
AshallowB
Fracturing of cap? Draw-down
10 7 (Õ)
Liquid-dominated
Not seen, remote
Liquid-dominated, later drawn-down by exploitation, then partially recovered following reduction in draw-off
In urban area, damage to houses but no loss of life
Brotheridge et al. Ž1995. Donaldson Ž1985., SKM unpublished notes
Burial and man-made modification of vent may have contributed to eruptions in 1998– 2000 (continued on next page)
P.R.L. Browne, J.V. Lawlessr Earth-Science ReÕiews 52 (2001) 299–331
304
Table 1 Ž continued . Location
Date
Mokai, NZ
1996
Tikitere, NZ Lake City California, USA ŽMud Volcano. reference also made to eruptions at Gerlach Hot Springs, NV Tiwi, RP ŽNaglagbong.
1966 1951
1980–1981 Žthree episodes.
Crater
Ejecta
Diameter Žm.
Depth Žm.
20
5
60, several ) 3 vents within 210-m diameter
Max. radius Žm.
Max. thickness Žm.
0.3 100, plus ) 4 minor airfall to 7200
Area Žm2 .1
Vol. Žm 3 . 2
1.3 = 10 5
1.7 = 10 5
Max. clast Žm.
10
50
2
5500
1300
Tongonan, RP 1983 ŽBao.
10
8
20
0.5
900
AsmallB
0.1
Yellowstone, 1963 USA ŽSeismic Geyser.. Eruptions of Sapphire Pool Ž1959–1961., Excelsior Geyser Ž1881., Link Geyser Ž1957–1958., Steamboat Geyser Žseveral. also referred to ŽPorkchop 1989 geyser.
) 12
)6
10?
220
AsmallB
1.0
66
5400
La Soufriere ` de Guadaloupe, Lesser Antilles Iwo Jima
1797, 1809– 1810, 1836– 1837, 1956, 1976–1977 1957 Žalso 1922.
33
Dieng, Indonesia
1979
3: largest ; 300
Min. ejection velocity Žmrs.
0.3 0.3
25
14
Height Žm.
1.88
30
30–150
14
100
150 plus lahars to 4000
7
7600
8 = 10 5
1
0.4
40
P.R.L. Browne, J.V. Lawlessr Earth-Science ReÕiews 52 (2001) 299–331
Min. focal depth Žm.
- 90 30?
Basis
Stratigraphy geometry
AshallowB
Estimated energy ŽkJ. 3
1.6 = 10 8 Ž k ., 2.7 = 10 9 Ž t .
10 7 (Õ)
AshallowB
AshallowB
Initiating mechanism
Alteration, morphology
10 8 (Õ)
305
Reservoir characteristics
Commentsr loss of life
Reference
Local steam-heated zone over outflow Steam-heated Steam-heated
No damage or loss of life
KML Ž1998.
Steam cap over water draw-down following exploitation
No damage or loss of life
Espanola Ž1974. White Ž1955.
Grindley Ž1982., personal observation ŽJVL., Yuhara Ž1995. Personal observation ŽJVL., PNOC Ž1982. Marler and White Ž1975., White et al. Ž1988.
Drought
Local steam heated zone over outflow
Seismic fracturing of silica cap
Over pressured liquid-dominated
Seasonal hydrological changes Tectonic fracturing
Liquid-dominated, overpressured
Fournier et al Ž1991.
Steam over liquid, close magmatic connection
Zlotnicki et al. Ž1992.
Artificial burial of natural vents? Earthquake
Steam zone, probably magmatic component
Satellite crater formed by collapse
Corwin and Foster Ž1959.
High pressure, vapour cap
142 killed but by gas
Le Guern et al. Ž1982.
(continued on next page)
P.R.L. Browne, J.V. Lawlessr Earth-Science ReÕiews 52 (2001) 299–331
306
Table 1 Ž continued . Location
Date
Crater
Ejecta
Diameter Žm.
Depth Žm.
Max. radius Žm.
200? 40
5
130
Agua Shuca, Ahuachapan, El Salvador Nakano-yu, Japan
1868? 1990
Nisyros, Greece
Prehistoric, At least up to 30 1871, 1873 23, largest Žseveral. 1887 350
Max. thickness Žm.
1995
5.9 = 10 6 Žsingle unit.
3.7 = 10 6 Žsingle unit. 10 6 2 = 10 7
1
10 7
3
1.6 = 10 5
1
10 6
3
) 1300
4 8
Whakarewa42,000 rewa, Rotorua ŽNgawha Crater. Yellowstone 23–25,000 Žmany.
100
4
100?
250– 300
11 Žlargest single unit., 40 Žtotal.
200
1.0
1.3 = 10 5
Kawerau Ž2.
- 1800– 16,000
4000– 6000 3 = 10 5
13
Rotokawa Ž8?. 3700–20,000
Height Žm.
9 Žthickest unit.
2500
300–500
Max. clast Žm.
500 including debris flows
) 20
9000–14,500
Vol. Žm 3 . 2
550
130
5
1.3 = 10 7 Žsingle unit., 1.5 = 10 7 Žtotal. 7 = 10 5 Žlargest single unit.
50–150
150–200 2
30
350–1600 ) 35
1200
20
10 6
1.4 = 10 7
ŽMary Bay.
- 13,650
) 2500
100
2000
Inyo Craters, USA Ž12. Te Kopia area, NZ Žseveral. Ngawha, NZ Žat least 6.
650,000– 550,000 ) 2000
200
52
1000
13
10 6
1.2 = 10 9
) 0.7
200
2
10 5
10 4
0.5
?
100–200
Whangairorohea, NZ Tikitere, NZ
?
40–75
14,000
Min. ejection velocity Žmrs.
1600
B. Prehistoric eruptions (age BP) Waiotapu 900–15,000 60–250 Žseveral.
Orakei Korako Ž11.
Area Žm2 .1
300 ) 10
100
P.R.L. Browne, J.V. Lawlessr Earth-Science ReÕiews 52 (2001) 299–331
Min. focal depth Žm.
Basis
Estimated energy ŽkJ. 3
307
Initiating mechanism
Reservoir characteristics
Commentsr loss of life
Reference
26 deaths
4.6 = 10 10 Ž k . , 6.3 = 10 12 Ž t . 10 9 (Õ)
Excavation
Liquid-dominated vapour cap, drawn down by exploitation Magmatic?
Bruno et al Ž1992., Goff and Goff Ž1997. Yuhara Ž1997.
Seismic
Liquid-dominated?
Chiodini et al. Ž1992., Marini et al Ž1993.
Silica sealing
Now mainly liquiddominated
Cross Ž1963., Hedenquist and Henley Ž1985. Nairn and Wiradiradja Ž1980.
Damage to houses and road, four killed
100–350
Alteration, stratigraphy
2.3 = 10 9 to 1.4 = 10 11 Ž k .
190
Stratigraphy
5 = 10 12 Ž t .
450?
Stratigraphy
10 11 (Õ)
Some may be magmatic?
) 120, possibly - 20
Stratigraphy
10 9 (Õ)
Faulting?
Now mainly liquiddom inated, som e steam heated features
Lloyd Ž1972.
Change in lake level?
Mainly liquiddominated, now drawn-down
Lloyd Ž1975.
Glacial fluctuations in groundwater
Now overpressured, mainly liquiddominated
Faulting
Maybe phreatomagmatic Steam over water
Muffler et al. Ž1971.,Wold et al. Ž1977., White et al. Ž1988. Mastin Ž1991.
Draining lake?
Highly overpressured
Faulting?
Liquid-dominated
42
Stratigraphy
450
Stratigraphy
4 = 10 9 – 4 = 10 11 Ž k . 10 10 –8 = 10 11 Žt. 10 10 Ž k ., 10 12 Žt. 10 8 (Õ)
Now mainly liquiddominated, some excess enthalpy Now mainly liquiddominated, some excess enthalpy
Collar Ž1985., Collar and Browne Ž1985.
Browne et al. Ž1994., Clark Ž1999. Browne et al. Ž1981., Lawless Ž1988. Youngman Ž1996. Espanola Ž1974. (continued on next page)
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308
Table 1 Ž continued . Location
Date
Crater Diameter Žm.
Atiamuri, NZ
Ejecta Depth Žm.
Max. radius Žm.
13,000
Wairakei, NZ ? ŽRautehuia Breccia.
Max. thickness Žm.
Area Žm2 .1
Vol. Žm 3 . 2
3 ?
Max. clast Žm.
Height Žm.
Min. ejection velocity Žmrs.
0.5
229 m? Žseveral units ?.
3 = 10 6 ?
Various data and estimates from the original sources are presented in this table in normal font, verbatim, with no modification except for conversion of units, where we have estimated derived parameters on the basis of the data given, these are in italics, using the following assumptions: Ž1. Where no more detailed data are available, areas of deposits are estimated from the maximum radius of deposition and assuming an elliptical distribution with an obliquity of 0.7. Ž2. Where no more detailed data are available, volumes of deposits are estimated from the area and maximum thickness by Eqs. 2–7 of Wohletz and Heiken Ž1992., assuming r h s one-fifth of the maximum ejection radius. Ž3. For previous estimates of energy involved in eruptions, k s mechanical energy required to transport ejecta on a kinetic or ballistic basis, t s total available energy. Estimates of t are usually much larger than k. Where no more detailed data are available, we have estimated the energy of eruption from the volume of ejecta Ž Õ ., by assuming 10 Jrcm3 is required.
eruption mechanism as to make this method of doubtful utility. Collar and Browne Ž1985. depicted vents at Rotokawa as overlapping and funnel-shaped rather than cone-shaped. Vents commonly ejected clasts, which were brecciated during earlier eruptions but then fell back into their vent. Many vents are not recognisable because: Ž1. much of the ejected material fell back into the crater and remains there; Ž2. many became sites of intense geothermal activity that masks their morphology; andror Ž3. inward slumping of the crater walls disguises the crater margins. However, the locations of some craters can be recognised from the raised apron of breccia surrounding them, by the distribution and thickness of the breccia and especially by the maximum size of its clasts. This also helps to distinguish them from the many crater-like features in geothermal areas that are not produced by hydrothermal eruptions but by other processes, such as ground collapse following the dissolution of near-surface rocks by acid waters. A good example of a discussion of whether certain craters are of hydrothermal origin is provided by White et al. Ž1988.. Pike Ž1974. provides criteria for using systematic numerical taxonomic methods to help reveal their origin.
The width of vent craters differs greatly. Examples are listed in Table 1, with the larger craters, such as those at Kawerau, Rotokawa and Ngawha, being 200–500 m across, whereas smaller ones, such as those at Ngatamariki, and Tongonan in the Philippines, are 10–20 m in diameter. The largest one reported if, indeed, it is of hydrothermal origin, is at Mary Bay, Yellowstone and is more than 2500 m across ŽWold et al., 1977.. 3.2. Breccia deposits Nelson and Giles Ž1985. and Sillitoe Ž1985. have discussed the distinguishing features of hydrothermal eruption breccias. Drawing on the above examples, we make the following comments: Ž1. Extent: The distribution of the products from a hydrothermal eruption obviously depends upon the magnitude of the eruption, the nearby topography, the geometry of the vent, the nature of the ejected material, whether or not the eruption occurred through water and the post depositional erosion history of the breccia. However, even the largest hydrothermal eruptions produce deposits that are several orders of magnitude smaller in volume than those deposited from a typical silicic pyroclastic
P.R.L. Browne, J.V. Lawlessr Earth-Science ReÕiews 52 (2001) 299–331
Min. focal depth Žm.
Basis
Estimated energy ŽkJ. 3
Initiating mechanism
Reservoir characteristics
Activity extinct now Magmatic?
eruption. Lahar deposits are more likely to occur within valley floors. Breccias deposited from the largest hydrothermal eruption at Rotokawa ŽFig. 2. covered an area of about 12 km2 , with a maximum thickness of 11 m ŽCollar and Browne, 1985. and had a volume possibly as great as 10 7 m3 Žit has been eroded considerably.. The Lower Eruption Breccia at Kawerau is estimated to have a volume of 2 = 10 7 m3 ŽNairn and Wiradiradja, 1980.. Five of the hydrothermal eruptions younger than 1800 years old at Waiotapu ŽLloyd, 1959; Hedenquist and Henley, 1985. produced ejecta that covered areas of between 2 and 6 km2 ; one deposit has a maximum thickness close to its vent of 13 m and a volume of 3.7 = 10 6 m3. The thickness of this, and most other eruption deposits, decreases exponentially away from their craters. These very large deposits are, however, exceptional and the great majority of hydrothermal eruptions probably produce deposits with volumes less than 10 5 m3. The distribution of ejecta around a vent can vary from uniform to quite asymmetrical, depending upon vent geometry and wind conditions. The most extreme historical example of the latter is probably the directional eruption blast at Waimangu in 1917 ŽMorgan, 1917., when some ejecta, described as
Now a shallow steam zone over a liquiddominated resource
Commentsr loss of life
309
Reference
Browne and Lloyd Ž1986. ECNZ Ž1992., Bogie et al. Ž1995.
AmudB, reputedly travelled 3200 m from the vent, and debris from a house destroyed by the blast was transported 1600 m. Fine air-fall material from the eruption cloud at the Lake City eruption ŽWhite, 1955. travelled 7200 m from the vent. Ž2. Textural Characteristics: Typical hydrothermal eruption breccias are very poorly sorted and invariably matrix-supported Žimplying rapid settling velocities with little opportunity for sorting.. Ejected material ranges from clay size to clasts several meters in diameter. Vertical and lateral grading of clasts is usually poorly developed although the largest clasts usually lie closest to their craters. Bedding forms are rare, unlike in base surge deposits. Probably because many clasts were milled within their vents during the course of an eruption before being ejected, they are typically subround. Larger blocks sometimes impact into underlying material and this is one way to distinguish hydrothermal eruptions from mudflow deposits, which they may superficially resemble. Some hydrothermal eruption breccias show evidence that they were deposited wet, such as the presence in them of accretionary lapilli or the vesicular texture of their matrix Že.g. Mastin, 1991.; some may even be wet enough to generate lahars Že.g. Marini et al., 1993.. Similar textures present in subsurface brec-
310
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clasts present in hydrothermal eruption breccias may, thus, differ from those in debris flows and lahars, both of which have clasts that are more likely to be monomict and altered differently Žif at all.. For example, it is unlikely that clasts in a lahar of volcanic origin would contain alteration minerals, such as epidote, adularia or perhaps zeolites, produced at moderate temperatures by near-neutral pH waters. Nor would products, such as vein quartz and platy calcite Žwhich have been recognised in hydrothermal eruption breccias., indicative of shallow two phase conditions in a convective reservoir, usually occur in a volcanic lahar whose alteration is more likely to indicate acid andror vapour-dominated conditions, and comprise kandites, sulphur and alunite. If an eruption originated within a secondary steam-heated aquifer, the alteration of the ejected clasts may resemble those of volcanic origin, but probably without very abundant sulphur and the
Fig. 1. Idealised development of hydrothermal veins and breccias.
cias, which are arguably of hydrothermal origin, were noted by Hulen et al. Ž1999.. In other eruptions the deposits may be quite dry, however, such as the historic eruptions at Karapiti, New Zealand, where the mobile phase was steam derived from a vapourdominated zone in the top of the reservoir. Thus, in terms of the eruption types discussed by Mastin Ž1995., it is important to appreciate that hydrothermal eruptions derived from a steam zone can fall into his category of Agas eruptionsB even at well below magmatic temperatures. Similarly, consideration of comments, such as those of Sheridan and Wohletz Ž1981. regarding waterrmagma ratios in hydrovolcanic eruptions could lead one to conclude that hydrothermal eruptions should always be wet, but this is clearly not the case. Ž3. Lithology and alteration of clasts: Much of the material ejected derives from the geothermal reservoir and, thus, reveals at the surface the types of rocks that comprise it. Many clasts are hydrothermally altered, so that they can be studied to interpret hydrological conditions in the reservoir itself using standard mineralogical techniques. The lithology of
Fig. 2. Distribution of 6060-year-old hydrothermal eruption breccia at the Rotokawa geothermal field. Isopachs are in metres. Also shown are deduced vent locations Žfrom Collar, 1985; Collar and Browne, 1985..
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phosphorous-bearing phases, such as woodhouseite, that appear to be diagnostic of primary magmatic volatiles ŽBogie and Lawless, 2000.. Possibly breccias deposited from an eruption originating within a steam-dominated reservoir could be distinguished by the occurrence of clasts with phases such as pyrrhotite, and the absence of phases indicating liquid conditions, such as abundant vein quartz. However, in practice, such observations tend to be equivocal. For reasons discussed below, large, deepfocussed eruptions from steam reservoirs are believed to be uncommon. 3.3. Amount of energy released Estimates of energy involved in eruptions can be based on a number of principles, but fundamentally they fall into two categories: those that assess mechanical energy required to produce the effects, in terms of cratering and ejecta, and those that look at energy supply and the thermodynamics of eruptive processes. In the first category, some methods are empirical, based on measurement made with large artificial explosions Že.g. Murphy and Vortman, 1961.. Others are ballistic, based on the distance that large clasts are ejected Že.g. Sherwood, 1967; Wilson 1972, 1980; Steinberg, 1977.. An alternative is to look at the amount of energy required to transport the mass of ejecta Že.g. Muffler et al., 1971.. In the second category are analyses of thermal energy available in hydrothermal systems and the thermodynamics of the resulting eruptions, Že.g. Muffler et al., 1971; Nelson and Giles, 1985; Mastin, 1995., or various volcanic processes Že.g. Self, Wilson and Nairn, 1970.. All of these methods have been applied to assessing the energy involved in hydrothermal eruptions, with varying degrees of sophistication. Some of the resulting estimates for individual eruptions are quoted in Table 1 and discussed below. Caution must be used in comparing these estimates, since the different methods will yield very different results. Some of the difficulties are as follows: Methods based on ballistics of large ejecta are not applicable where they are mainly fine-grained material or mud, as in some hydrothermal eruptions. Also, while maximum distances of ejecta are often given, and maximum clast size, what is needed for v
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an analysis of this type is the ejection distance of a particular clast of known size. The differences in scale, temperature, mechanisms and fluid properties between steam-driven eruptions and volcanic, nuclear or high-explosive blasts mean that analyses based on the thermodynamics of volcanic or artificial explosions are of doubtful applicability. Some estimates based on total ejecta mass or available energy assume isentropic decompression, whereas others are isenthalpic. This is discussed by Mastin Ž1995.. Estimates based on energy required to transport ejecta will usually be much smaller than estimates based on available thermal energy, because not all of the energy in the clasts is transferred to the fluid medium. Estimates of the efficiency of such heat transfer vary widely Že.g. Nairn and Wiradiradja, 1980; Sheridan and Wohletz, 1981.. Some estimates of the difference between kinetic and total energy are as great as 10 3 times ŽMastin, 1991.. Some of the more significant estimates that have been made for hydrothermal eruptions include the following. A common rule of thumb is that the energy required to form a crater varies according to the third power of the crater’s dimensions ŽWohletz and Heiken, 1992., although this breaks down for very large explosions, and the depth of focus of the explosion is also important in determining the radius of the crater ŽMurphy and Vortmann, 1961.. Muffler et al. Ž1971. calculated that the energy required Žisenthalpically, but including a component for comminution; Charles, 1957. for a hydrothermal eruption at Yellowstone, which ejected material from a depth of 70 meters was about 8.4 Jrcm3 and provided curves for estimating eruptive energy based on crater radius. Their method probably seriously under-estimates the energy required to form larger craters, since they assumed a constant depth and height of eruption. This compares with a range of 1.3 to 210 Jrcm3 for large crater-producing artificial explosions ŽMurphy and Vortmann, 1961.. Thermodynamic analysis by Mastin Ž1995. showed that the maximum energy available for hydrothermal eruptions is about 250 kJrkg of fluid–rock mixture Žabout 500 Jrcm3 ., and that maximum clast velocities are unlikely to exceed 400 mrs, which is consistent with ballistic observations. For the larger erupv
v
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tions, such as those at Rotokawa, with a volume of 10 7 m3, an energy requirement of 10 Jrcm3 corresponds to 10 11 kJ. For comparison, Hedenquist and Henley Ž1985. compared fall velocities of large clasts at Waiotapu to their probable trajectories following Keiffer Ž1977.. They then estimated that the kinetic energy required was between 2.3 = 10 9 hydrothermal eruption at the Kawerau field released Žnot requ ) and 1.4 = 10 11 kJ. Similar analyses by Nthey areairn and Wiradiradja Ž1980. estimated that one prehistoric ired. 5 = 10 12 kJ of thermal energy. These are of similar magnitude to son and Gthe estimates made for Yellowstone. Nelson and Giles Ž1985. approached the question of eruptive energy from the point of view of energy storage in a hydrothermal system and reached the conclusions that there was more than enough energy available in them to generate hydrothermal eruptions. White Ž1955. and Mastin Ž1991. estimated the kinetic energy of the South Inyo Crater eruption to be of the order of 10 10 kJ, and its total energy to be 10 11 –10 12 kJ, although there is some doubt whether that eruption was hydrothermal or phreato-magmatic. Some large hydrothermal systems in the Taupo Volcanic Zone are reported to have natural heat fluxes in the range 100–300 MW Žthermal: hereinafter designated MWth . ŽHochstein et al., 1993.. While noting that estimates based on surface heat flow measurements can be too low by a factor of three because they neglect the importance of subsurface boiling Že.g. Grant, 1985: cf. KRTA, 1986., a value of 200 MWth can be regarded as typical. The larger eruptions mentioned above, therefore, required energies equivalent to between hours and months of the natural heat flux of these systems. A similar observation was made by Hedenquist and Henley Ž1985., who stated that the eruptions at Waiotapu required energy equivalent to a few days of the natural heat flux of the system. Thus, even allowing that eruption processes are poor means to transfer energy ŽSheridan and Wohletz, 1981., it is evident that hydrothermal eruptions need not be rare events within hydrothermal systems lasting several hundred thousands of years Že.g. Browne, 1979.. The energy depletion, by itself, would leave no identifiable trace within the reservoir rocks in terms of hydrothermal alteration overprinting. These estimates clearly show
that hydrothermal eruptions can occur without additional heat being added to a geothermal system whose reservoir fluid is near boiling. 3.4. Fluid state in hydrothermal systems Although many geothermal systems have been drilled and tested, there are few reliable measurements of the pressures and temperatures prevailing in the upper few hundred meters Ži.e. the region where hydrothermal eruptions occur.. Conditions here usually need to be inferred, with a few exceptions, as noted below. The Lihir gold mine in Papua New Guinea Žcommissioned in 1996., where progressive dewatering and associated monitoring of an active hydrothermal system to permit mining is required, will offer a unique opportunity to collect such data ŽMoyle et al., 1990; Clayton, 1999.. A liquid-only hydrothermal system, without any dissolved gases, and discharging to the surface represents the simplest case. Here, the temperatures of the heat source and the boiling temperature of water at the ground surface Žassuming conditions are everywhere subcritical. limit the two ends of its temperature gradient. In the simplest situation, water in this system is just slightly below boiling temperature throughout. Many of the New Zealand systems, in their undisturbed state, have down-well temperature profiles that are close to the boiling-point-for-depth gradient with a piezometric surface close to the ground surface. Assuming that the pressure is determined by the weight of the overlying water column, pressures in such a system will everywhere be below that of cold hydrostatic and certainly far below lithostatic pressure. In the undisturbed state, hydrothermal eruptions will not occur; but even a very slight disturbance that causes a drop in confining pressure at any depth can initiate subsurface boiling, which may then trigger a hydrothermal eruption. As pointed out by Donaldson et al. Ž1981., where convection occurs, the resulting pressure gradient will be hydrodynamic due to frictional pressure drop. Parts of such geothermal systems may, therefore, be AoverpressuredB and artesian ŽCathles, 1977.. The overpressure is derived from the balance between the descending column of cold water providing recharge to the system, and the upflowing, less dense hot
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water — the so-called thermo-artesian effect ŽElder, 1968.. Similarly Hayba and Ingebritsen Ž1997. concluded, from numerical modelling of convective hydrothermal fluid flow, that overpressures of 50 % above hydrostatic could occur. Careful and crucial measurements made in some research drillholes at Yellowstone National Park by White et al. Ž1975. showed temperatures in this hot water reservoir tended to follow the boiling curve at depth but conductive gradients existed in the upper few tens of metres. Pressure gradients were as much as 47% above that of cold hydrostatic. This condition was ascribed to self-sealing ŽWhite et al., 1988.. Smith Ž1970. reported pressures 0.9 MPa in excess of cold hydrostatic at shallow depths at Kawerau. At Rotokawa the measured pressure exceeds the expected hydrostatic pressures by about 1 bar per 100 m depth Ži.e. by about 10%, Grant, 1985.. Well AkicksB Žback-flow from overpressured zones. are also common during drilling in geothermal systems, showing that, locally, pressure is above that of cold hydrostatic. In this situation, hydrothermal eruptions can more easily be initiated. The situation is complicated where increasing amounts of dissolved gases cause boiling to occur at progressively lower temperatures and greater depths within a reservoir, e.g. Ohaaki ŽSutton and McNabb, 1977; Mahon et al., 1980., and recharge is from a similar, or higher, elevation to the ground surface over the upflow zone. Systems with high gas contents, such as Ngawha in New Zealand and Lardarello in Italy, have thick vapour-rich zones above a liquid only phase. Unlike steam, gas that separates from deeper boiling water will not condense Žalthough some may redissolve in perched waters. as it moves to cooler regions. Condensation of steam may allow AnoncondensibleB gases to accumulate beneath rocks of low permeability creating a gas pocket that tends to spread laterally and vertically until it escapes. It can be an effective transmitter of high pressures from below. Pressures could become sufficiently high for a hydrothermal eruption to occur. Extrapolation of deep pressures to near-surface at Ngawha show that the pressure is potentially sufficient today to permit a hydrothermal eruption by lifting off a lithostatic load of about 130 m of rock ŽFig. 3.. That this does not happen is probably due to the capping of about 500 m of low-permeability
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sedimentary rocks, which do not permit the gas pressures to be transmitted close to the surface. By contrast, geothermal systems in steep terrain, such as most in Indonesia and the Philippines, have deep zones where the temperatures are close to boiling but their piezometric surfaces are as much as 1000 m below the ground surface; this is because of the lower elevations of the recharge zones. Where vertically extensive steam zones exist their pressures usually correspond to the saturation value at the maximum enthalpy point of steam, about 34 bars ŽRogers and Mayhew, 1980.. They could, therefore, theoretically, lift off a lithostatic load equivalent to about 180 m of rock. However, the steam zones do not extend closely enough to the surface to permit this to happen, presumably because they take a long time to develop. The upper boundary of steam zones in this situation may be a thick layer of low-permeability alteration products produced by moderate temperature, acid–sulphate–bicarbonate waters and characterised by clays and, in some cases, anhydrite ŽBogie et al., 1987.. The fact that vertically extensive steam zones are isothermal and, therefore, isobaric means that hydrothermal eruptions caused by fluid overpressuring cannot originate very deep within such systems. Note
Fig. 3. Pressure–depth profiles in explored New Zealand geothermal fields Žmodified after Browne et al. 1981..
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also that at temperatures below critical point, a given volume of reservoir containing a single liquid phase will have more energy to provide to a hydrothermal eruption than will a steam-dominated reservoir of the same volume. For a reservoir with a porosity of 30% and water saturation of 30% by volume, this difference is about 20% ŽFig. 4.. The effect increases as the temperature increases. Although the enthalpy of steam per unit mass is greater than that of water, it is almost invariant with temperature in the subcritical range and the enthalpy per unit Õolume is much less than that of water. In fact, in terms of stored heat per reservoir volume, more energy is stored in the residual liquid water in a steam zone Žat say 30% by volume water saturation. than in the accompanying steam. The contribution of energy from the reservoir rocks will be the same in each case Žthough perhaps the rate of energy transfer will not.. In terms of fluid flow through a porous reservoir to a fracture over a short time, it is the distance that the fluid has to flow and, hence, the reservoir Õolume supplying the fluid that matters, not the mass.
4. Physical mechanisms of hydrothermal eruptions When attempting to understand the physical mechanisms of hydrothermal eruptions, and hope-
Fig. 4. Amount of energy in fluid per unit reservoir volume in a geothermal reservoir at varying degrees of water saturation by volume. Curves are shown for a typical near-surface steam zone at 1508C, a typical deeper steam zone at 2408C, characteristic of those exploited for geothermal energy, and a zone at 2708C more typical of a deep, boiling two-phase source zone. In all cases, porosity s 30%.
fully to arrive at a model which has some predictive value, it is necessary to recognise that not all hydrothermal eruptions are the same. There are several different types, but some require special circumstances and so probably occur infrequently. Some mechanisms may be common to more than one type. The following types can be distinguished and are presented here in the order of requiring progressively lower subsurface fluid pressures: 4.1. Pressures exceeding lithostatic The simplest model of a hydrothermal eruption is one whereby a field wide ‘cap’ or cover rock allows pressures within the reservoir to increase until they exceed lithostatic pressure, plus the tensile strength Žif any. of the rock, at some place. When this happens there is a single, short eruption Žthough the eruption may then continue driven, through other mechanisms., originating within the reservoir that brecciates the host rocks, then ejects the resulting clasts. This mechanism was demonstrated to be a factor for the 1976 phreatic eruption of La Soufriere ` de Guadaloupe, which was preceded by ground tilt, indicating subhorizontal volcanic hydrofracturing occurred Žsummarised by Wohletz and Heiken, 1992, p. 97.. This mechanism is probably most common in the case of very small, near-surface eruptions, but it does not account for the following observations and features: Ž1. Hydrothermal eruptions occur in some geothermal fields where measured pressures may only slightly exceed hydrostatic. Hydrothermal eruptions have occurred at all of the fields shown in Fig. 3, except Ohaaki, despite the fact that only one of the systems ŽNgawha. is significantly overpressured with respect to lithostatic pressure. Eruptions have occurred within both the liquid and vapour-dominated portions of such systems. Ž2. Very few active hydrothermal fields have field-wide lithological cap rocks, otherwise they could not discharge any steam or water derived from the reservoir. Condensation of steam would usually make such a situation short-lived. An absence of surface activity above a hydrothermal system is usually due to hydrological factors Že.g. steep terrain., not because it has a lithological AlidB. The hydrological importance of near-surface low permeability
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caps, where they do occur, is mainly to keep cold water out of a reservoir rather than hot water in. Ž3. Since hydrothermal eruptions are typical features of so many geothermal systems, it should not be necessary to invoke complex or unusual mechanisms to explain their occurrence. They are probably most frequent when a geothermal system is young, as is the case at Waimangu, or perhaps under stress, as may have been the condition at Waiotapu following injection of magmatic gases into the reservoir fluids ŽHedenquist and Henley, 1985.. The latter were magmatic-hydrothermal eruptions following, perhaps, the injection of dykes ŽLawless, 1988.. However, even long-established systems, such as those at Orakeikorako and Kawerau, have been the sites of large hydrothermal eruptions; the latter system, for example, has been active, in some form or other, for the past 200,000 years ŽBrowne, 1979.. Any model of hydrothermal eruption mechanisms must also account for the frequent occurrence of eruptions in exploited systems, where there has very rarely been observed any chemical changes consistent with a sudden input of magmatic volatiles. The question which then arises is whether or not there are any genetic differences between the small hydrothermal eruptions, which occur in both the exploited and nonexploited fields and those of much greater magnitude, whose effects penetrate to several hundred meters depth. There is no evidence that hydrothermal eruptions, which occur in exploited geothermal areas are any different, except in their magnitudes, from those that take place during a geothermal system’s natural evolution. Ž4. Observations made by Allis Ž1986. of hydrothermal eruptions at the Craters of the Moon area ŽWairakei. show that these events lasted from 15 minutes to several hours. During this time, material was ejected Žand on falling back reejected., i.e. these events were not over instantly, which would probably have been the case were they single eviscerating eruptions that originated deep within a reservoir with almost all the energy being released at once. According to Warbrick Ž1934., the 1917 hydrothermal eruption at Waimangu lasted for almost 3 days, although activity declined during this time. Ž5. Hydrothermal eruptions reoccur repeatedly at the same sites, e.g. at the Puarenga Stream, Rotorua, in some instances after less than a year. This is true
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of both small and larger eruptions and requires that the hydrology of the field be restored nearly to its previous condition between eruptions. The extreme case was the Waimangu AGeyserB, which was a cyclical hydrothermal eruption with a periodicity averaging 36 h, but on occasion as short as 8 minutes Žquoted by Keam, 1962.. It is hard to see that super-lithostatic pressures could be built up this quickly and repeatedly. 4.2. Accumulation of steam and r or gas Eruptions can occur when ascending steam reaches a depth where its pressure exceeds that of lithostatic. The shallower the steam zone extends, the lower the initiation pressure, that is to say that cooler steam can be a problem in terms of eruption risk if it reaches shallow levels ŽFig. 5.. The only precursory condition is that there is subsurface boiling occurring at any depth, and there is sufficient permeability for the steam to ascend. This is the mechanism that is thought to cause hydrothermal eruptions in exploited fields. Here, declining water levels permit higher steam pressures to develop close to the ground surface as boiling conditions descend deeper into the reservoir ŽFig. 6.. Also, water draining from the formations generates a higher steam flux as it boils, deriving its energy to do so from the rocks. Most commonly, such an accumulation of steam occurs only at shallow depths Ždown to a few tens of metres., due to near-surface dewatering. It would usually be preceded by some surface emissions of steam. Less commonly, higher-pressure accumulations of steam could occur at depth, due to deeper changes within the reservoir. These may lead to larger hydrothermal eruptions, with no warning at the surface. The collection of carbon dioxide gas, near the surface but below a seal, may accentuate the effect, as unlike steam, it will not condense. 4.3. Subsurface pressure release If pressures are released suddenly at depth in a geothermal system, steam will form and gas separated from the liquid phase will be released, whereupon the eruption follows the same course as that described above. A pressure release could be due to
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some minor hydrothermal eruptions, were due to the effects of subsurface hydrofracturing. Marini et al. Ž1993. identified the same mechanism as the cause of the 1873 hydrothermal eruption at Nisyros, Greece. Zlotnicki et al. Ž1992. tracked progressive migration of hydrothermal fluid using a variety of techniques prior to the 1976–1977 eruptions of La Soufriere ` de Guadeloupe. They considered that opening of permeable channels through argillised aquicludes, with associated seismicity, was a factor leading to these phreatic eruptions. The necessary condition for an eruption of this type to occur is that water at the local boiling temperature exists at some depth. This water does not have to reach the surface initially, but simply be close enough to connect with it when a fracture opens. Fig. 5. Depth of possible eruption focus due to lithostatic unroofing based on depth of steam generation.
subsurface hydraulic fracturing ŽGrindley and Browne, 1976., or result from local tectonic dilatancy ŽSibson, 1989., or be caused by removal of overburden through erosion, landsliding, draining of a lake ŽMuffler et al., 1971., or lowering of the water table due to dry weather. If subsurface fracturing connects permeable channels between zones of higher and lower pressures, permitting fluid to flow rather than flash, then this may trigger an eruption immediately, or it may delay it until enough fluid accumulates. The best known example of an eruption of this type, with some delay, was in 1959 in Yellowstone, USA. A large earthquake Žthe Hegben Lake earthquake. was followed, within 24 hours, by eruptions of 289 springs as geysers, some of which ejected solid material and so can be classed as hydrothermal eruptions. One hundred and sixty of the springs had no previous record of eruption ŽMarler and White, 1975.. By contrast, one new feature created in that earthquake became a fumarole and then a geyser, which later ejected solid material. This evolution took place over more than 2 years. White et al. Ž1988. also concluded that the episodic and frequently seasonal disturbances of surface thermal activity in Norris Geyser Basin, Yellowstone, including
4.4. Addition of magmatic heat or gas Addition of heat to an active geothermal system could trigger a large eruption, but as discussed above, this should strictly be classed as a magmatic-hydrothermal eruption, not a hydrothermal eruption per se. A more subtle effect might be for an input of magmatic gas to cause boiling and, hence, an eruption. In
Fig. 6. Pressure–depth relations in a hydrothermal system, showing that a decline in water level and a drop in deep pressure can cause shallow pressures to rise due to the formation of a steam zone.
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practice, unless there was some distinctive hydrothermal mineralogy formed in the process it would be very hard to distinguish this event from a AnormalB hydrothermal eruption. If such an event occurred within an exploited system it could be expected to cause observable chemical changes; that such changes have rarely been observed in fluid composition suggests that this mechanism is not the most common. 4.5. ProgressiÕe flashing: the A top-downB model This is the mechanism that we consider is the most common cause of hydrothermal eruptions, especially in fields with low subsurface pressures. All the causes suggested earlier require a fluid near boiling temperature, a sudden lowering of pressure and the formation of steam. McKibbin Ž1989. pointed out that the material is ejected by the lifting power of the steam formed, and he modelled mathematically the resultant changes within a reservoir once an eruption started, and showed that a fluid flashing front descends ahead of the eruption surface. This work was extended by Smith and McKibbin Ž1999., who demonstrated analytically that eruptions could be initiated with a fluid pressure as little as 10% in excess of hydrostatic. They also demonstrated a physical model that confirmed the effectiveness of the process. The same general concepts were advanced by Civetta et al. Ž1974. in discussing the role of fluidisation in forming maars. The gas content of the fluid is not very critical to the mechanism, except for its control on the temperature and thus depth, of initial flashing ŽMcKibbin, 1996., although as Nelson and Giles Ž1985. pointed out, dissolved gas adds to the energy available in a geothermal fluid. We propose here that most natural hydrothermal eruptions start at the ground surface, or very close to it, and penetrate downwards into the reservoir. With this mechanism there is no need for any confining pressure and, indeed, it is possible that an eruption of this type could start from a free water surface, as at the Waimangu AGeyserB. The conditions necessary for an eruption of this type to occur are that boiling water exists at, or close to, the surface and this is underlain by water at a boiling point for depth temperature gradient.
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We think it unlikely that large eruptions begin at great depth within a geothermal reservoir since brecciating and lifting, almost instantaneously, a coneshaped mass of rock above a single focal point deep within a reservoir requires a large amount of energy. This energy would be needed to overcome a combination of the tensile strength Žif any. of the overlying rocks and the high lithostatic pressures they impose. It is energetically much easier for a hydrothermal eruption to begin within a meter or so of the ground surface below a very thin cap ŽFig. 7.. The initial steam burst ejects the covering materials together with entrained water and mud. Because the initial eruptive phase reduces pressures still further within the reservoir, more steam then forms from any residual meteoric or thermal water present. This steam then provides the lift required to brecciate and disperse more rocks. The result is the flashing front and brecciation surface move progressively downward within the reservoir followed by the eruption front. Water present in joints or cracks adjacent to the developing crater also flashes to steam as pressures reduce suddenly, causing the sides of the enlarging vent to brecciate and implode. This may occur more readily where the host rocks are brittle, perhaps through their being silicified. Ductile rocks or sediments are less likely to shatter and brecciate, rather they absorb much of the energy released by the formation of the steam. Most of the erupted material Žrock, mud and water. probably falls back into the crater, to be reerupted more than once. This results in breccia deposits outside and within the crater that are mixed with respect to clast rock type, i.e. the resultant deposits do not show a vertical sequence of clast lithology that is the inverse of the stratigraphic sequence of the reservoir rocks. The very earliest-deposited material, however, must be from nearest the ground surface. The hydrothermal eruption continues until the steam forms too slowly to provide sufficient lifting power to eject rocks from the crater, although some steam may continue to discharge for several years or longer. The maximum depth of disruption within the reservoir, or the apparent focal depth of the eruption, depends on several factors. These include the physical properties of the host rocks, especially the presence of near-vertical permeable pathways, such as
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fractures, the depth of the piezometric surface, the availability of meteoric water, and the amount of energy available from the host rocks, which must also be at temperatures close to boiling. It has long been recognised that thermal equilibrium may not be rapidly achieved in the erupted material Že.g. Mastin, 1995., but the significance of heat transferred from wall rock to fluid in fractures may have been underestimated in the past ŽScott and Watanabe, 1998; Henley and Hughes, 2000.. The hydrothermal eruptions that penetrate to the greatest depths are those which have the most permeable reservoir rocks. This is because they can
provide large amounts of water that can flash to steam to sustain and provide lift for the ejecta. The size of a hydrothermal eruption, therefore, largely depends upon the volume and supply rate of nearboiling water. Where the consequences of a hydrothermal eruption, such as rapid cooling, hydraulic fracturing and brecciation, penetrate to considerable depths, highly permeable conditions extend to the surface and sudden, severe changes to the hydrology of a field may result in, for example, mixing between condensate, meteoric and thermal fluids. Because of the enhanced permeability and steep thermal gradients,
Fig. 7. Ža–f. The development and course of a hydrothermal eruption and postulated change in position of its piezometric surface in a hot water field with a steam cover. Note that brecciation of the host rocks occurs where water changes to steam. This steam provides the energy needed to lift rocks out of the vent. Final sketch is of a situation when the piezometric surface has restored and hydrothermal alteration causes sealing. Scale is not given but may be from 5 to 300 m over the vertical distance shown.
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Fig. 7 Ž continued ..
hydrothermal mineral deposition in the brecciated zone is fast, and sealing of cracks and cementing of
clasts proceeds rapidly. This eventually produces horizons of brittle, coherent rocks that may partici-
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pate in hydrothermal eruptions that occur after the hydrology of the field has been more or less restored ŽFacca and Tonani, 1967.. This has happened at Kawerau, for instance, where a period of 5500 years separated large hydrothermal eruptions from the same site ŽNairn and Wiradiradja, 1980.. When an eruption ceases, the sides of the craters commonly collapse as a result of slumping, or later from acid dissolution of surrounding rocks. Some craters develop lakes Že.g. at Waiotapu., but the craters of hydrothermal eruptions are seldom spectacular in appearance or long lasting. Those older than about 1800 years in the Taupo Volcanic Zone, for example, have little expression and have been recognised almost entirely from the distribution and nature of the deposits they ejected. 4.5.1. Initiating mechanisms The trigger, which initiates a hydrothermal eruption of the Atop-downB type, is uncertain except in cases where overburden is removed. Seismic activity is a likely trigger ŽMarler and White, 1975. but does not always occur. The occurrence of eruption vents aligned in a northeast direction at Rotokawa ŽCollar, 1985. indicates a strong structural control that here parallels the regional fault pattern. Many hydrothermal eruptions are probably initiated by small and subtle events, such as a reduction in atmospheric pressure ŽAllis, 1986. that affect a reservoir filled with water very close to boiling temperatures. 4.5.2. Terminating mechanisms Hydrothermal eruptions that result from local and shallow overpressuring will cease within a few seconds. By contrast, hydrothermal eruptions that occur in the manner described in our model may last for hours or more and their level of intensity declines slowly. In this case, they are analogous to well blow outs whose effects continue for some time and also gradually diminish in their activity. The case of well WK 204 at Wairakei is instructive in this regard ŽThompson, 1976.: following a blow-out, it discharged episodically for 13 years, ejecting much solid material and forming a crater about 15 m across, before the discharge gradually became wetter and spontaneously ceased. The most obvious cause for a cessation of activity is for the supply of hot water to run out or to become
insufficient to generate enough steam to brecciate and lift any more rocks clear of the vent. It is notable that vents at the Craters of the Moon continued to discharge dry steam, but not rocks, after observed eruptions there ended Žour observations.; this is consistent with our model. Eruptions will also end when the vents become flooded with ground water, effectively quenching them as in the case of WK 204 described above. Hydrothermal eruptions originating below a lake, for example, will cease when their vents are flooded with cold lake water. Vents may become blocked as their sides collapse. A slower, but effective, mechanism is one whereby deposition of hydrothermal minerals blocks channels supplying fluid to a vent, in a way analogous to the manner in which geothermal wells can block by mineral deposition, sometimes within a matter of days.
5. Predicting and preventing hydrothermal eruptions 5.1. Predicting eruptions Both shallow and deep focus hydrothermal eruptions are potentially damaging to life and property ŽBixley and Browne, 1988; Bromley and Mongillo, 1994.. They are difficult to predict since there is no limiting pressure or depth below which they will not penetrate. Rather, it is a matter of identifying danger signs and comparing the nature of a reservoir with others where hydrothermal eruptions have occurred. Danger signs could include the following: Evidence of previous hydrothermal eruptions, such as craters and eruption breccias. Where possible these should be dated. Identifying past eruptions as having been phreatomagmatic or magmatic-hydrothermal, rather than strictly hydrothermal, is particularly important in this regard, since hydrothermal eruptions Žsensu stricto. may take place on a more or less regular Žand repeatable. time scale Ždepending on the rate of energy throughput of the convective hydrothermal system and possibly shallow self-sealing.. By contrast, the occurrence of the other types of eruptions is more stochastic within the time scale of human activities ŽLawless, 1988. and these may be triggered by external events, such as an earthquake or sudden unloading, but such occurrence can v
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be expected to be less frequent. For example, dating of the large eruption breccia at Awibengkok as being older than 8400 years ŽStimac and Sugiaman, 2000., together with recognition that it contained clasts with alteration atypical of current shallow reservoir conditions, were factors in deciding that it represented an unusual event, which was unlikely to be repeated within the lifetime of the power project ŽJVL, personal observation.. Evidence for extensive self-sealing, such as the occurrence of silica aprons, or other near-surface impermeable formations, such as clay-rich lacustrine sediments. Liquid-dominated systems with pressure profiles that equal, or exceed, boiling temperatures for depth from the surface downwards. Superheated steam emissions, or shallow steam and gas accumulations. However, to monitor these comprehensively would require very many wells with a range of depths at each location. In practice, a compromise would probably be reached by specifying a finite number of monitoring places, having regard to what is known of the near-surface hydrology. Shallow well AkicksB during drilling. Reservoir fluids with high gas contents, say over 1 wt.%. Changes in thermal activity following exploitation, especially falling water levels or drying up of previously boiling springs, higher chemical geothermometer temperatures Že.g. Fournier et al., 1991., or a change in water composition from neutral chloride to acid sulphate. Evidence of fluctuating groundwater levels. Signs of slope instability in thermal areas that could lead to sudden removal of overburden. The general nature of reservoir response to exploitation can be predicted by numerical simulation modelling, but this will not make unambiguous predictions of the occurrence of individual eruptions or their locations. v
v
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have been from sites of previous thermal activity, albeit apparently extinct. In the sole case that we know of where this was not so, namely the third recorded eruption at Tiwi Ž1982., its site lay along the strike of a fault known from previous thermal activity and eruptions. A simple precaution for avoiding effects from future eruptions, therefore, is to not site any facilities on, or close to, areas of present or past thermal activity. In view of the small size of eruptions observed in exploited fields, a safety zone of 200 m should be adequate. If disturbing sites of present or past thermal activity is unavoidable, then it is better not to excavate the ground surface, so as not to lower overburden pressure and thereby initiate an eruption, as happened at Nakano-yu ŽYuhara, 1997.. If ground disturbance is essential, then using gravel pack fill will allow easy escape for steam. This has been done successfully at geothermal projects in Indonesia and the Philippines. Installing large areas of asphalt or concrete slabs should not be done. It is noteworthy that there are three high-temperature, liquid-dominated geothermal fields in the world, which have been exploited on a large scale with only partial, or no, reinjection of spent thermal fluid, namely Wairakei, Tiwi and Ahuachapan. All have suffered large pressure declines within the liquid zone of their reservoirs Ž) 10 bars.. It is probably not coincidental that post-exploitation hydrothermal eruptions have occurred at all three. Reinjection of spent thermal fluid is now being practised at all of them.
v
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5.2. AÕoiding eruptions It is noteworthy that almost all known historic hydrothermal eruptions, in both undisturbed and exploited fields Žexcept those with a clear magmatic character, such as the activity at Waimangu that followed the Tarawera volcanic eruption in 1886.,
5.3. PreÕenting eruptions If shallow monitor wells start to record dangerously high steam pressures, then it may be possible to vent the steam. To the best of our knowledge, however, this has not yet been done anywhere on a regular or systematic basis. An alternative approach is to inject cool or cold water to prevent steam from accumulating. A scheme for selective reinjection of separated water and condensate in this way was recently proposed for a geothermal development at Tauhara, New Zealand ŽMercury Geotherm, 1996; Menzies and Lawless, 2000..
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It may be possible to prevent eruptions by flooding the site to raise the hydrostatic pressures. This method was used in an unsuccessful attempt to stop the eruption of well WK 204 at Wairakei ŽThompson, 1976.. It was also done at Tokaanu, New Zealand, where the tailrace from a hydroelectric power station comprises an artificial pond located and designed to suppress eruptions from a thermal area there. 6. Discussion and concluding remarks Hydrothermal eruptions do not all require a fieldwide cap or cover rock, which allows reservoir pressures to exceed those of lithostatic or the tensile strength of this rock Žthough they can occur this way.. Rather, they most commonly occur in waterdominated reservoirs with water nearly at its boiling temperature, so that any local depressurisation permits it to boil. In both cases, water flashes to steam and a flashing front descends into the reservoir. Energy for boiling comes principally from the fluid, although heating by wallrocks may also contribute. The large specific volume change of water when it boils provides the mechanism for the steam, thus, generated to brecciate and eject the reservoir rocks so that a zone of rock brecciation accompanies the descending flashing front. Both precede penetration of the eruption surface itself. Eruptions may continue for as long as several hours to days, ceasing only when steam is produced at a rate insufficient to lift any more rock fragments. A large proportion of the erupted material falls back into the vent, where it is commonly later cemented and altered by steam andror hot water. Where an eruption crater erodes its former presence may be recognised by the occurrence of altered clasts within a usually circular zone of breccia deposits. Acknowledgements We thank Arnold Watson, Ian Bogie, Phil White, Brian Barnett, Tony Cartwright, Colin Harvey and Stuart Simmons for their helpful suggestions and comments. Material on the 1999 eruptions at Rotorua was kindly provided by Peter Barnett. Trevor Hunt provided much useful information including a partial
translation of the paper by Yuhara Ž1997., made by Toshi Tosha of the Geological Survey of Japan. The paper benefited substantially from reviews by two anonymous referees for AEarth Science ReviewsB. Appendix A. Descriptions of selected hydrothermal eruptions A.1. Selected examples of historic hydrothermal eruptions For convenience, we consider separately hydrothermal eruptions that have occurred as part of the ‘natural’ evolution of a geothermal system and those that clearly result from its exploitation. However, there is no evidence that there is any difference between these except in their magnitudes, as discussed below. Excluded from discussion are phreatomagmatic eruptions, such as those that occurred at Crater Lake, Ruapehu ŽNairn et al., 1979., and at Rotomahana– Waimangu in 1886 ŽLloyd and Keam, 1965; Nairn, 1979.. Similarly, the eruption at Suoh, South Sumatra in 1933 ŽStehn, 1934., has previously been quoted as an example of a very large hydrothermal eruption, but recent petrology has shown this to be of magmatic-hydrothermal origin ŽKingston Morrison, unpublished data 1994.. We select only eruptions that have been comparatively well documented, which are known to us from personal observations and have not been reported previously, or which have particularly informative characteristics. Other examples are listed in Table 1. The data in Table 1 are as presented in the original sources: no attempt has been made to assess the accuracy of parameters estimated by others. Units have, however, been converted for consistency. In addition, where the data permit, we have provided estimates of ejecta area, volume and eruptive energy. Our estimates are given in italics. Where no better data are available, to estimate areas we have assumed that the ejecta has an elliptical distribution with an obliquity of 0.7. Ejecta volumes have been calculated by assuming that the thickness drops off exponentially from the vent ŽWohletz and Heiken, 1992, Eqs. 2–7., and that the half-thickness occurs at one-fifth of the maximum radius. Analysis of the 11 examples where sufficient isopach data are available
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to determine the ratio of half-thickness radius to total radius gives a range from 2.1 to 26 Žthough with only one estimate of over 10, namely Tauhara 1981 ŽScott and Cody, 1982., which was a lateral blast and also may be unduly influenced by minor air-fall material. with a mode of 3.5 and a mean of 6, but the better-documented examples tend to support the larger ratios. Where the crater size is large compared to the radius of the ejecta, this method may tend to overestimate the volume of the ejecta. Where there are multiple events at the same location, it is the area and volume for the largest single identifiable unit that is given, not the total, since it is the size of individual eruptions that is of the most interest in this context. For estimates of eruptive energy required, we have simply used a factor of 10 Jrcm3 of ejecta. These estimates are admittedly crude, but they do give a consistent basis of comparison and the completeness of the data in most cases does not justify any more detailed analysis. A.1.1. Historical ‘natural’ hydrothermal eruptions in undisturbed fields In areas that are frequently visited by tourists, numerous small hydrothermal eruptions have been observed. Eruptions of the same magnitude in more remote areas would scarcely be noticed, as their products are quickly eroded or else covered by vegetation. For example, a small hydrothermal eruption occurred a few hundred meters away from the main tourist route at Waimangu in 1981, but this event went unrecognised until several months afterwards ŽR F. Keam, personal communication.. Very small hydrothermal eruptions have occurred at Orakeikorako, where a silica sinter terrace ŽAArtist’s PaletteB . has been locally cracked or broken from below to form broken blister forms or linear features, resembling pressure ridges, that extend laterally for only a meter or so. Larger hydrothermal eruptions occurred at Waimangu, New Zealand ŽFig. 8. in 1915, 1917, August 1924, February 22 1973, several in the period June 1978– November 1979, 2–11 May and possibly late in 1981. Other AdisturbancesB in 1975 and 1978, which disrupted the usual activity, were not large enough to be unequivocally described as eruptions. The largest eruption, in 1917, caused two fatalities as a result of a laterally directed blast. The
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Waimangu ‘Geyser’, which erupted between 1901 and 1904, also showed many features characteristic of hydrothermal eruptions ŽWarbrick, 1934, Lloyd and Keam, 1974.. It is the only known historical example of a hydrothermal eruption with a repetitive, short-period cycle of about 36 h, although there are other examples of repeat eruptions at much longer and irregular intervals from the same site, e.g. Karapiti, Rotorua, and repeated eruptions interspersed with geysering at Steamboat Geyser, Yellowstone ŽWhite et al., 1988.. The hydrothermal eruption of February 22, 1973 is the best studied one at Waimangu, although not the largest. It occurred through a hot Žabout 608C. lake in early morning darkness and lasted for at least 15 min. People sleeping 850 m away were not awakened, so it must have been a quiet event ŽLloyd and Keam, 1974.. Ejecta fell over an area of approximately 3000 m2 to a maximum thickness of 0.45 m. Undoubtedly most of the ejecta fell back into the lake but much vegetation was scalded, stripped of foliage, or killed by hot
Fig. 8. Location map of Taupo Volcanic Zone showing geothermal systems where hydrothermal eruptions have occurred and other places mentioned in the text.
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water, steam, mud, stones and blocks. Lloyd and Keam Ž1974. calculated the erupted volume as being 970 " 150 m3. The ejected material consisted of a mixture of hydrothermally altered angular blocks, up to 0.3 m diameter, some hotter than 1008C when they landed; other ejecta fell as wet mud, probably at the lake temperature. Some material was ejected to heights of at least 40 m. Lloyd and Keam Ž1974. concluded that the depth of rock disruption extended to some 90 m below the lake bottom, which itself is not more than 30 m deep. The lake increased in area by 285 " 15 m2 as a result of the eruption and subsequent slumping. Precursors to the eruption were few and feeble. About 5 weeks before it occurred, two small springs appeared at the then lake edge, and boiling muddy water splashed up to 0.5 m from them; nearby, 50-mm-diameter stones were dislodged from enlarging vents. Other new springs appeared, some hot Ž99.58C. and turbulent, but some thermal activity on-land ceased, being replaced by that offshore, where gas discharges increased. Changes in thermal activity in this young, active geothermal field are common, and it is only with hindsight that these events were recognised as being possible precursors to the February 22nd eruption. Lloyd and Keam Ž1974. believe that the eruption occurred after a fluid flow path with higher permeability was created in rocks below the lake. A small hydrothermal eruption occurred at the Ohineraki thermal area in the northern part of the Mokai ŽNew Zealand. field in 1996 ŽSam Andrews, personal communication; Kingston Morrison, 1998.. It changed the shape of the main crater but ejecta were no longer recognisable in mid-1997. A hydrothermal eruption at Ngatamariki ŽNew Zealand. occurred in 1948 and produced a crater about 10 m in diameter ŽBrotheridge et al., 1995.. This eruption was not seen but its deposits partly surround the crater. Large eruptions occurred from three adjacent craters at the Dieng field, Java, in 1979 ŽLe Guern et al., 1982.. These occurred prior to power production from the field and resulted in large loss of life Ž142 persons., which was, however, caused by the accompanying gas release rather than the impact of ejecta or thermal effects. Most people died more than a kilometre from the craters. The eruption continued
for slightly less than one day and it is likely that the gas flowed downslope from the craters. The largest crater also emitted a lahar that flowed further than 3 km. Several earthquakes occurred between 1 and 3 h before the eruption. A.1.2. Hydrothermal eruptions that haÕe occurred in exploited fields Hydrothermal eruptions are very frequent in geothermal fields undergoing exploitation. These have been mainly very small events, lasting up to a few hours and produce craters up to 50 m across ŽTable 1.. A key question in this case is whether exploitation has in some way caused the eruptions Žsince that offers clues to understanding their mechanisms and whether mitigation is feasible., or whether they are AnaturalB events, which were reported because they occur in much visited areas. Some of the observed eruptions described below, such as those at Rotorua and Tongonan, cannot be directly linked to exploitation and their occurrence during a period of exploitation appears to be coincidental. In other cases, such as at Wairakei, there is a clear link between the eruptions and exploitation-induced pressure changes in the reservoir, or modification of the overburden through landslides or excavation. Hydrothermal eruptions have occurred at both Wairakei and Tauhara ŽFig. 8. ŽAllis, 1981, 1983. and these events were undoubtedly a consequence of the exploitation of these hydrologically linked systems. Two eruptions occurred in the Tauhara field at the Taupo Pony Club, in 1974 and 1981, 7 km from the Wairakei Power Station, and products ejected by the latter have been described in detail by Scott and Cody Ž1982.. Allis Ž1979, 1984, 1986. gives detailed accounts of the Craters of the Moon ŽKarapiti. area, within the Wairakei field, where at least 20 hydrothermal eruptions have taken place in the past 20 years. The most recent events were in March 2000 ŽC.J. Bromley, personal communication.. The craters produced are only about 2–20 m in diameter and depth, but ejected mud and pumice rose to heights of about 30 m. Eruptions in exploited fields are of short duration Žhours at longest. and of shallow focus Ža few meters.. Ejecta travels only 100 m or so from their vents, although a blast may be directed laterally; for example, the eruption in 1981 at the Taupo Pony
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Club ejected material which has an asymmetric distribution around its vent. The deposit consisted of silts, muds, and pumice blocks up to 40 cm in diameter ŽScott and Cody, 1982.. Numerous small hydrothermal eruptions have occurred at the Rotorua geothermal field. The main events are documented by Donaldson Ž1985. as occurring in 1894, 1895Ž2., 1897, 1901, 1902, 1906, 1931, 1932, and 1945. Others have occurred in 1998–2000. They are included in this section as the Rotorua field has been exploited by numerous shallow wells, but many of the eruptions took place before significant exploitation began, implying exploitation was not the cause of these eruptions. Some interesting features about the eruptions at Rotorua are as follows: Ž1. Numerous eruptions have occurred from the same sites Žespecially at the mouth of the Puarenga Stream.. Ž2. Several eruptions Žalthough not well-documented. occurred as part of a general upsurge in thermal activity in the few weeks after the brief 1886 volcanic eruption at Mt. Tarawera ŽKeam, 1988.. The volcanic activity at Mt Tarawera was clearly related to emplacement of a basaltic dyke along the NE–SW structural trend of the Taupo Volcanic Zone ŽNairn, 1979 .. Concurrently, large phreatic, phreatomagmatic and magmatic-hydrothermal eruptions occurred along the strike to the SW at Rotomahana and Waimangu. However, Rotorua is too far away Ž25 km., and in a direction normal to the structural trend for this same dyke to affect thermal activity at Rotorua. Possibly a discrete en-echelon dyke was injected at Rotorua or, more probably, there was simultaneous structural dilatancy at both sites, which permitted dyke injection at Tarawera and a hydrological change permitting increased fluid flow at Rotorua. R.F. Keam Ž1998, personal communication. suggested an interesting alternative explanation, namely that slumping of lake-shore sediments at Lake Rotorua, caused by several small felt earthquakes preceding the eruption at Tarawera, either reduced the overburden or increased near-surface permeability within the hydrothermal system, thus stimulating the activity. Ž3. The series of eruptions in 1998–2000 were small and did not cause any loss of human life, but they did render some houses uninhabitable. They are
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the only examples known of eruptions occurring as a results of cessation of exploitation. The water level in the Rotorua geothermal system was drawn-down by some 7 m due to exploitation by 1987, causing a drastic reduction in natural thermal activity, including the geysers, which are a major tourist attraction. In that year, a large number of geothermal bores were compulsorily closed, and fluid withdrawal since then has been strictly regulated. This has had the effect of allowing reservoir pressures to recover by about half of the previous draw-down, and the geysers are now very active again, but it also led to hydrothermal eruptions near Kuirau Park, damaging houses, which had been built close to the thermal areas during the period of inactivity. Three hydrothermal eruptions occurred at Tiwi ŽPhilippines. in 1980–1981 ŽGrindley, 1982. following its large-scale exploitation, which began in 1977, without reinjection, caused a large drop in liquid-zone pressures. This situation was closely analogous to the Karapiti eruptions at Wairakei. One difference, however, was that at Tiwi the first eruption took place at an area ŽNaglagbong. of geysering alkali-chloride springs, which ceased to discharge and became fumarolic some weeks before the eruption. At Karapiti, the site of the eruption was always fumarolic. The significance of the Tiwi events is that the presence of alkali-chloride springs at the site of the eruptions shows that there was very little hydraulic resistance to the upflow of primary thermal fluids there prior to exploitation. Hence, there was very little physical containment of steam prior to the eruption. Three small hydrothermal eruptions occurred over a period of 6 weeks at the same site in the Bao Valley, Tongonan geothermal field, Leyte, Philippines, in August 1981 ŽJVL, personal observation; PNOC, 1982.. They left a crater about 10 m across blocking a project road, and ejecting mud to a height of 10–20 m and a radius of 20–30 m. This occurred shortly before the Tongonan power station was commissioned and there had not been any drilling or well testing within 2 km of the eruption site for several years. Hence, it is unlikely that exploitation had any relation to these eruptions. The eruptions occurred after unusually dry weather, a factor that Allis Ž1984. considered also provoked the eruptions at Karapiti. An eruption occurred near the Nakano-yu hot springs, Japan, in 1995 ŽYuhara, 1997.. There were
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four fatalities. It is not clear if the victims were buried by eruption debris per se, or by an accompanying landslide. The eruption occurred during excavation for a road, at the bottom of a 4-m-deep trench. This must be classed as an eruption due to human disturbance, presumably due to overburden removal, although not from geothermal energy exploitation. Sulphur dioxide gas Žalong with H 2 S. was detected at the site three days after the explosion, suggesting a direct magmatic volatile component was involved in the event ŽMiyake and Ossaka, 1998.. Yuhara Ž1995. refers to other historic eruptions at Hakone, Noboribetsu, Unzen, Kirishima and Aso in Japan, but insufficient information is available to justify including these in Table 1. A landslide causing damage and 23 fatalities occurred at the Zunil geothermal field in Guatemala in 1991 ŽGoff and Goff, 1997.. This caused a steam eruption due to damage of a geothermal wellhead, in an area of thermal activity. Probably, however, the cause of the landslide was slope failure of hydrothermally altered rocks rather than a hydrothermal eruption, although it has often been cited as an exploitation induced eruption. A hydrothermal eruption from the Agua Shuca thermal area at the Ahuachapan geothermal field, El Salvador in 1990 ŽBruno et al., 1992, Goff and Goff, 1997., caused 25 fatalities. Historical accounts show that there had been considerable variability in the thermal activity there and hydrothermal eruptions occurred at the same place in about 1868 and at another thermal area at Ahuachapan at an earlier but unknown date. It is possible, therefore, that the Agua Shuca eruption was a purely natural phenomenon. However, the Ahuachapan geothermal field had been under production, with only minor reinjection, since 1975, which had caused a pressure drop within the reservoir of 12 bars by 1984 ŽVides-Ramos, 1985., and this probably contributed to the cause. A.2. Examples of some prehistoric hydrothermal eruptions Inevitably only larger prehistoric hydrothermal eruption can be recognised. There have been many large, prehistoric eruptions at several New Zealand fields ŽFig. 8.. At Kawerau, 14,500 and 9000 years ago ŽNairn and Wiradiradja, 1980.; at Whakare-
warewa ŽRotorua., about 42,000 years ago ŽLloyd, 1975.; at Te Kopia where there have been at least 2 ŽBrowne et al., 1994.; on at least four occasions at Orakeikorako during an 8000-year period ŽLloyd, 1972.. Eruptions also occurred at Waiotapu, more than 13,450 years ago, and at least five, probably more violent, ones in the past 1800 years ŽLloyd, 1959; Hedenquist and Henley, 1985.. At Upper Atiamuri there were eruptions about 14,000 years ago ŽBrowne and Lloyd, 1986.; at Tikitere ŽEspanola, 1974.; and at Wairakei ŽBogie et al., 1995.. Some of these may have been phreatic or magmatic-hydrothermal rather than strictly hydrothermal, however. The prehistoric eruptions are recognised mainly from the deposits they ejected, and there have undoubtedly been many eruptions of which no geological record survives, or whose deposits have yet to be recognised. In many places Že.g. Rotokawa., the craters have been completely filled, and can only be located by mapping the distribution of ejecta. Others are of very subdued morphology, for example, a large crater at the Te Kopia field is best seen on air photos. It is also likely that there have been hydrothermal eruptions at the Ngawha field in Northland ŽSkinner, 1981; Sinclair Knight Merz, 2000.. There are six large craters there, up to 300 m in diameter, plus several smaller ones. Mining for mercury and kauri gum, and slumping of the walls have modified some. A notable feature of the Ngawha craters is that all of the large ones are about the same size, are in a generally similar state of preservation Žhaving regard to local differences in erosion rate due to hydrology, or human modification., and they do not overlap. It is probable, therefore, that all of these craters are about the same age and may have formed in a single episode in response to pressure release following the rapid draining of an extensive lake Ž2 = 5 km., possibly as recently as the Holocene. This once covered the area and deposited lacustrine sediments up to 10 m thick ŽDavey and van Moort, 1974; Fleming, 1945.. At least eight, and probably as many as thirteen, large hydrothermal eruptions have occurred in the Rotokawa area ŽFig. 8.. Probably seven took place between 11,000 and 22,700 years ago; three between 9000 and 9700 years ago; at least one took place 6060 " 60 years ago; one between 4000 and 4500 years ago, and one about 3700 years ago ŽCollar,
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1985; Collar and Browne, 1985.. It is also possible, but not proven, that there were some hydrothermal eruptions prior to 22,600 years ago. There is no evidence, however, that any large hydrothermal eruptions have occurred there in the last 1,800, and possibly the last 3400s years. The eruption events at Rotokawa were dated because their deposits are interbedded with rhyolitic tephras of regional extent ŽFig. 9. whose ages are known from 14 C dating ŽWilson and Walker, 1985.. In addition a tree buried by a hydrothermal eruption at Rotokawa was 14 C
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dated as being 6060 q 60 years old. Collar Ž1985. located the vents for the hydrothermal eruptions by measuring the sizes of clasts present in their deposits; the largest clasts occur closest to vents. This is a better guide to deducing vent positions than isopach mapping since eruption breccias are readily, but unevenly, eroded and their present day thickness is not the same as their initial thickness. Breccias are invariably poorly sorted, matrix-supported, and consist of angular to subangular fragments of various types of tuffs, rhyolite lava, and mudstone present in
Fig. 9. Composite stratigraphic section at Rotokawa Žfrom Collar, 1985; Collar and Browne, 1985.. Tephra, ignimbrite and lapilli units are of regional distribution.
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a fine-grained matrix. The 6060-year-old breccia at Rotokawa also contains logs of wood and occasional lumps of sulphur. Breccia deposits are typically massive; some contain clasts that have their longest axes vertical, showing that they impacted into soft mud; some large blocks following ballistic trajectories deformed underlying tephra on impact ŽCollar, 1985.. Many ejected clasts had been hydrothermally altered and hydraulically brecciated prior to eruption, showing that they had once been AstewingB within the reservoir. No juvenile magmatic material has been recognised in the Rotokawa deposits, indicating they are not of phreatomagmatic origin; but since the country rocks are themselves young pyroclastics, it is not certain that such material would be easily recognised. The very large volume and deep focus of these eruptions suggests they could be of phreatic or magmatic-hydrothermal origin. There is no direct evidence for this, but this would be consistent with certain features of the hydrothermal system suggesting Rotokawa has a closer magmatic affinity than similar systems in the Taupo Volcanic Zone. These features are: Ž1. the youthful nature of the system, as suggested by the persistence of low temperature Ždisequilibrium. mineral assemblages within its hotter parts; Ž2. spring waters with the highest Cl contents of any hydrothermal system in the TVZ apart from Tokaanu; and Ž3. a near-surface sulphur deposit, which is larger than the current flux of H 2 S through the system could possibly deposit in the time available ŽKrupp and Seward, 1987; KRTA, 1986.. Deposits from the eruptions cover about 12 km2 , and together they locally total 40 m in thickness. The 6060-year-old eruption ŽFig. 2. vented from the lake and ejected a mass of country rock, mud, sulphur, water and steam, which slurped out of the Rotokawa Basin into the Waikato River. This must have been a very destructive event, causing major flooding for a long distance down the river. The other hydrothermal eruptions produced deposits of more local distribution, but these also undoubtedly affected the Waikato River. The very much smaller Ngatamariki eruption in 1948, for example, reportedly discoloured the Waikato River for several days ŽR.F. Keam, personal communication.. The craters formed by the hydrothermal eruptions at Rotokawa are deduced to have been located along
a northeast-oriented structural feature extending from within the lake itself to a position about 1.5 km distant ŽFig. 2.. However, these craters, which may have been up to 250 m in diameter ŽCollar, 1985., are now completely filled with young tephra, notably Taupo Ignimbrite, which deposited in the second century AD ŽWilson and Walker, 1985.. The existence, magnitude and nature of these hydrothermal eruptions are now only evident from study of the deposits that they produced. It is, therefore, possible that there could have been several very much smaller hydrothermal eruptions of which no record is now visible. References Allis, R.G., 1979. Thermal history of the Karapiti area, Wairakei. N. Z. Dep. Sci. Ind. Res., Geophys. Div. Rep. 137. Allis, R.G., 1981. Changes in heat flow associated with exploitation of Wairakei geothermal field, New Zealand. N. Z. J. Geol. Geophys. 24, 1–19. Allis, R.G., 1983. Hydrologic changes at Tauhara geothermal field, New Zealand. N. Z. Dep. Sci. Ind. Res., Geophys. Div. Rep. 193. Allis, R.G., 1984. The 9 April 1983 steam eruption of Craters of the Moon thermal area, Wairakei. N. Z. Dep. Sci. Ind. Res., Geophys. Div. Rep. 196. Allis, R.G., 1986. Physical effects of exploitation of Wairakei Geothermal Field. Tour Guide, TVZ. N. Z. Geol. Surv. Rec. 11, 166–169. Bixley, P.R., Browne, P.R.L., 1988. Hydrothermal eruption potential in geothermal development. Proc. 10th N. Z. Geotherm. Workshop, pp. 195–198. Bogie, I., Lawless, J.V., 1987. Controls on the hydrology of large volcanically hosted geothermal systems: implications for exploration for epithermal mineral deposits. Proc. 1st PACRIM Congr., pp. 57–60. Bogie, I., Lawless, J.V., Pornuevo, J.B., 1987. Kaipohan: an apparently non-thermal manifestation of hydrothermal systems in the Philippines. J. Volcanol. Geotherm. Res. 31, 281–292. Bogie, I., Lawless, J.V., MacKenzie, K.M., 1995. Geological results from drilling in the Poihipi Žwestern. sector of the Wairakei geothermal field, New Zealand. Proc. 17th N. Z. Geotherm. Workshop, pp. 55–60. Bogie, I., Lawless, J.V., 2000. Application of mineral deposit concepts to geothermal exploration. Proc. World Geotherm. Congr. Žin press.. Bromley, C.J., Mongillo, M.A., 1994. Hydrothermal eruptions — a hazard assessment. Proc. 16th N. Z. Geotherm. Workshop, pp. 45–50. Brotheridge, J.M., Browne, P.R.L., Hochstein, M.P., 1995. The Ngatamariki geothermal field, New Zealand: surface manifestations past and present. Proc. 17th N. Z. Geotherm. Workshop, pp. 61–66.
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