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Delayed deposition of plinian pumice during phreatoplinian volcanism: the 1800-yr-B.P. Taupo eruption, New Zealand
ELSEVIER
Journal of Volcanologyand Geothermal Research 67 ( 1995) 22 l-226
Delayed deposition of plinian pumice during phreatoplinian volcanism: the 1800-yr-B.P. Taupo eruption, New Zealand R.T. Smith ‘,* , B.F. Houghton b a Geology Department, University of Canterbury, Christchurch, New Zealand ’ Institute of Geological and Nuclear Sciences, Wairakei Research Centre. Taupo, New Zealand
Received 12 December 1994; accepted 19 December 1994
Abstract The 1800-yr-B.P. Taupo eruption was one of the most violent and complex rhyolitic explosive eruptions worldwide, in the last 5000 years. The highly energetic Taupo ignimbrite was preceded by 5 phases of plinian and phreatomagmatic fall. The Phase 2 plinian Hatepe Lapilli underlies the phreatoplinian Hatepe and Rotongaio ashes (phases 3 and 4), distinguished by their fine grain size and poor sorting. In a number of proximal localities, outsized plinian pumices occur either within the Rotongaio Ash or at the Rotongaio Ash/Hatepe Ash contact. We interpret these as due to two mechanisms of delayed deposition of pumice originally erupted in Phase 2. A third contrasting type of locality involving delayed deposition of Phase 3 ash during Rotongaio volcanism results from adhering phreatoplinian ash falling or being blown or washed from branches of trees.
units 2 and 5, two coarse plinian horizons (Fig. 1). The
1. Introduction
Coeval deposition of earlier erupted fine ash with coarser-grained particles erupted later in an eruption sequence is well known (Fierstein and Hildreth, 1992) and can be readily explained in terms of the longer transport times of the finer particles. Here, in contrast, we describe earlier erupted plinian pumices found in later fine-grained phreatoplinian ash from the 1800-yrB.P. Taupo eruption in New Zealand. This eruption sequence is notable for the alternation of units of markedly contrasting grain size. Unit 1, the Initial ash, is a small, locally dispersed deposit (Wilson and Walker, 1985) which will not be discussed further here. Units 3 and 4 of the eruption (the Hatepe and Rotongaio ashes, respectively) were described as type examples of phreatoplinian fall deposits (Self and Sparks, 1978; Walker, 1981), and sit enclosed within * Corresponding
author
0377-0273/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSD10377-0273(95)00004-6
phreatoplinian deposits are characterised by both wide dispersal and extremely fine grain size, particularly Unit 4, samples from which have median diameters of between 32 and 64 pm (4-5 4) in even the most proximal sections. We present below evidence for the occurrence of outsized pumice clasts in Unit 4 in two types of proximal depositional setting (sites A, B, and C on Fig. 1 ), and interpret these as due to two separate mechanisms of delayed deposition of plinian pumice erupted in Phase 2. A third type of locality involved delayed deposition during Phase 4 volcanism of ash initially erupted during Phase 3 (Fig. 1) . In cases where earlier erupted clasts are incorporated into later deposits, it is important to distinguish between the time when a clast is first erupted and the time at which it becomes permanently included in the eruption products. We therefore use Phase and Unit as time and
222
R.T. Smith, B.F. Houghton/Journal
a
0,
7
of Volcanology and Geothermal Research 67 (1995) 221-226
1,o
km
LOCALITY TYPES 0 l
lake-borne pumices tree-lodged pumices --I eous ash pods
“D
. ~
Fig. I. (a) Map showing proximal localities and vent areas for the phreatopliniau ashes. New vent sites postulated for the eruption of Phases 3 and 4 are shown, as is the Horomatangi Reefs site (HR) previously thought to be the vent for the entire 1800-yr-B.P. eruption. (b) Summary stratigraphy for the deposits of the 1800-yr-B.P. Taupo eruption, after Walker ( 1981). The columns headed wafer and magma represent relative quantities in the eruption column. Note the evidence for gullying and erosion at the Unit 3/Unit 4 boundary and within Unit 4, as a result of rain of water expelled from the vent.
stratigraphic subdivisions, respectively. A pumice erupted in Phase 2 may, for example, be incorporated into Unit 3 or 4. 2. Large isolated pumice Outsized pumice clasts at a single proximal location occur widely dispersed along the Unit 3/Unit 4 (Hatepe/Rotongaio) contact (site C) . Features of these clasts (Fig. 2a) are that they are remarkably uniform in size with average diameters of 80-120 mm, and are not accompanied by aerodynamically equivalent dense lithics. There are no lithic clasts present at this level and, even above and below this level, the largest Ethics (3-5 mm) are in hydraulic equivalence with the much finer grained Phase 3 and Phase 4 juvenile clasts. The grain size of the outsized pumices shows no overlap at all with the size distribution of Phase 4 clasts at the site, but they are identical to the coarsest clasts in Unit 2 (Fig. 3). Phase 2 represents the only possible source for these clasts which are larger than any form of juvenile or lithic clast known to have been erupted during Phase 4. We model these clasts as the
coarse tail of the largest and least dense pumices of Phase 2, which were lodged in tree branches throughout the period of deposition of Unit 2, through the switch to phreatomagmatic volcanism and during deposition of Unit 3. Abundant moulds of tree branches up to 10 cm in diameter, are present in units 2 to 4 of the 1800 yr B.P. sequence at this site. Several factors combined to make the Phase 3/Phase 4 transition a favourable time for dislodgement of the pumices. The transition was marked by a break in deposition accompanied by intense gullying associated with water expelled from the vent region, which rained out from the eruption column (Walker, 198 1) . This was followed by rapid accumulation of the wet, extremely cohesive Unit 4 ash. Secondly, this transition was marked by a previously unrecognised shift in vent position (Fig. 1), and this translation was probably accompanied by seismicity and ground motion enhancing the possibility of dislodging the pumice. A partial analog to this process is at Mount St. Helens, where plinian pumice from the May 18, 1980 eruption was observed to lodge and then fall from trees (Waitt and Dzurisin, 1980) _
R. T. Smith. B. F. Houghton/Journal
of Volcanology
and Geoihermal
Research 67 (1995) 221-226
223
Fig. 2. (a) Photograph of site C showing a large isolated pumice (P) at the Unit 3/Unit 4 boundary. (b). Photograph of site D showing; !enses (t.m.) in the underlying units.
of Phase 3 ash (p.~.) within Unit 4. Note the abundance and location of tree moulds
2.1. Punziceous ash interculutions Al: more distal localities, this - 64 size fraction, that appe ars to have been optimal for lodgement, ceased to
be a component of the Phase 2 plinian fall, and we do
not see evidence for delayed plinian pumice depcxition in Units 3 and 4 at these localities. However, tlhere is abundant evidence for a variation on the mechan lism of
224
R.T. Smith, B. F. Houghton/Journal
of Volcanology and Geothermal Research 67 (I 995) 221-226
T-L
Unit 3 Unit 2
-8
-4
0
size
4
8
6
Fig. 3. Plot of grain size distribution for proximal samples of Units 2 (crosses) and 4 (black),and maximum pumice diameters for Units 2 and 3 and for the isolated pumices (T-L).
tree-lodgement. Numerous small lenses and pods of creamy pumiceous ash, are found interbedded with the dark grey Unit 4 at localities marked in Fig. 1, often concentrated at specific partings (Fig. 2b). We interpret these lenses of pumiceous ash within the Unit 4 ash to be derived from ash accumulated on branches during Phase 3 volcanism. Walker (1981) has previously described the phenomenon of coatings of cohesive Units 3 and 4 on the upper surface of branch moulds buried within the upper portions of the Taupo sequence. Ground motion at the localities we describe may have been sufficiently strong, or the rain out of water during the later intra-Phase 4 gullying episodes so intense, that Phase 3 ash had only a very transient residence time on branches before being shaken or blown (or washed) off the branches and incorporated into Unit 4. The absence of well developed rilling and reworked Phase 4 ash at these sites (Fig. 2b) suggests ground shaking or wind action were the dominant mechanisms.
3. Lake-borne pumice Two additional proximal sites (A and B in Fig. 1) reveal a complex history for the syneruptive ancestral Lake Taupo, and coeval reworking of Phase 2 pumice and primary deposition of Phase 4 tephra at the lake margin. The two localities show a striking intercalation of well-sorted coarse, ‘plinian-style’ pumice beds with the grey Phase 4 ash. At site A, Unit 4 consists of an
alternation of grey Phase 4 fall ash, and white matrixand lithic-free pumice lapilli beds characterised by little or no grading and slight rounding of the pumice. The lower, grey fine-ash beds are characterised by bed forms indicating shallow water deposition but an upward change from low angle cross-stratification to thicker, massive beds with mud lump textures records a rapid transition to subaerial deposition (Fig. 4). We envisage that the setting for these deposits was the very edge of the ancestral Lake Taupo. This has important implications for paleohydrology because it indicates a third point in the eruption sequence when the geometry of ancestral Lake Taupo is known, Wilson and Walker ( 1985) having already established the margins of the lake prior to eruption and at the time of the Phase 6 Taupo ignimbrite. It is also important because even at Phase 4, after the eruption of 5.8 km3 of magma and perhaps 2-3 km” of water, the eastern side of the lake extended beyond the limits of the modern lake. Even more importantly, the lake was choked by a collar of Phase 2 pumice through phases 3 and 4 of the eruption. We picture the Unit 4 sequence at site A, forming by an alternation of rain out of primary Phase 4 phreatoplinian ash settling in shallow water with the arrival of seiches, possibly related to hydrothermal or phreatomagmatic explosions, bearing waterlogged lake-rafted Phase 2 (and possibly Phase 3) pumice. The whole sequence shoaled rapidly and ultimately became subaerial at which stage the involvement of the floated Phase 2 pumice ceased. 4. Conclusions The proximal exposures at Taupo reveal two situations where final deposition of plinian pumice was delayed by perhaps days or weeks, leading to its incorporation in younger deposits of contrasting eruptive style. These mixed deposits represent a trivial proportion of the total volume of the Taupo eruption, and even a trivial percentage of total outcrops, but they indicate some of the subtleties of deposition processes in even sustained explosive events.
Acknowledgements This research was funded by the Foundation Of Research, Science and Technology and the University
R. T. Smith, B. F. Houghton /Journal of Volcanology and Geothermal Research 67 (I 995) 221-226
225
-b
(a)
c
2
e-f
3
OS
9 -
2
h i -
1
i
! -4
0
L metres
8
o lake-borne pumice
0 subaerial Unit 4
0 subaerial Unit 2
Q subaqueous Unit 4
I
Fig. 4. (a) Sketch of the Unit 4 stratigraphy at site A (beds b to i)and plot of median diameter versus Inman sorting coefficient for samples of Unit 2 and Unit 4. The lake-borne pumice horizons in Unit 4 cannot be derived in any way from the much finer-grained primary Phase 4 material but can be derived from primary Phase 2 plinian fall by removal of lithic clasts and ash-sized matrix. (Letters are used to cross reference beds to Fig. 4b). (b) Photograph of Unit 4 at site A showing the alternation of lake-borne pumice layers and primary Phase 4 ash.
226
R.T. Smith. B. F. Houghton/Journal
of Volcanology and Geothermal Research 67 (1995) 221-226
Vice Chancellors’ Committee. It builds on the innovative volcanological studies at Taupo by G.P.L. Walker and C.J.N. Wilson. We are grateful for constructive reviews by K.A. Hodgson, T. Thordarson, and C.J.N. Wilson. References Fierstein. J. and Hildreth, E.W., 1992. The plinian eruptions of 1912 at Novarupta, Katmai National Park, Alaska. Bull. Volcanol., 54: 64&684.
Self, S. and Sparks, R.S.J., 1978. Characteristics of widespread pyroelastic deposits formed by the interaction of silicic magma and water. Bull. Volcanol., 41: 196212. Waitt, R.B. and Dzurisin, D., 1980. Proximal fall deposits from the May 18 eruption --stratigraphy and field sedimentology. U.S. GeoLSurv., Prof. Pap., 1250: 601-616. Walker, G.P.L., 1981. Characteristics of two phreatoplinian ashes and their water-flushed origin. J. Volcanol. Geotherm. Res., 9: 395-407. Wilson, C.J.N. and Walker, G.P.L., 1985. The Taupo eruption, New Zealand. 1. General aspects. Philos. Trans. R. Sot. London, Ser. A, 314: 199-228.
Journal of Volcanologyand Geothermal Research 67 ( 1995) 22 l-226
Delayed deposition of plinian pumice during phreatoplinian volcanism: the 1800-yr-B.P. Taupo eruption, New Zealand R.T. Smith ‘,* , B.F. Houghton b a Geology Department, University of Canterbury, Christchurch, New Zealand ’ Institute of Geological and Nuclear Sciences, Wairakei Research Centre. Taupo, New Zealand
Received 12 December 1994; accepted 19 December 1994
Abstract The 1800-yr-B.P. Taupo eruption was one of the most violent and complex rhyolitic explosive eruptions worldwide, in the last 5000 years. The highly energetic Taupo ignimbrite was preceded by 5 phases of plinian and phreatomagmatic fall. The Phase 2 plinian Hatepe Lapilli underlies the phreatoplinian Hatepe and Rotongaio ashes (phases 3 and 4), distinguished by their fine grain size and poor sorting. In a number of proximal localities, outsized plinian pumices occur either within the Rotongaio Ash or at the Rotongaio Ash/Hatepe Ash contact. We interpret these as due to two mechanisms of delayed deposition of pumice originally erupted in Phase 2. A third contrasting type of locality involving delayed deposition of Phase 3 ash during Rotongaio volcanism results from adhering phreatoplinian ash falling or being blown or washed from branches of trees.
units 2 and 5, two coarse plinian horizons (Fig. 1). The
1. Introduction
Coeval deposition of earlier erupted fine ash with coarser-grained particles erupted later in an eruption sequence is well known (Fierstein and Hildreth, 1992) and can be readily explained in terms of the longer transport times of the finer particles. Here, in contrast, we describe earlier erupted plinian pumices found in later fine-grained phreatoplinian ash from the 1800-yrB.P. Taupo eruption in New Zealand. This eruption sequence is notable for the alternation of units of markedly contrasting grain size. Unit 1, the Initial ash, is a small, locally dispersed deposit (Wilson and Walker, 1985) which will not be discussed further here. Units 3 and 4 of the eruption (the Hatepe and Rotongaio ashes, respectively) were described as type examples of phreatoplinian fall deposits (Self and Sparks, 1978; Walker, 1981), and sit enclosed within * Corresponding
author
0377-0273/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSD10377-0273(95)00004-6
phreatoplinian deposits are characterised by both wide dispersal and extremely fine grain size, particularly Unit 4, samples from which have median diameters of between 32 and 64 pm (4-5 4) in even the most proximal sections. We present below evidence for the occurrence of outsized pumice clasts in Unit 4 in two types of proximal depositional setting (sites A, B, and C on Fig. 1 ), and interpret these as due to two separate mechanisms of delayed deposition of plinian pumice erupted in Phase 2. A third type of locality involved delayed deposition during Phase 4 volcanism of ash initially erupted during Phase 3 (Fig. 1) . In cases where earlier erupted clasts are incorporated into later deposits, it is important to distinguish between the time when a clast is first erupted and the time at which it becomes permanently included in the eruption products. We therefore use Phase and Unit as time and
222
R.T. Smith, B.F. Houghton/Journal
a
0,
7
of Volcanology and Geothermal Research 67 (1995) 221-226
1,o
km
LOCALITY TYPES 0 l
lake-borne pumices tree-lodged pumices --I eous ash pods
“D
. ~
Fig. I. (a) Map showing proximal localities and vent areas for the phreatopliniau ashes. New vent sites postulated for the eruption of Phases 3 and 4 are shown, as is the Horomatangi Reefs site (HR) previously thought to be the vent for the entire 1800-yr-B.P. eruption. (b) Summary stratigraphy for the deposits of the 1800-yr-B.P. Taupo eruption, after Walker ( 1981). The columns headed wafer and magma represent relative quantities in the eruption column. Note the evidence for gullying and erosion at the Unit 3/Unit 4 boundary and within Unit 4, as a result of rain of water expelled from the vent.
stratigraphic subdivisions, respectively. A pumice erupted in Phase 2 may, for example, be incorporated into Unit 3 or 4. 2. Large isolated pumice Outsized pumice clasts at a single proximal location occur widely dispersed along the Unit 3/Unit 4 (Hatepe/Rotongaio) contact (site C) . Features of these clasts (Fig. 2a) are that they are remarkably uniform in size with average diameters of 80-120 mm, and are not accompanied by aerodynamically equivalent dense lithics. There are no lithic clasts present at this level and, even above and below this level, the largest Ethics (3-5 mm) are in hydraulic equivalence with the much finer grained Phase 3 and Phase 4 juvenile clasts. The grain size of the outsized pumices shows no overlap at all with the size distribution of Phase 4 clasts at the site, but they are identical to the coarsest clasts in Unit 2 (Fig. 3). Phase 2 represents the only possible source for these clasts which are larger than any form of juvenile or lithic clast known to have been erupted during Phase 4. We model these clasts as the
coarse tail of the largest and least dense pumices of Phase 2, which were lodged in tree branches throughout the period of deposition of Unit 2, through the switch to phreatomagmatic volcanism and during deposition of Unit 3. Abundant moulds of tree branches up to 10 cm in diameter, are present in units 2 to 4 of the 1800 yr B.P. sequence at this site. Several factors combined to make the Phase 3/Phase 4 transition a favourable time for dislodgement of the pumices. The transition was marked by a break in deposition accompanied by intense gullying associated with water expelled from the vent region, which rained out from the eruption column (Walker, 198 1) . This was followed by rapid accumulation of the wet, extremely cohesive Unit 4 ash. Secondly, this transition was marked by a previously unrecognised shift in vent position (Fig. 1), and this translation was probably accompanied by seismicity and ground motion enhancing the possibility of dislodging the pumice. A partial analog to this process is at Mount St. Helens, where plinian pumice from the May 18, 1980 eruption was observed to lodge and then fall from trees (Waitt and Dzurisin, 1980) _
R. T. Smith. B. F. Houghton/Journal
of Volcanology
and Geoihermal
Research 67 (1995) 221-226
223
Fig. 2. (a) Photograph of site C showing a large isolated pumice (P) at the Unit 3/Unit 4 boundary. (b). Photograph of site D showing; !enses (t.m.) in the underlying units.
of Phase 3 ash (p.~.) within Unit 4. Note the abundance and location of tree moulds
2.1. Punziceous ash interculutions Al: more distal localities, this - 64 size fraction, that appe ars to have been optimal for lodgement, ceased to
be a component of the Phase 2 plinian fall, and we do
not see evidence for delayed plinian pumice depcxition in Units 3 and 4 at these localities. However, tlhere is abundant evidence for a variation on the mechan lism of
224
R.T. Smith, B. F. Houghton/Journal
of Volcanology and Geothermal Research 67 (I 995) 221-226
T-L
Unit 3 Unit 2
-8
-4
0
size
4
8
6
Fig. 3. Plot of grain size distribution for proximal samples of Units 2 (crosses) and 4 (black),and maximum pumice diameters for Units 2 and 3 and for the isolated pumices (T-L).
tree-lodgement. Numerous small lenses and pods of creamy pumiceous ash, are found interbedded with the dark grey Unit 4 at localities marked in Fig. 1, often concentrated at specific partings (Fig. 2b). We interpret these lenses of pumiceous ash within the Unit 4 ash to be derived from ash accumulated on branches during Phase 3 volcanism. Walker (1981) has previously described the phenomenon of coatings of cohesive Units 3 and 4 on the upper surface of branch moulds buried within the upper portions of the Taupo sequence. Ground motion at the localities we describe may have been sufficiently strong, or the rain out of water during the later intra-Phase 4 gullying episodes so intense, that Phase 3 ash had only a very transient residence time on branches before being shaken or blown (or washed) off the branches and incorporated into Unit 4. The absence of well developed rilling and reworked Phase 4 ash at these sites (Fig. 2b) suggests ground shaking or wind action were the dominant mechanisms.
3. Lake-borne pumice Two additional proximal sites (A and B in Fig. 1) reveal a complex history for the syneruptive ancestral Lake Taupo, and coeval reworking of Phase 2 pumice and primary deposition of Phase 4 tephra at the lake margin. The two localities show a striking intercalation of well-sorted coarse, ‘plinian-style’ pumice beds with the grey Phase 4 ash. At site A, Unit 4 consists of an
alternation of grey Phase 4 fall ash, and white matrixand lithic-free pumice lapilli beds characterised by little or no grading and slight rounding of the pumice. The lower, grey fine-ash beds are characterised by bed forms indicating shallow water deposition but an upward change from low angle cross-stratification to thicker, massive beds with mud lump textures records a rapid transition to subaerial deposition (Fig. 4). We envisage that the setting for these deposits was the very edge of the ancestral Lake Taupo. This has important implications for paleohydrology because it indicates a third point in the eruption sequence when the geometry of ancestral Lake Taupo is known, Wilson and Walker ( 1985) having already established the margins of the lake prior to eruption and at the time of the Phase 6 Taupo ignimbrite. It is also important because even at Phase 4, after the eruption of 5.8 km3 of magma and perhaps 2-3 km” of water, the eastern side of the lake extended beyond the limits of the modern lake. Even more importantly, the lake was choked by a collar of Phase 2 pumice through phases 3 and 4 of the eruption. We picture the Unit 4 sequence at site A, forming by an alternation of rain out of primary Phase 4 phreatoplinian ash settling in shallow water with the arrival of seiches, possibly related to hydrothermal or phreatomagmatic explosions, bearing waterlogged lake-rafted Phase 2 (and possibly Phase 3) pumice. The whole sequence shoaled rapidly and ultimately became subaerial at which stage the involvement of the floated Phase 2 pumice ceased. 4. Conclusions The proximal exposures at Taupo reveal two situations where final deposition of plinian pumice was delayed by perhaps days or weeks, leading to its incorporation in younger deposits of contrasting eruptive style. These mixed deposits represent a trivial proportion of the total volume of the Taupo eruption, and even a trivial percentage of total outcrops, but they indicate some of the subtleties of deposition processes in even sustained explosive events.
Acknowledgements This research was funded by the Foundation Of Research, Science and Technology and the University
R. T. Smith, B. F. Houghton /Journal of Volcanology and Geothermal Research 67 (I 995) 221-226
225
-b
(a)
c
2
e-f
3
OS
9 -
2
h i -
1
i
! -4
0
L metres
8
o lake-borne pumice
0 subaerial Unit 4
0 subaerial Unit 2
Q subaqueous Unit 4
I
Fig. 4. (a) Sketch of the Unit 4 stratigraphy at site A (beds b to i)and plot of median diameter versus Inman sorting coefficient for samples of Unit 2 and Unit 4. The lake-borne pumice horizons in Unit 4 cannot be derived in any way from the much finer-grained primary Phase 4 material but can be derived from primary Phase 2 plinian fall by removal of lithic clasts and ash-sized matrix. (Letters are used to cross reference beds to Fig. 4b). (b) Photograph of Unit 4 at site A showing the alternation of lake-borne pumice layers and primary Phase 4 ash.
226
R.T. Smith. B. F. Houghton/Journal
of Volcanology and Geothermal Research 67 (1995) 221-226
Vice Chancellors’ Committee. It builds on the innovative volcanological studies at Taupo by G.P.L. Walker and C.J.N. Wilson. We are grateful for constructive reviews by K.A. Hodgson, T. Thordarson, and C.J.N. Wilson. References Fierstein. J. and Hildreth, E.W., 1992. The plinian eruptions of 1912 at Novarupta, Katmai National Park, Alaska. Bull. Volcanol., 54: 64&684.
Self, S. and Sparks, R.S.J., 1978. Characteristics of widespread pyroelastic deposits formed by the interaction of silicic magma and water. Bull. Volcanol., 41: 196212. Waitt, R.B. and Dzurisin, D., 1980. Proximal fall deposits from the May 18 eruption --stratigraphy and field sedimentology. U.S. GeoLSurv., Prof. Pap., 1250: 601-616. Walker, G.P.L., 1981. Characteristics of two phreatoplinian ashes and their water-flushed origin. J. Volcanol. Geotherm. Res., 9: 395-407. Wilson, C.J.N. and Walker, G.P.L., 1985. The Taupo eruption, New Zealand. 1. General aspects. Philos. Trans. R. Sot. London, Ser. A, 314: 199-228.