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Mt. Niragongo: renewed activity of the lava lake
Journal of Volcanology and Geothermal Research, 20 (1984) 267--280
267
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
Mt. NIRAGONGO: RENEWED ACTIVITY OF THE LAVA LAKE
H. T A Z I E F F
15 Quai de Bourbon, Paris, 75004 (France) (Received February 21, 1983; revised and accepted July 8, 1983)
ABSTRACT Tazieff, H., 1984. Mt. Niragongo: renewed activity of the lava lake. J. Volcanol. Geotherm. Res., 20: 267--280. On the 21st of June, 1982, Mt. Niragongo ended a period of dormancy that had begun on January 11, 1977, and fresh lava began to flow into the 800-m-deep crater. On October 3, a huge lava lake, wider and deeper than any previously observed (500 m across and close to 400 m deep) rose to within 440 m of the crater rim. The observed activity consisted of a large, central upweUing fountain of very fluid lava from which concentric lava waves expanded radially; numerous small, relatively viscous lava flows creeped over the surrounding thin solidified crust, that covered about 95% of the lake area. These observed features seem to characterize the upper part of a large convective system. The persistence of such an extraordinarily large steady~state lava lake may be due to the equaily exceptional fluidity of the magma rising at the intersection of four different tectonic trends of fractures in the subvolcanic basement.
INTRODUCTION
Together with its close neighbour, Nyamlagira, as well as Kilauea in Hawaii, Erta'Ale in the Afar depression, and Mt. Erebus in Antarctica, Mr. Niragongo belongs to the very exceptional kind of volcano with permanent (or steady-state) lava lakes. Unlike Kilauea and Erta'Ale volcanoes, Niragongo and Erebus are intracontinental. They are also unusual, because of their respective rock types: Mt. Erebus is the only volcano presently erupting Kenites (Kyle and Cole, 1974), while Niragongo is the only one in the world to emit leucite-, melilite-, and nepheline-bearing basanitic lavas (Sahama, 1962, 1973). The chemical compositions of these very undersaturated basanites accounts for their unusually high fluidity. The first reference to persistent lava activity in the deep crater of Mt. Niragongo dates from 1928. The lava lake was discovered only in 1948 (Tazieff, 1949) and has since been observed and studied for close to thirty years (Fig. 1). On January 19th 1977, the volcanic edifice split open and the lava lake was suddenly drained, together with magma in the feeding fractures above the lowest levels of the new erupting fissures, about 1700 m a.s.1. The 0377-0273/84]$03.00
O 1984 Elsevier Science Publishers B.V.
268
:o_f_lakeedward ~----~,~ )
...'
Fig. 1. Virunga volcanoes range. The four tectonic trends observed on Mt. Niragongo do appear on this general map: the N--W one linking Niragongo to Nyamlagira, as well as Karisimbi to Mikeno, the N--S one of the Great African Rift (to the west of lake Edward and the alignment of Niragongo and its two main satellites), the W--NW one (the 1948 fracture) and the EW one (Muhavura--Sabinyo and Visoke--Mikeno). h y d r o s t a t i c pressure b e t w e e n t h e lake level, 3 3 0 0 m a.s.l., and these l o w e r outlets, t o g e t h e r w i t h the magma's e x c e p t i o n a l fluidity, a c c o u n t f o r the p o w e r f u l f o r c e and high v e l o c i t y (> 100 k m / h ) with which t h e m a g m a r u s h e d o u t (Tazieff, 1977). T h e s u d d e n e m p t y i n g o f b o t h t h e lake and its large system o f feeding fractures caused t h e i m m e d i a t e collapse o f t h e wide c o n c e n t r i c terraces t h a t had b e e n built u p during t h e previous c e n t u r y (or centuries) w i t h i n t h e m o r e t h a n o n e k i l o m e t e r - w i d e crater. T h e resulting chasm, a b o u t 8 0 0 m deep, consisted o f a sub-cylindrical u p p e r part, a p p r o x i m a t e l y 500 m deep, and a f u n n e l - s h a p e d l o w e r part, a b o u t 300 m deep, with walls consisting o f huge b l o c k y a p r o n s resulting f r o m t h e collapse o f the terraces. T h e l o w e r m o s t level o f t h e c r a t e r was e s t i m a t e d t o be 2 6 7 0 m a.s.1. (Fig. 2). T h e s e d e p t h s have been calculated p h o t o g r a m m e t r i c a l l y b y M. Bacchus ( I n s t i t u t Geograp h i q u e National, Paris) and p r o v e to be 50 to 100 m shallower t h a n t h o s e
269
Fig. 2. The Niragongo chasm in January 1977. The white dyke is to the south. Remnants of the upper terrace mark the 3270 m level. The huge conical fallen debris apron starts at an altitude ca. 2800 m. e s t i m a t e d in 1 9 7 7 (Tazieff, 1 9 7 7 ) . All t h e depths, widths, and v o l u m e s o f t h e p r e s e n t lava lake have b e e n o b t a i n e d t h a n k s t o M. B a c c h u s ' calculations. ACTIVITY SINCE 1977 F o l l o w i n g t h e 1 9 7 7 e r u p t i o n , all activity ceased f o r over five years. It res u m e d in 1 9 8 2 , t h e first n o t i c e c o m i n g o n J u n e 21st, w h e n a m o l t e n lava p o n d and a single s p a t t e r - c o n e were seen f r o m t h e rim t o be near the b o t t o m o f t h e c r a t e r w h e r e a f l o o r of rain-washed debris had started t o develop (Fig. 3) (E. G a u t h i e r , pers. c o m m u n . , 1982). On J u n e 21, a lava lake 3 0 0 m across, fed b y t w o eruptive vents l o c a t e d respectively 2 8 9 0 m and 2 9 9 0 m a.s.l., was developing at 2 8 0 0 m a.s.1. (Fig. 4). B y J u l y 5, t h e lava lake was 4 6 0 m across and 2 9 0 0 m a.s.1. On J u l y 11, a wide lava lake was observed at a p p r o x i m a t e l y t h e same level. Its area was abou.t 1 2 5 , 0 0 0 m 2, t h e main p a r t ( a b o u t 95%) o f w h i c h was e n c r u s t e d b y a t h i n slab o f solidified lava. A central p o n d , a b o u t 2 0 , 0 0 0 m 2 in area, d i s p l a y e d an i n c a n d e s c e n t surface (M. Pieper, pers. c o m m u n . , 1 9 8 2 ) . A t t h a t date, t h e lake was a p p r o x i m a t e l y 200 m d e e p and c o n t a i n e d a b o u t 9 X 106 m 3 o f m o l t e n lava. On A u g u s t 4th, the lake was entirely c r u s t e d over e x c e p t f o r t w o large f o u n t a i n s c o r r e s p o n d i n g t o the initial vents
270
Fig. 3. The lower part of the chasm in early 1982 showing huge 1977 collapse debris aprons, 300 m high, and a small debris-covered floor about 30 m thick, formed within 4 years. (and spatter-cones). Its surface, 3050 m a.s.l., was a b o u t 370,000 m 2, and its volume a b o u t 44 × 106 m 3 (Fig. 5). F r o m August 5th to August 13 or 14, t h e activity apparently s t oppe d (A. de Munck and M. Pieper, pers. c o m m u n . , 1982). On O c t o b e r 3rd, when the a u t h o r revisited Mt. Niragongo t o g e t h e r with the latter two informants, t he lake occupied t he whole width of the crater up to a b o u t 380 m below the crater rim (i.e.: a b o u t 3090 m a.s.1.) (Fig. 6). T he d e p t h and volume of the lava were thus of the order of 390 m and 65 × 106 m 3 respectively. This volume is far larger than any observed before. Jaggar (1917) measured a de pt h of 40 feet for the Halemaumau lava lake. (The volume o f the lakes o f Erebus, Erta'Ale and the f o r m e r lakes of Nira-
271
Fig. 4. The 1982 lava lake in its early stage (June 21st). Two eruptive vents, approx. 180 m apart, with spatter-built cones about 40 m high are located at both ends of an eruptive fracture hidden by the new lava flows. The lake was then ca. 340 m across. (photo C. Peyer). g o n g o can n o t be c o m p u t e d , b e c a u s e t h e i r d e p t h s c o u l d n o t be m e a s u r e d or even estimated.} T h e average lava o u t p u t d u r i n g t h e t h r e e first w e e k s o f r e n e w e d activity h a d t h u s b e e n o f t h e o r d e r o f 9 × 106 m 3, i.e. a b o u t 4.5 × 10 s m 3 p e r d a y ; d u r i n g t h e t h r e e f o l l o w i n g w e e k s it was a b o u t 1.4 × 106 m 3 p e r d a y and d r o p p e d t o 3.5 × 10 s m 3 p e r d a y d u r i n g t h e n e x t t w o m o n t h s . T h e daily average f r o m t h e s t a r t t o early O c t o b e r 1 9 8 2 was 6.5 × l 0 s m 3. T h e activity o b s e r v e d o n J u l y 5 t h c o n s i s t e d o f c o n t i n u o u s lava-fountaining w i t h i n t h e lake a n d t w o s p o r a d i c a l l y s p i t t i n g s p a t t e r - c o n e s . T h e degassing f o u n t a i n s w e r e a b o u t 50 m across. On J u l y l l t h , t h e l o w e r o f t h e t w o s p a t t e r - c o n e s h a d d i s a p p e a r e d b e l o w t h e s u r f a c e o f t h e rising lake, b u t t h e
272
/ /
(
4august1982 3020
~lk~~ / ~ " , ,
Fig. 5. The lava lake on Aug. 4th 1982, as drawn by M. Bacchus from a photo by M. Bebrone.
Lavaemission 215.82
PhotosHchPEYER,5isengi 25./,.82et 21.6.82 Fig. 6. The successive levels of the 1982 lava lake, as plotted (from oblique photographs) by M. Bacchus.
273 fountains were located at t he same place and were of the same magnitude as before. T h e apparently excentric location o f the violently bubbling fountains p h o t o g r a p h e d in early July was due to the presence of exceptionally large debris aprons along t he southwestern floor of the circular crater-wall (Fig. 3). On O c t o b e r 3rd, no spatter cones could be seen nor any visible activity that could be related to these d r o w n e d features. As before, fountains, 40 to 50 m across, were located in the central part of the lake, the level of which was n o w above the u p p e r apex o f the ramps of fallen rocks. The continuous bubbling was violent enough to hurl masses of m ol t en magma weighing several tons to heights of up to 100 m (Fig. 7). The continuous gas o u t p u t was o f the same or der of magnitude as t hat measured in 1972, i.e. 284 000 t o n s / d a y , (77,000 tons H20, 184,000 tons CO2 and 23,000 tons SO2) (Le Guern, 1980).
Fig. 7. The lava lake on Oct. 3rd 1982. The molten part was about 200 m across, the boiling fountains being ca. 50 large. The outer rim of the very thinly encrusted surface is thickened to several meters by squeezed out, comparatively viscous, lava flows. The t e m p e r a t u r e of the bubbling lava was 1100 -+ 30°C. Using the figures o f B o n n e t (1960), w ho had calculated the heat o u t p u t of the 1959 Niragongo lava lake (930 MW), the o u t p u t on O c t o b e r 4th, 1982, was estimated at a b o u t 35 × 103 MW, i.e. ca. f o r t y times greater. Measurements carried of the lava lake o f Erta'Ale ( Z e t t w o o g et al., 1972), giving heat flows Q1 = 14
274 W cm -2 for red lava surfaces and Q2 = 2.1 W cm -2 for black surfaces. If the same rates hold for Niragongo, they indicate a total heat o u t p u t of ca. 25 × 103 MW, i.e. of the same order of magnitude that obtained by Bonnet's method. In October 1982, no currents could be seen but the continuous, violent fountaining engendered large, concentrically expanding, waves, several meters high with a wave-length of ca. 60 m. Notwithstanding the fact that no currents were actually observed, it seems obvious that a radial transfer of molten lava is continuously occurring, as indicated by the numerous flows, a few meters wide and a few tens of meters long, that, during our stay, were continuously emitted here and there along the edge of the huge encrusting slab covering the main outer part of the lake. These flows consisted of degassed lavas, cooled down to about 960 +- 30°C, and were short-lived, stopping and solidifying after moving for five or ten minutes. A circular band, a few tens of meters wide, of these solidified small lava flows, adds several meters the otherwise very thin crust, which is initially of the order of a few centimeters thick. The o u t p u t of these small outflows was estimated at 20,000 or 30,000 m3/day (Fig. 8). This comparatively modest outflow is nevertheless important because of the evidence it provides of outwardly oriented, radial currents below the outer crust of the lake. We had no opportunity to observe large overflows of lava, but the appearance of the lake's encrusted surface suggests that swift
Fig. 8. The outer rim of the lake, thickened by squeezed-out flows. (photo Besseville).
275
and expansive overflows characterize the present activity, as they did dozens of times during our three-weeks-long residence in the crater during 1972. They lasted only a few minutes b u t poured over the rim of the lake because of the rapid upwelling of the feeding magma. The extraordinarily fluid lavas rapidly covered a large part o f the solidified crust, and on many occasions, its total surface. The uprise of the present lake-level most probably occurs in the same way, the fluidity of the lava being of the same order of magnitude as in 1972. The lava lake evolution observed during the 1982 activity has been traced photogrammetrically by M. Bacchus (Figs. 9 and 10).
9
1st platform
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2900
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2800
/
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~J2700 2670 2500
Fig. 9. Map of the crater. Showing successive lake levels, and remnants of the 3267 m. a.s.1, platform, that collapsed in 1977.
N A T U R E OF THE LAVA LAKE
Permanent lava lakes are continuously fed with fresh magma and yet, their volume does not normally increase (and frequently even decreases, as observed at Niragongo from 1948 to 1959). The conclusion is that, in a steadystate lava-lake, the volumes of degassed, cooled, and therefore heavier lavas that sink are equal to the volumes of gas-rich, hot, and therefore comparatively light magma that rise from depth. In other words, a permanent, steady-state lava lake is governed by two-phase convection. This hypothesis has been accepted by most volcanologists for many years, b u t it seems that little material evidence of such a mechanism has so far been obtained. The peripheral location of the small flows that creep over the superficial crust may be considered such evidence, because the explanation for their location must lie in the present morphology of the crater. A 45 de-
276
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-
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Fig. 10. Cross-cutting of the Niragongo
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277 gree slope of the funnel-shaped lower half of the original floor restricts the downward motion of the large current of lava. Because of the continuous lateral stresses exerted radially from the central feeding fountains, an upward c o m p o n e n t is developed at the lakes' periphery where the radial stresses meet the resistance due to the restricted size of the lower part of the crater As a result, lavas are extruded through cracks in the surficial crust. It is probable that the first stage of the present activity did not involve convection at all b u t consisted merely of infilling through one or more comparatively narrow vents {Fig. 3) in the lower half of the chasm. This would account for the swiftness of this infilling. Convection may have started when a hydrostatic balance was reached between the fresh magma and ponded lava. During this early stage, both melting of rocks of the crater floor and possibly mechanical abrasion probably enlarged the feeding vent enough to allow deep-rooted convection to start. Such a convection cell, working in a considerable width and depth, would not be established in comparatively narrow, cylindrical feeding chimneys of the normal type; a channel wide enough to allow steady-state convection would be necessary. By the same reasoning, the feeding fractures underneath a volcano could not narrow abruptly with depth, for, if they did, convection could not develop. Since convection is observed, large feeding channels must exist beneath any volcano with a steady-state lava-lake. NATURE OF THE FEEDING SYSTEM Similar structural controls seem to affect all three of the volcanoes containing lava lakes with which I am familiar, Niragongo, Erta'Ale, and Erebus. Mt. Erebus, in Antarctica, is located at the intersection of three tectonic trends: the E--W fracture line that links it with dormant Mt. Terror, the N--S fracture line linking it with Mt. Bird, and the SW--NE fracture line which, extending in the direction of Mt. Discovery, is dotted by the series of small volcanoes of the H u t Point peninsula (Fig. 11). Erta'Ale, in the Afar Depression of Ethiopia, shows two trends, the main one of the WNW--ESE Red Sea rift and a N--S one, quite obvious in the volcanic pile itself (Fig. 12). If three, or even two, intersecting sets of open fractures suffice to provide wide enough channels to allow deep-rooted convection, it is clear that the spot where multiple tectonic trends intersect should favor a steady-state lava lake to appear and persist. This is the case of Niragongo, which is located above the following systems: (1) The main N--S trend of the western branch of the Great Rift Valley system. In its sector extending from lake Tanganyika to Mt. Ruwenzori this trend is locally marked, on the one hand, by the alignment of Niragongo itself with its two large satellites, Shareru and Baruta (Fig. 1), and on the other hand by the main eruptive fissure that split Niragongo on January 10th, 1977 (Tazieff, 1977). (2) The general E--W trend of the Virunga volcanic range taken as a whole
278
- TPj~
~
~~MCMURDO
SC 77°30
,
~5OE
167"
,
110
, 2o , 3okras 169 °
Fig. 11. Ross Island, Antarctica. Mt. Erebus, with its permanent lava lake activity stands at the cross-point o f three main volcanotectonic trends: an E--W, a N-15-W and a S-15-W Olles.
is also marked by the Muhavura--Gahinga and the Visoke--Mikeno couples. The importance of this trend became apparent during the 1977 eruption of Niragongo (Tazieff, 1977). (3) The regional NW--SE trend of fractures linking Niragongo with its close neighbour Nyamlagira. The same trend, some kilometers eastwards, links Mt. Karisimbi to Mt. Mikeno (Fig. 1). (4) A SW--NE trend, which is far less conspicuous, but from 1948 to 1959 appeared to control the shape of the former lava lake and, in July 1982, the nascent lava lake and two spatter-cones aligned with it on the same trend (Figs. 4 and 5). Such a set of intersecting fractures must favor a wide feeding system that allows convection to extend deep enough to maintain the permanently active lava lake in which molten, gas-rich, deep-seated magma comes directly to the surface. Whether the magma reservoir is located only a few kilometers below the volcanic pile, or, as seems more likely, in the lower crust or upper mantle, convection could extend to such depths only if a wide set of open fractures connected it with the surface. A narrow chimney of the kind normally visualized could not maintain such a system.
279
Fig. 12. The large as well as the narrower fissures observed on the northern slopes of Erta'Ale's cone are not radial but are tectonic ones, parallel to a N--S trend.
VOLCANIC HAZARDS F r o m t h e p r a g m a t i c s t a n d p o i n t o f e r u p t i v e hazards, t h e p r e s e n t activity s h o u l d be c a r e f u l l y m o n i t o r e d in o r d e r t o w a r n t h e p o p u l a t i o n s living at t h e v o l c a n o ' s f o o t s h o u l d t h e necessity arise. On O c t o b e r 3rd, 1 9 8 2 , t h e v o l u m e o f t h e m o l t e n lava lake was still growing. This g r o w t h will i n e v i t a b l y s t o p , e i t h e r w h e n t h e volcanic pile yields u n d e r t h e h y d r o s t a t i c pressure e x e r t e d o n its t h i n walls o r w h e n t h e b a l a n c e will be r e a c h e d b e t w e e n upwelling m a g m a a n d d o w n - e n g u l f e d lava. T h e s e walls are c o n s i d e r a b l y t h i n n e r since t h e 1 9 7 7 inner t e r r a c e s collapsed. Failure o f t h e walls w o u l d a l l o w lava and n e w gas-rich
280
magma to escape and threaten numerous villages and two large towns at the foot of the volcano. During the half-a-century between 1928 and 1977, a balance was reached between deep-seated mechanisms and surface conditions, and steady-state activity of the lava lake may be attained again during the present cycle. The eruptive risk will then be minimized at least for a period of years or decades. Nevertheless, owing to the comparative weakness of the Niragongo edifice after the 1977 outbreak, the eruptive hazards will be higher than before. PRESENT
CONDITIONS
During November 1982 the activity stopped. On December 10th, the surficial crust was subsiding through innumerable partial collapses accompanied by strong cracking sounds. The general level of the solidified lake was several meters below its former m a x i m u m (J. Besseville, pets. commun., 1982). This type of interruption is c o m m o n in the activity of lava lakes and is not necessarily the end of the activity.
REFERENCES
Barberi, F. and Varet, J., 1970. The Erta'Ale volcanic range (Danakil depression, Northern Afar, Ethiopia) Bull. Volcanol., 34: 848--917. Delsemme, A., 1960. Premiere contribution a l'~tude du d~bit d'~nergie du Niragongo. Bull. Acad. R. Sci. O.M., 6(4): 699--707. Jaggar, T.A., 1917. Volcanological investigationsat Kilauea. A m . J. Sci.,43: 255--288. Kyle, P.R. and Cole, J.W., 1974. Structural control of volcanism in the M c Murdo volcanic group, Antarctica. Bull. Volcanol., 38: 16--25. Le Guern, F., Tazieff, H. and Carbonnelle, J., 1977. Heat and gas transfer from Niragongo lava lake. EOS, Trans. A m , Geophys. Union, 58:921 (abstr.). Sahama, T.G., 1962. Petrology of Mt. Niragongo: a review. Trans. Edinburgh Geol. Soc., 19: 1--28. Sahama, T.G., 1973. Evolution of the Niragongo magma. J. Petrol., 14(1): 33--48. Tazieff, H., 1949. Premiere exploration du volcan Niragongo. Bull. Soc. Beige G~ol.~ 58: 165--172. Tazieff, H., 1977. A n exceptional eruption: Mt. Niragongo, Jan. 10th 1977. Bull. Volcanol., 40: 189--200. Zettwoog, P., Carbonnelle, J. Le Guern, F. and Tazieff, H., 1972. Mesures de transferts d'~nergie et de transfertsde masse au volcan Erta'Ale (Afar, Ethiopie). C.R. Acad. Sci. Paris, 274: 1265--1268.
267
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
Mt. NIRAGONGO: RENEWED ACTIVITY OF THE LAVA LAKE
H. T A Z I E F F
15 Quai de Bourbon, Paris, 75004 (France) (Received February 21, 1983; revised and accepted July 8, 1983)
ABSTRACT Tazieff, H., 1984. Mt. Niragongo: renewed activity of the lava lake. J. Volcanol. Geotherm. Res., 20: 267--280. On the 21st of June, 1982, Mt. Niragongo ended a period of dormancy that had begun on January 11, 1977, and fresh lava began to flow into the 800-m-deep crater. On October 3, a huge lava lake, wider and deeper than any previously observed (500 m across and close to 400 m deep) rose to within 440 m of the crater rim. The observed activity consisted of a large, central upweUing fountain of very fluid lava from which concentric lava waves expanded radially; numerous small, relatively viscous lava flows creeped over the surrounding thin solidified crust, that covered about 95% of the lake area. These observed features seem to characterize the upper part of a large convective system. The persistence of such an extraordinarily large steady~state lava lake may be due to the equaily exceptional fluidity of the magma rising at the intersection of four different tectonic trends of fractures in the subvolcanic basement.
INTRODUCTION
Together with its close neighbour, Nyamlagira, as well as Kilauea in Hawaii, Erta'Ale in the Afar depression, and Mt. Erebus in Antarctica, Mr. Niragongo belongs to the very exceptional kind of volcano with permanent (or steady-state) lava lakes. Unlike Kilauea and Erta'Ale volcanoes, Niragongo and Erebus are intracontinental. They are also unusual, because of their respective rock types: Mt. Erebus is the only volcano presently erupting Kenites (Kyle and Cole, 1974), while Niragongo is the only one in the world to emit leucite-, melilite-, and nepheline-bearing basanitic lavas (Sahama, 1962, 1973). The chemical compositions of these very undersaturated basanites accounts for their unusually high fluidity. The first reference to persistent lava activity in the deep crater of Mt. Niragongo dates from 1928. The lava lake was discovered only in 1948 (Tazieff, 1949) and has since been observed and studied for close to thirty years (Fig. 1). On January 19th 1977, the volcanic edifice split open and the lava lake was suddenly drained, together with magma in the feeding fractures above the lowest levels of the new erupting fissures, about 1700 m a.s.1. The 0377-0273/84]$03.00
O 1984 Elsevier Science Publishers B.V.
268
:o_f_lakeedward ~----~,~ )
...'
Fig. 1. Virunga volcanoes range. The four tectonic trends observed on Mt. Niragongo do appear on this general map: the N--W one linking Niragongo to Nyamlagira, as well as Karisimbi to Mikeno, the N--S one of the Great African Rift (to the west of lake Edward and the alignment of Niragongo and its two main satellites), the W--NW one (the 1948 fracture) and the EW one (Muhavura--Sabinyo and Visoke--Mikeno). h y d r o s t a t i c pressure b e t w e e n t h e lake level, 3 3 0 0 m a.s.l., and these l o w e r outlets, t o g e t h e r w i t h the magma's e x c e p t i o n a l fluidity, a c c o u n t f o r the p o w e r f u l f o r c e and high v e l o c i t y (> 100 k m / h ) with which t h e m a g m a r u s h e d o u t (Tazieff, 1977). T h e s u d d e n e m p t y i n g o f b o t h t h e lake and its large system o f feeding fractures caused t h e i m m e d i a t e collapse o f t h e wide c o n c e n t r i c terraces t h a t had b e e n built u p during t h e previous c e n t u r y (or centuries) w i t h i n t h e m o r e t h a n o n e k i l o m e t e r - w i d e crater. T h e resulting chasm, a b o u t 8 0 0 m deep, consisted o f a sub-cylindrical u p p e r part, a p p r o x i m a t e l y 500 m deep, and a f u n n e l - s h a p e d l o w e r part, a b o u t 300 m deep, with walls consisting o f huge b l o c k y a p r o n s resulting f r o m t h e collapse o f the terraces. T h e l o w e r m o s t level o f t h e c r a t e r was e s t i m a t e d t o be 2 6 7 0 m a.s.1. (Fig. 2). T h e s e d e p t h s have been calculated p h o t o g r a m m e t r i c a l l y b y M. Bacchus ( I n s t i t u t Geograp h i q u e National, Paris) and p r o v e to be 50 to 100 m shallower t h a n t h o s e
269
Fig. 2. The Niragongo chasm in January 1977. The white dyke is to the south. Remnants of the upper terrace mark the 3270 m level. The huge conical fallen debris apron starts at an altitude ca. 2800 m. e s t i m a t e d in 1 9 7 7 (Tazieff, 1 9 7 7 ) . All t h e depths, widths, and v o l u m e s o f t h e p r e s e n t lava lake have b e e n o b t a i n e d t h a n k s t o M. B a c c h u s ' calculations. ACTIVITY SINCE 1977 F o l l o w i n g t h e 1 9 7 7 e r u p t i o n , all activity ceased f o r over five years. It res u m e d in 1 9 8 2 , t h e first n o t i c e c o m i n g o n J u n e 21st, w h e n a m o l t e n lava p o n d and a single s p a t t e r - c o n e were seen f r o m t h e rim t o be near the b o t t o m o f t h e c r a t e r w h e r e a f l o o r of rain-washed debris had started t o develop (Fig. 3) (E. G a u t h i e r , pers. c o m m u n . , 1982). On J u n e 21, a lava lake 3 0 0 m across, fed b y t w o eruptive vents l o c a t e d respectively 2 8 9 0 m and 2 9 9 0 m a.s.l., was developing at 2 8 0 0 m a.s.1. (Fig. 4). B y J u l y 5, t h e lava lake was 4 6 0 m across and 2 9 0 0 m a.s.1. On J u l y 11, a wide lava lake was observed at a p p r o x i m a t e l y t h e same level. Its area was abou.t 1 2 5 , 0 0 0 m 2, t h e main p a r t ( a b o u t 95%) o f w h i c h was e n c r u s t e d b y a t h i n slab o f solidified lava. A central p o n d , a b o u t 2 0 , 0 0 0 m 2 in area, d i s p l a y e d an i n c a n d e s c e n t surface (M. Pieper, pers. c o m m u n . , 1 9 8 2 ) . A t t h a t date, t h e lake was a p p r o x i m a t e l y 200 m d e e p and c o n t a i n e d a b o u t 9 X 106 m 3 o f m o l t e n lava. On A u g u s t 4th, the lake was entirely c r u s t e d over e x c e p t f o r t w o large f o u n t a i n s c o r r e s p o n d i n g t o the initial vents
270
Fig. 3. The lower part of the chasm in early 1982 showing huge 1977 collapse debris aprons, 300 m high, and a small debris-covered floor about 30 m thick, formed within 4 years. (and spatter-cones). Its surface, 3050 m a.s.l., was a b o u t 370,000 m 2, and its volume a b o u t 44 × 106 m 3 (Fig. 5). F r o m August 5th to August 13 or 14, t h e activity apparently s t oppe d (A. de Munck and M. Pieper, pers. c o m m u n . , 1982). On O c t o b e r 3rd, when the a u t h o r revisited Mt. Niragongo t o g e t h e r with the latter two informants, t he lake occupied t he whole width of the crater up to a b o u t 380 m below the crater rim (i.e.: a b o u t 3090 m a.s.1.) (Fig. 6). T he d e p t h and volume of the lava were thus of the order of 390 m and 65 × 106 m 3 respectively. This volume is far larger than any observed before. Jaggar (1917) measured a de pt h of 40 feet for the Halemaumau lava lake. (The volume o f the lakes o f Erebus, Erta'Ale and the f o r m e r lakes of Nira-
271
Fig. 4. The 1982 lava lake in its early stage (June 21st). Two eruptive vents, approx. 180 m apart, with spatter-built cones about 40 m high are located at both ends of an eruptive fracture hidden by the new lava flows. The lake was then ca. 340 m across. (photo C. Peyer). g o n g o can n o t be c o m p u t e d , b e c a u s e t h e i r d e p t h s c o u l d n o t be m e a s u r e d or even estimated.} T h e average lava o u t p u t d u r i n g t h e t h r e e first w e e k s o f r e n e w e d activity h a d t h u s b e e n o f t h e o r d e r o f 9 × 106 m 3, i.e. a b o u t 4.5 × 10 s m 3 p e r d a y ; d u r i n g t h e t h r e e f o l l o w i n g w e e k s it was a b o u t 1.4 × 106 m 3 p e r d a y and d r o p p e d t o 3.5 × 10 s m 3 p e r d a y d u r i n g t h e n e x t t w o m o n t h s . T h e daily average f r o m t h e s t a r t t o early O c t o b e r 1 9 8 2 was 6.5 × l 0 s m 3. T h e activity o b s e r v e d o n J u l y 5 t h c o n s i s t e d o f c o n t i n u o u s lava-fountaining w i t h i n t h e lake a n d t w o s p o r a d i c a l l y s p i t t i n g s p a t t e r - c o n e s . T h e degassing f o u n t a i n s w e r e a b o u t 50 m across. On J u l y l l t h , t h e l o w e r o f t h e t w o s p a t t e r - c o n e s h a d d i s a p p e a r e d b e l o w t h e s u r f a c e o f t h e rising lake, b u t t h e
272
/ /
(
4august1982 3020
~lk~~ / ~ " , ,
Fig. 5. The lava lake on Aug. 4th 1982, as drawn by M. Bacchus from a photo by M. Bebrone.
Lavaemission 215.82
PhotosHchPEYER,5isengi 25./,.82et 21.6.82 Fig. 6. The successive levels of the 1982 lava lake, as plotted (from oblique photographs) by M. Bacchus.
273 fountains were located at t he same place and were of the same magnitude as before. T h e apparently excentric location o f the violently bubbling fountains p h o t o g r a p h e d in early July was due to the presence of exceptionally large debris aprons along t he southwestern floor of the circular crater-wall (Fig. 3). On O c t o b e r 3rd, no spatter cones could be seen nor any visible activity that could be related to these d r o w n e d features. As before, fountains, 40 to 50 m across, were located in the central part of the lake, the level of which was n o w above the u p p e r apex o f the ramps of fallen rocks. The continuous bubbling was violent enough to hurl masses of m ol t en magma weighing several tons to heights of up to 100 m (Fig. 7). The continuous gas o u t p u t was o f the same or der of magnitude as t hat measured in 1972, i.e. 284 000 t o n s / d a y , (77,000 tons H20, 184,000 tons CO2 and 23,000 tons SO2) (Le Guern, 1980).
Fig. 7. The lava lake on Oct. 3rd 1982. The molten part was about 200 m across, the boiling fountains being ca. 50 large. The outer rim of the very thinly encrusted surface is thickened to several meters by squeezed out, comparatively viscous, lava flows. The t e m p e r a t u r e of the bubbling lava was 1100 -+ 30°C. Using the figures o f B o n n e t (1960), w ho had calculated the heat o u t p u t of the 1959 Niragongo lava lake (930 MW), the o u t p u t on O c t o b e r 4th, 1982, was estimated at a b o u t 35 × 103 MW, i.e. ca. f o r t y times greater. Measurements carried of the lava lake o f Erta'Ale ( Z e t t w o o g et al., 1972), giving heat flows Q1 = 14
274 W cm -2 for red lava surfaces and Q2 = 2.1 W cm -2 for black surfaces. If the same rates hold for Niragongo, they indicate a total heat o u t p u t of ca. 25 × 103 MW, i.e. of the same order of magnitude that obtained by Bonnet's method. In October 1982, no currents could be seen but the continuous, violent fountaining engendered large, concentrically expanding, waves, several meters high with a wave-length of ca. 60 m. Notwithstanding the fact that no currents were actually observed, it seems obvious that a radial transfer of molten lava is continuously occurring, as indicated by the numerous flows, a few meters wide and a few tens of meters long, that, during our stay, were continuously emitted here and there along the edge of the huge encrusting slab covering the main outer part of the lake. These flows consisted of degassed lavas, cooled down to about 960 +- 30°C, and were short-lived, stopping and solidifying after moving for five or ten minutes. A circular band, a few tens of meters wide, of these solidified small lava flows, adds several meters the otherwise very thin crust, which is initially of the order of a few centimeters thick. The o u t p u t of these small outflows was estimated at 20,000 or 30,000 m3/day (Fig. 8). This comparatively modest outflow is nevertheless important because of the evidence it provides of outwardly oriented, radial currents below the outer crust of the lake. We had no opportunity to observe large overflows of lava, but the appearance of the lake's encrusted surface suggests that swift
Fig. 8. The outer rim of the lake, thickened by squeezed-out flows. (photo Besseville).
275
and expansive overflows characterize the present activity, as they did dozens of times during our three-weeks-long residence in the crater during 1972. They lasted only a few minutes b u t poured over the rim of the lake because of the rapid upwelling of the feeding magma. The extraordinarily fluid lavas rapidly covered a large part o f the solidified crust, and on many occasions, its total surface. The uprise of the present lake-level most probably occurs in the same way, the fluidity of the lava being of the same order of magnitude as in 1972. The lava lake evolution observed during the 1982 activity has been traced photogrammetrically by M. Bacchus (Figs. 9 and 10).
9
1st platform
3270
3075
%%• ~'~
tt
Floating island (1959) ~1
ti
~ 2 n d
II
,
platform/( -~ - -
3000
3.10.82
3090
4.8.82
3050
early 7.82
2900
27.6.82
2800
/
///,1 ~ ~o.\
/ ~ o~'
~J2700 2670 2500
Fig. 9. Map of the crater. Showing successive lake levels, and remnants of the 3267 m. a.s.1, platform, that collapsed in 1977.
N A T U R E OF THE LAVA LAKE
Permanent lava lakes are continuously fed with fresh magma and yet, their volume does not normally increase (and frequently even decreases, as observed at Niragongo from 1948 to 1959). The conclusion is that, in a steadystate lava-lake, the volumes of degassed, cooled, and therefore heavier lavas that sink are equal to the volumes of gas-rich, hot, and therefore comparatively light magma that rise from depth. In other words, a permanent, steady-state lava lake is governed by two-phase convection. This hypothesis has been accepted by most volcanologists for many years, b u t it seems that little material evidence of such a mechanism has so far been obtained. The peripheral location of the small flows that creep over the superficial crust may be considered such evidence, because the explanation for their location must lie in the present morphology of the crater. A 45 de-
276
Remaining
in 1982
Platform DIsappeared
lssv
between
Nov.77
3267m
and 1982
-
Lava lake
(I(4
F”” line :
c&ee_apro”;
\
Dashed
0
Fountains
-3
Crater
‘!xQ
line
: Nov.77
‘82
(estimated
InFebruary
77 there
was no scree apron
into the shaft of the crater
at N-E
bottom
inApril
position
Fig. 10. Cross-cutting of the Niragongo
with
crater.
an accuracy
of 50 m)
connecting
the bottom
crater
277 gree slope of the funnel-shaped lower half of the original floor restricts the downward motion of the large current of lava. Because of the continuous lateral stresses exerted radially from the central feeding fountains, an upward c o m p o n e n t is developed at the lakes' periphery where the radial stresses meet the resistance due to the restricted size of the lower part of the crater As a result, lavas are extruded through cracks in the surficial crust. It is probable that the first stage of the present activity did not involve convection at all b u t consisted merely of infilling through one or more comparatively narrow vents {Fig. 3) in the lower half of the chasm. This would account for the swiftness of this infilling. Convection may have started when a hydrostatic balance was reached between the fresh magma and ponded lava. During this early stage, both melting of rocks of the crater floor and possibly mechanical abrasion probably enlarged the feeding vent enough to allow deep-rooted convection to start. Such a convection cell, working in a considerable width and depth, would not be established in comparatively narrow, cylindrical feeding chimneys of the normal type; a channel wide enough to allow steady-state convection would be necessary. By the same reasoning, the feeding fractures underneath a volcano could not narrow abruptly with depth, for, if they did, convection could not develop. Since convection is observed, large feeding channels must exist beneath any volcano with a steady-state lava-lake. NATURE OF THE FEEDING SYSTEM Similar structural controls seem to affect all three of the volcanoes containing lava lakes with which I am familiar, Niragongo, Erta'Ale, and Erebus. Mt. Erebus, in Antarctica, is located at the intersection of three tectonic trends: the E--W fracture line that links it with dormant Mt. Terror, the N--S fracture line linking it with Mt. Bird, and the SW--NE fracture line which, extending in the direction of Mt. Discovery, is dotted by the series of small volcanoes of the H u t Point peninsula (Fig. 11). Erta'Ale, in the Afar Depression of Ethiopia, shows two trends, the main one of the WNW--ESE Red Sea rift and a N--S one, quite obvious in the volcanic pile itself (Fig. 12). If three, or even two, intersecting sets of open fractures suffice to provide wide enough channels to allow deep-rooted convection, it is clear that the spot where multiple tectonic trends intersect should favor a steady-state lava lake to appear and persist. This is the case of Niragongo, which is located above the following systems: (1) The main N--S trend of the western branch of the Great Rift Valley system. In its sector extending from lake Tanganyika to Mt. Ruwenzori this trend is locally marked, on the one hand, by the alignment of Niragongo itself with its two large satellites, Shareru and Baruta (Fig. 1), and on the other hand by the main eruptive fissure that split Niragongo on January 10th, 1977 (Tazieff, 1977). (2) The general E--W trend of the Virunga volcanic range taken as a whole
278
- TPj~
~
~~MCMURDO
SC 77°30
,
~5OE
167"
,
110
, 2o , 3okras 169 °
Fig. 11. Ross Island, Antarctica. Mt. Erebus, with its permanent lava lake activity stands at the cross-point o f three main volcanotectonic trends: an E--W, a N-15-W and a S-15-W Olles.
is also marked by the Muhavura--Gahinga and the Visoke--Mikeno couples. The importance of this trend became apparent during the 1977 eruption of Niragongo (Tazieff, 1977). (3) The regional NW--SE trend of fractures linking Niragongo with its close neighbour Nyamlagira. The same trend, some kilometers eastwards, links Mt. Karisimbi to Mt. Mikeno (Fig. 1). (4) A SW--NE trend, which is far less conspicuous, but from 1948 to 1959 appeared to control the shape of the former lava lake and, in July 1982, the nascent lava lake and two spatter-cones aligned with it on the same trend (Figs. 4 and 5). Such a set of intersecting fractures must favor a wide feeding system that allows convection to extend deep enough to maintain the permanently active lava lake in which molten, gas-rich, deep-seated magma comes directly to the surface. Whether the magma reservoir is located only a few kilometers below the volcanic pile, or, as seems more likely, in the lower crust or upper mantle, convection could extend to such depths only if a wide set of open fractures connected it with the surface. A narrow chimney of the kind normally visualized could not maintain such a system.
279
Fig. 12. The large as well as the narrower fissures observed on the northern slopes of Erta'Ale's cone are not radial but are tectonic ones, parallel to a N--S trend.
VOLCANIC HAZARDS F r o m t h e p r a g m a t i c s t a n d p o i n t o f e r u p t i v e hazards, t h e p r e s e n t activity s h o u l d be c a r e f u l l y m o n i t o r e d in o r d e r t o w a r n t h e p o p u l a t i o n s living at t h e v o l c a n o ' s f o o t s h o u l d t h e necessity arise. On O c t o b e r 3rd, 1 9 8 2 , t h e v o l u m e o f t h e m o l t e n lava lake was still growing. This g r o w t h will i n e v i t a b l y s t o p , e i t h e r w h e n t h e volcanic pile yields u n d e r t h e h y d r o s t a t i c pressure e x e r t e d o n its t h i n walls o r w h e n t h e b a l a n c e will be r e a c h e d b e t w e e n upwelling m a g m a a n d d o w n - e n g u l f e d lava. T h e s e walls are c o n s i d e r a b l y t h i n n e r since t h e 1 9 7 7 inner t e r r a c e s collapsed. Failure o f t h e walls w o u l d a l l o w lava and n e w gas-rich
280
magma to escape and threaten numerous villages and two large towns at the foot of the volcano. During the half-a-century between 1928 and 1977, a balance was reached between deep-seated mechanisms and surface conditions, and steady-state activity of the lava lake may be attained again during the present cycle. The eruptive risk will then be minimized at least for a period of years or decades. Nevertheless, owing to the comparative weakness of the Niragongo edifice after the 1977 outbreak, the eruptive hazards will be higher than before. PRESENT
CONDITIONS
During November 1982 the activity stopped. On December 10th, the surficial crust was subsiding through innumerable partial collapses accompanied by strong cracking sounds. The general level of the solidified lake was several meters below its former m a x i m u m (J. Besseville, pets. commun., 1982). This type of interruption is c o m m o n in the activity of lava lakes and is not necessarily the end of the activity.
REFERENCES
Barberi, F. and Varet, J., 1970. The Erta'Ale volcanic range (Danakil depression, Northern Afar, Ethiopia) Bull. Volcanol., 34: 848--917. Delsemme, A., 1960. Premiere contribution a l'~tude du d~bit d'~nergie du Niragongo. Bull. Acad. R. Sci. O.M., 6(4): 699--707. Jaggar, T.A., 1917. Volcanological investigationsat Kilauea. A m . J. Sci.,43: 255--288. Kyle, P.R. and Cole, J.W., 1974. Structural control of volcanism in the M c Murdo volcanic group, Antarctica. Bull. Volcanol., 38: 16--25. Le Guern, F., Tazieff, H. and Carbonnelle, J., 1977. Heat and gas transfer from Niragongo lava lake. EOS, Trans. A m , Geophys. Union, 58:921 (abstr.). Sahama, T.G., 1962. Petrology of Mt. Niragongo: a review. Trans. Edinburgh Geol. Soc., 19: 1--28. Sahama, T.G., 1973. Evolution of the Niragongo magma. J. Petrol., 14(1): 33--48. Tazieff, H., 1949. Premiere exploration du volcan Niragongo. Bull. Soc. Beige G~ol.~ 58: 165--172. Tazieff, H., 1977. A n exceptional eruption: Mt. Niragongo, Jan. 10th 1977. Bull. Volcanol., 40: 189--200. Zettwoog, P., Carbonnelle, J. Le Guern, F. and Tazieff, H., 1972. Mesures de transferts d'~nergie et de transfertsde masse au volcan Erta'Ale (Afar, Ethiopie). C.R. Acad. Sci. Paris, 274: 1265--1268.