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The submarine eruption and erosion of Surtla (Surtsey), Iceland
Journal of Volcanology and Geothermal Research, 19 (1983) 239--246 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
239
THE SUBMARINE ERUPTION AND EROSION OF SURTLA (SURTSEY), ICELAND
B. PETER KOKELAAR and GRAHAM P. DURANT
School of Environmental Sciences, UTster Polytechnic, Shore Road, Newtownabbey, Co. Antrim BT37 OQB (Northern Ireland) Department of Geology, The University, Glasgow G12 8QQ (Scotland) (Received June 29, 1982; revised and accepted February 4, 1983)
ABSTRACT Kokelaar, B.P. and Durant, G.P., 1983. The submarine eruption and erosion of Surtla (Surtsey), Iceland. J. Volcanol. Geotherm. Res., 19: 239--246. Surtla is the site of a short-lived submarine vent which built basaltic clastic deposits almost to sea level, in 1963, early in the eruption of Surtsey. Since then wave and current activity have eroded the volcanic pile such that in July 1981 its top was a fairly level plateau 45 m below sea level, and its surface comprised a lag deposit of sparse blocks of lava in a bed mainly of glass granules. This winnowed layer was underlain by a nonreworked, poorly sorted and finer deposit of glassy clasts formed by a combination of disruption by magmatic volatiles, steam explosions and quench brecciation. During the eruption, the explosion violence and associated comminution increased as the pile built up to shallower water depths. It is argued that at times of continuous effusion a cupola of steam was situated over the vent, as indicated by scoriaceous spatter which shows agglutination and "bread-crust" features that can only have developed in conditions more akin to subaerial than hitherto envisaged in a subaqueous eruption.
INTRODUCTION The basaltic Surtsey v o l c a n o is in the V e s t m a n n a e y j a r ( W e s t m a n n Islands) volcanic s y s t e m at the s o u t h e r n e x t r e m i t y o f I c e l a n d ' s Eastern Volcanic Zone ( J a k o b s s o n , 1979). I t is f o u n d e d on the Icelandic insular shelf, w h i c h is c o m p o s e d m a i n l y o f lithified marine basaltic v o l c a n o g e n i c sediments ( A l e x a n d e r s s o n , 1972). Surtla (20 ° 33.35'W 63 ° 1 8 . 5 5 ' N ) is the m o s t distal o f the three satellite v e n t s o f the volcano. It was active d u r i n g the early explosive phase o f Surtsey ( S u r t u r 1), b u t unlike its successors, Syrtlingur and Jolnir, never e m e r g e d above sea level (Thorarinsson, 1964, 1966, 1 9 6 8 ; T h o r a r i n s s o n et al., 1964). This s t u d y results f r o m several dives, with scuba e q u i p m e n t , o n t o Surtla in J u l y 1981. T h e details o f the e r u p t i o n and erosion o f Surtla are pieced t o g e t h e r f r o m c o n t e m p o r a r y a c c o u n t s , u n d e r w a t e r observations and the
0377-0273/83/$03.00
© 1983 Elsevier Science Publishers B.V.
240 interpretation of sampled material. The petrology of Surtla basalts is presented separately (Kokelaar and Durant, 1983). THE HISTORY OF ERUPTION AND EROSION Surtsey appeared as a volcanic island on November 14th 1963, after one week of submarine eruption. On December 28th 1963, during continuous explosive activity on Surtsey (Surtur 1), turbulence in the sea 2.3 km to the ENE was reported and on the following day three active vents could be seen, 4--6 m below the sea surface, along a 250--300 m NE-trending fissure. There were red flashes at half-second intervals from incandescent magma, and concentric wave patterns were generated synchronously. Intermittent columns of condensed steam rose from the site and black tephra was ejected explosively up to 50 m. On January 6th, after only 17 days of activity, this eruption ended and only a few pieces of tephra floated in the vicinity. The vent had been entirely submarine and the underwater pile was named Surtla (Thorarinsson, 1964; Thorarinsson et al., 1964). During the Surtla eruption, the volcanic pile was elongated NE over its source fissure and almost reached sea level from its base, which is about 120 m below sea level. The crest of this short ridge was reduced, following the eruption, to 23 m below sea level by February 14th 1964, and to around 34 m in July 1967. By June 1968 the top of the pile was a slightly undulating level plain at 40 m. This plain was 350 m across and showed an indistinct surface patterning. A northerly current of 1 m/sec was recorded (Norrman, 1970). A fairly level plateau at 45 m below sea level was determined in the present study, by echo sounder and divers' depth guages. Two sites approximately 100 m apart were examined, photographed and sampled. SUBMARINE OBSERVATIONS AND SAMPLE ANALYSES The plateau consisted of basalt blocks distributed sparsely in a monotonous, indistinctly rippled and winnowed deposit of variously vesicular glassy basalt clasts, dominantly of granule size. This surface layer was underlain by similar material with a larger proportion of fines and free crystals of olivine and plagioclase. The presence of small blocks resting in the upper 5 cm indicated the general absence of deeper reworking, although some large blocks were associated with deeper current-scours and were avoided in sampling. A small quantity of fines was lost in sampling. Table I shows grain-size distribution parameters (Inman, 1952) of samples of the clastic deposits, exclusive of blocks, from each site. Sample SA1 includes material from the winnowed surface layer; SA2 was collected from non-reworked deposits between 5 and 15 cm beneath the surface. The deposits have a near log-normal grain-size distribution and poor sorting, contrasting strongly with the very poorly sorted, finer and fine-skewed products of Surtsey's explosive activity. In terms of median diameter versus sorting, the Surtla
241 TABLE I Size distribution parameters (Inman, 1952) of Surtla and Surtsey tephra Sample
Mean diameter (M~)
Sorting (o~) Skewness (o~)
SA1 (5.7 kg) SA2(2.1kg) Surtsey
-1.69 -0.91 1.19 -+ 0.71
1.19 1.25 2.75 +- 0.37
0.008 -0.087 0.27 -+ 0.11
*SA1 and SA2 are from Surtla dive sites; Surtsey is the average of 9 analyses of Surtsey tephra from Sheridan (1972).
samples are in the field of Strombolian deposits (see Walker and Croasdale, 1972, fig. 2). Morphometric parameters of grains from sample SA2 are recorded {Fig. 1) in terms of relative percentages of planar (P), convex (V) and concave {C) grain perimeters {"roundness" according to Szadeczky-Kardoss {1933)). The Surtla grains show a wide range of forms and there is a tendency for coarser grains to plot with P > 20%. The grains range from highly scoriaceous to very sparsely or non-vesicular. The blocks, up to 1 m in diameter and generally spaced between 2 and 5 m apart, were mostly angular or subangular, with some subrounded. The majority of blocks were polyhedral and of variably vesicular basalt, some p
3
3
-2 g
2
0
0
0
0 0
..3 2 2-2
0 2 J
_3
3
2
U
L[-2
0 0
-2
p= o 3
3
2 2
3
C
3
v
Fig. 1. Ternary diagram of planar (P), concave (C) and convex (V) perimeters of grains from sample SA2. The phi diameter of each grain is plotted.
242 with concentric zones of vesicles and an extremely fine-grained chilled margin. No glassy selvedges were found. Other blocks were irregularly shaped scoriaceous basalt. One such block, 22 cm across, was a tightly folded slab. Its lower surface was agglutinated with smaller basalt clasts, whereas the upper side had a crude " br e a d- cr us t " surface with fibres of basaltic glass extending across the irregular tapering cracks. This block was evidently plastic when the smaller clasts were agglutinated and its vesicular core expanded to fracture its upper chilled surface. Some blocks were almost completely buried in the granular deposits but most were well exposed. The larger well-exposed blocks caused adjacent current-scour depressions up to 10 cm deep and containing lag deposits of large granules and pebbles. On the down-current side, fine granules and sand built tapering ridges. The sense of current direction was consistently from the west and, although no natural clast transport was observed, currents of around 0.5 to 1.0 m/sec from the west were measured and clouds of very fine basalt clasts were suspended and transported if the subsurface deposit was disturbed manually. Additionally, some blocks were heavily encrusted in epifauna (mainly serpulids, hydroids, bryozoans and alcyonarians), some were less densely populated and some quite clean. The cleanest blocks were the most angular. DISCUSSION Surtla was reduced, in 17~/~ years, from near sea level to 45 m below sea level. Although tectonic subsidence c a n n o t be ruled out entirely, studies of Surtsey (Tryggvason, 1972) show this to be very small there; 30--40 cm between 1967 and 1970, and since then even slower. In the six weeks following the eruption, erosion down to 23 m was due mainly to wave action. Thereafter the rate of erosion decreased, such that in 13 years only 5 m was removed. The flat top of the pile suggests that wave action was the main agent of erosion, although the sedimentary structures and direct observations of water currents indicate that strong current activity also played a significant role. The relative importance of current erosion as a factor in height reduction increased as the pile was lowered beneath the wave base of all but the most violent storms. The bed load of currents must have been deposited over the edge of Surtla's plateau, where it then crept or rolled down a stable slope or slumped to generate debris flows and turbidity currents which spread the basalt clasts ont o the surrounding shelf. Because of the cont i nued erosion of Surtla, its surface deposit does not directly reflect the primary volcanic deposit. As the finer material was eroded from the pile, the blocks must have remained more or less in situ, simply being lowered as they were undermined. Consequently, the observed distribution of blocks is a lag concent r a t i on of all the blocks originally distributed t h r o u g h o u t the 40 m of the pile that has been removed. This is borne out by the surfaces of sampled blocks. Those densely populated by epifauna are also
243
the most rounded and must have been exposed first, near the top of the pile, where they suffered considerable attrition, particularly in the first few weeks of strong wave action. The blocks less encrusted with epifauna are correspondingly more angular, and must have been exposed later. The granules (2 to 4 mm) of the primary deposit (sample SA2) have an average specific gravity of 2.15 (the average of the finer grains is 2.43). In the successively shallower deposits of the complete pile this value probably decreased, because vesicularity would have increased as hydrostatic confining pressure became less. Some clasts floated during the visible eruption. Thus the overall density of clasts would have been less at the shallower levels and their erosion and transport relatively enhanced. Honnorez and Kirst (1975) suggest that basaltic volcaniclastic particles in a PVC analysis (Fig. 1) with P > 20% reflect non-explosive quench brecciation and constitute hyaloclastites, and those with P ~ 20% reflect explosive fragmentation and constitute hyalotuffs. However, the P = 20% discriminant boundary was derived by comparing aqueous quench granulates with subaerial vesicular pyroclastic material and it can be quite misleading in interpretation of aquagene deposits. For example, on Surtsey where an incandescent pahoehoe tongue was washed by waves which rapidly produced quench granulates non-explosively, the products plot with P <: 20%, that is as hyalotuffs, mostly because the quenched lava was vesicular (see Honnorez and Kirst, 1975, fig. 2C). Because of the vesicularity and presence of tachylitic grains, Honnorez and Kirst (1975) attributed the fragmentation partly to subaerial explosive activity (p. 451) and then, in interpretation of the PVC plot, to phreatic explosions (p. 460). Although minor rootless steam explosions did occur, they could not produce the vesicularity observed, and tachylitic grains occur in entirely submarine clastic deposits. Clearly in this case, quench granulation of the vesicular lava could not produce the required percentage of planar grain boundaries, and misinterpretation as hyalotuff was the result. On the other hand, as will be shown, a high degree of planarity (P > 20%) does not necessarily reflect quiet quench granulation, as it may also result from steam explosions. It is argued here that the wide range of grain forms of the Surtla deposits should be expected and may be characteristic of submarine explosive eruptions. Simple contact of sea water with degassed or non-vesiculating magma does not produce explosive activity, because the steam generated at the initial contact acts as an insulating layer and limits further violent interactions (see Tepley and Moore, 1974). However, when magma is explosively erupted into water as a result of accelerating expansion of exsolved magmatic volatiles, further explosive activity can result initially from the practically instantaneous quenching and concommitant evolution of steam. Also, when magma rises rapidly through wet unconsolidated clastic deposits, some are incorporated and the ensuing expansion of trapped steam causes violent explosions (Kokelaar, 1983). Although the expansion and coalescence of vesicles must play some part in clast formation, the effects of the explosive trans-
244 formation of liquid water to steam almost certainly predominate. This is evident in the greatly reduced violence and increased median grainsize resuiting when Surtseyan explosions revert to Hawaiian or Strombolian in response to the exclusion of water from the crater (Kokelaar, 1983 ). Clast~forming processes attributable to steam explosion include the shat~ tering effect of the shock wave, attrition and abrasion. The shock wave tends to produce planar fracture surfaces and the other processes can produce both rounding (convexity), and, with breakage of clasts, angularity (planarity). Also, blebs of magma or hot glass, forced by explosion momentarily into contact with water, produce hyaloclastic quench breccias with planar grain perimeters. Clearly, steam explosivity dominates clast-forming processes in subaqueous explosive eruptions and tends to produce planar grain boundaries. But, as with the Surtsey lava (above), the expression of this planarity in the PVC plot depends on the vesicularity and vesicle distribution in the original magma. Vesiculation is a prerequisite at least for the initiation of steam explosive activity, and the distribution of vesicles is bound to be irregular. Therefore, the wide range of Surtla grain forms is hardly surprising. At Surtla, Surtseyan explosions only developed when the vent was close to sea level, and then rather weakly. Thus the characteristic fine comminution shown by Surtsey tephra analyses (Table I) probably never occurred. The observed early turbulence in the sea suggests there were suppressed explosions that failed to break the sea surface. Underwater explosions are suppressed by the combined effects of hydrostatic pressure, the inertia of the containing water and its chilling effects. At 45 m below sea level the expansion of liquid water to steam at say 500°C is approximately 1 to 1000 and at sea level it is approximately 1 to 3566. Because suppression of explosions lessens with decreasing depth, the degree of clast comminution and the range of clast trajectories in successive explosions increases. During the Surtla eruption some of the finest material was probably suspended and transported away by water currents, but most clasts must have been rapidly dumped close to the vent with little chance for sorting. This material would have tended to slump and form mass-gravity flows carrying tephra to greater depths. The shelf around Surtsey consists largely of lithified basaltic turbidites derived from numerous Surtsey-like volcanoes (Alexandersson, 1972). A significant proportion of these should constitute doubly graded sequences formed contemporaneously with shallowing eruptions. Sparse blocks of vesicular basalt at Surtla represent rare lava flow-units, possibly of pillow or pahoehoe form, which were broken by explosions or jointing, and admixed with the finer deposits. Scoriaceous blocks, however, appear to be parts of bombs and highly plastic spatter. The agglutination and "bread crust" surfaces, which are similar to those of cauliflower bombs from Surtseyan deposits (Lorenz, 1974) and scoriaceous bombs from littoral cones (Fisher, 1968), can only have developed in conditions more akin to subaerial than hitherto envisaged in subaqueous eruptions. To explain this it is envisaged that during phases of continuous eruption a cupola of steam of
245 variable width and height persisted over the vent, within which the rate of quenching was lower than in water. Steam in the cupola was generated from steam explosions, along hot-lava--water interfaces and from within the pile. Magmatic volatiles would also have contributed. Condensation of the steam against the relatively cold water of the cupola's upper surface would have prevented the rise and dissipation of the cupola simply as a bubble, unless the overlying water approached the boiling temperature appropriate to the depth. The volume of the cupola would have been controlled by the thermal input and the hydrostatic head. We conclude that the Surtla eruption entailed a complex combination of extrusive volcanic processes, with explosion violence, and thereby comminution of various kinds, increasing with decreasing water depth. Hyaloclastic brecciation developed in a variety of circumstances and relatively degassed magma was extruded as lava. Three vents were active along the Surtla fissure so a thorough mixture of materials and clast forms in the pile was inevitable. However, according to the processes outlined, it is interpreted that the primary clastic deposits of the original pile must have been generally relatively coarse, unbedded and poorly sorted below, passing upwards into more poorly sorted, finer and more fine-skewed, with sparse, isolated fragments of lava (perhaps whole pillows) and scoriaceous bombs throughout. Strong magnetisation measured at Surtla in 1965 (see Kjartansson, 1967, p. 59) suggests that pillow lavas may form the deepest part of the pile. Such sequences are c o m m o n in the shallow levels of subaqueous volcanoes; for example, in the intraglacial serrated ridges and table mountains of Iceland (Jones, 1969). ACKNOWLEDGEMENTS We gratefully acknowledge financial support from the Royal Society Maurice Hill Research Fund, the Carnegie Trust for the Universities of Scotland, and the Ulster Polytechnic. We thank Sveinn Jakobsson and Sigurdur Thorarinsson for encouragement and advice, and members of the British Sub-Aqua Club 1981 Iceland Expedition, in particular the leader Gordon Ridley, for logistical support in the " f i e l d " work. Also, we are indebted to Pat Andrews for assistance with the various analyses, and to Malcolm Howells and John Phillips for critically reviewing the manuscript, which was deciphered and typed by Jennifer Larkin.
REFERENCES
Alexandersson, T., 1972. The sedimentary xenoliths from Surtsey: Turbidites indicating shelf growth. Surtsey Res. Progress Rep., 6: 101--116. Fisher, R.V., 1968. Puu Hou littoral cones, Hawaii. Geol. Rundsch., 57: 837--864. Honnorez, J. and Kirst, P., 1975. Submarine basaltic volcanism: Morphometric parameters for discriminating hyaloclastites from hyalotuffs. Bull. Volcanol., 39: 441--465.
246 Inman, D.L., 1952. Measures for describing the size distribution of sediments. J. Sediment. Petrol., 22: 125--145. Jakobsson, S.P., 1979. Petrology of Recent basalts of the Eastern Volcanic Zone, Iceland. Acta Nat. Isl., 26: 1--103. Jones, J.G., 1969. Intraglacial volcanoes of the Laugarvatn region, southwest Iceland -- I. Q. J. Geol. Soc. London, 124: 197--211. Kjartansson, G., 1967. Volcanic forms at the sea bottom. In: S. Bj6rnsson {Editor), Iceland and Mid-Ocean Ridges. Reykjavik, Soc. Sci. Isl., 38: 53--66. Kokelaar, B.P., 1983. The mechanism of Surtseyan volcanism. J. Geol. Soc. London, 140. Kokelaar, B.P. and Durant, G.P., 1983. The petrology of basalts from Surtla (Surtseyl. leeland. J. Volcanol. Geotherm. Res., 19: 247--253. Lorenz, V., 1974. Studies of the Surtsey tephra deposits. Surtsey Res. Progress Rep., 7: 72--79. Norrman, J.O., 1970. Trends in postvoleanic development of Surtsey Island. Progress report on geomorphological activities in 1968. Surtsey Res. Progress Rep., 5: 95--112. Sheridan, M.F., 1972. Textural analysis of Surtsey tephra: a preliminary report. Surtsey Res. Progress Rep., 6: 150--151. Szadeezky-Kardoss, E.V., 1933. Die Bestimmung des Abrollungsgrades. Zentralbl. Mineral., Geol. Paliiontol., B: 389--401. Tepley, L. and Moore, J.G., 1974. Fire under the sea; the origin of pillow lavas (16 mm sound-motion picture). Moonlight Productions, Mountain View, California. Thorarinsson, S., 1964. Surtsey, the new island in the North Atlantic. Almenna Bokafelagid, Reykjavik, 63 pp. Thorarinsson, S., 1966. The Surtsey eruption: Course of events and the development of Surtsey and other new islands. Surtsey Res. Progress Rep., 2: 117--123. Thorarinsson, S., 1968. The Surtsey eruption: Course of events during the year 1967. Surtsey Res. Progress Rep., 4: 143--148. Thorarinsson, S., Einarsson, T., Sigvatdason, G. and Elisson, G., 1964. The submarine eruption off the Vestmann Islands 1963--64. A preliminary report. Bull. Volcanol., 27: 435--445. Tryggvason, E., 1972. Precision levelling in Surtsey. Surtsey Res. Progress Rep., 6: 158--162. Walker, G.P.L. and Croasdale, R., 1972. Characteristics of some basaltic pyroelasties. Bull. Volcanol., 35: 303--317.
239
THE SUBMARINE ERUPTION AND EROSION OF SURTLA (SURTSEY), ICELAND
B. PETER KOKELAAR and GRAHAM P. DURANT
School of Environmental Sciences, UTster Polytechnic, Shore Road, Newtownabbey, Co. Antrim BT37 OQB (Northern Ireland) Department of Geology, The University, Glasgow G12 8QQ (Scotland) (Received June 29, 1982; revised and accepted February 4, 1983)
ABSTRACT Kokelaar, B.P. and Durant, G.P., 1983. The submarine eruption and erosion of Surtla (Surtsey), Iceland. J. Volcanol. Geotherm. Res., 19: 239--246. Surtla is the site of a short-lived submarine vent which built basaltic clastic deposits almost to sea level, in 1963, early in the eruption of Surtsey. Since then wave and current activity have eroded the volcanic pile such that in July 1981 its top was a fairly level plateau 45 m below sea level, and its surface comprised a lag deposit of sparse blocks of lava in a bed mainly of glass granules. This winnowed layer was underlain by a nonreworked, poorly sorted and finer deposit of glassy clasts formed by a combination of disruption by magmatic volatiles, steam explosions and quench brecciation. During the eruption, the explosion violence and associated comminution increased as the pile built up to shallower water depths. It is argued that at times of continuous effusion a cupola of steam was situated over the vent, as indicated by scoriaceous spatter which shows agglutination and "bread-crust" features that can only have developed in conditions more akin to subaerial than hitherto envisaged in a subaqueous eruption.
INTRODUCTION The basaltic Surtsey v o l c a n o is in the V e s t m a n n a e y j a r ( W e s t m a n n Islands) volcanic s y s t e m at the s o u t h e r n e x t r e m i t y o f I c e l a n d ' s Eastern Volcanic Zone ( J a k o b s s o n , 1979). I t is f o u n d e d on the Icelandic insular shelf, w h i c h is c o m p o s e d m a i n l y o f lithified marine basaltic v o l c a n o g e n i c sediments ( A l e x a n d e r s s o n , 1972). Surtla (20 ° 33.35'W 63 ° 1 8 . 5 5 ' N ) is the m o s t distal o f the three satellite v e n t s o f the volcano. It was active d u r i n g the early explosive phase o f Surtsey ( S u r t u r 1), b u t unlike its successors, Syrtlingur and Jolnir, never e m e r g e d above sea level (Thorarinsson, 1964, 1966, 1 9 6 8 ; T h o r a r i n s s o n et al., 1964). This s t u d y results f r o m several dives, with scuba e q u i p m e n t , o n t o Surtla in J u l y 1981. T h e details o f the e r u p t i o n and erosion o f Surtla are pieced t o g e t h e r f r o m c o n t e m p o r a r y a c c o u n t s , u n d e r w a t e r observations and the
0377-0273/83/$03.00
© 1983 Elsevier Science Publishers B.V.
240 interpretation of sampled material. The petrology of Surtla basalts is presented separately (Kokelaar and Durant, 1983). THE HISTORY OF ERUPTION AND EROSION Surtsey appeared as a volcanic island on November 14th 1963, after one week of submarine eruption. On December 28th 1963, during continuous explosive activity on Surtsey (Surtur 1), turbulence in the sea 2.3 km to the ENE was reported and on the following day three active vents could be seen, 4--6 m below the sea surface, along a 250--300 m NE-trending fissure. There were red flashes at half-second intervals from incandescent magma, and concentric wave patterns were generated synchronously. Intermittent columns of condensed steam rose from the site and black tephra was ejected explosively up to 50 m. On January 6th, after only 17 days of activity, this eruption ended and only a few pieces of tephra floated in the vicinity. The vent had been entirely submarine and the underwater pile was named Surtla (Thorarinsson, 1964; Thorarinsson et al., 1964). During the Surtla eruption, the volcanic pile was elongated NE over its source fissure and almost reached sea level from its base, which is about 120 m below sea level. The crest of this short ridge was reduced, following the eruption, to 23 m below sea level by February 14th 1964, and to around 34 m in July 1967. By June 1968 the top of the pile was a slightly undulating level plain at 40 m. This plain was 350 m across and showed an indistinct surface patterning. A northerly current of 1 m/sec was recorded (Norrman, 1970). A fairly level plateau at 45 m below sea level was determined in the present study, by echo sounder and divers' depth guages. Two sites approximately 100 m apart were examined, photographed and sampled. SUBMARINE OBSERVATIONS AND SAMPLE ANALYSES The plateau consisted of basalt blocks distributed sparsely in a monotonous, indistinctly rippled and winnowed deposit of variously vesicular glassy basalt clasts, dominantly of granule size. This surface layer was underlain by similar material with a larger proportion of fines and free crystals of olivine and plagioclase. The presence of small blocks resting in the upper 5 cm indicated the general absence of deeper reworking, although some large blocks were associated with deeper current-scours and were avoided in sampling. A small quantity of fines was lost in sampling. Table I shows grain-size distribution parameters (Inman, 1952) of samples of the clastic deposits, exclusive of blocks, from each site. Sample SA1 includes material from the winnowed surface layer; SA2 was collected from non-reworked deposits between 5 and 15 cm beneath the surface. The deposits have a near log-normal grain-size distribution and poor sorting, contrasting strongly with the very poorly sorted, finer and fine-skewed products of Surtsey's explosive activity. In terms of median diameter versus sorting, the Surtla
241 TABLE I Size distribution parameters (Inman, 1952) of Surtla and Surtsey tephra Sample
Mean diameter (M~)
Sorting (o~) Skewness (o~)
SA1 (5.7 kg) SA2(2.1kg) Surtsey
-1.69 -0.91 1.19 -+ 0.71
1.19 1.25 2.75 +- 0.37
0.008 -0.087 0.27 -+ 0.11
*SA1 and SA2 are from Surtla dive sites; Surtsey is the average of 9 analyses of Surtsey tephra from Sheridan (1972).
samples are in the field of Strombolian deposits (see Walker and Croasdale, 1972, fig. 2). Morphometric parameters of grains from sample SA2 are recorded {Fig. 1) in terms of relative percentages of planar (P), convex (V) and concave {C) grain perimeters {"roundness" according to Szadeczky-Kardoss {1933)). The Surtla grains show a wide range of forms and there is a tendency for coarser grains to plot with P > 20%. The grains range from highly scoriaceous to very sparsely or non-vesicular. The blocks, up to 1 m in diameter and generally spaced between 2 and 5 m apart, were mostly angular or subangular, with some subrounded. The majority of blocks were polyhedral and of variably vesicular basalt, some p
3
3
-2 g
2
0
0
0
0 0
..3 2 2-2
0 2 J
_3
3
2
U
L[-2
0 0
-2
p= o 3
3
2 2
3
C
3
v
Fig. 1. Ternary diagram of planar (P), concave (C) and convex (V) perimeters of grains from sample SA2. The phi diameter of each grain is plotted.
242 with concentric zones of vesicles and an extremely fine-grained chilled margin. No glassy selvedges were found. Other blocks were irregularly shaped scoriaceous basalt. One such block, 22 cm across, was a tightly folded slab. Its lower surface was agglutinated with smaller basalt clasts, whereas the upper side had a crude " br e a d- cr us t " surface with fibres of basaltic glass extending across the irregular tapering cracks. This block was evidently plastic when the smaller clasts were agglutinated and its vesicular core expanded to fracture its upper chilled surface. Some blocks were almost completely buried in the granular deposits but most were well exposed. The larger well-exposed blocks caused adjacent current-scour depressions up to 10 cm deep and containing lag deposits of large granules and pebbles. On the down-current side, fine granules and sand built tapering ridges. The sense of current direction was consistently from the west and, although no natural clast transport was observed, currents of around 0.5 to 1.0 m/sec from the west were measured and clouds of very fine basalt clasts were suspended and transported if the subsurface deposit was disturbed manually. Additionally, some blocks were heavily encrusted in epifauna (mainly serpulids, hydroids, bryozoans and alcyonarians), some were less densely populated and some quite clean. The cleanest blocks were the most angular. DISCUSSION Surtla was reduced, in 17~/~ years, from near sea level to 45 m below sea level. Although tectonic subsidence c a n n o t be ruled out entirely, studies of Surtsey (Tryggvason, 1972) show this to be very small there; 30--40 cm between 1967 and 1970, and since then even slower. In the six weeks following the eruption, erosion down to 23 m was due mainly to wave action. Thereafter the rate of erosion decreased, such that in 13 years only 5 m was removed. The flat top of the pile suggests that wave action was the main agent of erosion, although the sedimentary structures and direct observations of water currents indicate that strong current activity also played a significant role. The relative importance of current erosion as a factor in height reduction increased as the pile was lowered beneath the wave base of all but the most violent storms. The bed load of currents must have been deposited over the edge of Surtla's plateau, where it then crept or rolled down a stable slope or slumped to generate debris flows and turbidity currents which spread the basalt clasts ont o the surrounding shelf. Because of the cont i nued erosion of Surtla, its surface deposit does not directly reflect the primary volcanic deposit. As the finer material was eroded from the pile, the blocks must have remained more or less in situ, simply being lowered as they were undermined. Consequently, the observed distribution of blocks is a lag concent r a t i on of all the blocks originally distributed t h r o u g h o u t the 40 m of the pile that has been removed. This is borne out by the surfaces of sampled blocks. Those densely populated by epifauna are also
243
the most rounded and must have been exposed first, near the top of the pile, where they suffered considerable attrition, particularly in the first few weeks of strong wave action. The blocks less encrusted with epifauna are correspondingly more angular, and must have been exposed later. The granules (2 to 4 mm) of the primary deposit (sample SA2) have an average specific gravity of 2.15 (the average of the finer grains is 2.43). In the successively shallower deposits of the complete pile this value probably decreased, because vesicularity would have increased as hydrostatic confining pressure became less. Some clasts floated during the visible eruption. Thus the overall density of clasts would have been less at the shallower levels and their erosion and transport relatively enhanced. Honnorez and Kirst (1975) suggest that basaltic volcaniclastic particles in a PVC analysis (Fig. 1) with P > 20% reflect non-explosive quench brecciation and constitute hyaloclastites, and those with P ~ 20% reflect explosive fragmentation and constitute hyalotuffs. However, the P = 20% discriminant boundary was derived by comparing aqueous quench granulates with subaerial vesicular pyroclastic material and it can be quite misleading in interpretation of aquagene deposits. For example, on Surtsey where an incandescent pahoehoe tongue was washed by waves which rapidly produced quench granulates non-explosively, the products plot with P <: 20%, that is as hyalotuffs, mostly because the quenched lava was vesicular (see Honnorez and Kirst, 1975, fig. 2C). Because of the vesicularity and presence of tachylitic grains, Honnorez and Kirst (1975) attributed the fragmentation partly to subaerial explosive activity (p. 451) and then, in interpretation of the PVC plot, to phreatic explosions (p. 460). Although minor rootless steam explosions did occur, they could not produce the vesicularity observed, and tachylitic grains occur in entirely submarine clastic deposits. Clearly in this case, quench granulation of the vesicular lava could not produce the required percentage of planar grain boundaries, and misinterpretation as hyalotuff was the result. On the other hand, as will be shown, a high degree of planarity (P > 20%) does not necessarily reflect quiet quench granulation, as it may also result from steam explosions. It is argued here that the wide range of grain forms of the Surtla deposits should be expected and may be characteristic of submarine explosive eruptions. Simple contact of sea water with degassed or non-vesiculating magma does not produce explosive activity, because the steam generated at the initial contact acts as an insulating layer and limits further violent interactions (see Tepley and Moore, 1974). However, when magma is explosively erupted into water as a result of accelerating expansion of exsolved magmatic volatiles, further explosive activity can result initially from the practically instantaneous quenching and concommitant evolution of steam. Also, when magma rises rapidly through wet unconsolidated clastic deposits, some are incorporated and the ensuing expansion of trapped steam causes violent explosions (Kokelaar, 1983). Although the expansion and coalescence of vesicles must play some part in clast formation, the effects of the explosive trans-
244 formation of liquid water to steam almost certainly predominate. This is evident in the greatly reduced violence and increased median grainsize resuiting when Surtseyan explosions revert to Hawaiian or Strombolian in response to the exclusion of water from the crater (Kokelaar, 1983 ). Clast~forming processes attributable to steam explosion include the shat~ tering effect of the shock wave, attrition and abrasion. The shock wave tends to produce planar fracture surfaces and the other processes can produce both rounding (convexity), and, with breakage of clasts, angularity (planarity). Also, blebs of magma or hot glass, forced by explosion momentarily into contact with water, produce hyaloclastic quench breccias with planar grain perimeters. Clearly, steam explosivity dominates clast-forming processes in subaqueous explosive eruptions and tends to produce planar grain boundaries. But, as with the Surtsey lava (above), the expression of this planarity in the PVC plot depends on the vesicularity and vesicle distribution in the original magma. Vesiculation is a prerequisite at least for the initiation of steam explosive activity, and the distribution of vesicles is bound to be irregular. Therefore, the wide range of Surtla grain forms is hardly surprising. At Surtla, Surtseyan explosions only developed when the vent was close to sea level, and then rather weakly. Thus the characteristic fine comminution shown by Surtsey tephra analyses (Table I) probably never occurred. The observed early turbulence in the sea suggests there were suppressed explosions that failed to break the sea surface. Underwater explosions are suppressed by the combined effects of hydrostatic pressure, the inertia of the containing water and its chilling effects. At 45 m below sea level the expansion of liquid water to steam at say 500°C is approximately 1 to 1000 and at sea level it is approximately 1 to 3566. Because suppression of explosions lessens with decreasing depth, the degree of clast comminution and the range of clast trajectories in successive explosions increases. During the Surtla eruption some of the finest material was probably suspended and transported away by water currents, but most clasts must have been rapidly dumped close to the vent with little chance for sorting. This material would have tended to slump and form mass-gravity flows carrying tephra to greater depths. The shelf around Surtsey consists largely of lithified basaltic turbidites derived from numerous Surtsey-like volcanoes (Alexandersson, 1972). A significant proportion of these should constitute doubly graded sequences formed contemporaneously with shallowing eruptions. Sparse blocks of vesicular basalt at Surtla represent rare lava flow-units, possibly of pillow or pahoehoe form, which were broken by explosions or jointing, and admixed with the finer deposits. Scoriaceous blocks, however, appear to be parts of bombs and highly plastic spatter. The agglutination and "bread crust" surfaces, which are similar to those of cauliflower bombs from Surtseyan deposits (Lorenz, 1974) and scoriaceous bombs from littoral cones (Fisher, 1968), can only have developed in conditions more akin to subaerial than hitherto envisaged in subaqueous eruptions. To explain this it is envisaged that during phases of continuous eruption a cupola of steam of
245 variable width and height persisted over the vent, within which the rate of quenching was lower than in water. Steam in the cupola was generated from steam explosions, along hot-lava--water interfaces and from within the pile. Magmatic volatiles would also have contributed. Condensation of the steam against the relatively cold water of the cupola's upper surface would have prevented the rise and dissipation of the cupola simply as a bubble, unless the overlying water approached the boiling temperature appropriate to the depth. The volume of the cupola would have been controlled by the thermal input and the hydrostatic head. We conclude that the Surtla eruption entailed a complex combination of extrusive volcanic processes, with explosion violence, and thereby comminution of various kinds, increasing with decreasing water depth. Hyaloclastic brecciation developed in a variety of circumstances and relatively degassed magma was extruded as lava. Three vents were active along the Surtla fissure so a thorough mixture of materials and clast forms in the pile was inevitable. However, according to the processes outlined, it is interpreted that the primary clastic deposits of the original pile must have been generally relatively coarse, unbedded and poorly sorted below, passing upwards into more poorly sorted, finer and more fine-skewed, with sparse, isolated fragments of lava (perhaps whole pillows) and scoriaceous bombs throughout. Strong magnetisation measured at Surtla in 1965 (see Kjartansson, 1967, p. 59) suggests that pillow lavas may form the deepest part of the pile. Such sequences are c o m m o n in the shallow levels of subaqueous volcanoes; for example, in the intraglacial serrated ridges and table mountains of Iceland (Jones, 1969). ACKNOWLEDGEMENTS We gratefully acknowledge financial support from the Royal Society Maurice Hill Research Fund, the Carnegie Trust for the Universities of Scotland, and the Ulster Polytechnic. We thank Sveinn Jakobsson and Sigurdur Thorarinsson for encouragement and advice, and members of the British Sub-Aqua Club 1981 Iceland Expedition, in particular the leader Gordon Ridley, for logistical support in the " f i e l d " work. Also, we are indebted to Pat Andrews for assistance with the various analyses, and to Malcolm Howells and John Phillips for critically reviewing the manuscript, which was deciphered and typed by Jennifer Larkin.
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