Abstract
The 13-day-long Gjálp eruption within the Vatnajökull ice cap in October 1996 provided important data on ice–volcano interaction in a thick temperate glacier. The eruption produced 0.8 km3 of mainly volcanic glass with a basaltic icelandite composition (equivalent to 0.45 km3 of magma). Ice thickness above the 6-km-long volcanic fissure was initially 550–750 m. The eruption was mainly subglacial forming a 150–500 m high ridge; only 2–4% of the volcanic material was erupted subaerially. Monitoring of the formation of ice cauldrons above the vents provided data on ice melting, heat flux and indirectly on eruption rate. The heat flux was 5–6×105 W m-2 in the first 4 days. This high heat flux can only be explained by fragmentation of magma into volcanic glass. The pattern of ice melting during and after the eruption indicates that the efficiency of instantaneous heat exchange between magma and ice at the eruption site was 50–60%. If this is characteristic for magma fragmentation in subglacial eruptions, volcanic material and meltwater will in most cases take up more space than the ice melted in the eruption. Water accumulation would therefore cause buildup of basal water pressure and lead to rapid release of the meltwater. Continuous drainage of meltwater is therefore the most likely scenario in subglacial eruptions under temperate glaciers. Deformation and fracturing of ice played a significant role in the eruption and modified the subglacial water pressure. It is found that water pressure at a vent under a subsiding cauldron is substantially less than it would be during static loading by the overlying ice, since the load is partly compensated for by shear forces in the rapidly deforming ice. In addition to intensive crevassing due to subsidence at Gjálp, a long and straight crevasse formed over the southernmost part of the volcanic fissure on the first day of the eruption. It is suggested that the feeder dyke may have overshot the bedrock–ice interface, caused high deformation rates and fractured the ice up to the surface. The crevasse later modified the flow of meltwater, explaining surface flow of water past the highest part of the edifice. The dominance of magma fragmentation in the Gjálp eruption suggests that initial ice thickness greater than 600–700 m is required if effusive eruption of pillow lava is to be the main style of activity, at least in similar eruptions of high initial magma discharge.
Similar content being viewed by others
References
Alsdorf DE, Smith LC (1999) Interferometric SAR observations of ice topography and velocity changes related to the 1996, Gjálp subglacial eruption, Iceland. Int J Remote Sensing 20:3031–3050
Bacon CR (1977) High temperature heat content and heat capacity of silicate glasses: experimental determination and a model for calculation. Am J Sci 277:109–135
Bemmelen RW van, Rutten MG (1955) Table mountains of northern Iceland. EJ Brill, Leiden, pp 1–217
Björnsson H (1975) Subglacial water reservoirs, jökulhlaups and volcanic eruptions. Jökull 25:1–11
Björnsson H (1986) Surface and bedrock topography of ice caps in Iceland mapped by radio-echo soundings. Ann Glaciol 8:11–18
Björnsson H (1988) Hydrology of ice caps in volcanic regions. Societas Scientiarum Islandica, 45, Reykjavík, pp 1–139
Björnsson H, Einarsson P (1990) Volcanoes beneath Vatnajökull, Iceland: evidence from radio-echo sounding, earthquakes and jökulhlaups. Jökull 40:147–168
Björnsson H, Gudmundsson MT (1993) Variations in the thermal output of the subglacial Grímsvötn Caldera, Iceland. Geophys Res Lett 20:2127–2130
Björnsson H, Pálsson F, Gudmundsson MT (1992) Vatnajökull, northwestern part, 1:100,000, subglacial surface map. National Power Company and Science Institute, Reykjavík
Björnsson H, Rott H, Gudmundsson S, Fischer A, Siegel A, Gudmundsson MT (2001a) Glacier–volcano interactions deduced by SAR interferometry. J Glaciol 47:58–70
Björnsson H, Pálsson F, Flowers GE, Gudmundsson MT (2001b) The extraordinary 1996 jökulhlaup from Grímsvötn, Vatnajökull, Iceland. American Geophysical Union Fall Meeting. Eos Trans AGU 82:47
Blankenship DD, Bell RE, Hodge SM, Brozena JM, Behrendt JC, Finn C (1993) Active volcanism beneath the West Antarctic ice sheet and implications for ice-sheet stability. Nature 361:526–529
Bödvarsson R, Rögnvaldsson ST, Slunga R, Kjartansson E (1999) The SIL acquisition system— at present and beyond year 2000. Phys Earth Planet Inter 113:89–101
Clarke GKC (1982) Glacier outburst flood from "Hazard Lake", Yukon Territory, and the problem of flood magnitude prediction. J Glaciol 28:3–21
Einarsson P, Saemundsson K (1987) Earthquake epicentres 1982–1985 and volcanic systems in Iceland. Map 1:750,000. Menningarsjóður, Reykjavík
Einarsson P, Brandsdóttir B, Gudmundsson MT, Björnsson H, Grönvold K, Sigmundsson F (1997) Center of the Icelandic hotspot experiences volcanic unrest. Eos 78:369–375
Grönvold K, Jóhannesson H (1984) Eruption in Grímsvötn: course of events and chemical studies of the tephra. Jökull 34:1–11
Gudmundsson A (1983) Form and dimensions of dykes in eastern Iceland. Tectonophysics 95:295–307
Gudmundsson MT, Björnsson H (1991) Eruptions in Grímsvötn 1934–1991. Jökull 41:21–46
Gudmundsson MT, Björnsson H, Pálsson F (1995) Changes in jökulhlaup sizes in Grímsvötn, Vatnajökull, Iceland, 1934–1991, deduced from in situ measurements of subglacial lake volume. J Glaciol 41:263–272
Gudmundsson MT, Sigmundsson F, Björnsson H (1997) Ice–volcano interaction of the 1996 Gjálp subglacial eruption, Vatnajökull, Iceland. Nature 389:954–957
Gudmundsson MT, Pálsson F, Björnsson H, Högnadóttir T (2002) The hyaloclastite ridge formed in the subglacial 1996 eruption in Gjálp, Vatnajökull, Iceland: present-day shape and future preservation. In: Smellie JL, Chapman M (eds) Ice–volcano interaction on Earth and Mars. Geological Society, London Spec Publ. 202, pp 319–335
Gudmundsson S, Gudmundsson MT, Björnsson,H, Sigmundsson F, Rott H, Carstensen JM (2002) Three-dimensional glacier surface motion maps at the Gjálp eruption site, Iceland, inferred from combining InSAR and other ice-displacement data. Ann Glaciol 34: 315–322
Hickson CJ (2000) Physical controls and resulting morphologic forms of Quaternary ice-contact volcanoes in western Canada. Geomorphology 32:239–261
Höskuldsson Á, Sparks RSJ (1997) Thermodynamics and fluid dynamics of effusive subglacial eruptions. Bull Volcanol 59:219–230
Jones JG (1969) Intraglacial volcanoes of the Laugarvatn region, southwest Iceland, I. Q J Geol Soc Lond 124:197–211
Jones JG (1970) Intraglacial volcanoes of the Laugarvatn region, southwest Iceland, II. J Geol 78:127–140
Kamb B (1987) Glacier surge mechanism based on linked cavity configuration of the basal water conduit system. J Geophys Res 92:9083–9100
Kjartansson G (1943) Árnesingasaga: The geology of Árnessýsla district. Árnesingafélagid, Reykjavik, pp 1–250
Kristmannsdóttir H, Björnsson A., Pálsson S, Sveinbjörnsdóttir ÁE (1999) The impact of the 1996 subglacial volcanic eruption in Vatnajökull on the river Jökulsá á Fjöllum, North Iceland. J Volcanol Geotherm Res 92:359–372
Langley K (2000) A morphological investigation of volcanic activity beneath Vatnajökull, Iceland, interpreted from radio– echo sounding data. MS Thesis, University of Iceland, pp 1–129
Larsen G, Gudmundsson MT, Björnsson, H (1998) Eight centuries of periodic volcanism at the center of the Iceland Hot Spot revealed by glacier tephrostratigraphy. Geology 26:943–946
Major JJ, Newhall CH (1989) Snow and ice perturbation during historical volcanic eruptions and the formation of lahars and floods. Bull Volcanol 51:1–27
Mathews WH (1947) "Tuyas." Flat topped volcanoes in northern British Columbia. Am J Sci 245:560–570
Nye JF (1957) The distribution of stress and velocity in glaciers and ice sheets. Proc R Soc Lond Ser A 239:113–133
Nye JF (1976) Water flow in glaciers: jökulhlaups, tunnels and veins. J Glaciol 76:181–207
Paterson WSB (1994) The physics of glaciers. Pergamon/Elsevier, Kidlington, pp 1–480
Röthlisberger H (1972) Water pressure in intra- and subglacial channels. J Glaciol 62:177–203
Shreve RL (1972) Movement of water in glaciers. J Glaciol 62:205–214
Sigmarsson O, Karlsson HR, Larsen G (2000) The 1996 and 1998 subglacial eruptions beneath the Vatnajökull ice sheet in Iceland: contrasting geochemical and geophysical inferences on magma migration. Bull Volcanol 61:468–476
Smellie JL (1999) Lithostratigraphy of Miocene-Recent, alkaline volcanic fields in the Antarctic Peninsula and eastern Ellsworth Land. Antarct Sci 11:347–363
Sparks RSJ, Bursik MI, Carey SN, Gilbert JS, Glaze LS, Sigurdsson H, Woods AW (1997) Volcanic Plumes. Wiley, London, pp 1–574
Spera FJ (2000) Physical properties of magma. In: Sigurdsson H (ed) Encyclopaedia of volcanoes, Academic Press, New York, pp 171–190
Steinthorsson S, Hardarson BS, Ellam RM, Larsen G, (2000) Petrochemistry of the Gjálp 1996 subglacial eruption, Vatnajökull, SE Iceland. J Volcanol Geotherm Res 98:79–90
Thorarinsson S (1967) Hekla and Katla. The share of acid and intermediate lava and tephra in the volcanic products through the geological history of Iceland. In: Björnsson S (ed) Iceland and the Mid-Ocean Ridges. Soc Sci Islandica 38:190–197
Thorarinsson S (1974) Vötnin Stríð. Saga Skeiðarárhlaupa og Grímsvatnagosa [The swift flowing rivers: the history of Grímsvötn jökulhlaups and eruptions]. Menningarsjóður, Reykjavík, pp 1–254
Tuffen H, Pinkerton H, McGarvie DW, Gilbert, JS (2002) Melting at the glacier base during a small-volume subglacial rhyolitic eruption: evidence from Bláhnúkur, Iceland. Sediment Geol 149:183–198
Weertman J (1972) General theory of water flow at the base of a glacier or ice sheet. Rev Geophys Space Phys 10:287–333
Wilson L, Head JW (2002) Heat transfer and melting in subglacial basaltic volcanic eruptions: implications for volcanic deposit morphology and meltwater volumes. In: Smellie JL, Chapman M (eds) Ice–volcano interaction on Earth and Mars. Geological Society, London Spec Publ. 202, pp 5–26
Wohletz KH (1983) Mechanisms of hydrovolcanic pyroclast formation: grain-size scanning electron microscopy, and experimental studies. J Volcanol Geotherm Res 17:31–64
Wolfe CJ, Bjarnason ITh, VanDecar JC, Solomon SC (1997) Seismic structure of the Iceland mantle plume. Nature 385:245–247
Acknowledgements
We would like to thank Gudrún Larsen, Finnur Pálsson, John Smellie and Tómas Jóhannesson for their useful discussions. Constructive and helpful comments by reviewers Jim Head and Joe Walder improved the quality of this paper. The aerial field observations during and after the eruption were made with the assistance of the Iceland Civil Aviation Authority, especially chief pilot Snaebjörn Gudbjörnsson, and the helicopter service Þyrluþjónustan and pilot Jón K. Björnsson. Fieldwork on the glacier in 1997, 1998 and later was done with the aid of the Iceland Glaciological Society and the National Power Company of Iceland. Financial support for this project was obtained by a special grant from the Icelandic Government and the Icelandic Public Road Administration.
Author information
Authors and Affiliations
Corresponding author
Additional information
Editorial responsibility: J. Donnelly-Nolan
Rights and permissions
About this article
Cite this article
Gudmundsson, M.T., Sigmundsson, F., Björnsson, H. et al. The 1996 eruption at Gjálp, Vatnajökull ice cap, Iceland: efficiency of heat transfer, ice deformation and subglacial water pressure. Bull Volcanol 66, 46–65 (2004). https://doi.org/10.1007/s00445-003-0295-9
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s00445-003-0295-9