SUMMARYType: caldera |
GEOLOGIC HISTORYGrímsvötn volcano is located directly above the inferred Icelandic hot-spot and is among the largest and most productive volcanoes in Iceland. Grímsvötn, Iceland's most frequently active volcano in historical time, lies largely beneath the vast Vatnajökull icecap. The caldera lake is covered by a 200-m-thick ice shelf and only the southern rim of the 6 x 8 km caldera is exposed. The geothermal area in the caldera causes frequent jokulhlaups (glacier outburst floods) when melting raises the water level high enough to lift its ice dam. More subglacial eruptions are known from this caldera than from any other volcano in the world. Historic Activity: The first historical eruption (of nearly 50) was in 1332. Grímsvötn's 1938 eruption emitted on the order of 400 x 106 m3; of volcanic material, perhaps the third largest eruption in Iceland this century. Recent Activity: A subglacial volcanic eruption occurred beneath the western part of the ice cap in late May 1983. An oval shaped melt lake formed in the glacier and explosions were observe in the lake at varying time intervals. From the evening of Sept. 30 until Oct. 13, 1996, a subglacial eruption occurred along part of the East Rift Zone that traverses beneath the NW side of Vatnajokull, Europe's largest continental glacier. The eruption was preceded by an unusual sequence of earthquakes. The eruption melted through the ice cap and the resulting meltwater drained into Grimsvotn caldera raising the ice shelf above the caldera lake. Current Activity: An eruption began within the caldera of the subglacial Grimsvotn volcano, Vatnajokull ice cap in Central Iceland, at 09:20 Dec. 18, 1998. The eruption vents were on a short fissure located on the southern caldera fault, similar to the 1983 and 1934 eruptions. A continuous eruption plume extended to elevation of 10 km. Westerly wind deflects the plume to the east causing tephra fallout on the surface of the glacier. Tephra has not yet been detected outside the glacier. The eruption was preceded by a mild increase in seismic activity during the last few weeks. A small earthquake swarm occurred at 22 h on Dec. 17, and a sharp increase in earthquake activity began at 03:30 on Dec. 18. This activity was replaced by continuous tremor at 09:20 marking the beginning of the eruption. The eruption ended on Dec. 28, 1998. Data Sources
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ADDITIONAL INFORMATION
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Last Update: 2/12/00 |