Categories:
Grimsvötn volcano is situated near the center of the Vatnajökull ice cap in central Iceland and is one of Iceland's most active volcanoes. It has a complex of calderas and a subglacial caldera lake sustained by geothermal heat. Small eruptions have occurred at the volcano in 1983 and 1998. In 1996, the Gjálp subglacial eruption occurred north of the volcano and yet again in the fall of 2004.
The Grimsvötn centre is the more active of the two with an eruption frequency during past centuries close to one eruption per decade. The last eruption occurred in 2004. As Bardarbunga, the Grimsvötn centre is a part of a a fissure system which includes the Laki fissure, which in 1783 produced about 12-14 km3 of basaltic lava. Within the ice filled Grimsvötn caldera intense geothermal activity continuously melts the ice to form a subglacial lake, which at intervals of 5 to 10 years is emptied along subglacial channels to create large floods (jökulhlaup) on the sandur plain, Skeidararsandur, on the Icelandic south coast.
The subglacial Grimsvötn volcano, started erupting on November 1, 2004 around 22 GMT. An intense swarm of volcanic earthquakes that started about 3 hours earlier changed at that time to continuous low frequency tremor, indicating onset of an eruption. Weather conditions prohibited direct observations of the beginning of the eruption at this remote volcano situated near the center of Europe's largest ice cap, Vatnajökull.
The eruption was preceded by both long-term and short-term precursors, and finally triggered by release of overburden pressure associated with a glacial outburst flood (jökulhlaup), originating from the Grimsvötn subglacial caldera lake. Accumulation of magma in a shallow magma chamber under the Grimsvötn caldera has been ongoing since its last eruption in 1998. GPS measurements show uplift of 5-10 cm/year in the caldera center, and horizontal displacements away from the caldera. Earthquake activity increased in middle of 2003, at about the same time uplift exceeded its 1998 maximum. Pressure in the Grimsvötn magma chamber is likely to have exceeded its pre-eruption level from 1998 at this time. Additional uplift and expansion of the volcano since then suggested approaching failure of the volcano. Earthquake activity increased further in late October, 2004. Geothermal heat sustains a lake in the caldera that intermittently causes glacial outburst floods. On October 26 high frequency seismic tremor indicated increased water flow from the caldera lake and suggested that a glacial outburst flood was about to begin. On October 29 discharge increased in river Skeidara. The outburst flood was caused by high water level in the Grimsvötn caldera lake from ice melting by geothermal activity. The release in overburden pressure associated with the outburst flood triggered the eruption. The drop in water level in the Grimsvötn caldera at the onset of the eruption is uncertain, but is probably on the order of 10-20 meters, corresponding to a pressure change of 0.1-0.2 MPa on the volcano surface. This modest pressure change triggered the eruption because internal pressure in the Grimsvötn shallow magma chamber was high after continuous inflow of magma to the volcano since 1998. |
