Release of overburden pressure triggers an eruption of Grimsvotn volcano, Iceland, November 1, 2004

Release of overburden pressure triggers an eruption of Grimsvotn volcano, Iceland, November 1, 2004

The subglacial Grimsvotn volcano, Iceland, 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, Vatnajokull.

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 (jokulhlaup), originating from the Grimsvotn subglacial caldera lake. Accumulation of magma in a shallow magma chamber under the Grimsvotn caldera has been ongoing since its last eruption in 1998 (Sturkell et al., 2003; Sigmundsson et al., 2004). 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 Grimsvotn 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 Grimsvotn 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 Grimsvotn 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 Grimsvotn shallow magma chamber was high after continuous inflow of magma to the volcano since 1998.
Background: Grimsvotn volcano (N 64.41°, W 17.33°), situated near the center of the Vatnajokull ice cap in central Iceland, is one of Iceland’s most active volcanoes. It has a complex of calderas (Gudmundsson and Milsom, 1997), and a subglacial caldera lake sustained by geothermal heat. Small eruptions have occurred at the volcano in 1983 and 1998 (around 0.1 km³). In 1996, the Gjalp subglacial eruption occurred north of the volcano (Gudmundsson et al., 1997). The most recent eruption triggered by a pressure release as the current eruption occurred in 1934.
Selected references:

Einarsson, P, and B. Brandsdottir, Seismic activity preceding and during the 1983 volcanic eruption in Grimsvotn, Iceland, Jokull, 34, 13-23, 1984.

Gudmundsson, M. T., and J. Milsom, Gravity and magnetic studies of the subglacial Grimsvotn volcano, Iceland: Implications for crustal and thermal structure, J. Geophys. Res., 102, 7691-7704, 1997.
Gudmundsson, M. T., F. Sigmundsson, and H. Bjornsson, Ice-volcano interaction of the 1996 Gjalp subglacial eruption, Vatnajokull, Iceland, Nature, 389, 954-957, 1997.
Sigmundsson, F., Sturkell, E., Pinel, V., Einarsson, P., Pedersen, R., Geirsson, H., Gudmundsson, M. T., Bjornsson, H., Pagli, C., Deformation and Eruption Forecasting at Volcanoes under Retreating Ice Caps: Discriminating Signs of Magma Inflow and Ice Unloading at Grimsvotn and Katla volcanoes, Iceland, Eos Trans. AGU, 85(47), Fall Meet. Suppl., submitted abstract, 2004.
Sturkell, E., and P. Einarsson, F. Sigmundsson, S. Hreinsdottir, and H. Geirsson, Deformation of Grimsvotn volcano, Iceland: 1998 eruption and subsequent inflation, Geophys. Res. Letters, 30, 1182, doi: 10.129/2002GL016460, 2003.
Information contacts:

Freysteinn Sigmundsson, Pall Einarsson, Magnus Tumi Gudmundsson, Institute of Earth Sciences, University of Iceland, Reykjavik, Iceland ([e-mail], palli@hi.s, [e-mail]).

Steinunn Jakobsdottir, Matthew Roberts, Kristin Vogfjord, Ragnar Stefansson, Icelandic Meteorological Office, Reykjavik, Iceland ([e-mail], [e-mail], [e-mail], [e-mail]).