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 (fs@hi.is, palli@hi.s, mtg@hi.is).
Steinunn Jakobsdottir, Matthew Roberts, Kristin Vogfjord, Ragnar Stefansson, Icelandic
Meteorological Office, Reykjavik, Iceland (ssj@vedur.is, matthew@vedur.is,
vogfjord@vedur.is, ragnar@vedur.is).