The volcanic eruption in Grímsvötn in 2004 was relatively small and limited in scope. Recent Eruptions in Grímsvötn have been short and not caused great harm. In what follows we describe the 2004 eruption. The description can be read in full on the website: Global volcanism program.
Grímsvötn. Photo Páll Stefánsson/Iceland Review
According to scientists from the Institute of Earth Sciences at the University of Iceland and the Icelandic Meteorological Office, an eruption began at the subglacial Grímsvötn volcano in the Vatnajökull ice cap, Iceland, on 1 November 2004 around 2100, and was declining by 5 November. The eruption, preceded by both long- and short-term precursors, was triggered by the release of overburden pressure associated with a glacial-outburst flood (jökulhlaup) originating from the subglacial caldera lake. The jökulhlaup reached a maximum on the afternoon of 2 November. At that time the peak discharge from affected rivers on the coastal plain at Skeidararsandur was 3,000-4,000 m3/s (based on information from the Icelandic Hydrological Service). Discharge declined quickly after the peak. No damage occurred to roads or bridges. The total volume of the jökulhlaup was ~ 0.5 km3.
Seismicity increased at the volcano in mid-2003, about the same time that uplift exceeded a maximum reached in 1998. Tthe last eruption at Grímsvötn occurred within the caldera beginning on 18 December 1998 and resulted in a catastrophic flood. Additional uplift and expansion of the volcano since mid-2003 heralded the latest activity. Seismicity further increased in late October 2004, and on 26 October high-frequency tremor indicated increased water flow from the caldera lake and suggested that a glacial outburst flood was about to begin. On 29 October, the amount of discharge increased in the Skeidara River. About 3 hours before the eruption an intense swarm of volcanic earthquakes started, changing to continuous low-frequency tremor at the onset of the eruption.
The release in overburden pressure associated with the outburst flood triggered the eruption. The amount of drop in water level in the caldera at the onset of the eruption was uncertain, but was probably on the order of 10-20 m, corresponding to a pressure change of 0.1-0.2 MPa at the volcano’s surface. This modest pressure change triggered the eruption because pressure in the shallow magma chamber was high after continuous inflow of magma since 1998.
Figure 5 shows the epicenters from 18 October to 1 November 2004, along with preliminary locations of the eruption site. In the early morning of 1 November, an earthquake swarm began beneath Grímsvötn. By 1400 there were 12 earthquakes; at 0651 the largest, an event of M 3 occurred. At 2010 on 1 November an eruption warning was sent to the Civil Defense, earthquake magnitudes had increased and around that time the swarm intensified. About 160 earthquakes with magnitudes up to 2.8 were recorded during the next 2 hours.