An eruption began within the caldera of the subglacial Grímsvötn
volcano, Vatnajökull ice cap in Central Iceland, at 09:20 on December
18. Ten minuets later, around 9:30, an eruption plume was observed.
The eruption penetrated through the Grímvötn subglacial caldera
lake and its ice shelf, however not from great depth. The eruption
vents were on a 1300 m long east-west eruptive fissure located on the southern
caldera fault, similar as eruption in 1934 and 1983. On the initial day
of the eruption, a continuous eruption plume extended to elevation of 10
km. On that day, the eruption plume could be seen from large distances.
It was well observed from Reykjavik, the capital of Iceland, located 200
km from the eruptive site.
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 December 17, and a sharp increase in earthquake activity began at 03:30 on December 18. This activity was replaced by continuous tremor at 09:20 marking the beginning of the eruption.
Eruption tremor stopped around 14:00 on December 28, and this is considered to mark the end of the eruption. No activity has been observed after that.
The eruption occurred 10 km south of the 1996 eruption in Vatnajökull (Gudmundsson et al., 1997) that caused a catastrophic outburst flood from the Vatnajökull glacier. This time no major flood is anticipated, because water is not flowing towards the Grímsvötn caldera lake; rather a small amount of the Grímsvötn ice shelf near the eruption site melted.