The following was received by the Smithsonian's Global Volcanism Network. This report is preliminary and subject to change as the event is studied in more detail. We will forward additional significant information as it becomes available.
A subglacial eruption near the Bardarbunga and Grimsvotn volcanoes in Iceland.
An eruption broke out beneath the Vatnajokull glacier in Central Iceland in the evening of September 30. The eruption was preceded by an unusual sequence of earthquakes, beginning on September 29 with a magnitude 5 event in the Bardarbunga caldera. Similar earthquakes have occurred in Bardarbunga many times during the last 22 years, without eruptive activity and almost without aftershocks. This time the earthquake was followed by an intense swarm of small earthquakes, that spread out to the neighbouring volcanoes Grimsvotn and Hamarinn. A warning of a possible eruption was issued to air traffic on September 30 at 19 h. Later that evening the earthquake activity near Grimsvotn decreased markedly, while that of Bardarbunga continued. At about 22 h the seismograph at Grimsvotn began recording continuous eruption tremor with small amplitude. The amplitude increased very slowly during the next hours and reached a maximum at about 6 h in the morning of October 1. A large depression in the glacier ice of Vatnajokull was discovered from overflying aeroplanes in the early hours.
The glacier is apparently being melted by an eruption on a 4 km long fissure beneath the glacier, which is 400-600 m thick in this location. The fissure has a NNE-SSW strike and is located on the northern flank of the Grimsvotn central volcano. The melt water is draining into the Grimsvotn caldera and raising the ice shelf on the caldera lake. The eruption continues at the time of writing (14 h UTC, October 1).
The present eruption is a continuation of a remarkable series of seismic and magmatic events in the Vatnajokull area that began in 1995: July 1995: A glacier flood from a subglacial geothermal area on Loki Ridge (Eastern Cauldron) NW of Grimsvotn. The draining of the water reservoir was followed by a distinct tremor episode, presumably a small eruption triggered by the pressure release. This event was followed by a general increase in seismicity during the next few months.
February 1996: An intense earthquake swarm centered on the Hamarinn Volcano, lasted a week.
August 1996: A glacier flood from a subglacial geothermal area on Loki Ridge (Western Cauldron) NW of Grimsvotn. The draining of the water reservoir was followed by a tremor episode, presumably a small eruption triggered by the pressure release. This event was followed by increased seismicity during the next few weeks.
September 1996: Present activity: Large earthquake, intense earthquake swarm and an eruption N of Grimsvotn.
Pall Einarsson Science Institute, University of Iceland