On September 29, 1996 at 10:48 an earthquake of magnitude 5 on the Richter scale was detected within the Vatnajökull icecap in SE Iceland. This event was followed by an intense earthquake swarm with a large number of small events with intermittent larger quakes of magnitude 3-4 on the Richter scale. The activity continued at similar level until shortly before midnight on September 30th when volcanic tremor was recorded and earthquake shocks became less frequent. In the afternoon of September 29 a warning was issued to national and international aviation authorities that an explosive (phreatic) eruption might be expected within the near future and an ash cloud might interfere with the dense air traffic across the Atlantic.
In the morning of October 1st an over flight discovered a subsidence bowl in the glacier surface at a location where an eruption had occurred in 1938. More or less continuous over flights during the day observed continuous increase in the size and depth of the initial subsidence bowl as three additional bowls formed on a line N30E indicating intensive melting at the base of the glacier along a fissure 5-6 km long.
Simultaneously the ice cover on the 10 km diameter Grimsvötn caldera some 15 km to the south of the active subglacial fissure started to rise indicating that melt water from the eruption was flowing into the caldera depression and lifting its ice cover.
On Oct 2nd, in the early morning, an over flight observed that the eruption had broken through the ice. Rhythmic explosions resulted in black ash clouds rising to a height of 500 meters while the buoyant eruption column rose to 3000 meters before being deflected by a southerly wind at a velocity of 30-40 knots. Visibility and flying conditions were extremely poor and allowed observation for only a few minutes.
The Vatnajökull glacier in Europe is a temperate glacier covering about 8300 km2 in the SE part of Iceland. Volcanic fissure systems of the Mid- Atlantic Ridge plate boundary are partly covered by the western part of the ice sheet. Two major volcanic centers lie beneath the ice, the Bardarbunga volcanic centre and the Grimsvötn volcanic centre both with large subglacial caldera depressions. The Bardarbunga centre is a part of a fissure system extending over 100 km to the south and some 50 km to the north of the glacier. The last eruption within the Bardarbunga centre occurred in 1910, but eruptions on the fissure system have occurred in 871 AD, 1477 AD and 1862 AD, all producing substantial amounts of lava.
The Grimsvötn centre is the more active of the two with an eruption frequency during past centuries close to one eruption per decade. The last eruption occurred in 1983. As Bardarbunga the Grimsvötn centre is a part of a a fissure system which includes the Laki fissure, which in 1783 produced about 12-14 km3 of basaltic lava. Within the ice filled Grimsvötn caldera intense geothermal activity continuously melts the ice to form a subglacial lake, which at intervals of 5 to 10 years is emptied along subglacial channels to create large floods (jökulhlaup) on the sandur plain, Skeidararsandur, on the Icelandic south coast. The lake was last emptied in 1996 and the water level is presently low.
The present eruption fissure is located between these two volcanic centres with a direction parallel to the regional tectonic lineament. The subglacial topography directs meltwater from the erupting fissure toward the Grimsvötn caldera which is rapidly filling. By the evening of Oct. 1st the ice cover above the subglacial lake had risen 10-15 meters. A rise of the water level by additional 35 meters will trigger a flood.