Two cauldrons above an erupting fissure beneath the Vatnajökull glacier, located in Central Iceland. An eruption started beneath the glacier during the evening of September 30. When this image was taken on October 1, part of the glacier's surface had subsided to form two cauldrons, each about 1-2 km wide. This part of the glacier, located on the north flank of Grímsvötn volcano, was about 400-600 m thick before the eruption. A 4-km-long fissure eruption beneath the glacier quickly melted the ice; the resulting meltwater drained into the Grímsvötn caldera. During one 4-hour period, scientists observed the surface subside by about 50 m! Note the many fractures in the ice on the margins of the cauldrons.
 

View of eruption crater and ash-covered Vatnajökull glacier about 36 hours after the eruption had broken through the ice. By this time, the area of subsidence had grown to about 9 km long and 2-3 km wide. The eruption continued for about another 10 days, and meltwater from the glacier flowed into the Grímsvötn caldera. On October 1, water level in the caldera's subglacial lake was about 1410 m; by October 16, the water level had risen to 1504 m, an increase of 94 m! According to scientists monitoring the activity, lava erupting from the fissure was piled up on the ground beneath the glacier, "forming a mountain ridge which in places is expected to be 200 m high."

On October 16, scientists stated that the meltwater, which had been accumulating under the ice shelf in the Grímsvötn caldera lake, could begin draining at any time to trigger a jökulhlaup (glacial outburst flood). On November 5 the expected jökulhlaup began.