The Icelandic term “jökulhlaup” has been international­ly adopted for glacial outburst floods that occur frequently at many locations in Iceland. Jökulhlaups can be extremely dangerous and a significant hazard for settlements. They occur through steady melting of ice above geothermal areas, melting by magma–ice interactions during volcanic eruptions, or through the release of water stored in marginal lakes dammed by glaciers. Jökulhlaups maintain the globally unique sandur plains, to the north and south of Vatnajökull ice cap, as well as rapidly evolving canyons. Jökulhlaups have had a significant role in land­scape evolution north of the Vatnajökull ice cap. The 28 km long Jökulsárgljúfur canyon formed during the early Holocene by a few huge jökulhlaups that originated from Vatnajökull (either in Kverkfjöll or Bárðarbunga volcanoes) and flooded the path of Jökulsá á Fjöllum glacial river.

The Grímsvötn subglacial caldera lake, maintained by geothermal heat, is the source of the most widely known jökulhlaups. More than 40 jökulhlaups from Grímsvötn have been reported since the Middle Ages. They were released into the Skeiðará river after travelling 50 km along the bed of Skeiðarár­jökull outlet glacier. In the largest jökulhlaups, almost all the Skeiðarársandur outwash plain, ca. 1000 km² in area was flooded. Skeiðarársandur is probably the world’s largest sandur, or outwash plain, in front of an active glacier. The 100–250 m thick sediments have piled up during the last 10,000 years from the gener­al action of glacial rivers as well as jökulhlaups.

Jökulhlaups have also regu­larly occurred in the river Skaftá, from the Skaftárkatlar subglacial lakes. Skaftárkatlar (the Skaftá cauldrons) northwest of Grímsvötn are depressions in the glacier surface, surrounded by circular crevasses, formed by geothermal melting at the glacier bed. Geothermal activity causes meltwater to be collected in subglacial lakes at two locations beneath the cauldrons in the surface of the glacier, which is 350–400 m thick in the area of the lakes. When the lake water is released, the surface of the ice cover is lowered by 70–100 m. The ice surface then rises again between jökulhlaups.

Map showing the origin of jökulhlaups in Vatnajökull. Red: subglacial eruptions, blue: ice-dammed lakes (present and previous), green: subglacial geothermal activity, however Gengissig lake in Kverkfjöll,  is also ice-dammed. Source: Richard Williams and Oddur Sigurðsson, in press.

Floodwater in Skaftá from Skaftá cauldrons on the 2^nd of October 2015. Photo: Tómas Jóhannesson.

Locations of the Grímsvötn volcanic caldera and the Skaftárkatlar geothermal fields in western Vatnajökull and the flow paths of jökulhlaups from the subglacial lakes at these sites. Source: Icelandic Meteorological Office and Institute of Earth Sciences, University of Iceland.

Flooded Skeiðarársandur on the 6^th of November 1996. Photo: Oddur Sigurðsson.