Vatnajökull (Vatnajokull) glacier is the largest glacier in Iceland and it is also the largest glacier mass in Europe. Classified as an ice cap type of glacier, it covers an area of roughly 8300 km2, and it’s about 950 m thick at its thickest point. Its average thickness is 400-600 m, and the total ice volume of Vatnajokull is probably in the vicinity of 3300 cubic km. The Equilibrium-Line Altitude (ELA), the level at which accumulation and ablation are in balance, lies at approximately 1100 m a.s.l. along its southern margins, at 1200 m along its western part, and at 1300 m in its northern part. About 60% of the glacier surface is above the ELA. The ice cap covers a highland plateau, generally reaching 600-800 m altitude, but dissected by numerous broad and narrow subglacial valleys.

Vatnajokull is named after subglacial lakes located in a very volcanically active region in its centre. The subglacial landscape is an undulating plateau with valleys and gorges. The icecap rises between 1400 and 2000 m above sea level. The ablation elevation is slightly different: 1100 m in the south, 1200 m in the west and 1300 m in the north. A great number of glacier snouts of different sizes flow down onto the lower lying areas. At its lowest point, the glacier base reaches 300 m below sea level.

No glacier in Iceland has been researched more thoroughly than Vatnajokull. The research started in 1934 when the lake region erupted, and has been carried out continuously since 1950 when the Iceland Glaciological Society was founded. The Society owns huts in several places on the ice cap. The first confirmed trip across the ice cap from the south and back was accomplished in 1875 by an Englishman and a few Icelanders. They were the first to see the Askja eruption the same year and report it to the people living around Lake Mývatn (Myvatn).

The hidden volcanoes
A number of large volcanoes, most of them active, are concealed beneath the colossal Vatnajokull ice cap. The volcanoes Bárðarbunga (Bardarbunga) (2020 m) and Grímsvötn (Grimsvotn) (1725 m) are located in the western part. Together with Hekla, Grimsvotn volcano has been Iceland's most active volcano since the Middle Ages. Both Bardarbunga and Grimsvotn contain large subglacial caldera depressions. The Bardarbunga centre is 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 latest eruptions of the lake region took place in 1996, 1998, 2004 and 2011. 

Kverkfjöll (Kverkfjoll) is a large glaciated central volcano on the northern edge of Vatnajokull, with a powerful high temperature area featuring mud-holes, steam blowholes and a luke-warm lake. Underneath the glacier there is an ice cavern system several km long.

The southernmost part of Vatnajokull encloses the stratovolcano Öræfajökull (Oraefajokull), and Iceland´s highest peak, Hvannadalshnjúkur (Hvannadalshnjukur) rises 2110 metres above Iceland´s south shore.

The 1996 Gjálp subglacial eruption 
Late in the evening of September 30, 1996, seismometers detected the beginning of an eruption under the Vatnajokull ice cap, from a fissure called Gjálp (Gjalp), located between the Grimsvotn caldera and Bardarbunga. Late on October 1, the day after the eruption had started, the surface of the ice over the caldera, in which a subglacial lake had accumulated, had risen ten to fifteen metres. The next day, the eruption broke through the surface of the ice, emitting an ash cloud 10 km high. The volcano quietened on the 13th, but the ice continued to melt and overflow the Grimsvotn lake. More than three cubic kilometres of ice melted, but little was emitted through normal runoff points. Since an ice dam and the caldera itself held the melt back, the jokulhlaup did not occur until November, or at least one month later.

At 7:20am on the fifth of November, the meltwater burst vertically from two kilometres above the tongue of the glacier. By four that afternoon, the jokulhlaup was fully realized. A mixture of sediment, meltwater, and ice moved at 10 km per hour from the full twenty-kilometre width of the glacier's terminus across Skeidararsandur, forming standing waves three and four metres high. The total flow peaked at over fifty thousand cubic metres per second in the five outwash channels, making it briefly the second largest river of the world. The flood obliterated a 376-metre-long bridge, most of a second bridge 900 metres in length, twelve kilometres of roadway and twenty-three power-line towers, causing fourteen million USD in damage while adding seven square kilometres to the area of Iceland. There were no fatalities or injuries, and the flood did not reach any nearby settlements.

The impact of the1996 subglacial eruption at Vatnajokull ice cap was felt around the world. The phenomenon of a subglacial volcanic eruption is not often encountered; therefore, numerous journalists and scientists from around the world met in Iceland during the eruption to study the events and ramifications of these occurrences. The study of volcanoes, especially active volcanoes located under glaciers, has since broadened its scope of research into the distinct volcanism of glacial ice-volcano interactions, or glaciovolcanism, in order to unlock more of the secrets of our planet. The insights gained can be applied to other fields, including plate tectonics and the behaviour of land on other planets.

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