its jökulhlaup (Reykjavik Helicopter©) and plume (Earth Observatory, NASA)
Figure I.8. Sheep disturbed by the ash from the eruption (Flickr©)
The glaciers are located on volcanic edifices, considered to be at least Quaternary. The largest ice cap, the Vatnajökull, rests on some of the most active volcanoes of the island, located above the summit of a deep magma plume.
Bárðarbunga (Figure I.9) is one of the volcanoes found above the Icelandic hotspot and is located on the western margin of the present Vatnajökull ice cap.
Figure I.9. Digital terrain model of Vatnajökull (black line: current cap boundary) completed with the flood drainage positioning and the potential extension (to a depth of 200 km) of the Icelandic mantle plume (in gray) (source: H. Björnsson, 2009)
The most recent eruption of this volcano (August 2014–February 2015; Figure I.10) was linked to the draining of a magma chamber located 12 km below the caldera, following the climate driven melting of the cap (about 1 m/year).
To the northwest, the most impressive lava flow since the 18th century, the Holhurhaun flow, occurred along a fracture line, in association with swarms of earthquakes that stretched to the Askja volcano in the north. The previous eruption, that of Veiðivötn, had flown toward the south in 1747, awakening the Torfa volcano at the same time.
The Bárðarbunga is also a source for jökulhlaups or megafloods, resulting from the melting of the glaciers by the heat of the lava emitted and which mostly flow toward the north.
Figure I.10. (A and B) Views of the Bárðarbunga caldera obliterated by ice during the 2014 eruption with a melting cauldron to the west (A and black arrow) (photo: mbl.is/RAX). (C) Satellite image of the emersion of the Holhurhaun flow (star) on August 13, 2014 at the foot of the Bárðarbunga (B) (photo: TerraSAR-X)
Another major volcanic structure is located in the center of the ice cap, directly above the top of the mantle plume: it is the triple caldera of Grímsvötn (Figures I.9 and I.11), which emitted the vast majority of basaltic tephra that hide the glaciers and reach the lands surrounding the North Atlantic.
The most famous is the Saksunarvatn tephra splayed around 10,200 years cal BP. This volcano is never at rest; its current eruptive frequency is about 10 years and it also remained continuously active during the Ice Age, but with a lower frequency.
It is mainly responsible for the formation of subglacial lakes and is at the origin of most of the jökulhlaups that gully the emissaries of the Vatnajökull cap (Figures I.12 and I.13). At present, these floods mainly destroy road infrastructures such as the Main Highway (N1).
Figure I.11. Rim of the northern caldera of Grìmsvötn (Ragnar Sigurdsson©)
In northern Iceland, volcanic activity is also significant, in association with the northern rift. Many geothermal fields are exploited there, such as the Krafla field northeast of Lake Myvatn (Figures I.14 and I.15).
This volcanic activity also occurs at sea, both in the north in the Kolbeinsey Ridge and its intermittent island (white point in Figure I.16(A)) and in the southwest along the Reykjanes Ridge, or in the Vestmann Islands, a southern extension of the East Volcanic Zone.
Figure I.12. The Grìmsvötn volcano. (A) Initiation of the northward flow associated with a collapse of the ice mass (sun to the west), which led to the great jökulhlaup of November 1996 (Oddur Sigurðsson©). (B) Grimsvötn crater at the end of the 2011 eruption (Dima Moiseenko©). (C) Interstratified and deformed basaltic tephras in the terminal glacier tongue of Brúarjökull (LMIs)
Figure I.13. (A) The jökulhlaups of the Skafta River from the Grimsvötn in 1996 (M.T. Gudmundsson©) and (B) Main Highway (N1) in 2011 (Veðurstofa Íslands©), frequently repaired since 1970, with (C) the jökulhlaup memorial of November 1996: two enormous pieces of the metal deck of the old bridge, twisted like common wires (Françoise Bergerat©)
Figure I.14. (A) Fissural eruption of Krafla in 1980, along fractures arranged en échelon. (B and C) The geothermal power plant (C) narrowly escaped destruction by lava flows (flow with white arrow) (B-C: Brigitte Van Vliet-Lanoë©)
Figure I.15. Eruption of Krafla in 1980: hornitos on fractures and lava flows in 1997 (Brigitte Van Vliet-Lanoë©)
The latter were the locus of a first submarine eruption in 1963 (building of the Surtsey volcano), then of a fissural eruption (followed by a strombolian
