Iceland Within the Northern Atlantic, Volume 2. Группа авторов

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 phase) partially destroying the town of Vestmanayer on the main island of Heimaey in 1973.

In relation to volcanic activity and especially to tectonic activity, seismicity is permanent in Iceland and major earthquakes have regularly occurred, particularly in the northern peninsulas (Húsavík region) and in the whole south of the island. When crossing the lava fields between Hveragerði and the Hekla, many remarkably preserved traces of major historical earthquakes (M > 6) can be observed (Figure I.17).

Figure I.16. (A) The submarine ridges of Kolbeinsey and (B) of Reykjanes (multibeam echosounder images, HAFRO.is). (C) The eruption of Heimaey, building the “Mountain of Fire” (Eldfell) in 1973

Figure I.17. Trace of the Réttarnes seismic fault (1294 or 1732) in the Rangavellir: South Iceland Seismic Zone (Françoise Bergerat©)

      If in the north of the island the current earthquakes occur mainly offshore, the Húsavík and Kopasker agglomerations are however far from being sheltered from a significant seismic event, and in the south, several major earthquakes have occurred very recently (M_w 6–7; June 2000, May 2008).

While Icelandic houses are relatively insensitive to earthquakes (Figure I.18), the same cannot be said for road infrastructure or greenhouses. The temporary rise or fall of water tables or lakes is frequent, reactivating or deactivating geysers and causing fluid escapes. This is particularly the case in the Hveragerði or Geysir region: Strokkur is currently the most active and Great Geysir is currently intermittent (Figure I.19).

The cold pole of Iceland is represented by its glaciers, currently relatively little extended but which covered practically all the island at the time of the last glaciation, inhibiting the activity of a great majority of the volcanoes. Most of the time, they settled at the top of the volcanic edifices constituting the high points of the island such as Vatnajökull (2,009 m at Bárðarbunga) or Hofsjökull (1,765 m at Habunga; Figure