Читать книгу Iceland Within the Northern Atlantic, Volume 2 - Группа авторов - Страница 12

Brigitte VAN VLIET-LANOË and René MAURY

Figure I.1. Iceland from Space (document Geographical Institute of Iceland/Landmælingar Ísland [LMIs])

Iceland (Figure I.1), a young and isolated island in the middle of the Atlantic Ocean, has only very recently been discovered in terms of the scale of human history. Irish monks (the Papar) passed from island to island in their curraghs (Figure I.2) via the Shetland Islands and the Faroe Islands to evangelize the legendary Hyperborea. These journeys took place as early as the 6th century, a period with cold volcanic winters.

The Papar discovered a world of fire and ice, the gates of hell. They settled in round peat-covered huts and dug shovel caves in the consolidated sandy interglacial formations in the south of the island.

Figure I.2. (A) Icelandic stamp illustrating the discovery of Iceland by Irish monks (Papar). (B) The glacial lake Jökulsarlón dominated by the volcano Oræfajökull (Brigitte Van Vliet-Lanoë©)

Two hundred years later, the Vikings, warriors but also more than anything farmers in search of cultivable land (Figure I.3), settled in the south and west of the island from 860 AD (for Anno Domini, year 1 of the Christian calendar) on wooded land made fertile by thick layers of volcanic loess. This is the landmana of the Icelandic sagas. They installed their parliament, the Alþing, around 900 AD, in a remarkable site (Figure I.4), which became a high place of plate tectonics, the Þingvellir graben, the boundary between the European and American plates.

These fertile lands were surmounted for at least 400,000 years by a fire monster, the Hekla volcano (Figure I.5). Its Plinian eruption in 1104 AD (H1, volcanic explosivity index of 5) destroyed many Viking settlements in the Rangavellir, not only by falling pumice and gas but also by the associated glacial megafloods, the jökulhlaups, submerging the Þjorsárdalur with a wave of muddy water more than 25 m high. At that time, the Hekla must have been more ice-covered than it is today.

Figure I.3. Traditional sheepfolds in southern Iceland with the volcano Hekla in the background (Rangavellir) (Brigitte Van Vliet-Lanoë©)

Figure I.4. (A) The Þingvellir graben, seen from an airplane (Þingvellir National Park Web site). The Alþing site is located in front of the white buildings (chapel). (B) Reconstruction of the Alþing in the Middle Ages by W.G. Collingwood (1897)

Figure I.5. (A) The Hekla volcano (Brigitte Van Vliet-Lanoë©) and (B) its cartographic representation on the map Islandia of Ortelius (1585)

Despite the island’s long isolation from continental Europe, there is a lot of information about its history. Indeed, Icelanders have jealously preserved their language and ancient books, including the famous sagas, and have often proved to be great writers and avid readers, even on isolated farms.

In addition to their literary and historical interest, the sagas represent a source of exceptional paleo-environmental information on a period whose climatic evolution was very complex: the Medieval Optimum and the climatic degradation that followed. The University of Iceland was founded in 1911 and, due to its special nature, Iceland is the country with the highest proportion of geologists and especially volcanologists among its population.

Figure I.6. Typical landscape of ancient basalts on the eastern coast of Iceland (Skridalur) (Brigitte Van Vliet-Lanoë©)

Iceland is a land of fire and ice, still sparsely populated (about 350,000 inhabitants in 2020), prized by tourists for its “unspoiled”, photogenic character and its many natural wonders, although Viking colonization quickly made the forest disappear. But recent tourist development has also caused an invasion of 4×4 vehicles, brand new hotels and vacation huts, raising the standard of living of the population, but gradually destroying a natural heritage – including the geological heritage – surprisingly well preserved until the early 21st century. Industrial development (geothermal, hydroelectricity and electrometallurgy) kept the population in the peripheral sectors of the island and above all modified the landscape of the coastal zones. Whatever one does or looks at in Iceland is de facto connected to the geological history of the island (Figure I.6).

Despite its remoteness, Iceland is a land that directly influences Western Europe through its position in the north-central Atlantic, as a beacon of the Gulf Stream and thermohaline circulation, or through its meteorological depression. But it is also a land consisting mainly of layered basaltic piles, still active from a tectonic and volcanic point of view. We were reminded of it by the last eruption of the Eyjafjallajökull (March–October 2010) with its plume of ash that invaded Europe and disrupted intercontinental commercial flights (Figures I.7 and I.8).

Figure I.7. The eruption of the Eyjafjallajökull in 2010: 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 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 I.20).

Figure I.18. Destruction caused during the earthquakes of June 2000 in Bitra: (A and B) farm buildings, (C) main highway (N1) (A-B-C Françoise Bergerat©), and at the end of May 2008 in Hveragerði: (D) dislocated pipes and damaged greenhouses (Brigitte Van Vliet-Lanoë©)

Figure I.19. Successive phases of an explosion of the Strokkur geyser, Geysir geothermal field (Brigitte Van Vliet-Lanoë©)

Figure I.20. The Hofsjökull. Document made from radar images (CNES©). The caldera is located at the top left of the picture

These glaciers have profoundly carved the island since the Neogene, with deep glacial valleys, ice-smoothed or striated rocks, countless drumlins and large areas of abandoned glacial sediments on the central plateau, especially around the Kerlingarfjöll (Figure I.21). Some volcanoes have typically subglacial morphologies, such as tabular volcanoes or tuyas, the best known of which is Herðubreið (Figure I.22). Others form alignments of ridges, the tindar, which formed at the margin of the melting caps (Figure I.23).

The waters from these glaciers have also shaped canyons with huge waterfalls, on powerful, gray and loaded water rivers, the jökullsá (Figures I.23 and I.24). These waters are currently collected for an important hydroelectric production with mainly industrial purposes (aluminum and rare metals extracted from imported ores). This resource accounts for 72% of Iceland’s electricity production.

Various cap outlets are currently being developed and managed, with water stored in very large dams, generally superimposed on the same course and designed to resist jökulhlaups of interglacial rank. Global warming in recent decades and potentially induced volcanism are likely to call this policy into question.

Figure I.21. (A) The Kerlingarfjöll surrounded by its glacial desert. (B) Perched upper cirque and (C) ice-smoothed rocks of the eastern fjords (Mjóifjörður, south of Seiðifjörður) (Brigitte Van Vliet-Lanoë©)

Figure I.22. A subglacial tabular volcano: the Herðubreið, north of Vatnajökull, North Volcanic Zone (Brigitte Van Vliet-Lanoë©)

Figure I.23. Jökulsá á Kreppa north of Vatnajökull with hyaloclastite or tindar ridges (Brigitte Van Vliet-Lanoë©)

Figure I.24. A key Icelandic resource: water. (A) Bruarjökull outlet (the glacier is at the bottom of the photograph) (LMIs©). (B) One of the Dettifoss waterfalls (Jökulsá á Fjöllum). (C) The Haslsón dam on the Jökulsá á Brú. (D) The Fannahlið aluminum plant (Hvalfjörður) (photos B, C and D: Brigitte Van Vliet-Lanoë©)

On land, Iceland’s only important and renewable resources are its water and, as a result, its hydroelectricity, as well as its many geothermal sites related to the presence of the hot spot.

In this two-volume book, we will present the geological and glacial history of this island, its current tectonic and volcanic activity and the impact of its formation on the climatic evolution of the last few millions of years. Volume 1 replaces Iceland within the geological framework of the North Atlantic, and describes its tectonic and geodynamic evolution. This second volume is dedicated to the study of the interactions between Icelandic volcanism and external geodynamics, i.e. with glaciations and the climatic evolution of the Atlantic zone during the Neogene and the Quaternary.

For color versions of the figures in this Introduction see, www.iste.co.uk/vanvliet/iceland1.zip.