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Iceland plume
The Iceland plume is a postulated upwelling of anomalously hot rock in the Earth's mantle beneath Iceland. Its origin is thought to lie deep in the mantle, perhaps at the boundary between the core and the mantle at ca. 2880 km depth. Opinions differ as to whether seismic studies have imaged such a structure. In this framework, the volcanism of Iceland is attributed to this plume, according to the theory of W. Jason Morgan.〔W.J. Morgan (1971): Convection plumes in the lower mantle, ''Nature'' 230,42-43〕
== Geological history ==
According to the plume model, the source of Icelandic volcanism lies deep beneath the center of the island. The earliest volcanic rocks attributed to the plume are found on both sides of the Atlantic. Their ages have been determined to lie between 58 and 64 million years. This coincides with the opening of the north Atlantic in the late Paleocene and early Eocene, which has led to suggestions that the arrival of the plume was linked to, and has perhaps contributed to the breakup of the North Atlantic continent. In the framework of the plume hypothesis, the volcanism was caused by the flow of hot plume material initially beneath thick continental lithosphere and then beneath the lithosphere of the growing ocean basin as rifting proceeded. The exact position of the plume at that time is a matter of disagreement between scientists,〔L.A. Lawver, R.D. Müller (1994):Iceland hotspot track, ''Geology'' 22,311-314〕〔R.S. White, D.P. McKenzie (1989): Magmatism at rift zones: The generation of volcanic continental margins and flood basalts, ''J. Geophys. Res.'' 94(B6),7685-7729〕 but was probably beneath central Greenland;, as is whether the plume is thought to have ascended from the deep mantle only at that time or whether it is much older and also responsible for the old volcanism in northern Greenland, the Ellesmere Island Volcanics on Ellesmere Island, and in the Arctic Ocean (Alpha Ridge).〔D.A. Forsyth, P. Morel-a-l'Huisser, I. Asudeth, A.G. Green (1986): Alpha Ridge and Iceland - products of the same plume?, ''J. Geodyn.'' 6, 197-214〕
As the northern Atlantic opened to the east of Greenland during the Eocene, North America and Eurasia drifted apart; the Mid-Atlantic Ridge formed as an oceanic spreading center and a part of the submarine volcanic system of mid-oceanic ridges. The initial plume head may have been several thousand kilometers in diameter and it erupted volcanic rocks on both sides of the present ocean basin to produce the North Atlantic Igneous Province. Upon further opening of the ocean and plate drift, the plume and the mid-Atlantic Ridge are postulated to have approached one another, and finally met. The excess magmatism that accompanied the transition from flood volcanism on Greenland, Ireland and Norway to present-day Icelandic activity was the result of ascent of the hot mantle source beneath progressively thinning lithosphere, according to the plume model, or a postulated unusually productive part of the mid-ocean ridge system.
An extinct ridge exists in western Iceland, leading to the theory that the plume has shifted east with time. The oldest crust of Iceland itself is more than 20 million years old and was formed at an old, now extinct oceanic spreading center in the western fjord (Vestfirðir) region. The westward movement of the plates and the ridge above the plume and the strong thermal anomaly of the latter caused this old spreading center to starve 15 million years ago and lead to the formation of a new one in the area of today's peninsulas Skagi and Snæfellsnes; in the latter there is still some activity in the form of the Snæfellsjökull volcano. The spreading center, and hence the main activity, have shifted eastward again 7–9 million years ago, however, and formed the current volcanic zones in the southwest (WVZ; Reykjanes–Hofsjökull–Vatnajökull) and northeast (NVZ; Vatnajökull–Tjörnes). Presently, a slow decrease of the activity in the WVZ takes place, while the volcanic zone in the southeast (Katla–Vatnajökull), which was initiated 3 million years ago, develops.〔K. Sæmundsson (1979): Outline of the geology of Iceland, ''Jökull'' 29,7-28〕 The reorganisation of the plate boundaries in Iceland has also been attributed to microplate tectonics.〔
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