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Gaussberg (or Schwarzen Berg, Mount Gauss) is a extinct volcanic cone in East Antarctica fronting on Davis Sea immediately west of Posadowsky Glacier. It is ice-free and conical in nature, having formed subglacially about 55,000 years ago. The current edifice is thought to be the remains of a once-larger mountain that has been reduced by glacial erosion and subaerial erosion. The volcano has produced lamproite magmas, and is the youngest volcano to have produced such magmas on Earth.
Owing to its peculiar composition, Gaussberg has been intensively researched. The mountain was investigated in 1912 by the 1911-1914 Australasian Antarctic Expedition, by the Soviet Antarctic Expedition in 1956–1957, by Australian expeditions in 1977, 1981, 1987 and by an expedition linked to an entity "K.D.C" in 1997. Regional krill stocks in turn were named after the mountain. Owing to its peculiar composition and isolated location, the volcano has an importance out of proportion to its actual size. The mineral gaussbergite is named after the volcano.
It consists of a , cone located between the East Antarctic Ice Sheet on three sides and the sea on the fourth. It is the only nunatak in the region, with rocky outcrops at the summit and on the northern flank. The edifice covers an area of about and has a volume of . Most of the edifice is made out of with radii of and thick crusts. The volcano is covered with lava fragments resembling lapilli which may have formed through erosion. Gaussberg has no volcanic crater, rather having a ridge at the summit. The volcano has several terraces of undetermined origin and may have formed as a shield volcano with multiple vents. The rocks were probably emplaced subglacially, although the occurrence of pahoehoe lava is possible. There are on the southern, northwestern and northeastern foot of the volcano, and and glacial striations are evidence that the volcano was formerly glaciated.
Its activity has been related to the Kerguelen Plateau, but the Kerguelen volcanoes have yielded different magma compositions and there is no major geological structure linking the two other than the so-called "Kerguelen-Gaussberg Ridge", thus a connection between the two is unproven. A graben system in the region, which may have formed in Gondwana and may be correlated to tectonic structures on the Indian Peninsula, has been christened the "Gaussberg Rift"; the volcano rises on a horst on the rift but its relation to the rift is unclear. Finally, the 90° E Fault that separates regional tectonic structures may have influenced volcanism at Gaussberg.
The source of the Gaussberg lamproites is unclear, as the processes usually proposed for the formation of such magmas do not easily apply to the Gaussberg rocks. The magma may have formed through the incomplete melting of phlogopite-rich mantle and further chemical processes such as crystal fractionation that raised the potassium/aluminium ratio above 1. Deep mantle structures that formed through subduction billions of years ago and remained isolated since then have been proposed as the source of Gaussberg lamproites. The Kerguelen plume may or may not have played a role.
Gaussberg was probably constructed in a single eruptive episode but there is evidence that the present-day edifice formed on an older, eroded volcano. Gaussberg formed under much thicker ice than there is today in the area, and the ice deposited moraines on its summit. There are different views on how erosion affected Gaussberg; some think that it was largely spared and others that erosion wore down the initially much larger edifice to its current size; the latter theory is the preferred view of the Global Volcanism Program and is supported by aeromagnetic data which suggest an initial size of . Dust layers in the Siple Dome ice core may come from wind-driven erosion of Gaussberg rocks.
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