rocks were probably emplaced subglacially, although the occurrence of pahoehoe lava is possible. There are moraines on the southern, northwestern and northeastern foot of the volcano, and erratic blocks and glacial striations are evidence that the volcano was formerly glaciated. Geology Gaussberg is an extremely isolated volcano although an ice rise a few kilometres southwest of Gaussberg and aeromagnetic surveys suggest that within there are other small volcanoes in the area. It is the only Antarctic volcano situated on the Antarctic Shield, where the thickest crust of Antarctica is found. Why it formed about 50,000 years ago on a stable continental margin is unclear; either a mantle plume, an instability of the East Antarctic continent or lateral flow of mantle plume material are possible. The basement underneath Gaussberg is formed by gneisses of Archean to Proterozoic age. The lithosphere under Gaussberg is over thick and has an unusually high heat flow. 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. Composition The volcano has a uniform chemical composition consisting of lamproite (originally identified as leucitite), which defines a potassium-rich mafic rock suite. The rocks are almost free of visible crystals but contain numerous vesicles. Phenocrysts include clinopyroxene, leucite and olivine. The Gaussberg suite is the youngest lamproite known on Earth. The rocks are rich in volatiles including carbon dioxide and water. There are xenoliths, mostly granites coming from the Precambrian basement, and zircons recovered from the rocks are up to several billion years old. Palagonite, salt and native sulfur deposits have been found. 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