is caused by a remnant plume head underneath the continent. The basement crops out along the coast and consists of granitoids and metamorphic sediments left by a Devonian-Cretaceous volcanic arc. This volcanism manifests itself with 18 large and numerous smaller volcanoes, which occur in groups, rows or as solitary systems in Marie Byrd Land. The larger centres have produced phonolite, rhyolite, trachyte and rocks with intermediate compositions, and reach heights of over above sea level. The smaller centres are found at the foot of the larger centres, as parasitic vents on their slopes or along the coast. These vents have produced alkali basalt, basanite and hawaiite. Composition Basalt occurs at all four volcanoes. Phonolite and trachyte are found at Mount Rees and Mount Steere, the former also at Mount Frakes; Mount Rees also features rhyolite. Phenocrysts include clinopyroxene, magnetite, olivine and plagioclase. The magma erupted in the Crary Mountains originated in the mantle and underwent fractional crystallization after formation. Geologic history The Crary Mountains were active between 9.3 and 0.04 million years ago during the Miocene and Pliocene. The youngest dates have been obtained by argon-argon dating on Mount Frakes, and imply an eruption 35,000±10,000–32,000±10,000 years ago. These ages were obtained on English Rock, which has also yielded ages of 826,000±79,000–851,000±36,000 and 1.62±0.02 million years ago. Tephra deposits in ice cores recovered at Byrd Station may have originated at Marie Byrd Land volcanoes such as these of the Crary Mountains. The maximum age of each volcano decreases in southeastward direction, from 9.34±0.24 million years old Mount Rees to 2.67±0.39 million years old Boyd Ridge. The pattern of volcanism migrating along the chain has been observed at other mountain ranges such as the Executive Committee Range, where it takes place at a pace of like at the Crary Mountains. It is directed away from the centre of the Marie Byrd Land volcanic province and may reflect the propagation of a fracture in the crust. West Antarctica has been subject to glaciation since the Oligocene, where a perhaps local ice cap or snow deposit existed at Mount Petras. Volcanoes erupting through ice leave specific geologic structures which can be used to reconstruct the timing and extent of past glaciations. Geologic evidence at the Crary Mountains implies that a substantial West Antarctic Ice Sheet existed during the Miocene, and that fluctuations in its size may have stressed the crust and modulated the activity