Editing Crary Mountains (section)

== Geology ==

[[Cenozoic]] volcanism in Marie Byrd Land is related to the [[West Antarctic Rift]] and has been explained by the activity of a [[mantle plume]]. This plume either underlies Marie Byrd Land and its volcanoes, or it rose to the surface before Antarctica separated from [[New Zealand]] during the middle [[Cretaceous]] and induced volcanism across the continental borderlands of the [[Southwest Pacific]]. In the latter theory, the Marie Byrd Land volcanism is caused by a remnant plume head underneath the continent.{{sfn|Panter|Hart|Kyle|Blusztanjn|2000|p=216}} The [[basement (geology)|basement]] crops out along the coast and consists of [[granitoid]]s and [[metamorphic]] sediments left by a [[Devonian]]-[[Cretaceous]] [[volcanic arc]].{{sfn|Panter|Hart|Kyle|Blusztanjn|2000|p=218}}

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 {{convert|3000|m}} above sea level.{{sfn|Panter|Hart|Kyle|Blusztanjn|2000|p=217}} The smaller centres are found at the foot of the larger centres, as [[parasitic vent]]s on their slopes or along the coast. These vents have produced [[alkali basalt]], [[basanite]] and [[hawaiite]].{{sfn|Panter|Hart|Kyle|Blusztanjn|2000|p=218}}

=== 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. [[Phenocryst]]s include [[clinopyroxene]], [[magnetite]], [[olivine]] and [[plagioclase]].{{sfn|Panter|Hart|Kyle|Blusztanjn|2000|p=219}} The magma erupted in the Crary Mountains originated in the [[Mantle (geology)|mantle]] and underwent [[Fractional crystallization (geology)|fractional crystallization]] after formation.{{sfn|Panter|Hart|Kyle|Blusztanjn|2000|p=224}}

=== Geologic history ===

The Crary Mountains were active between 9.3 and 0.04 million years ago{{sfn|Panter|Hart|Kyle|Blusztanjn|2000|p=216}} during the Miocene and [[Pliocene]].{{sfn|Wilch|McIntosh|Panter|Dunbar|1993|p=7}} 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.{{sfn|Wilch|McIntosh|2002|p=243}} These ages were obtained on English Rock, which has also yielded ages of 826,000±79,000–851,000±36,000{{sfn|Wilch|McIntosh|2002|p=248}} and 1.62±0.02 million years ago.{{sfn|Wilch|McIntosh|Panter|2021|p=541}} [[Tephra]] deposits in [[ice core]]s recovered at [[Byrd Station]] may have originated at Marie Byrd Land volcanoes such as these of the Crary Mountains.{{sfn|Gow|Williamson|1971|p=213}}

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 {{convert|7|mm/year|in/year}} 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 (geology)|crust]].{{sfn|Panter|Hart|Kyle|Blusztanjn|2000|p=219}}

[[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.{{sfn|Wilch|McIntosh|2002|p=237}} 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 of volcanoes in its area.{{sfn|Wilch|McIntosh|2002|p=251}} Before its formation the Crary Mountains might have been islands.{{sfn|American Geophysical Union|1971|p=30}} In the Crary Mountains, ice occurred either in the form of slope ice when the mountains were erupting{{sfn|Wilch|McIntosh|2002|p=252}} or as a thick continental [[ice sheet]].{{sfn|American Geophysical Union|2001|p=71}} The glaciers were cold-based and thus did not produce [[tillite]]s or glacial surfaces,{{sfn|Haywood|Smellie|Ashworth|Cantrill|2008|p=423}} and were probably thin.{{sfn|Wilch|McIntosh|Panter|2021|p=541}} Glacial erosion took place mainly between 8.55 and 4.17 million years ago; it formed the cirques in Mount Rees and Mount Steere{{sfn|Wilch|McIntosh|Panter|Dunbar|1993|p=9}} and transported [[glacial erratic]]s on the mountains.{{sfn|Wilch|McIntosh|Panter|2021|p=545}}