page_title: Alexander Island
section_title: Geology
hierarchical_section_title: Alexander Island / Geology
caption_attribution_description: English: Cross Section of the Antarctic Peninsula's Subduction Zone.
context_page_description: Alexander Island, which is also known as Alexander I Island, Alexander I Land, Alexander Land, Alexander I Archipelago, and Zemlja Alexandra I, is the largest island of Antarctica. It lies in the Bellingshausen Sea west of Palmer Land, Antarctic Peninsula from which it is separated by Marguerite Bay and George VI Sound. George VI Ice Shelf entirely fills George VI Sound and connects Alexander Island to Palmer Land. The island partly surrounds Wilkins Sound, which lies to its west. Alexander Island is about 390 kilometres long in a north-south direction, 80 kilometres wide in the north, and 240 kilometres wide in the south. Alexander Island is the second largest uninhabited island in the world, after Devon Island.
context_section_description: According to Hole, "The geology of Alexander Island can be attributed mainly to processes associated with the subduction of proto-Pacific oceanic crust along the western margin of the Antarctic Peninsula, from latest Triassic to Late Tertiary times."  The LeMay Group accretionary prism complex, along with plutonic and volcanic rocks, are prevalent along the western portion of the island.  The LeMay Group consists of variably-deformed and metamorphosed sedimentary and igneous rocks.  Although it is dominated by deformed arkosic sedimentary rocks, it includes turbiditic greywackes, black mudstones, and conglomerates.  The 4 km thick Upper Jurassic to Lower Cretaceous Fossil Bluff Group sedimentary rocks outcrop as a 250 km long by 30 km wide belt along the eastern coast.  This Fossil Bluff Group consists of a basal deep-marine assemblage 2200 m thick, overlain by a mudstone assemblage up to 950 m thick, followed by a shallow-marine assemblage of coarsening upward sandstones.  Alkali basalts erupted after the cessation of subduction.  These range in age from the tephrites at Mount Pinafore (5.5-7.6 Ma), to the basanites at Rothschild Island (5.5 Ma) and Hornpipe Heights (2.5 Ma), to the alkali and olivine basalts on Beethoven Peninsula (<1-2.5 Ma).
  The LeMay Range Fault trends N-S, parallel to the George VI Sound, and the Fossil Bluff Formation is downfaulted to the east of this fault against the LeMay Group.  Sand dykes are found against this fault zone and in many other parts of the Fossil Bay Formation.  Fossils within the Fossil Bluff Formation include ammonites, belemnites, bivalves, and serpulids.
caption: Antarctic Peninsula's tectonic movement
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