[Named for Great Sitkin Volcano which forms northern half of Great Sitkin Island in central part of Aleutian Islands located in northeast part of Adak 1:250,000 quad, AK.] [no type designated] Forms main cone of Great Sitkin Volcano (including rocks of cone itself, newly formed crater dome, five older plugs, basalt flow of Sitkin Valley, and parasitic tuff cone on northwest flank of main cone). Consists of (composite) main cone of flows and tuff beds with flows predominating, at least, in upper part of cone. Flow rocks predominantly of porphyritic andesite and basalt. Crater occupied by dome composed of black, glassy porphyritic basalt. Basalt flow near head of Sitkin Creek extruded from flank of cone, is only fairly young flow known, which is 100 to 125 feet thick with prominent columnar jointing. Unconformably overlies Sand Bay volcanics (new). Most of main cone buried under thin cover of pumice and rock fragments. Age is considered late Tertiary and Quaternary [based on regional stratigraphic relations].

Source: GNU records (USGS DDS-6; Menlo GNULEX).