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Mount Drum collapsed dome (Pleistocene). Commonly pale-red, but locally medium-gray where unoxidized, porphyritic hornblende andesite that contains phenocrysts of plagioclase (5-15%, as large as 6 mm), hornblende (2-5%, as large as 2 mm), clinopyroxene (1-3%, 1-2 mm), and hypersthene (0-3%, 0.5-4 mm), in pilotaxitic to glassy groundmass. Dome is mostly a pile of andesite rubble that consists of angular to subrounded blocks as much as tens of meters in diameter, but that average less than 1 m, in pinkish-gray, andesitic, sand- to dust-sized matrix. Part of dome apparently was not disrupted (ruled area). Dome may have been destroyed by violent phreatic explosive activity shortly after emplacement and is probable source of Sanford debris flow (Richter and others, 1988), which is exposed as far as 50 km to northwest where it is interbedded with glacial deposits in Copper River basin. * * *Plagioclase from dome yielded K-Ar age of 0.67+/-0.01 Ma. |