Back scattered electron image of sample AT-3788. This sample is tephra from the Ugashik caldera forming eruption which occurred between 40-171 thousand years ago.
From Miller (2004): "The only isotopically-dated volcanic rocks are dacite from the south rim of the caldera which yielded a single whole-rock K-Ar age determination of 171,000+/-22,000 yrs (table 2 on plate 2 [in original text])."