40K/40Ar whole-rock age determinations of trachyte from Observation Hill give a mean age of 1.18 ± 0.03 m.y., establishing a minimum age for the onset of Ross Island volcanism. This age is within the Matuyama reversed geomagnetic polarity epoch and agrees with the reversed magnetization previously reported for trachytes from the same locality by Cox (1966). The trachyte outcrops have been glaciated, and the new age data reinforce a previously proposed maximum age of 1.2 m.y. for the Ross Sea glaciations.

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