Geologic setting, petrology, and age of Pliocene to Holocene volcanoes of the Stepovak Bay area, western Alaska Peninsula: A section in Geologic studies in Alaska by the U.S. Geological Survey, 1988

Five volcanoes are situated north of Stepovak Bay on the western Alaska Peninsula; only one of these, Kupreanof volcano, had been reported prior to USGS reconnaissance geologic mapping of the area between 1982 and 1986. The volcanoes overlie sedimentary rocks of Eocene to late Miocene age and welded(?) tuff deposits of uncertain, but presumably late Tertiary, age. All five of the volcanoes are aligned northeast-southwest at the southeast end of the well defined Chignik segment of the Aleutian arc. The group shows evidence of eruptive activity beginning approximately 4.7 Ma and continuing into Holocene time and represents one of the longest lived active volcanic centers on the Alaska Peninsula. Associated rocks are generally tholeiitic andesite in composition and have a limited range of mineralogy and texture. Extensive upper Tertiary and Quaternary andesite lava flows cap ridges and fill valleys in the vicinity of the volcanoes and, in some cases, extend to near sea level. Holocene eruptive activity, which occurred on four of the five Stepovak Bay group volcanoes, resulted in three small debris flows, two short lava flows, and a small cinder cone. Active fumaroles exist today on two of the volcanoes.