Post-caldera eruptions at Okmok Volcano, Umnak Island, Alaska, with emphasis on recent eruptions from cone A

Abstract

Okmok volcano has undergone two caldera-forming eruptions, 12,000 and 2050 BP, and has been quite active in historic time. The historic eruptive record has been compiled and augmented with descriptions and photographs of recent eruptions. Eruptions in 1958 and 1997 produced the first post-caldera lava flows to traverse most of the caldera floor. The source of these flows, Cone A, has been constructed largely during the 20th century. Major element analysis of lavas from eight major intracaldera cones reveals two chemically and spatially separate trends, which suggest two separate magma sources beneath the caldera, one feeding an arc of cones extending from the west to north margin of the caldera, the other feeding an arc running from southwest to east. Recent geodetic results by other workers show a single active inflation source related to Cone A but located beneath the center of the caldera. A rheologic study of the 1997 lava flow was undertaken to determine how viscosities calculated from flow morphology compare with viscosities and eruption temperatures obtained from petrology. This may be a useful tool for constraining composition of new flows observed by satellite imagery, and for constraining eruptive conditions for older flows when chemistry is known.