ERUPTIVE STYLE AND HISTORY OF MOUNT HOOD VOLCANO, OREGON

The Mount Hood edifice grew through long-lived, recurrently active andesitic volcanism over the past 1.3 million years. The radial distribution of lava flows around the present cone suggests a focused central vent since at least 0.5 Ma. During the growth of the present cone, basaltic andesite lava flows erupted from satellitic vents on all flanks of the volcano except the south and southeast sectors.

Mount Hood lavas range from andesite to low-silica dacite, and the majority of products range from 60.5 to 63.5 wt.% SiO2. Lava chemistry shows no systematic change through time, and bulk-rock analyses of Hood lavas overlap those of the 1.3-Ma andesitic edifice known as the Sandy Glacier volcano, which underlies Hood’s west flank. Limited Nd and Sr isotopic data for lavas of the past 200 ka show only minor variation in ratios (143Nd/144Nd=0.51291-0.51300; 87Sr/86Sr=0.7031-0.7034) and no trend with age.

The lack of widespread pumiceous tephra or pumiceous pyroclastic-flow deposits suggests that Mount Hood has never produced voluminous explosive eruptions. Rather, lava flows, growth of lava domes, and block-and-ash pyroclastic flows produced by dome collapse characterize its eruptive style. Similarly, most tephra-fall deposits appear to originate from ash clouds associated with dome collapses. Such deposits, although quite thick (up to 1 m) locally, are geographically restricted in distribution. Mount Hood has also produced several notable (on the order of 0.1 to 1 km3) debris avalanches in its history, the largest known of which transformed into a lahar that traveled down Hood River valley about 100 ka and at least temporarily filled the Columbia River valley to a depth of at least 30 m.

Mount Hood’s edifice chiefly comprises lava flows, but its dominant eruptive style for the past 30 ka has been dome growth and collapse. It is difficult to say whether the activity since 30 ka represents a fundamental shift in eruptive style or simply that evidence of older domes and block-and-ash pyroclastic flows are poorly preserved in the geologic record. On the basis of the recent past, we expect the next eruption of Mount Hood to include lava-dome growth and collapse with attendant block-and-ash flows, ash clouds, and lahars.