Recent eruptions of Mount Adams, Washington Cascades, USA |
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Authors: | Wes Hildreth Judy Fierstein |
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Institution: | (1) Volcano Hazards Team, U. S. Geological Survey, Menlo Park, California 94025, USA, US |
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Abstract: | The postglacial eruption rate for the Mount Adams volcanic field is ∼0.1 km3/k.y., four to seven times smaller than the average rate for the past 520 k.y. Ten vents have been active since the last main
deglaciation ∼15 ka. Seven high flank vents (at 2100–2600 m) and the central summit vent of the 3742-m stratocone produced
varied andesites, and two peripheral vents (at 2100 and 1200 m) produced mildly alkalic basalt. Eruptive ages of most of these
units are bracketed with respect to regional tephra layers from Mount Mazama and Mount St. Helens. The basaltic lavas and
scoria cones north and south of Mount Adams and a 13-km-long andesitic lava flow on its east flank are of early postglacial
age. The three most extensive andesitic lava-flow complexes were emplaced in the mid-Holocene (7–4 ka). Ages of three smaller
Holocene andesite units are less well constrained. A phreatomagmatic ejecta cone and associated andesite lavas that together
cap the summit may be of latest Pleistocene age, but a thin layer of mid-Holocene tephra appears to have erupted there as
well. An alpine-meadow section on the southeast flank contains 24 locally derived Holocene andesitic ash layers intercalated
with several silicic tephras from Mazama and St. Helens. Microprobe analyses of phenocrysts from the ash layers and postglacial
lavas suggest a few correlations and refine some age constraints. Approximately 6 ka, a 0.07-km3 debris avalanche from the southwest face of Mount Adams generated a clay-rich debris flow that devastated >30 km2 south of the volcano. A gravitationally metastable 2-to 3-km3 reservoir of hydrothermally altered fragmental andesite remains on the ice-capped summit and, towering 3 km above the surrounding
lowlands, represents a greater hazard than an eruptive recurrence in the style of the last 15 k.y.
Received: 24 June 1996 / Accepted: 6 December 1996 |
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Keywords: | Cascade arc Stratovolcano Postglacial eruptions Lava flows Tephra Volcano hazards |
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