Mt. Hood Eruption and Atmosphere

Each of three major eruptive periods at Mount Hood (12,000-15,000), 1,500-1,800, and 200-300 years ago) produced dacite domes, pyroclastic flows, and mudflows, but virtually no pumice. Most of the fine lithic ash that mantles the slopes of the volcano and the adjacent mountains fell from ash clouds that accompanied the pyroclastic flows. Widely scattered pumice lapilli that are present at the ground surface on the south, east, and north sides of Mount Hood may have been erupted during the mid-1800's, when the last known activity of the volcano occurred (REPORT: Recent Eruptive History..., 1998).

The geologically recent history of Mount Hood suggests that the most likely eruptive event in the future will be the formation of another dome, probably within the present south-facing crater. The principal hazards that could accompany dome formation include pyroclastic flows and mudflows moving from the upper slopes of the volcano down the floors of valleys. Ash clouds which accompany pyroclastic flows may deposit as much as a meter of fine ash close to their source, and as much as 20 centimeters at a distance of 11 kilometers downwind from the pyroclastic flows. Other hazards that could result from such eruptions include laterally directed explosive blasts that could propel rock fragments outward from the sides of a dome at high speed, and toxic volcanic gases. The scarcity of pumiceous ash erupted during the la

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anic eruptions can be challenged (THR DYNAMICS ....). The use of eruption chronologies to prove that volcanic eruptions are the major independent variable in the generation of transient but powerful "fluctuations" such as cooler temperatures, damaged tree-rings, disturbed atmospheric circulation patterns, famine and social unrest, remains problematic. The mere association of an eruption and an environmental phenomenon thought worth recording in the past is not proof of a dependent relationship. That being said, the only way to hazard a guess is to inspect the recent records of similar volcanic types and consider the aftermath of their eruptions in light of atmospheric change. The use of eruption chronologies to prove that volcanic eruptions are the major independent variable in the generation of transient but powerful "fluctuations" such as cooler temperatures, damaged tree-rings, disturbed atmospheric circulation patterns, famine and social unrest, remains problematic. The mere association of an eruption and an environmental phenomenon thought worth recording in the past is not proof of a dependent relationship. An alternative measure of volcanic eruptions was proposed by Newhall and Self [1982] who suggested that it was th
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Approximate Word count = 1924
Approximate Pages = 8 (250 words per page)

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