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The eruptive history of Mount Hood prior to about 15,000 years ago is chiefly
recorded by the volcanic rocks that form the volcano. These rocks were
described by Wise (1969), who divided them into four main groups. From oldest
to youngest these are olivine andesite lava flows that form the base of
the volcano, long flows of pyroxene andesite that extend down former
canyons that headed on the volcano, flows of pyroxene andesite that form the
part of the volcano above about 1,800 meters (5,900 feet), and a remnant of a
round dome of dacite that forms Crater Rock (Wise, 1969). The bulk of the
volcano is made up of andesite in which the silicon dioxide (SiO2) content
ranges from about 57 to 61 percent. Rocks at Mount Hood that were described by
Wise (1969) as dacites contain 62-63 percent SiO2 and are limited to the
products of the geologically most recent eruptions, including the deposits of
the Polallie, Timberline, and Old Maid eruptive periods described in this
report. These dacites generally contain the ferromagnesian minerals hypersthene
or hornblende, or both, and may include small amounts of augite. At some other
volcanoes the presence or absence of these and other minerals has been used to
distinguish volcanic deposits of different ages, but this was not found to be
possible during the present (1980) study of Mount Hood
Several volcanic vents on the lower flanks of Mount Hood or beyond its base have erupted olivine andesite lavas which are chemically similar to one another, but which do not seem to be genetically related to Mount Hood (Wise, 1969, p.994, 999-1000). One vent is at The Pinnacle on the north flank of the volcano, another is near Cloud Cap Inn on the northeast flank, and a third lies in the valley of the Middle Fork Hood River 11.5 kilometers northeast of the summit of Mount Hood. Wise believed that the lava flows from the vent at The Pinnacle were erupted before the Fraser Glaciation, but regarded those from the vent near Cloud Cap Inn as of post-Fraser age. Lava flows from The Pinnacle are locally overlain by glacial deposits of Fraser age. Soil profiles on deposits that overlie the lavas from the vent at Cloud Cap Inn indicate that these lavas, too, are pre-Fraser and probably are older than the Hayden Creek Drift. The olivine andesite lava flow in the Middle Fork Hood River valley extends down the valley floor about 6 kilometers from a vent south-southwest of Parkdale (The length of the lava flow was mistakenly given as 6 miles by Wise (1969)). The flow is about 60 meters thick (Wise, 1969) and as much as 1.2 kilometers wide. Charcoal from a soil beneath the lava flow had a radiocarbon age of 6,890 +/- 130 years (Harris, 1973, p.66-67). |
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