d. An older pre-Mount Hood volcano, the Sandy Glacier volcano (basaltic andesite and andesite), is exposed on the west side of Mount Hood and is partly covered by Mount Hood rocks. The summit area of Mount Hood is comprised of several andesite or dacite domes and has been weakened by alteration and is susceptible to a potentially catastrophic slope failure. Major volcanic mudflow
deposits of altered Mount Hood andesite are know to occur up to 40 km from the volcano (REPORT: Recent Eruptive History..., 1998).Four eruptive periods have been recognized at Mt. Hood during the last 15,000 yrs. The eruptive periods have spilled pyroclastic block and ash flows and lahars into the four river systems that drain the volcano.
* Polallie Eruptive Period (15,000-12,000 yrs ago). Deposits of
this period were formed chiefly by pyroclastic flows and debris
flows, occur on all sides of Mt. Hood, thus indicating a vent at
* Timberline Eruptive Period (1,800-1,400 yrs ago). This period
produced between 0.7 and 1.1 cu km of pyroclastic flows and debris flows. The vents for Timberline and all subsequent eruptions were high on the southwest flank, perhaps at the vent now filled by the Crater Rock dome. Deposits of Timberline age are mostly restricted to the southwest flank of the volcano, where they form a broad volcaniclastic fan. Some debris flows, however, extend down the Sandy River or 80 km to its mouth at the Columbia River.
* Zigzag Eruptive Period (600-400 yr ago). These deposits are of
limited volume and have been found only in the valleys of the
* Old Maid Eruptive Period (250-180 yr ago). The deposits of this period include a dome (Crater Rock), a pyroclastic flow, and many debris flows in the White and Sandy River valleys. Crater Rock is a dacite dome 300-400 m across at its base and about
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