Oregon

Mt. Hood
       7 ½’ quadrangle: Mt. Hood South
       Clackamas and Hood River CountiesCounties
       Mt. Hood National Forest, Mt. Hood Wilderness
       Cascade Range. Pacific Crest Trail passes on the west side.

Volcanic
11,239 ft (3,426 m)


Closer view of small glacier on shelf at 8600 feet, above Sandy Glacier.
(Photo by Cyndy T. Hall, 2003)

Bedrock: Mt. Hood Dacite

Quaternary (Late Pleistocene)

Dacite erupted in 1760 to form Crater Rock, the remains of a collapsed lava dome, but the summit itself is made up of older dacite lavas and pyroclastic debris. Some of the dacites bear olivine phenocrysts, some of them pyroxene, and some hornblende. A hornblende dacite plug dome on the south slope has been dated at 2000 years BP. The Mt. Hood lavas apparently did not all evolve from one parent magma. The volcano was active during the period from 1760 to 1805, and steam emissions were reported in 1859, 1865, and 1907. The common route to the summit passes near Devil’s Kitchen, a fumarole stinking of hydrogen sulfide.

Lava domes typically grow in episodes, alternately growing and collapsing, which can result in pyroclastic flows like Mt. St. Helens has been producing since 1980. Dacite lava flows tend to be very viscous and would not advance far down the slopes, whereas pyroclastic flows and mudflows (lahars) can travel far down river valleys. Deposits on the south and southwest flanks of Mt. Hood indicate a past history of pyroclastic flows and lahars, so these are the most likely hazards in the future. In 1806 Lewis and Clark observed abundant sediments, derived from the 1760-1805 eruptive episode, in what they named Sandy River where it empties into the Columbia River.

For a discussion of all the volcanic highpoints, click here.

Surficial Geology: As viewed from a distance, Mt. Hood has the classic Fuji-like shape of a stratovolcano. Glaciation during the last 29,000 years has transformed the top of the cone into a more irregular shape, with horns and arêtes. The route to the summit crosses a bergshrund crevasse, where ice under the lower snowfield is pulling away from the underlying rock, due to the force of gravity. Beyond the crevasse the hiker passes between rock walls called the Pearly Gates to reach the upper snowfields.

Selected References:

  • Crandell, D.R., 1980, Recent eruptive history of Mt. Hood, Oregon and potential hazards from future eruptions: U.S. Geological Survey Bulletin 1492, 81p
  • Harris, Stephen L., 1988, Fire Mountains of the West: The Cascade and Mono Lake Volcanoes : Mountain Press, Missoula, MT, p.172-187.
  • Wise, W.S., 1969, Geology and petrology of the Mt. Hood area: A study of High Cascade volcanism: Geological Society of America Bulletin, v.80, p.969-1006.

Other suggested sources of information:


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