Eruption History
The glacially eroded summit area consists of several andesitic or dacitic lava domes; Pleistocene collapses produced avalanches and lahars (rapidly moving mudflows) that traveled across the Columbia River to the north. The eroded volcano has had at least four major eruptive periods during the past 15,000 years.
The last three at Mount Hood occurred within the past 1,800 years from vents high on the southwest flank and produced deposits that were distributed primarily to the south and west along the Sandy and Zigzag Rivers. The last eruptive period took place around 170 to 220 years ago, when dacitic lava domes, pyroclastic flows and mudflows were produced without major explosive eruptions. The prominent Crater Rock just below the summit is hypothesized to be the remains of one of these now-eroded domes. This period includes the last major eruption of 1781–82 with a slightly more recent episode ending shortly before the arrival of Lewis and Clark in 1805. The latest minor eruptive event occurred in August 1907.
The glaciers on the mountain's upper slopes may be a source of potentially dangerous lahars when the mountain next erupts. There are vents near the summit that are known for emitting noxious gases such as carbon dioxide and sulfur dioxide. Prior to the 1980 eruption of Mount St. Helens, the only known fatality related to volcanic activity in the Cascades occurred in 1934 when a climber suffocated in oxygen-poor air while exploring ice caves melted by fumaroles in Coalman Glacier.
Since 1950, there have been several earthquake swarms each year at Mount Hood, most notably in July 1980 and June 2002. Seismic activity is monitored by the USGS Cascades Volcano Observatory located in Vancouver, Washington, which issues weekly updates (or daily updates when significant eruptive activity is occurring at a Cascades volcano).
The most recent evidence of volcanic activity at Mount Hood consists of fumaroles near Crater Rock and hot springs on the flanks of the volcano.
Read more about this topic: Mount Hood
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