Mount Hood Historical Activity

Present thermal activity is in fumarole fields near Crater Rock, at the apex of a semi-circular zone of fumaroles and hydrothermally-altered, heated ground. In summer 1987, maximum ground temperatures were near 85 degrees C and maximum fumarole temperatures were about 92 degrees C (Cameron, 1988), slightly above the boiling point of water at 3100 m. Many of the fumaroles are actively precipitating crystalline sulfur. Comparison of modern and historical photographs shows that the amount of perpetually snow-free ground surrounding the fumarole fields has been increasing since last century. Until the 1980 eruption of Mount St. Helens, the only volcanically related human fatality in the Cascades occurred in the thermal area at Mount Hood in 1934, when a climber exploring ice caves in Coalman Glacier melted by fumaroles suffocated in the oxygen-poor gas.

Jökulhlaups (glacial-outburst floods) have been recorded from the Zigzag, Ladd, Coe, and White River Glaciers. In 1922, a dark debris flow issued from a crevasse high on Zigzag Glacier and moved 650 meters over the ice before entering another crevasse; this event initiated a scare that Mount Hood was erupting (Conway, 1921). The Ladd Glacier jokulhlaup in 1961 destroyed sections of the road around the west side of the mountain and partly undermined a tower of a major powerline (Birch, 1961). The Coe Glacier outburst occurred around 1963, causing a section of trail to be abandoned and the "round-the-mountain" trail to be rerouted farther from the glacier. Jokulhlaups from White River Glacier were reported in 1926, 1931, 1946, 1949, 1959, and 1968; the Highway 35 bridge over the White River was destroyed during each episode. The more frequent outbursts from White River Glacier may be due in part to an increase in size of the fumarole field at the head of the glacier at Crater Rock (Cameron, 1988).

A rainfall-induced debris flow on Polallie Creek on Christmas 1980 killed one and destroyed an 8-kilometer section of Highway 35. The flow started as a moderate slope failure of only 3800 cubic meters but rapidly bulked up and deposited over 76,000 cubic meters of debris at the mouth of Polallie Creek (Gallino and Pierson, 1984). The debris dammed the East Fork Hood River, creating a temporary lake; the dam breached, and flooding destroyed the highway.

Felt earthquakes on Mount Hood occur every 2 years on the average. Seismic monitoring, in effect since 1977 (Weaver et.al., 1982), indicates a generalized concentration of earthquakes just south of the summit area and 2-7 kilometers below sea level. A seismic swarm in July 1980, during which nearly 60 earthquakes (mostly 5-6 km deep with a maximum bodywave magnitude of 2.8) recorded in a 5-day period (Rite and Iyer, 1981), prompted development of an emergency response plan to coordinate local authorities in the event of future eruption.

Geodetic surveillance of the volcano was initiated in 1980, and 30 EDM lines and several tilt stations were resurveyed in 1983 and 1984 (Chadwick et.al., 1985; Cascades Volcano Observatory, unpub.data). Observed changes are within the range of expected error.