Volcanic Activity

A volcano is a vent in the earth’s crust through which molten rock, rock fragments, gases, or ashes are ejected from the earth’s interior. There are a wide variety of hazards related to volcanoes and volcano eruption, and these hazards are typically distinguished by the different ways in which volcanic materials and other debris flow from the volcano.

Mount Hood is a potentially active volcano close to rapidly growing communities and recreation areas that could have significant effects on the daily lives of Wasco County residents. It is located 10 miles west of the Wasco County border, and 20-35 miles from population centers in the County that include The Dalles, Dufur, Tygh Valley, Pine Hollow, Wamic, Maupin, and Mosier.

The most likely widespread and hazardous consequence of a future eruption will be from lahars (rapidly moving mudflows) sweeping down the entire length of the Sandy (including the Zigzag) and White River valleys. The White River runs through the Mt Hood National Forest, past a wide variety of agricultural areas in South County, and near the town of Tygh Valley on its way to the Deschutes River. Lahars can be generated by hot volcanic flows that melt snow and ice or by landslides from the steep upper flanks of the volcano. Structures close to river channels are at the greatest risk of being destroyed. The degree of hazard decreases as height above a channel increases, but large lahars can affect areas more than 30 vertical meters (100 vertical feet) above river beds.

Ashfall could also have a significant impact across the entire County. The prevailing winds in the area are out of the West. During the 1980 Mt St Helens eruption (65 miles northwest of The Dalles), areas of Wasco County were covered by up to an inch or more of ash, and some vehicles could not operate as their air intakes were clogged. Depending on the season of the eruption, wildfires caused by falling ash, lahars down White River, or the blast itself could be impactful as well.

Cascade Range volcanoes in the U.S. have erupted more than 200 times during the past 12,000 years for an average of nearly two eruptions per century. At least five eruptions have occurred during the past 150 years. The most recent eruptions in the Cascade Range are the well-documented 1980-1986 eruptions of Mt. St. Helens, which claimed 57 lives and caused nearly a billion dollars in damage and response costs. The effects were felt throughout the northwest, and another Cascade Range volcanic eruption could significantly impact various aspects of life in Wasco County.

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