Volcanic Eruptions

Washington has five active volcanoes (Mt. Baker, Glacier Peak, Mt. Rainier, Mt. St. Helens and Mt. Adams). All are capable of producing the most violent type of eruptions that eject huge amounts of ash, rock, and gas and trigger rockfalls and mudflows. The 1980 Mt. St. Helens blast ejected one quarter cubic miles of material devastating many square miles around it, sending mudflows down the Toutle River and spreading ash over most of Eastern Washington.

As frightening as the power of our volcanoes are, most are not a direct threat to Seattle. Mt. Rainier is the most likely to affect us. The most probable way Seattle would be affected would be by ash and mud washing out of the Duwamish River in the days after an eruption. A big debris flow, called a lahar, would rush down into the Puget Lowlands. It is predicted, based on the size of past flows, to stop south of Seattle. The impact for Seattle comes when rain washes large amounts of this material downstream toward Elliott Bay.

Major Incidents

Issues to Note

Most volcanic eruptions are preceded by swarms of small earthquakes, giving warning to surrounding communities. The prevailing winds would most likely transport ash to the east making a large ashfall unlikely in Seattle. Mudflows are not likely a direct threat. Seattle has power generating dams close to Mt. Baker and Glacier Peak, but they are up stream from the peaks making them less likely to be impacted by a mudflow. However, they could receive ashfall.

The Cascade Volcano Observatory monitors volcanic activity in the Pacific Northwest. They develop hazard assessments and would assist local governments understand a threat in the event a volcano began an eruptive cycle.

On the Web

How to Prepare for Volcanoes (pdf). Information from the City of Seattle, King County and Washington State.

Volcano Hazards from Mount Rainer, Washington Revised 1998. Assessment from the Cascade Volcano Observatory.

Volcano World. Educational site from Public Outread project of the North Dakota and Oregon Space Grant Consortia.

What To Do If a Volcano Erupts. Information from the United States Geologic Survey.