Volcano Hazards including Lahars

Key Points

  • Washington State is home to five active volcanoes located in the Cascade Range east of Seattle: Mt. Baker, Glacier Peak, Mt. Rainier, Mt. Adams and Mt. St. Helens. Washington and California are the only states in the lower 48 to experience a major volcanic eruption in the past 150 years.

  • Major hazards caused by eruptions are blast, fast moving currents of gas and rock, mudflow and ash fall.

  • Seattle is too far from volcanoes, including Mt. Rainier, to receive damage from blast and gas and rock flows.

  • Ash falls could reach Seattle from any volcano, but prevailing weather patterns would probably blow ash away from Seattle. Mt. Rainier and Glacier Peak are closest volcanoes to Seattle and therefore the most likely ash sources. To underscore this uncertainty, ash deposits from a pre-historic eruption were recently found in Seattle. The deposit's source could not be identified.

  • The City of Seattle depends on power, water and transportation resources located in the Cascades and Eastern Washington where ash could fall. Seattle City Light operates dams directly east of Mt. Baker and in Pend Oreille County in eastern Washington and would be in the likely path of an ash cloud. Seattle water comes from two reservoirs located in the Central Cascades. They are outside the probable path of an ash cloud.

  • If ash were to strike Seattle it would create health problems, paralyze the transportation system, destroy many mechanical objects, endanger the utility networks and cost millions of dollars to clean up.

  • The United States Geological Survey defines lahars as "mudflows and debris flows that originate from the slopes of a volcano." Lahars contain high concentrations of rock debris and can travel tens of meters per second. Most, but not all, are preceded by volcanic and seismic activity.

  • Of the five Cascade volcanoes, Mt. Rainier is the most significant threat. Lahars from Mt. Rainier have buried the Kent Valley, but there is no evidence a lahar has reached Seattle in the past 10,000 years. An United States Geological Survey analysis states that it is possible for a lahar to reach Seattle, but would be extremely unlikely.

  • Although the risk of lahars seems quite small, some uncertainty exists because the last lahars occurred thousands of years ago before development. It is not understood whether or how the development will affect a lahar.

  • Seattle has a high probability of "post-lahar sedimentation." A lahar is likely to stop in the Kent Valley, then the next big storm transports loose materials from the lahar down the Green and Duwamish Rivers, causing problems for the maritime community.

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