Geology of the Pacific Northwest

Week 7:  LAHARS IN WASHINGTON

Lahars |  Resources

Lahars

Washington's Mt. Rainier, a stratovolcano which is over 14,410 feet, is the tallest volcano in the Cascade mountain range. It has been discharging lahars (mudflows) and landslides for a half million years but more often in recent years (1910, 1947, and 1963).

Deposits of at least 55 lahars emanating from Mount Rainier within the last 10,000 years have been identified in its larger valleys (http://seattlep-i.nwsource.com/mountsthelens/rain12.shtml). Two lahars added onto Puget Sound's shoreline–the Osceola and Paradise mudflows. The Osceola Mudflow occurred around 5,700 years ago when Mt. Rainier's northeast section slid down the mountain and traveled from Mt. Rainier to Enumclaw in the space of one and a half hours (It takes an hour to drive the same distance). Lava flows, mud flows, and eruptions of volcanic ash and debris filled in a crater created by the lahar.

Major casualities and damage would be sustained by more than 1.5 million people living within range of the volcano, were it to erupt again. Mt. Rainier is a threat for lahars because:

• the huge volume of ice and snow on Rainier's cone makes the the surrounding valleys and towns especially susceptible to devastating killer lahars.
• volcanic gases and hot water within the mountain precipitate the likelihood of an explosion that makes the threat of landslides and lahars unavoidable.
• the mountain is over 14,000 feet high and is unstable on its western slope where crumbly rotten rock could easily give way.
• Rainier has a long history of provoking two kinds of devasting lahars:
1. glacier outburst floods – a rapid input of rainwater or meltwater to the glacier's base; the smallest but most frequent of the debris flows from the Kautz, Nisqually, Winthrop, and South Tahoma glaciers.
2. mud flows – avalanche of debris and water when volcanic heat melts a glacier too rapidly. The Mt. Rainier-generated Osceola Mudflow of 5,700 years ago is an example.

Mt. Rainier harbors more ice than most of the other U.S. volcanos. Strong shaking could trigger avalanches, floods, and slides of rock, mud, and debris into numerous parks, streams, rivers, and cities, endangering thousands of people.

Since earthquakes are quite often the best indicator of a possible eruption, geologists closely monitor the seismicity of the volcano through the Pacific Northwest Seismograph Network, jointly operated by universities in Washington and Oregon. Scientists also conduct geochemical and geodetic studies in the area.

Fallout to the Seattle-Tahoma populations from Mt. Rainier eruptions include:

• volcanic gases that affect breathing
• falling ash
• acidic rain
• pyroclastic flows
• landslides
• mudslides
• lava flows
• lahars and floods from melted glaciers

RESOURCES

Send comments to Rob Viens (e-mail: rviens@bcc.ctc.edu) or call him at his Bellevue Community College office at (425) 564-3158. Office hours are by appointment.