Bridge for Kids - Orting, WA

Bridge for Kids and Mount Rainier

What is a lahar?

A lahar is a mudflow originating on a volcano. The mud and debris in a lahar typically follows a river path, or drainage, and is the consistency of wet concrete. This type of natural hazard can be triggered in a number of ways -- some with warning, such as seismic activity, and others without any warning at all. There is also a range of severity associated with lahars, from flows that only move a short distance from their origin, to those that travel over many miles.

Lahars can reach speeds of up to 50 miles per hour. Orting is only 30 miles away from Mount Rainier, making the estimated arrival time in town 40 minutes.

Lahar Classifications

Like hurricanes, tornadoes, floods and other disasters, lahars have a "ratings" system:

Case M events are the largest, occurring on a 10,000 year timeline. The most recent flow of this magnitude was the Osceola Mudflow, which covered 212 square miles.

Case 1 events are more common, occurring on a 500-1000 year timeline. Approximately 500 years ago, the Electron Mudflow buried an old growth forest in the Orting Valley. The mud from this could reach 18 feet deep.

Case 2 events are moderate in severity, recurring every 100-500 years. These are typically caused by an eruption swiftly melting ice or snow. There have been more than 12 such events in the last 6000 years.

Case 3 events are the most common of all and relatively small -- occurring anywhere from annually to every 100 years. They can be triggered by sudden release of water stored in glaciers, such as the 2001 outburst in Nisqually. Debris from events like this may not travel out of the National Park, but can cause flooding in lower areas as glacier water displaces river water.

For more information, check out the links below!

Basics

http://www.agiweb.org/geotimes/Apr04/feature_MountRainier.html

http://en.wikipedia.org/wiki/Lahar

http://volcanoes.usgs.gov/Products/Pglossary/lahar.html

In Depth Information

http://www.pdc.org/iweb/volacno_hazards.jsp

http://www.co.pierce.wa.us/pc/abtus/ourorg/dem/EMDiv/Mt%20Rainier%20VHRP.htm

http://volcanoes.usgs.gov/Hazards/What/Lahars/RuizLahars.html

http://www.ess.washington.edu/SEIS/PNSN/