Mount Rainier - Living Safely With a Volcano in Your Backyard

Mount Rainier - Living Safely With a Volcano in Your Backyard

An Active Volcano at Rest Between Eruptions


Mount Rainier, an active volcano currently at rest between eruptions, is the highest peak in the Cascade Range. Its edifice, capped by snow and 25 glaciers, has been built up by untold eruptions over the past 500,000 years. It last erupted in 1894-95, when small summit explosions were reported by observers in Seattle and Tacoma. Mount Rainier's next eruption might be of similar or larger size and could produce volcanic ash, lava flows, and avalanches of intensely hot rock and volcanic gases, called "pyroclastic flows."

Some of these events swiftly melt snow and ice and could produce torrents of meltwater that pick up loose rock and become rapidly flowing slurries of mud and boulders known as "lahars." In contrast to lava flows and pyroclastic flows that are unlikely to extend farther than 10 miles from the volcano's summit and remain within Mount Rainier National Park, the largest lahars can travel for tens of miles and reach Puget Sound.

Volcanic ash will be distributed downwind, most often toward the east, away from Puget Sound's large population centers. Airborne plumes of volcanic ash can greatly endanger aircraft in flight and seriously disrupt aviation operations. Although seldom life threatening, volcanic ash fallout on the ground can be a nuisance to residents, affect utility and transportation systems, and entail substantial clean-up costs.

How Hazardous is Mount Rainier?


Mount Rainier has erupted less often and less explosively in recent millennia than its well-known neighbor, Mount St. Helens. However, the proximity of large population centers in valleys susceptible to lahars from Mount Rainier makes it a far greater threat to life and property than Mount St. Helens for the following reasons:

Population and development at risk: About 80,000 people and their homes are at risk in Mount Rainier's lahar-hazard zones. Key infrastructure such as major highways and utilities cross through these zones, which also contain economically important businesses, hydroelectric dams, and major seaports.

Size and frequency of lahars: During the past several thousand years large lahars have reached the Puget Sound lowland on average at least once every 500 to 1,000 years. Smaller flows not extending as far as the lowland occurred more frequently. If future large lahars happen at rates similar to those of the past, there is roughly a 1-in-10 chance of a lahar reaching the Puget Sound lowland during an average human lifespan.

There may be little or no advance warning: Studies by U.S. Geological Survey (USGS) scientists show that at least one of Mount Rainier's recent large landslide-generated lahars may have occurred when the volcano was quiet and not providing the warning signs typical of a restless and erupting volcano. In such a rare case, the only warning could be a report that a lahar is already underway.

Monitoring and Emergency Planning Are Ongoing


The USGS, in cooperation with the Pacific Northwest Seismic Network at the University of Washington, continuously monitors Mount Rainier and assesses potential hazards stemming from volcanic activity. Volcanoes often show signs of unrest, such as increased seismicity (earthquakes) and emission of volcanic gases and swelling of the volcano, days to months in advance of an eruption. When unrest is detected, scientists will notify emergency-management officials and increase monitoring efforts.

The Mount Rainier Volcanic Hazards Response Plan, which was created by cooperating local, county, State, and Federal agencies, is on the Web. The plan describes the responsibilities of agencies and how they will communicate with each other and the public during a volcanic crisis.


What to Do if Threatened by a Lahar or Debris Flow


Know the signs of debris flows and lahars. Experience from around the world shows that moving to high ground off the valley floor is the only way to ensure safety during a lahar. When hiking in valleys on the slopes of Mount Rainier during late summer or during intense rainfall, be alert for the signs of an approaching debris flow-ground shaking and roaring sound-and move up the valley wall to higher ground. The same is true for lahars, but, because they affect much larger areas, people need to move out of threatened areas before lahars get close. Lahars are almost always preceded by volcanic unrest, so in most instances there will be time to warn people when there is an increased risk. Obtain a NOAA weather radio to receive alerts about possible lahars, as well as other natural hazards (for further information on the Web go to http://www. weather.gov/nwr.)