The Hazards of Mount Rainier
by Cory Zellers

Mount Rainier, Photo by David Meunch.

When Mount Rainier erupts it will have very devastating results. With Rainier being the largest volcano in the Cascades and located near large cities there is great potential for destruction. The potential hazards can be known by using Mt. St. Helens as an example and the stratigraphy of the surrounding landscape. These hazards include floods, lahars, landslides, pyroclastic flows, lava flows, and tephra fallout. These hazards can not only destroy an area, but also effect water quality and other health risks in places that survive.

Some of Mount Rainier's hazards are not associated with an eruption. Avalanches, landslides, floods, and lahars can be caused by any of the volcano's thirty earthquakes that occur each year. Avalanches happen frequently and are often caused by the melting of glaciers. Landslides and rock falls also occur with great regularity. Floods and lahars can occur at any time. These can be triggered by the steam escaping from the mountain and melting some of the nearly 4.5 billion cubic meters of snow and ice locked up in glaciers. Mount Rainier has more snow and ice than any other mountain in the lower 48 states.

The biggest dangers come when the mountain erupts. If Rainier erupts in the same fashion as Mt. St. Helens, there will be a large pyroclastic flow. A pyroclastic flow is a massive cloud of superheated ash and rock, up to 1,500 degrees fahrenheit, that can travel at speeds of up to 300 mph. The pyroclastic flow at Mt. St. Helens that was caused by the initial lateral blast affected an area reaching 13 miles from the summit. The first 3 to 4 miles of the blast destroyed everything in its path. The blast incinerated trees, leaving the landscape totally bare. The next 8 to 9 miles is known as the tree down zone. All of the trees in this area were either knocked down or carried elsewhere. If Mount Rainier has a side collapse triggering a lateral blast like this, much of the same damage can be expected. There has been at least one lateral blast from Rainier in the past. It triggered the largest mudflow the world has ever known.

Lava flows in Mount Rainier's past seem to have only effected areas on or very near the mountain. In some places ancient flows have been found up to 9 miles away from the base. If there is a large lava flow it will not effect any large cities. The first signs of volcanic activity at Mount Rainier appeared 2.9 million years ago. This was when a proto-Mount Rainier deposited the Lily Formation. This formation is a thick sequence of volcanoclastic debris west of the mountain. Early lava flows of the present Mount Rainier formed a small shield on a dissected surface of the basement rock, which has as much as 700 meters of relief. Approximately 270 cubic kilometers of lava have been erupted from the mountain in the last 1 million years.

Another hazard that can be shown in deposits surrounding the mountain is tephra fallout. Tephra is volcanic ash that is released during an eruption. A thick pumice layer northeast, east, and southeast of the volcano is interpreted to have erupted from Rainier between 70,000 and 30,000 years ago. During explosive eruptions in the Holocene, Mount Rainier produced 11 tephra beds totaling over 0.5 cubic kilometers. There are many factors in determining how devastating the effects of the tephra will be. Wind direction is the main consideration. One of the worst possibilities would be if the wind were blowing towards the Seattle and Tacoma areas. Tephra fallout in a big city would cause many water resource problems. After the Mt. St. Helens erupton the city of Spokane, Washington had a water shortage because of people washing off their cars and homes. Depending on building structure, tephra can cause roofs to cave in and buildings to collapse. 10 centimeters of tephra is enough to cause structural damage to a building. Tephra also clogs sewers, harms automobiles, and damages airplane engines.

Snow Covered Mt. Rainier, Photo by David Meunch.

The greatest threat from Mount Rainier comes from the lahars. A lahar is a large mudflow that looks and acts like wet concrete and travels down river valleys at speeds of up to 60 mph. The lahars from Mt. St. Helens extended nearly 50 miles from the summit. Due to the great steepness of Mount Rainier, lahars would travel further than the Mt. St. Helens lahars. Mount Rainier has 5 major river valleys for lahars to navigate. The upper river valleys have gradients of up to 800 feet per mile while the lower valleys are up to 400 feet per mile. Each valley is in a canyon up to 3000 feet below adjacent divides. This gives the mudslides excellent channels to flow through with great speed. Along with the height and steep slopes, Rainier has more glacial ice than any other volcano in the Cascades. Nearly 4.5 billion cubic meters of ice and snow reside atop the mountain. When the volcano erupts it will melt this ice sending the water and debris down the river valleys. This would cause much damage throughout Mount Rainier's heavily populated surroundings. If a lahar reached the city of Tacoma the results would be catastrophic. Since the lahars from Mt. St. Helens traveled 50 miles, lahars coming down the steep slopes of Mount Rainier could easily travel the 50 miles to Tacoma. This lahar would bring an overflowing river full of mud, trees, and other debris into the city. In the past lahars have reached present day Tacoma.

A Glacier on Mt. Rainier, Photo by David Meunch.

The largest mudflow in the world was the Osceola Mudflow. It originated atop Mount Rainier 5,600 years ago. Its deposits are estimated to be 10 billion cubic meters and cover an area of 212 square miles. Many small cities are built on these deposits including Orting, Buckley, Sumner, Puyallup, Enumclaw, and Auburn. At least six other large lahars have come down the valleys of Mount Rainier in the last 5,600 years. The Electron Mudflow occurred about 600 years ago. The flow was more than 30 meters deep where it entered the Puget Sound lowland at the community of Electron. Its deposits at Orting are as much as 6 meters thick and contain remanants of old-growth forest. The Electron Mudflow destroyed the mature old-growth forest that covered the Puyallup River flood plain. It is predicted that if the same size mudflow occurred here now, it would cover 40% more area due to the now deforested plains.

When Mount Rainier erupts the effects will be much more devastating than Mt. St. Helens. With the amount of ice and the shape of Mount Rainier, lahars will be the biggest danger. With large cities nearby and numerous smaller cities built on deposits of great mudflows many more people will be effected.


Foxworthy, Bruce and Hill, Mary. Volcanic Eruptions of 1980 at Mount St. Helens - The First 100 Days. United States Geological Survey Professional Paper; 1249

Hoblitt, R.P. Volcano Hazards from Mount Rainier, Washington. United States Geological Survey Open-File Rport 98-428.

Scott, K.M. Mount Rainier - Living with Perilous Beauty. United States Geological Survey Fact Sheet 065-97.

Scott, K.M. Sedimentology, Behavior, And Hazards Of Debris Flows At Mount Rainier. United States Geological Survey Professional Paper 1547.

All images were taken with permission from Meunch Photograpy at

This site was created on November 26, 2002, by

Cory Zellers
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copyright 2002 © Cory Zellers and Emporia State University. All rights reserved.