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Mount Rainier's Volcanic Activity
 
The Mount Rainier volcano dominates the landscape of a large part of western Washington. It stands nearly 3 miles higher than the lowlands to the west and 1.5 miles higher than the surrounding mountains. The base of the volcano spreads over an area of about 100 square miles, and lava flows that radiate from the base of the cone extend to distances of as much as 9 miles. The flanks of Mount Rainier are drained by five major rivers and their tributaries. Clockwise from the northwest the major rivers are the Carbon, White, Cowlitz, Nisqually, and Puyallup. Each river flows westerly through the Cascade Range and, with the exception of the Cowlitz, empties into Puget Sound near Tacoma, Washington. The Cowltiz joins the Columbia River in the southwestern part of the State to flow to the Pacific Ocean.

The volcano's past behavior is the best guide to possible future hazards. The written history of Mount Rainier encompasses the period since about A.D. 1820, during which time one or two small eruptions, many small debris flows, and several small debris avalanches have occurred. This time interval is far too brief to serve as a basis for estimating the future behavior of a volcano that is several hundreds of thousands of years old. Fortunately, prehistoric deposits record the types, magnitudes, and frequencies of past events, and show which areas were affected by them. At Mount Rainier, as at other Cascade volcanoes, deposits produced since the latest ice age (approximately during the past 10,000 years) are well preserved. Studies of these deposits reveal that we should anticipate potential hazards from some phenomena that only occur during eruptions and from others that may occur without eruptive activity. Tephra falls, pyroclastic flows and surges, ballistic projectiles, and lava flows occur only during eruptions. Debris avalanches, debris flows, and floods commonly accompany eruptions, but can also occur during dormant periods.

Because Mount Rainier is the highest and third most voluminous volcano in the Cascade Range, it is potentially the most dangerous volcano because of the large population living around its lowland drainages. These areas are at risk because of the mountain's great relief and the huge area and volume of ice and snow on the cone that could generate lahars during eruptions. Lahar is an Indonesian word describing mudflows and debris flows that originate from the slopes of a volcano. Both types of flows contain a high concentration of rock debris to give them the internal strength necessary to transport huge boulders as well as buildings and bridges and to exert extremely high impact forces against objects in their paths.
Lahars can be of any size. They may be as small as several centimeters wide and deep, flowing less than one meter per second. Steep, unvegetated slopes during a heavy rain are often good sites to observe such small flows. At the other extreme, they can be a few hundred meters wide, tens of meters deep, flow at several tens of meters per second, and travel over 100 kilometers from a volcano. Such catastrophic lahars are triggered by volcanic eruptions or by massive landslides such as the one that occurred on May 18, 1980 at Mount St. Helens volcano.

Lahars are commonly initiated by large landslides of water-saturated debris, heavy rainfall eroding volcanic deposits, sudden melting of snow and ice near a volcanic vent by radiant heat or on the flanks of a volcano by pyroclasitc flows, or breakout of water from glaciers, crater lakes, or from lakes dammed by volcanic eruptions. Lahars can travel over 50 miles downstream, commonly reaching speeds between 20 and 40 miles per hour. Sometimes they contain so much rock debris (60-90% by weight) that they look like fast-moving rivers of wet concrete. Close to the volcano they have the strength to rip huge boulders, trees, and houses from the ground and carry them down-valley. Further downstream they simply entomb everything in mud. Historically, lahars have been one of the most deadly volcanic hazards.