Rainier Eruption Odds Low, Impact High, Expert Says

September 25, 2003

National Geographic Ultimate Explorer: Volcano Alert airs in the United States on Saturday, September 27, at 8 p.m. ET/PT on MSNBC.

Mount Rainier provides a scenic backdrop to much of western Washington State and more people have moved into its associated valleys. But the mountain's serene, snow-covered summit belies an ominous fact: Rainier's status as an active volcano.

Some 60 volcanoes erupt around the world each year. Early warning of volcanic eruptions through modern technology has made these terrifying events less dangerous. Predicting eruptions is still far from an exact science.

Mount Rainier (14,410 feet/4,392 meters) has not erupted since the first half of the 19th century, and the mountain hasn't experienced a truly large eruption for about 1,000 years.

While experts say there are currently no signs of imminent danger from volcanic activity, the impact associated with an eruption or other volcanic event on the mountain is far greater today than in years past.

William Scott, a geologist with the United States Geologic Survey (USGS) in Vancouver, Washington, told National Geographic's Ultimate Explorer host Lisa Ling how the potential problem has grown more dangerous: "We have no sign that the volcano is doing anything different or is any more likely to erupt than it was 10 or 20 or 30 years ago. The issue is that people have moved in much, much closer."

Massive Slides

One key danger lies in powerful volcanic mudflows known as lahars. The concrete-like slides of rock, mud, and water form from volcanic landslides and earthquakes or when pyroclastic flows (a mix of lava fragments and gas) rapidly melt the mountain's cubic mile (four cubic kilometers) of glacial ice snow and ice.

Such slides move at surprising speed. At Mount Rainier, scientists have determined that lahars have traveled 45 to 50 miles per hour (70 to 80 kilometers per hour) at depths of 100 feet (30 meters) or more in confined valleys. Given the suddenness in which lahars can occur, residents living in their path would have little warning.

Studies of sediment layers indicate that, in the past, lahars have swept down Mount Rainier's flanks and through its river valleys. Today, four of the five major river systems in Mount Rainier's one hundred-square-mile (260-square-kilometer) base flow into the heavily-populated suburbs of Pierce County.

Many massive lahars have slid as far as the Puget Sound lowland, covering the sites of present-day communities as far as 45 miles (70 kilometers) from Mount Rainier, according to USGS research. Geologists have determined that slides of large magnitude have occurred every 500 to 1,000 years, based on radiocarbon dating of trees found buried in deposits left by the flows.

Today, some 150,000 people currently live in historic lahar slide paths like the 2,300-year-old National Lahar in the Nisqually Valley and the 500-year-old Electron Mudflow in the Puyallup Valley.

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