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What is the greatest hazard presented by Mount Rainier?

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Welcome to CoreFacts, where we're always short on time and big on science. I'm Steve Sobieszczyk. Let's get right to it, today's question is:

What is the greatest hazard presented by Mount Rainier?

Debris flows pose the greatest hazard to people near Mount Rainier. A debris flow is a mixture of mud and rock debris that looks and behaves like flowing concrete. Giant debris flows sometimes develop when large masses of weak, water-saturated rock slide from the volcano's flanks. Many of these debris flows cannot be predicted and may even occur independently of a volcanic eruption. Giant debris flows can also form during an eruption as hot rock fragments tumble down the volcano's slopes, eroding and melting snow and glacier ice. Although they happen infrequently, giant debris flows have the potential to inundate much of the southern Puget Sound lowland. Scientists estimate that debris flows can travel the distance between Mount Rainier and the Puget Sound lowland in as little as 30 minutes to a few hours. Hundreds of thousands of people now live in areas that have been buried by debris flows during the past few thousand years.

And now you know. Join us again tomorrow as we'll give you yet another new CoreFact. If you're interested in previous CoreFacts, or would like to check out CoreCast, our in-depth science podcast, browse over to If you would like to contact us here at the show or submit a question you think we should answer on the air, you can email us at or leave us a voicemail at 703-648-5600. Remember, long distance fees do apply.

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