Lahar. Sounds like a word that you’d find in the Klingon dictionary. I’ve lost my copy, but I do have handy the “Encyclopedia of Geological Terms”.
Lahar: A dense, fast flowing mixture of water, mud, ash, rock and debris. Lahars typically form on the steep slopes of volcanoes that experience a sudden influx of water from rapid glacier melt or heavy rain.
Contrasting the glaciers in my previous blog, lahars are bona fide hazards faced by many communities sitting within the shadow of volcanoes. In the United States, nowhere is awareness of these flows keener than on the flats west of Mt. Rainier. Some 100,000 people have homesteaded there on paleo-lahar deposits. These accumulations aren’t million-year old relics. The Osceloa flow 5000 years ago ran all the way ‘round the back side of the mountain, then 50 km to tide lands of Tacoma. Just five centuries past, a lahar buried Orting town site. Speeding at 50 miles per hour, flows might repeat the same anytime and give just 20 minutes notice.
To help you understand and appreciate the subject, I’ve made these computer simulations of a Mt. Rainier lahar. Similar to lava flows, when lahars stop they harden into a concrete-like layer entombing everything below. Distinct from lava flows, lahars both excavate topography and deposit on it. By digging up and incorporating material as the move --- “bulking” they call it ---lahars actually increase in volume and power with time.
“Lahar- Mt. Rainer”, the YouTube below, includes these movies and others.
Like hurricanes in Florida or tornados in Oklahoma, the lahars of Washington State are natural processes that require residents to understand the hazard, form an action plan, and then step up to a measured risk. “Gragmack” as a Klingon might say. (I’m pretty sure that it’s legal to use that expression online.)
Steven N. Ward Santa Cruz
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John Rundle is a Distinguished Professor of Physics and Geology at UC Davis and the Executive Director of the APEC Collaboration for Earthquake Simulations. He chaired the Board of Advisors for the Southern California Earthquake Center from 1994 to 1996. Read John's blog.