“Positively Glacial.” Does that expression ever come to your mind? Standing in bank teller lines, it comes to mine. Waiting for my teenage son to take out the trash, it comes to mine.
Just how glacial are glaciers anyway? Turns out, most glaciers move at several to many centimeters PER DAY. Not greased lighting for sure, but swiftness is relative. Seismologists like me are used to framing speeds in terms of the rates of tectonic plate motion. It’s these rates after all that source those pesky earthquakes. Plate velocities top out at a couple centimeters PER YEAR. Glaciers jackrabbit in comparison, but “Positively Tectonic” does not ring as clearly.
Let’s make a glacier growth simulation. Like dam break floods or lava flows, glaciers are driven by gravity and opposed by friction. Key aspects include the rate and location of ice formation, the rate of ice melting and the lay of the land. Instead of freezing some far off location, let’s home-grow a glacier at Mt. Rainier, Washington State. This simulation captures the glacier during 500 years of rapid expansion followed by 500 years of sluggish spreading and eventual retreat after I switched off ice making.
In their centuries of advance, icy rivers coalesce into a sheet, thicken to several hundred meters, flow 40 kilometers down the mountain and smother the flat lands where now folks live. As far as natural hazards go, I’m guessing that not many residents in sight of Mt. Rainier feel compelled to include glacier damage in their homeowner’s insurance policy, so I suppose “Positively Glacial” has good connotations too. Me? I’m still waiting for my son to take out the trash.
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.