Glacial Lakes

First, Michael and co. are moving on again today. I'm not sure where they'll be dropped off from, but Claire's Aunt Inez will be taking them down the road a bit and they hope to be in Portland in less than a week. Almost to the last state they have to ride through! (Though they're all flying out of the San Francisco area on Aug.20, so they still have to get there.)

There was a request to discuss how glacial lakes were formed. A glacial lake is a place in rock where a glacier erodes rock. I believe this is most common as a glacier advances or recedes, because as the glacier moves it has to push rock out of the way. The reason they can be so deep is because glaciers are really powerful! Although they aren't moving particularly quickly (by our standards), they have tons mass and thus large momentum and kinetic energy. A glacial lake is exposed as a glacier retreats, the ice melts, the melt water fills in the depression gouged out, and voila, you have a glacial lake.

In the particular case of the Idaho lakes, I believe that the general latitude in Idaho/Montana that Michael was staying is roughly where the Cordilleran Ice Sheet ended. This ice sheet covered much of Canada during the last ice age. As a result, as the ice advanced or retreated these large lakes could have been created by the movement of these huge glaciers. Smaller glacial lakes are very common in mountains where there were only small, local glaciers. Lake Pend Orielle (near where Michael was staying)is the 13th deepest lake in the world, so there is more going on here than normal glaciation.

A particular geologic phenomenon was responsible for the depth of the lakes. At the end of the last ice age a glacial dam was created near Lake Pend Orielle when a glacier blocked the Clark River's path. As a result a huge lake was created: Lake Missoula. This lake was huge, but the intense pressure on the ice dam from the water eventually caused some ice to melt and the dam to fail. There were catastrophic floods all the way from the lake to the ocean and these created the Columbia River Gorge that Michael and co will be riding in to Portland. Lake Missoula would flood, eventually the ice dam would recover and the cycle would start again. Wikipedia tells me that these floods happened ~40 times in a 2,000 year period. Having a catastrophic flood every 50 years would certainly create the gorge and also help gouge out the lakes. If you want to follow up here are 1 and 2 good links.

Another interesting note here has to do with fluid dynamics, something Michael is particularly interested in. The river flow was immensely high during these floods. Some geologists have estimated it as equal to or exceeding the total flow of every river in existence today (that means the Amazon, Nile, etc etc). That is a lot of moving water! When water moves this quickly something called Cavitation occurs. Fast moving liquid has very low pressure and as a result air bubbles in the liquid collapse rapidly and create shockwaves that are particularly effective in eroding rock. I believe Michael watched a documentary about this process while he was staying with Claire's aunt, so it seems particularly pertinent to this situation. Repeated extremely fast moving water near the dam's breaking point would have created cavitation and gouged particularly deep in rock. It is in these eroded spots that the lakes now lie.

Michael wants me to show you a movie he found on an MIT website about cavitation and that he thinks is really cool. Follow this link and click on the 'Cavitation' movie, second one down.