Why are there big lakes along the edge of the Canadian Shield?

November 16, 2011 9:55 AM

GeologyFilter: Why are there big lakes along the edge of the Canadian Shield?

See, for example, this map, which shows the Canadian Shield in red, and along its SW edge we have, from SE to NW, the Great Lakes, Lakes Winnipeg and Winnipegosis, Reindeer Lake (marginal example), Great Slave Lake, and Great Bear Lake.

If your answer is "whenever you see something like that, a glacier did it", please elaborate. How do glaciers form lakes? What makes this strip of land so susceptible to those processes? Etc.

posted by stebulus to Science & Nature (2 answers total)

Best answer: It is more than just a glacier, it is a legacy of the ice sheets. The bedrock around the shield is 'softer' or more likely to deform under the weight of the ice than the shield itself, creating depression and channels for the water to go to and collect in.

Also probably has to do with isostatic rebound. The weight of the ice in effect acts as another layer in the geologic strata, pushing the continental crust deeper into the mantle. After the ice is gone that part of the continental crust is lighter and the mantle pushes up on it raising that portion of the crust. Just like a boat in water. Anyway , because the shield is made of stronger, more coherent rock it rebounds faster and harder than the surrounding, softer rock (which is also deformed more, see above).

This rebound is probably the source of midplate earthquakes in the midwest and eastern US.
posted by bartonlong at 10:15 AM on November 16, 2011

Animation of the process for kids shows the main points in a simple way.
posted by Listener at 10:40 AM on November 16, 2011 [1 favorite]