Wednesday, July 21, 2010

Games Geologists Play

While teaching classes on western rivers, I often give descriptions of the upcoming rock units. After describing mappable units, grain size, shape, color and other macro-characterizations, I ask them to determine where the next rock unit appears as we float the river. The winner often receives an extra Oreo cookie at lunch. (A game needs a prize) Other games have included: find the river, and the ever popular where are we.

Can you find the break between the Ladore formation and the Madison limestone?
Last week we were traveling in Northern Colorado and found ourselves looking at some fantastic glacially polished rock. I was explaining to my ever-patient wife about the life and death of glaciers when we created a new game: Find the Equilibrium Line.

The equilibrium line divided the ice sheet into two parts. The upper section is where material was being added to the glacier as more snow was accumulating than melting. We can observe paleo-accumulation zones from the presence of erosional features- such as glacially polished rock.
The area below the equilibrium line is where more material was melting than being accumulated. Because there was an excess of material, ice moved from above the equilibrium line to below the equilibrium line. Paleo-ablation, or melting zones can be determined by observing depositional features such as a moraine. (The image below was shamelessly right-clicked from a google image search as it showed a moraine way better than any of my pictures.)The trick is to find the point where erosion ends and deposition begins.
Here, the elevation of the paleo-equilibrium line is is roughly 11,300 feet above sea level.

For extra cookies, or credit. a very rough estimate of the paleo-climate can be determined with some simple calculations. The difference between the "modern" equilibrium line ( approximately 14,000 feet (a number created a long time ago while in grad school) and the "paleo" equilibrium line is 2700 feet. The dry adiabatic rate is roughly 3.5o F/ 1000 feet, giving us a rough temperature difference between the ice age maximum and now would be 9o F.

I know that there are a multitude of factors not being taken in consideration, but this seems an easy exercise where students can observe geology, do some simple calculations and go home with an answer that makes sense.

And, of course the winner gets an extra Oreo cookie.

No comments: