A number of questions were raised about the driving force for plate motion. One of the best ways to assess causes of various phenomena is to look for correlations between different types of measured data. A good example of this is that if you plot (by state) the proportion of smokers versus the incidence of death by cancer, you get a straight line with a positive slope (I nabbed the data off the web and plotted it below). This provides support for the hypothesis that smoking causes cancer. Note that the correlations do not prove that smoking causes cancer - it merely provides circumstantial evidence. For example, it may be that both are related to economic class (poor people smoke; poor people get cancer), but smoking and cancer are not directly related. A good counter example is that if you plot gun ownership by state against the number of registered communists, you will get a negative trend. One interpretation is direct: if you own enough guns, you can keep the commies out of your backyard. A different interpretation might be that both relate to the social mindset or politics of a region: that people who don't tolerate those with wild political ideas also hunt a lot. Correlations also don't provide any evidence for what the mechanism might be (what it is about tobacco that causes cancer).

You as a group offered a few ideas as to what the driving force for plate tectonics is, including pushing away from the ridges, pulling by the sinking force of subduction, and drag by upwelling currents. I will also suggest to you that there is a resistive force: drag by the continents (continental lithosphere is known to be thicker than oceanic lithosphere, so it might act like a sail).

So important clues to each of these would be to compare a plate's velocity to the amount of ridge around a plate, the amount of a plate that is sinking into a subduction zone, the number of hotspots beneath a plate, and the amount of plate that is continent.

• Plot up these various parameters against plate velocity. I spent the morning with a ruler and a map, so have compiled the data for you. You can do this by hand on graph paper; if you happen to know how to drive excel, use that.
• Discuss your data and come up with an explanation for the driving force of plate tectonics.
• Come up with at least one other explanation that would explain the relationships that you observe.

This will be due before class on Wed., Oct. 4.