Now this is interesting. You know I differ in a number of ways from Neal. One way is I thi6nk the Earth expa6nded and contracted some hence perhaps the driving force for subduction. Look @ Mars. It expanded but did not subsequently contract. Probably more docile because smaller and/or further from the sun (less solar flux per cubic whatever)

http://fr.youtube.com/watch?v=d44Jj_...eature=related