Dom, I don't know the crustal thickness. But, when you say 8 km, I think you mean that it is 8 km to the magma chamber. The magma chambers are likely in the crust, and I'd GUESS that the crust in these areas is 20 or 30 km thick. I think some of the Long Valley magma chamber may be shallower-as little as 5 km, but I don't know what the latest is (I may have heard that number as long as 15 years ago...). Chris