CompuServe Messages

Hi, Doug and Kamron! >> The lithosphere is colder than the mantle below it because it is more able to lose its heat to the surface. This makes it denser, and given any opportunity, it will sink …<< I haven't been keeping really close tabs on this discussion, but it seems to me that one factor that's missing is the fact that both the oceanic crust (mostly basalt) and the mantle (mostly ultramafics) are both mineralogically denser than the continental crust (mostly granite and quartz/feldspar rich rocks). Basalt contains lots of pyroxenes and olivine, as well as lesser amounts of magnetite and amphiboles–these are all heavy minerals. Ultramafic (mantle) rocks are even heavier, being mostly olivine and pyroxene with other heavy stuff like garnet. Granite and quartz are relatively lighter. Thus you have a good reason for oceanic crust to subduct UNDER continents, quite independent of temperature factors. >> (Kamron): Greenland looks like it drifted north while Iceland drifted south. << >> (Doug): Iceland was not there in Pangaea; it spilled out of that hotspot at the ridge after that part of the Atlantic opened up << I agree with Doug…the only drifting Iceland has been doing is the east and west coasts drifting away from the middle of the island. Check out these old Scientific American articles on the subject: May 1972: Plate Tectonics, by John F. Dewey Mar 1975: The Earth's Mantle by Peter J. Wyllie Aug 1976: Hot Spots on the Earth's Surface, by K.C. Burke and J.Tuzo Wilson Nov 1976: Convection Currents in the Earth's Mantle, by D.P. McKenzie and F. Richter Apr 1978: Kimberlite Pipes by Keith G. Cox –Howard