CompuServe Messages

Kamron, Well, a few points. First, we know that plates are being subducted and that the subducted plates plunge into the asthenosphere at a near 45 degree angle. This is fact. How do we know this? Seismological sounding. This gets into some pretty complex geophysics but to greatly simplify things the sounding is like a radar. A good book on geophysics could give you all the details. Now that we know what *is* happening, we can take a stab at why. First you gotta think in geologic time. These motions are very slow. Given enough time and pressure, what we perceive as solid objects will undergo plastic deformation. Kinda like silly putty. Or window glass. Ever notice an really old window and how the glass is thicker at the bottom? With the time and pressures involved these "rigid" plates deform. The average time for a cooled subducted plate to be assimilated into the mantle is about 12 million years. What is the driving force? The jury is still out, but it is commonly believed to be convection. Seismology tells us the asthenosphere is partially molten (perhaps about 3 – 5 percent) and is capable of plastic flow. It is believed that large convection cells exist under the plates and we're pushed along like a raft in a current. The shapes of the plates has a lot to do with geometry. The movement of these plates over the earth cannot be appreciated on a flat map. The earth is spherical and this adds a new twist to understanding the movements of the lithospheric plates. In summary: The oceanic plates are relatively thin and dense and easily subducted. Continental plates are thicker and very buoyant. Convection drives it all. What you really need is a good college textbook on geology. All this stuff is explained in a good text with bibliographies for more info. Mike