CompuServe Messages

Hi, Kamron– Sea floor basalt is less dense than mantle material, but it is more dense than general continental rock. If a flat-topped iceberg meets a flat-bottomed boat, and they are moving too fast to simply stop, the boat is likely to push up on top of it, since it rides higher in the water – this will push the iceberg underwater, and if there is also a downward current in the water, it may get caught and be pulled still deeper (well, maybe not, but that is because the density difference is likely greater than that between seafloor and mantle) and "try" to drag down anything attached behind it. Unlike the iceberg, subducted mantle may later become denser yet still remain "attached", able to drag further seafloor behind it. I gather geologists are still arguing about subduction-pull versus spreading-push as the more important force moving the plates. If it is push from spreading, that is the irresistible force that cannot easily be "satisfied" by mere crumpling at the collision sites, which can only absorb a limited amount of crust. Something has to dive. Even if the force is "none of the above", the evidence for plate tectonic motions is strong enough to declare that it must exist, whatever it is. –Doug