CompuServe Thread

Doug >> Yes, it is, in cases of fold mountains like the Appalachians and Himalayas. We can measure the gravitational differences (and I think the seismic wave distortions) caused by the "roots" of major mountain chains where the crust extends unusually deep into the lithosphere (shallow mantle), counterbalancing the weight of the mountains (acting like a float to hold the mountains up).<< I would expect the base of the continents to be thicker in areas where there is major up lift. I have a major problem with the continent compressing though.(wouldn,t you know<g>) If we take the mean level of the ocean basin and compare the high parts of the continents we end up with a crust that should be at least 50,000 feet thick but probably more like 100,000 feet thick in order for it to float as high as it does. This block of solid granite would resist compression and the ocean basin on the other side would give way if the pressure on the pushed side were enough. How do you see the rigid block of crust compressing? I know that there has been upthrusted granitic intrusions through the crust in the rockies for example but even that large formation is dwarfed by the continent base which would seam to be a solid compression resistant mass. >>The entire plate (crust plus lithosphere) is being compressed when continents collide. If the lithosphere bulges into the outer mantle, I figure it simply becomes outer mantle rock, but the crustal bulge remains as fold mountains and their roots. I do not know if the roots are _precisely_ matched to the mountains (which would need some filling of spaces vacated by folding) or just generally matched, as when paper is crumpled (no filling needed).<< It would bulge downward to match the upward motion. But why would this occur rather than the thin ocean basin give way and buckle up? >>Fault block mountains have roots too, though they are created differently. Over time, I expect any mountain without a "float" underneath would simply sink into the crust, faster than it could erode. << I agree there are deep roots to float the mountains. The whole crust I see as very deep. This process again would seam to match more closely to what I would expect in a rapid event rather than a slow event. Slow motion would absorb and dissipate the heat and transmit the pressures to the other side better than a rapid event where the enormous inertias would help create the upthrusts. Kamron