Answer to Question 1
Answer: Most mountain belts form near convergent plate boundaries where crust thickens by compressional shortening and by intrusion and crystallization of magma. The high elevations in the western United States do not have a thick crustal root but are held up, instead, by unusually low-density mantle. Even as crustal thickening causes uplift of mountains, thrust faults weigh down adjacent lowlands to form deep basins. These basins fill with sediment eroded from the mountains and contain rich resources of oil, natural gas, and coal. Mountains rise vertically at rates of 3-10 millimeters per year, or only about one-tenth the speed of horizontal plate motion.
Answer to Question 2
Answer: Isostatic adjustments to changes in crustal weight and thickness cause vertical motion of the crust. Compressional stress that shortens the crust also thickens it, causing isostatic uplift of the mountains. Tensional stress thins crust, causing isostatic subsidence of sedimentary basins. Isostasy causes crust beneath eroding mountains to rise slowly even though the overall elevations decrease, because the weight of the crust decreases as the mountains erode. The eroded sediment accumulated in sedimentary basins, causing subsidence. Slow flexural isostatic adjustment to changing weight is well documented by the patterns of recent and ongoing uplift and subsidence in eastern North America, which are attributed to glacial rebound. Isostatic movement of the crust causes movement along old faults, thereby accounting for earthquakes that occur far from plate boundaries.
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