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Contents of 13.5 How and where do mountains form?

• Most mountain belts form near convergent boundaries where crust thickens by compressional shortening and intrusion of magma.
• The hot lower-crustal root beneath high mountains is too weak to support the mass above. The crust spreads horizontally, producing normal faults in parts of the mountain belt, while contraction occurs elsewhere.
• The high elevations in the western U.S. do not have a thick crustal root, but are instead held up by unusually low-density mantle.
• Even as crustal thickening causes mountain uplift, thrust faults weigh down adjacent lowlands to form basins. These fill with sediments eroded from the mountains and contain fossil fuel resources.
• Mountains rise vertically at rates of 3-10 mm/yr, or only about 1/10 the rate of horizontal plate motion.

13.5 How and where do mountains form?