The origin of mountains - implications for the behaviour of Earth's lithosphere

Mountains are an expression of buoyancy in the outermost part of the Earth, caused by density contrasts in the lithosphere and underlying asthenosphere, as described by the principle of isostasy. The biggest density contrast is between the lower density crust and more dense mantle, and so changes in crustal thickness are the main reason for the creation of the planet's highest and most extensive mountain ranges, in the Andes of South America, and the great ranges of Central Asia, in Tibet and the Himalaya. Crustal thickening is primarily a result of crustal shortening, driven by horizontal forces in the lithosphere. The response of the lithosphere to these forces depends on its thickness and rheology, and when tectonic plates converge, mountain ranges will form where the lithosphere is weak and thin, pushed up by the same forces that drive the plates themselves, though limited by the maximum force that can be transmitted across the plate interface. The smaller density contrast between the lithospheric mantle and underlying less dense asthenosphere will also play a subsidiary role in the elevation of mountain belts, depending on changes in lithospheric thickness. The nature of deformation during mountain building is strongly controlled by the interaction of lithosphere of different strengths