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How are mountains and volcanoes formed? It all has to do with plate tectonics. The outer shell of the earth's surface (lithosphere) is made up of separate plates that float on top of the liquid asthenosphere. These plates move relative to each other and it is at their boundaries where we find volcanoes, mountains and ocean trenches. The outer crust of the lithosphere is either oceanic crust or continental crust. The continental crust is between 35 and 40 kilometres thick but less dense than the oceanic crust, which is only 6-8 kilometres thick. The earth is a sphere with a constant surface area, therefore, if new crust is formed at places where plates diverge, then a similar amount of crust must also me "destroyed".