understand how plate tectonics works in detail, it helps to focus on the borders between tectonic plates. At divergent margins, like those described by Harry Hess, material rises from the mantle and pushes plates apart. Elsewhere, though, at convergent margins, plates are pushed together. If the two plates have about the same density—if, say, they both consist of granitic continental plates—then, like two bull walruses competing for mates, they will rear up. This is how the Himalayas formed; within the past fifty million years, the fast-moving Indian plate traveled north from Antarctica and
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