Conventional wisdom says that changes to crustal rocks pushed down deep when continents collide develop over millions of years. But it seems that some metamorphism may be caused by tectonic events lasting only a decade. The Bergen Arcs in Norway are famous for rare and rather beautiful rocks known as eclog-ites. Striking, coarse-grained, and characterized by large pink garnets and a green matrix rich in silicates known as pyroxene (Fig. 1), eclogites form at extremely high pressures, and are important indicators of the conditions in the deepest parts of mountain chains. In western Norway, they formed in a continental collision some 425 million years ago; since then, glaciers have ground and polished the rock surfaces to reveal the heart of the-ancient collision zone, creating a wonderful natural laboratory in which to study processes that occur deep below a mountain chain - processes that must be happening today some 50 kilometres below the Himalayas.
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