DAVID HARDY / SCIENCE PHOTO LIBRARY DAVID HARDY / SCIENCE PHOTO LIBRARY
Cut-away illustration of the Earth. Six main zones are seen: at the centre is the inner core, surrounded in turn by the outer core, lower mantle, a transition zone, the upper mantle and finally the thin crust. The top half shows convection patterns around plate margins. At centre is the Mid-Atlantic Ridge, a constructive spreading zone in the middle of the plate. At right is another spreading zone in the Indian Ocean. At left is a subduction zone, where the Nazca plate is being pushed under the South American plate. Such destructive margins create large chains of volcanos and mountain chains such as the Andes.
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