DAVID HARDY / SCIENCE PHOTO LIBRARY DAVID HARDY / SCIENCE PHOTO LIBRARY
Diagram of colliding tectonic plates. The tectonic plates are large-scale units of crust which move relative to one another in response to the thermal activity of the core and mantle. At top centre is a constructive margin of an oceanic plate, where material arrives at the surface and spreads out. The outer edges of the plate are thus forced under their neighbours. At left, the plate is subducted beneath a continental plate. This forms a vast coastal range of mountains rich in volcanic activity (such as the Andes). At right, the plate subducts beneath another oceanic plate, forming a chain of volcanic islands (eg Aleutians, Japan, Philippines, etc.).
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