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Title: Continental rifting and the formation of a new ocean basin.
Caption: A. Continental rifting is thought to occur where tensional forces stretch and thin the crust. As a result, molten rock ascends from the asthenosphere and initiates volcanic activity at the surface. B. As the crust is pulled apart, large slabs of rock sink, generating a rift valley. C. Further spreading generates a narrow sea. D. Eventually, an expansive ocean basin and ridge system are created.
Keywords: plate tectonics, tectonic plates, divergent plate boundaries, boundary, divergence, mid-ocean ridges, rift valleys, sea-floor spreading, magma, oceans