MIKKEL JUUL JENSEN / SCIENCE PHOTO LIBRARY MIKKEL JUUL JENSEN / SCIENCE PHOTO LIBRARY
Sea floor spreading. Artwork showing three stages in the process of sea floor spreading. The process takes millions of years and it is the process by which the continents move over the Earth. The first stage (top) occurs when underground forces (red arrows) pull the overlying crust apart. A shallow sea has formed between the volcanic plate margins. In the second stage (middle), the fracture zone has formed into a mid-ocean ridge with underwater volcanoes. At this ridge, molten rock from below the Earth's crust is forming new sea floor. At bottom, the underwater volcanoes have formed new land above the sea along the ridge. This occurs if the rate of volcanism exceeds the speed of the sea floor spreading.
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