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Our transcription: Oceanic crust comprises about 70 percent of Earth's surface. Most of this crust originated by injection and eruption of magma at divergent boundaries expressed as mid-ocean ridges capped by rift zones. This process of crustal growth it called "Sea Floor Spreading." Earth's volume has remained essentially the same for billions of years. As a result, plates can grow larger by sea floor spreading only if other plates are growing smaller. Plates are reduced in size or destroyed where they converge, creating some of the most dramatic topography anywhere on Earth. An important land form marking the collision between plates is a deep marine trench. Here, one plate slips beneath the other in a process called subduction. Some plates, however, are too buoyant to subduct and simply crumple together.
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