首页>英语角 > 听力 > 美音 > 课 程 > 科学 > 硕士>What Are Metamorphic Rocks?
Hi I’m Geologist John Swanson for About.com and today I am going to talk about metamorphic rocks. Heat and pressure working together are capable of powerful things. In fact, they can even change rocks. The name for rocks that has been transformed by heat and pressure are metamorphic rocks. Metamorphic comes from Greek words meaning, "change" and "form."More specifically, metamorphic rocks are formed by the physical or chemical modification by heat and pressure of an existing igneous or sedimentary material into that of a denser form. The force of plate tectonics, stress, compression, and shearing can, over long periods of time, deform and warp rocks into smaller and more compact forms. Once they’re compacted and warped, they take up a small volume of space.Not surprisingly, metamorphic rocks are always denser and more compact than the original form they began as. Being more compact means the rocks are now less susceptible to breakdown caused be erosional forces.The earth’s plates move very slowly and are measured on a geologic time scale, which is far longer than any human lifetime. Over time, a crustal plate, composed of igneous or sedimentary rock may become subducted under another plate.The force and weight of being subducted underneath the other plate can cause the rock composing the plate to undergo what is called metamorphism. Adding to the weight and pressure of subduction, heat from the Earth's interior can melt the rock slightly, in a process termed "contact metamorphism."After the forces have acted upon the rock and transformed it from one form to another, it is now referred to as a metamorphic rock. Hopefully you now have a better understanding of metamorphic rocks. Thanks for watching. If you’d like to learn more, please find us on the web at About.com.
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