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KEY CONCEPT Metamorphic rocks form as existing rocks change. STANDARDS 8–3.7 Illustrate the creation and changing of landforms that have occurred through geologic processes (including volcanic eruptions and mountain-building forces). VOCABULARY metamorphism p. 96 recrystallization p. 97 foliation p. 100 BEFORE, you learned NOW, you will learn • Igneous rocks form as molten rock cools • Sedimentary rocks form from earlier rocks • How a rock can change into another type of rock • How new minerals can grow in existing rocks THINK ABOUT How does a rock change into another kind of rock? Examine a sample of shale and a sample of schist (shihst). Shale, a sedimentary rock, can change into schist. Think about how this change could occur without the shale’s melting or breaking apart. Make a prediction about what process changes shale into schist. shale schist Heat and pressure change rocks. When you cook popcorn, you use heat to increase the pressure within small, hard kernels until they explode into a fluffy snack. Cooking popcorn is just one example of the many ways in which heat and pressure can change the form of things—even things like rocks. Rocks change into other rocks by the process of metamorphism. A similar word, metamorphosis, refers to what happens when a caterpillar changes into a butterfly. The process in which an existing rock is changed by heat or pressure—or both—is called metamorphism (MEHT-uh-MAWR-FIHZuhm). The original sedimentary or igneous rock is called the parent rock. The resulting rock is a metamorphic rock. Even a metamorphic rock can be a parent rock for another type of metamorphic rock. Many of the metamorphic rocks people use were once sedimentary rocks. Limestone is the parent rock of marble, which is used by builders and artists. Shale can be the parent rock of schist, which can be a source of the gemstone garnet. Some schists are a source of the mineral graphite, which is used in pencils. Give an example of a way people use metamorphic rocks. 96 Unit 1: Earth’s Surface