Chapter 4 Minerals,Rocks, and the Rock Cycle

Chapter 4 Minerals,Rocks, and the Rock Cycle - 2009 Allan...

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CHAPTER 4 © 2009 Allan Ludman and Stephen Marshak W.W. Norton & Company MINERALS, ROCKS, AND THE ROCK CYCLE PURPOSE To understand how relationships among grains differ in igneous, sedimentary, and metamorphic rocks, because of the way in which the rocks form. To use textures, and mineral identification to determine if a rock is igneous, sedimentary, or metamorphic MATERIALS NEEDED An assortment of igneous, sedimentary, and metamorphic rocks • Supplies to make artificial igneous, sedimentary, and metamorphic rocks • A magnifying glass or hand lens. • Mineral testing supplies 4.1 INTRODUCTION A rock is an aggregate of mineral grains, fragments of previously existing rock, or a mass of natural glass. Rocks can form in a variety of ways—some by freezing of a melt, others by cementing loose grains together, others by precipitation from water solutions, and still others from changes that happen in response to temperature and pressure underground. Geologists can identify rocks, and interpret aspects of Earth's history, by studying two principal characteristics of rocks: rock composition (meaning, the identity of minerals or glass that make up a rock) and rock texture (meaning, the dimensions and shape of grains, and the way in which grains are arranged, oriented, and held together). On our dynamic planet, rocks don’t survive forever—nature recycles materials, using those in one rock to form new rocks through a series of steps called the rock cycle. In this Chapter, we first introduce the three basic classes of rocks. Then we learn how to observe the characteristics of rocks and how to use these characteristics to classify rocks. Then, you will make “rocks” in the classroom and see how different processes produce different textures. By combining your mineral identification skills and 1
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knowledge of rock textures, you can face the challenge of determining how specimens of common rocks have formed. 4.2 THE THREE CLASSES OF ROCKS Geologists struggled for a century with the question of how to classify rocks and finally concluded that rocks can best be classified on the basis of how they formed. Rock composition and texture provide the basis for this classification, for these characteristics reflect the process of formation. In modern terminology, we distinguish three classes of rock: igneous, sedimentary, and metamorphic. Igneous rocks form by cooling and solidification of molten rock, formed by the melting of preexisting rock in the mantle or lower crust. We refer to molten rock below the Earth's surface as magma, and molten rock that has erupted onto the surface as lava. Some volcanoes erupt explosively, blasting fragments into the air, and when these fragments fall back to Earth, coalesce, and solidify, the resulting rock is also considered to be igneous.
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