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Chapter 5 the Formation of Rocks

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Chapter 5 the Formation of Rocks Chapter 5 The Formation of Rocks WHAT IS CLASSIFICATION? In Chapter 1 you learned that science is a way of making, or- ganizing, and using observations. One of the important skills of a scientist is the ability to classify. In the process of classi- fication, objects, ideas, or information is organized according to their properties. Things that are similar in some character- istic are grouped together. Things that do not have this char- acteristic are put into a different group. In this process, it is important to specify what properties are being used to group things. For example, in the last chapter you read about the classification of minerals according to those with metallic lus- ter and minerals with nonmetallic luster. ACTIVITY 5-1 CLASSIFICATION Your teacher will give you about a dozen objects to classify. Di- vide the objects into groups based on their observable properties. Start by listing all the objects. Each time you divide the objects into groups, state a single property that allows you to clearly de- cide to which group an object belongs. As you divide each group into smaller groups, limit the number of subgroups to two or three. 112 WHAT ARE ROCKS? 113 Continue the process until each object is alone in a group. Create a flowchart that will allow someone else to separate and/or iden- tify all the objects. WHAT ARE ROCKS? A rock is a substance that is or was a natural part of the solid Earth, or lithosphere. Rocks come in a great many vari- eties. Some rocks are unusual enough for geologists and peo- ple interested in geology to collect them. Rocks can also be colorful or attractive. Landscapers often make use of rocks in planning homes or parks. Most rocks are composed of a variety of minerals. In some rocks, you can recognize the minerals as variations in color or other mineral properties within the rock. Granite is a good example. If you look carefully at a sample of granite with a hand lens, you will probably observe some parts of the rock that are transparent. This is probably the mineral quartz. Other parts are white or pink with angular cleavage. These are properties of plagioclase feldspar and potassium feldspar. Dark mineral grains that occur in thin sheets are biotite mica. Dark minerals that occur in stubby crystals are proba- bly hornblende, the most common mineral in the amphibole family. Other minerals can occur in granite, but they are not as common as these four. Other rocks are composed of a single mineral. Some vari- eties of sandstone, particularly if they are very light in color, can be nearly 100 percent quartz. Very pure limestone is nearly all calcite. Rock salt can be nearly pure halite. ACTIVITY 5-2 MAKING A ROCK COLLECTION As you read through this chapter, collect rocks from around your community. If you wish, you can add rocks from your travels or rocks that you have obtained elsewhere. You may recall that when 114 CHAPTER 5: THE FORMATION OF ROCKS you worked with minerals you tried to use fresh, unweathered samples. It is important to follow the same rule in collecting rocks. Weathered rocks tend to crumble, and they are also harder to identify than fresh samples. This rock collection will have two purposes. First, it should help you discover the variety of rocks around you. The second purpose will be to use the information in this chapter to classify your rocks. Your samples should be divided into the three categories you will learn about in this chapter: igneous, sedimentary, and meta- morphic rocks. You may collect as many rocks as you wish, but the final submission will consist of six small rocks, each of which shows a properties of its category. Therefore, for each rock sub- mitted classify it as igneous, metamorphic, or sedimentary and tell why each sample fits into its group. Your samples may be placed in a half-dozen egg carton. You are encouraged to check your progress on this assignment with your teacher and to carefully observe the rock samples you use in class to clarify the properties used to classify rocks. Your teacher will collect your samples after the class has completed the chapter. Classification of Rocks In the previous chapter you learned that minerals are classi- fied on the basis of their observable properties such as luster, hardness, color, and cleavage. Geologists have found it more useful to separate rocks into three groups based on how they formed. The way rocks form is called their origin. When any material is heated enough, it will melt. This is true even of rocks. If the melted material cools enough, it will solidify.The process of solidification is sometimes called crys- tallization. You may recall that minerals were defined as crys- talline solids. The formation of a true solid involves atoms and molecules moving into fixed and ordered positions. While the networks of atoms and molecules may not be large enough to WHAT ARE ROCKS? 115 form visible crystals, it is the movement of atoms and mole- cules into these fixed and ordered positions that defines the formation of a solid. Therefore solidification is the formation of a crystalline solid. Igneous rocks form from hot, molten (liquid) rock mate- rial that originated deep within Earth. Only igneous rocks have this origin. Hot, liquid rock is called magma. (At Earth’s surface magma is known as lava.) In Chapter 2 you learned that Earth’s temperature increases as you go deeper within the planet. In some places within Earth, it is hot enough to melt rock. When this molten rock rises to or near Earth’s sur- face where it is cooler,the liquid rock material changes to solid rock. Igneous rocks are especially common around volcanoes and in places where large bodies of rock that have melted and then solidified underground have been pushed to the surface. In Chapter 7 you will learn that most of Earth’s interior is in the solid state. If temperatures underground are hot enough to melt rock, why is the interior not mostly liquid? The reason that Earth is mostly in the solid state is the increase of pressure with depth. While the increasing temperature tends to melt the rock, the increase in pressure prevents melting. Rocks weather and break down when they are exposed at Earth’s surface. The weathered rock material is usually washed away by rainfall and carried into streams. Eventu- ally the weathered material accumulates as layers of sedi- ment somewhere, usually in a large body of water such as an ocean. As more layers of weathered rock are deposited on top of the older layers, the lower layers are compressed by the weight of sediments accumulating above them. Furthermore, water circulating through them carries rock material into the spaces between the particles filling them with natural cement. Compression and cementing of weathered rock frag- ments or the shells of once living creatures is the origin of most sedimentary rocks. The second group of rocks is called sedimentary rocks. If sedimentary or igneous rocks are buried so deeply that heat and pressure cause new minerals to form, the result is metamorphic rocks. Metamorphic rocks are the only group that forms directly from other rocks (igneous or sedimen- tary). But if most metamorphic rocks form within Earth, how 116 CHAPTER 5: THE FORMATION OF ROCKS can they be found at the surface? The answer to this question involves two important Earth-changing processes. The first is uplift. Earth contains a great deal of heat energy. As heat es- capes from the interior of the planet, it sometimes pushes up rocks to form mountains. The second process is weathering and erosion. Weathering and erosion wear down the moun- tains exposing rocks at the surface that actually formed at depths of 10 km or more underground. Wherever you find a large mass of metamorphic rock at the surface, you are prob- ably looking at the core of an ancient mountain range. Not all rocks fit easily into one of these three categories. Some volcanoes throw great quantities of ash into the air.The ash falls, settles in layers, and hardens. The settling part of ash layers’ origin is similar to the processes that form sedi- mentary rock. However, because the material came from a volcano, volcanic ash is classified as an igneous rock. Another example is the gradual change from igneous or sedimentary rock to metamorphic rock. It may not be clear at what point the rock should no longer be classified as its original parent rock and when it should be called a metamorphic rock. In spite of these occasional difficulties, the classification of rocks by their origin has generally served geologists and Earth sci- ence students well. Figure 5-1 illustrates the way to classify rocks as igneous, sedimentary, or metamorphic by their ap- pearance. Rocks can be classified by their appearance be- cause the way they look offers clues to their formation. HOW ARE IGNEOUS ROCKS CLASSIFIED? Igneous rocks are classified by their color and texture. The colors in rocks come from the minerals that make up the rocks. Minerals rich in aluminum (chemical symbol, Al) are commonly light-colored, sometimes pink. These minerals are called felsic because the feldspars are the most common light-colored minerals. the word “felsic” comes from feldspar and silicon. Minerals rich in magnesium (chemical symbol, Mg) and iron (chemical symbol, Fe) such as olivine and py- HOW ARE IGNEOUS ROCKS CLASSIFIED? 117 Figure 5-1 Rocks can be classified by their appearance. roxene families are called mafic (MAY-fic) The word “mafic” comes from a combination of magnesium and ferric, which is used to describe iron.
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