4 Overview This section examines what happens when rock metamorphoses. Changes in heat or pressure can alter a rock s chemical nature and physical structure. Students will learn how different types of metamorphism cause changes in rock texture. Bellringer Ask students to write a brief description of how cookies are made. Ask them to consider how the mixture of raw ingredients is like sedimentary rock. Ask them to describe how cookie dough metamorphoses when it is baked in an oven. v -g Modeling Metamorphism Provide each student with pieces of red, yellow, green, and purple modeling clay. Have students flatten each piece, pile the pieces on top of each other, and press down on them firmly. Then, have students push inward on opposite sides of the stack or pull the stack gently so that the clay doesn t break apart. Explain that they will be learning how intense pressure and heat can cause rock to behave in similar ways. l Visual/Kinesthetic 4 READING WARM-UP Objectives Describe two ways a rock can undergo metamorphism. Explain how the mineral composition of rocks changes as the rocks undergo metamorphism. Describe the difference between foliated and nonfoliated metamorphic rock. Explain how metamorphic rock structures are related to deformation. Terms to Learn foliated nonfoliated READING STRATEGY Discussion Read this section silently. Write down questions that you have about this section. Discuss your questions in a small group. Figure 1 This metamorphic rock is an example of how mineral grains were aligned into distinct bands when the rock underwent metamorphism. CHAPTER RESOURCES Chapter Resource File CRF Lesson Plan Directed Reading A b Directed Reading B s Technology Transparencies Bellringer Regional and Contact Metamorphism Metamorphic Rock Have you ever watched a caterpillar change into a butterfly? Some caterpillars go through a biological process called metamorphosis in which they completely change their shape. Rocks can also go through a process called metamorphism. The word metamorphism comes from the Greek words meta, which means changed, and morphos, which means shape. Metamorphic rocks are rocks in which the structure, texture, or composition of the rock have changed. All three types of rock can be changed by heat, pressure, or a combination of both. Origins of Metamorphic Rock The texture or mineral composition of a rock can change when its surroundings change. If the temperature or pressure of the new environment is different from the one in which the rock formed, the rock will undergo metamorphism. The temperature at which most metamorphism occurs ranges from 50 C to 1,000 C. However, the metamorphism of some rocks takes place at temperatures above 1,000 C. It seems that at these temperatures the rock would melt, but this is not true of metamorphic rock. It is the depth and pressure at which metamorphic rocks form that allows the rock to heat to this temperature and maintain its solid nature. Most metamorphic change takes place at depths greater than 2 km. But at depths greater than 16 km, the pressure can be 4,000 times greater than the pressure of the atmosphere at Earth s surface. Large movements within the crust of the Earth cause additional pressure to be exerted on a rock during metamorphism. This pressure can cause the mineral grains in rock to align themselves in certain directions. The alignment of mineral grains into parallel bands is shown in the metamorphic rock in Figure 1. When rocks metamorphose under high temperature and pressure, they become plastic and can be easily deformed. It is not unusual for spherical pebbles in a conglomerate to be stretched into ellipses more than 30 times their original diameter! 106 Chapter 4 Rocks: Mineral Mixtures
Regional metamorphism Contact metamorphism Magma Figure 2 Metamorphism occurs over small areas, such as next to bodies of magma, and over large areas, such as mountain ranges. Sedimentary rock Using the Figure ---------b Analogies Ask students to think of some analogies for contact meta morphism (for example, an egg frying in a skillet). Have students draw a diagram of the process of contact metamorphism. Have the class think of some analogies for regional metamorphism (for example, making toast). Then, have students draw a diagram of the process of regional metamorphism. l Logical/Visual Writing Contact Metamorphism One way rock can undergo metamorphism is by being heated by nearby magma. When magma moves through the crust, the magma heats the surrounding rock and changes it. Some minerals in the surrounding rock are changed into other minerals by this increase in temperature. The greatest change takes place where magma comes into direct contact with the surrounding rock. The effect of heat on rock gradually decreases as the rock s distance from the magma increases and as temperature decreases. Contact metamorphism occurs near igneous intrusions, as shown in Figure 2. Regional Metamorphism When pressure builds up in rock that is buried deep below other rock formations or when large pieces of the Earth s crust collide with each other, regional metamorphism occurs. The increased pressure and temperature causes rock to become deformed and chemically changed. Unlike contact metamorphism, which happens near bodies of magma, regional metamorphism occurs over thousands of cubic kilometers deep within Earth s crust. Rocks that have undergone regional metamorphism are found beneath most continental rock formations. Reading Check Explain how and where regional metamorphism takes place. (See the Appendix for answers to Reading Checks.) Answer to Reading Check Regional metamorphism occurs when pressure builds up in rock that is buried deep below other rock formations or when large pieces of the Earth s crust collide. The increased pressure can cause thousands of square miles of rock to become deformed and chemically changed. Stretching Out 1. Sketch the crystals in granite rock on a piece of paper with a black-ink pen. Be sure to include the outline of the rock, and fill it in with different crystal shapes. 2. Flatten some plastic play putty over your drawing, and slowly peel it off. 3. After making sure that the outline of your granite has been transferred to the putty, squeeze and stretch the putty. What happened to the crystals in the granite? What happened to the granite? Is That a Fact! The largest expanse of exposed metamorphic rock in the world is the Canadian Shield, a huge horseshoeshaped region encircling Hudson Bay. Covering about half of Canada, it is about 4,586,900 km 2 and is the source of more than 70% of the minerals mined in Canada. M A T E R I A L S FOR EACH STUDENT black ink pen paper plastic play putty Answer 3. The crystals become stretched and deformed. The granite changes its shape because of the force applied to it. MISCONCEPTION ALERT Metamorphism As you discuss Figure 2, be sure students understand that the composition of a metamorphic rock and the heat and pressure it is subjected to determine how much it deforms. Stu dents should understand that the bulk composition of rock does not change during metamorphism unless fluids are introduced into the rock. However, the mineral composition of the rock may change as heat and pressure change. Section 4 Metamorphic Rock 107
Research -------------------------------------------g Metamorphic Minerals PORTFOLIO Several types of minerals found in metamorphic rock, such as garnet, tourmaline, and serpentine, are used in sculpture and in jewelry making. Encourage students to choose one of these minerals, research it, and create a poster illustrating how it forms and what its uses are. l Visual MISCONCEPTION ALERT Heat and Temperature Heat and temperature are not the same thing. Heat is the transfer of thermal energy from one object to another. Temperature is a measure of how hot (or cold) something is. Temperature is not a form of energy. When a rock comes into contact with magma, thermal energy is transferred from the magma to the rock because the magma is at a higher temperature than the rock. As a result, the temperature of the rock increases while the temperature of the magma decreases. But heating a rock does not always raise its temperature. If the rock is already so hot that it is on the verge of melting, additional heat will cause the rock to melt (and change state), but will not change the rock s temperature. Figure 3 The minerals calcite, quartz, and hematite combine and recrystallize to form the metamorphic mineral garnet. Making a Rock Collection With a parent, try to collect a sample of each class of rock described in this chapter. You may wish to collect rocks from road cuts or simply collect pebbles from your garden or driveway. Try to collect samples that show the composition and texture of each rock. Classify the rocks in your collection, and bring it to class. With other members of the class, discuss your rock samples and see if they are accurately identified. Figure 4 Scientists can understand a metamorphic rock s history by observing the minerals the rock contains. For example, a metamorphic rock that contains garnet formed at a greater depth and under greater heat and pressure than a rock that contains only chlorite. Calcite Quartz Hematite Composition of Metamorphic Rock Metamorphism occurs when temperature and pressure inside the Earth s crust change. Minerals that were present in the rock when it formed may not be stable in the new temperature and pressure conditions. The original minerals change into minerals that are more stable in these new conditions. Look at Figure 3 to see an example of how this change happens. Many of these new minerals form only in metamorphic rock. As shown in Figure 4, some metamorphic min erals form only at certain temperatures and pressures. These minerals, known as index minerals, are used to estimate the temperature, depth, and pressure at which a rock undergoes metamorphism. Index minerals include biotite mica, chlorite, garnet, kyanite, muscovite mica, sillimanite, and staurolite. Reading Check What is an index mineral? Magma Heat and pressure Chlorite 400 C 4 to 32 km Muscovite mica 700 C 5 to 34 km Garnet 700 C to 1,200 C 25 to 60 km Garnet Answer to Reading Check An index mineral is a metamorphic mineral that forms only at certain temperatures and pressures and therefore can be used by scientists to estimate the temperature, pressure, and depth at which a rock undergoes metamorphosis. h -g Making Models Have students make a model cross section of the Earth s crust. The model should include materials that represent magma, contact and regional metamorphic rocks, and sedimentary strata. e l Kinesthetic/Visual 108 Chapter 4 Rocks: Mineral Mixtures
Textures of Metamorphic Rock You have learned that texture helps scientists classify igneous and sedimentary rock. The same is true of metamorphic rock. All metamorphic rock has one of two textures foliated or nonfoliated. Take a closer look at each of these types of metamorphic rock to find out how each type forms. Foliated Metamorphic Rock The texture of metamorphic rock in which the mineral grains are arranged in planes or bands is called foliated. Foliated metamorphic rock usually contains aligned grains of flat minerals, such as biotite mica or chlorite. Look at Figure 5. Shale is a sedimentary rock made of layers of clay minerals. When shale is exposed to slight heat and pressure, the clay minerals change into mica minerals. The shale becomes a foliated metamorphic rock called slate. Metamorphic rocks can become other metamorphic rocks if the environment changes again. If slate is exposed to more heat and pressure, the slate can change into rock called phyllite. When phyllite is exposed to heat and pressure, it can change into schist. If metamorphism continues, the arrangement of minerals in the rock changes. More heat and pressure cause minerals to separate into distinct bands in a metamorphic rock called gneiss (NIES). Figure 5 The effects of metamorphism depend on the heat and pressure applied to the rock. Here you can see what happens to shale, a sedimentary rock, when it is exposed to more and more heat and pressure. Gneiss INCLUSION Strategies Schist Behavior Control Attention Deficit Disorder Gifted and Talented Have students conduct a scavenger hunt in their classroom to identify minerals that are used to make items that we use every day. Organize students into groups of 3 to 4 students. Assign each group an area of the classroom, and provide each group with self-stick labels to mark items made out of minerals. Each group should foliated the texture of metamorphic rock in which the mineral grains are arranged in planes or bands Sedimentary shale Phyllite Slate try to label the mineral(s) that make up each item. Items made of minerals that students may identify include light bulbs, electrical wiring, door knobs, locks, hinges, windows, plumbing fixtures, and pencils. Ask students what additional items made of minerals may be found in their homes. Talented and gifted students can do additional research on minerals used to make a light bulb and/or a e computer. l Visual/Logical Discussion --------------------------------------b Predicting Patterns Display metamorphic rocks in groups of foliated rocks (slate, phyllite, schist, and gneiss) and nonfoliated rocks (quartzite, marble, hornfels, and soapstone). Have students compare the rocks according to color, appearance, and composition. Explain that the temperature and pressure of regional metamorphism tend to produce foliated rocks, whereas the temperature of contact metamorphism tends to produce nonfoliated rocks. Ask students to predict what the terms foliated and nonfoliated mean and then read further to see if they were correct. l Visual/Logical CONNECTION v Real World -a Asbestos Removal Asbestos is an informal name for a group of fibrous minerals usually found in regionally metamorphosed rock. Manufacturers value these minerals because minerals resist burning and don t readily conduct thermal energy or electric current. However, some kinds of asbestos fracture into tiny needles that can become airborne. This dust is linked to a lung disease called asbestosis. As a result, asbestos has been removed from many public places at great expense. Have students find out about the many uses for asbestos and any cleanup projects that are occurring in your area. Have students summarize their findings in the form of a poster. l Logical Section 4 Metamorphic Rock 109
Reteaching -------------------------------------b Creating an Outline Have students select a type of foliated or nonfoliated metamorphic rock. Ask students to create an outline of the steps that occur in the formation of the rock. Some students may want to write their outline in the form of a recipe for making a metamorphic rock. l Logical Quiz ---------------------------------------------------------------------g 1. Why is marble considered a nonfoliated metamorphic rock? (Its mineral grains are not arranged in planes or bands.) 2. What does the composition of a metamorphic rock tell you about the rock s origin and formation? (Different metamorphic minerals indicate the temperature and pressure conditions that existed when the rock formed.) Alternative Assessment ---------------------------g Preparing a Lesson Have students prepare a lesson about this chapter to present to a secondgrade class. They will need to prepare vocabulary lists, illustrations, and worksheets to help the younger students understand the types of rock, the uses of rock, and the way rocks form. l Logical/Visual WRITING Metamorphosis SKILL The term metamorphosis means change in form. When some animals undergo a dramatic change in the shape of their body, they are said to have undergone a metamorphosis. As part of their natural life cycle, moths and butterflies go through four stages. After they hatch from an egg, they are in the larval stage in the form of a caterpillar. In the next stage, they build a cocoon or become a chrysalis. This stage is called the pupal stage. They finally emerge into the adult stage of their life, in which they have wings, antennae, and legs! Research other animals that undergo a metamorphosis, and summarize your findings in a short essay. nonfoliated the texture of metamorphic rock in which the mineral grains are not arranged in planes or bands Nonfoliated Metamorphic Rock The texture of metamorphic rock in which the mineral grains are not arranged in planes or bands is called nonfoliated. Notice that the rocks shown in Figure 6 do not have mineral grains that are aligned. This lack of aligned mineral grains is the reason these rocks are called nonfoliated rocks. Nonfoliated rocks are commonly made of one or only a few minerals. During metamorphism, the crystals of these minerals may change in size or the mineral may change in composition in a process called recrystallization. The quartzite and marble shown in Figure 6 are examples of sedimentary rocks that have recrystallized during metamorphism. Quartz sandstone is a sedimentary rock made of quartz sand grains that have been cemented together. When quartz sandstone is exposed to the heat and pressure, the spaces between the sand grains disappear as the grains recrystallize to form quartzite. Quartzite has a shiny, glittery appearance. Like quartz sandstone, it is made of quartz. But during recrystallization, the mineral grains have grown larger than the original grains in the sandstone. When limestone undergoes metamorphism, the same process that happened to the quartz happens to the calcite, and the limestone becomes marble. The calcite crystals in the marble are larger than the calcite grains in the original limestone. Figure 6 Two Examples of Nonfoliated Metamorphic Rock Marble and quartzite are nonfoliated metamorphic rocks. As you can see in the views through a microscope, the mineral crystals are not well aligned. Marble Quartzite h -a Investigate Your Area Have students look at stone buildings and houses around their town. Ask students to identify the rock used in construction as igneous, sedimentary, or metamorphic. Ask students to consider the following questions: Which rock type was most commonly used? Which rock type was used least? Why was a specific rock type used for a particular application? Encourage students to find out the origin of rock used in buildings in your community. l Visual 110 Chapter 4 Rocks: Mineral Mixtures
Metamorphic Rock Structures Like igneous and sedimentary rock, metamorphic rock also has features that tell you about its history. In metamorphic rocks, these features are caused by deformation. Deformation is a change in the shape of a rock caused by a force placed on it. These forces may cause a rock to be squeezed or stretched. Folds, or bends, in metamorphic rock are structures that indicate that a rock has been deformed. Some folds are not visible to the naked eye. But, as shown in Figure 7, some folds may be kilometers or even hundreds of kilometers in size. Reading Check to deformation? Review Summary Metamorphic rocks are rocks in which the structure, texture, or composition has changed. Two ways rocks can undergo metamorphism are by contact metamorphism and regional metamorphism. As rocks undergo metamorphism, the original minerals in a rock change into new minerals that are more stable in new pressure and temperature conditions. Foliated metamorphic rock has mineral crystals aligned in planes or bands, whereas nonfoliated rocks have unaligned mineral crystals. Metamorphic rock structures are caused by deformation. How are metamorphic rock structures related Using Key Terms 1. In your own words, define the following terms: foliated and nonfoliated. Understanding Key Ideas 2. Which of the following is not a type of foliated metamorphic rock? a. gneiss b. slate marble schist Answer to Reading Check Deformation causes metamorphic structures such as folds. Explain the difference between contact metamorphism and regional metamorphism. Explain how index minerals allow a scientist to understand the history of a metamorphic rock. For every 3.3 km a rock is buried, the pressure placed upon it increases 0.1 gigapascal (100 million pascals). If rock undergoing metamorphosis is buried at 16 km, what is the pressure placed on that rock? (Hint: The pressure at Earth s surface is.101 gigapascal.) Figure 7 These large folds occur in metamorphosed sedimentary rock along Saglet Fiord in Labrador, Canada. r Making Inferences If you had two metamorphic rocks, one that has garnet crystals and the other that has chlorite crystals, which one could have formed at a deeper level in the Earth s crust? Explain your answer. 7. Applying Concepts Which do you think would be easier to break, a foliated rock, such as slate, or a nonfoliated rock, such as quartzite? Explain. Analyzing Processes A mountain range is located at a boundary where two tectonic plates are colliding. Would most of the metamorphic rock in the mountain range be a product of contact metamorphism or regional metamorphism? Explain. For a variety of links related to this chapter, go to www.scilinks.org Topic: Metamorphic Rock SciLinks code: HSM0949 CHAPTER RESOURCES Chapter Resource File CRF Section Quiz g Section Review g Vocabulary and Section Summary g Reinforcement Worksheet b Critical Thinking a Technology Interactive Explorations CD-ROM Rock On! g Answers to Section Review 1. Sample answer: Foliated metamorphic rock consists of minerals that are arranged in planes or bands. The minerals in nonfoliated metamorphic rock do not appear to be arranged in a pattern. 2. c 3. Contact metamorphism is a type of metamorphism that occurs near igneous intrusions, where magma comes into direct contact with surrounding rock. Regional metamorphism is a type of metamorphism that occurs when large pieces of Earth s crust collide, causing rock to become deformed and chemically changed over large areas. 4. Because index minerals form only at certain temperatures and pressures, index minerals indicate the temperature, pressure, and depth at which a rock metamorphosed. 5. 16 km 3.3 km 4.84 km 0.1 gigapascal 0.484 gigapascal.101 gigapascal (atmospheric pressure).585 gigapascal 6. The rock with garnet crystals would probably have formed deeper in the Earth because the mineral garnet forms at a higher temperature and higher pressure than the mineral chlorite. 7. Because the grains in a foliated metamorphic rock are arranged in parallel bands, foliated metamorphic rock would be easier to break than a nonfoliated metamorphic rock. 8. Rock becomes deformed and chemically changed over large areas of Earth s crust by increases in temperature and pressure that occur when tectonic plates collide. Therefore, most of the rock in the mountain range would be the product of regional metamorphism. Section 4 Metamorphic Rock 111