2 Igneous Rock 2 Overview Where do igneous rocks come from? Here’s a hint: The word igneous comes from a Latin word that means “fire.” READI NG WARM-U P This section discusses the formation of igneous rock from the cooling of magma. Students learn about the difference between intrusive and extrusive igneous rock. Students also learn about the difference between felsic and mafic igneous rock and how the rate of cooling affects the texture of igneous rock. Objectives • • Describe three ways that igneous rock forms. Explain how the cooling rate of magma affects the texture of igneous rock. Distinguish between igneous rock that cools within Earth’s crust and igneous rock that cools at Earth’s surface. • Terms to Learn intrusive igneous rock extrusive igneous rock Bellringer READI NG STRATEGY Pose the following question to students: “Do you think rocks that cooled and solidified from lava on Earth’s surface would look different from those that cooled and solidified from magma inside the Earth? Why?” Reading Organizer As you read this section, make a table comparing intrusive rock and extrusive rock. Igneous rock forms when hot, liquid rock, or magma, cools and solidifies. The type of igneous rock that forms depends on the composition of the magma and the amount of time it takes the magma to cool. Origins of Igneous Rock Igneous rock begins as magma. As shown in Figure 1, there are three ways magma can form: when rock is heated, when pressure is released, or when rock changes composition. When magma cools enough, it solidifies to form igneous rock. Magma solidifies in much the same way that water freezes. But there are also differences between the way magma freezes and the way water freezes. One main difference is that water freezes at 0°C. Magma freezes between 700°C and 1,250°C. Also, liquid magma is a complex mixture containing many melted minerals. Because these minerals have different melting points, some minerals in the magma will freeze or become solid before other minerals do. Figure 1 The Formation of Magma Composition When fluids such as water combine with rock, the composition of the rock changes, which lowers the melting point of the rock enough to melt it. Discussion ----------------------------------g Volcanoes Ask students to discuss how volcanoes affect people. Discuss eruptions, lava flows, and ash clouds. Then, ask students about the benefits of volcanoes. Explain that lava and magma form land. Explain that volcanic soil is some of the most fertile soil in the world, which is why many populations are willing to live alongside potentially dangerous volcanoes. l Logical Pressure The high pressure deep inside the Earth forces minerals to remain solid. When hot rock rises to shallow depths, the pressure in the rock is released, and the minerals can melt. Temperature A rise in temperature can cause the minerals in a rock to melt. Different melting points cause some minerals to melt while other minerals remain solid. CHAPTER RESOURCES Chapter Resource File CRF • Lesson Plan • Directed Reading A b • Directed Reading B s Technology Transparencies • Bellringer 98 Chapter 4 • Rocks: Mineral Mixtures Surtsey is a volcanic island south of Iceland that people actually saw being born! In 1963, fishermen saw jets of spray, steam, and lava shooting more than 30 m out of the ocean. One month later, the volcano broke through the surface to form an island. By the time the eruptions ended, Surtsey covered an area of approximately 2.8 km2. Figure 2 Igneous Rock Texture Coarse-grained Using the Figure ---------b Fine-grained Felsic Granite Rhyolite Figure 3 The amount of time it takes for magma or lava to cool determines the texture of igneous rock. Mafic Gabbro Basalt Composition and Texture of Igneous Rock Look at the rocks in Figure 2. All of the rocks are igneous rocks even though they look different from one another. These rocks differ from one another in what they are made of and how fast they cooled. The light-colored rocks are less dense than the dark-colored rocks are. The light-colored rocks are rich in elements such as aluminum, potassium, silicon, and sodium. These rocks are called felsic rocks. The dark-colored rocks, called mafic rocks, are rich in calcium, iron, and magnesium, and poor in silicon. Figure 3 shows what happens to magma when it cools at different rates. The longer it takes for the magma or lava to cool, the more time mineral crystals have to grow. The more time the crystals have to grow, the larger the crystals are and the coarser the texture of the resulting igneous rock is. In contrast, the less time magma takes to cool, the less time crystals have to grow. Therefore, the rock that is formed will be fine grained. Fine-grained igneous rock contains very small crystals, or if the cooling is very rapid, it contains no crystals. Fast-cooling lava Fine-grained igneous rock Magma Slow-cooling magma Coarse-grained igneous rock ✓ Reading Check Explain the difference between felsic rock and mafic rock. (See the Appendix for answers to Reading Checks.) Answer to Reading Check Felsic rocks are light-colored igneous rocks rich in aluminum, potassium, silicon, and sodium. Mafic rocks are dark-colored igneous rocks rich in calcium, iron, and magnesium. Making Inferences Have students rank the rocks shown in Figure 2 by how fast they cooled. Tell students to pay careful attention to the grain size of each rock. (From fastest cooled to slowest cooled, the rocks are basalt, rhyolite, gabbro, and granite.) l Visual/Logical CONNECTION to Life Science -----------------------------------a Life Along a Rift Until 1977, biologists thought few lifeforms lived at ocean depths where sunlight does not reach. When scientists in the submersible Alvin explored the bottom of a deep ocean trench called the Galápagos Rift, they discovered structures called black smokers that release dissolved mineral compounds and heat the water. Scientists were amazed to discover an entire ecosystem that did not depend on photosynthesis for energy. This discovery has led some scientists to speculate that life may also have originated in the outer solar system—particularly in the oceans that may exist under the surface of Europa, one of Jupiter’s moons. Have students research the bizarre life-forms that scientists found living around black smokers. l Logical Is That a Fact! The Deccan Traps of India Between 68 and 64 million years ago, a hot spot under western India erupted over 1 million km3 of basaltic lava. These lava flows, called the Deccan Traps, are more than 2 km thick in some places. Mathematicians calculate that if the lava from the eruptions in western India were spread evenly over the entire Earth, it would cover the Earth with a layer more than 2 m thick! Section 2 • Igneous Rock 99 Igneous Rock Formations Igneous rock formations are located above and below the surface of the Earth. You may be familiar with igneous rock formations that were caused by lava cooling on the Earth’s surface, such as volcanoes. But not all magma reaches the surface. Some magma cools and solidifies deep within the Earth’s crust. For another activity related to this chapter, go to go.hrw.com and type in the keyword HZ5RCKW. Reteaching -------------------------------------b Word Meanings Compare the words intrusive and extrusive with the words interior and exterior. Have students brainstorm other words that use the prefixes in- and ex- to help them remember the meanings of the terms intrusive and extrusive. l Logical e Intrusive Igneous Rock When magma intrudes, or pushes, into surrounding rock below the Earth’s surface and cools, the rock that forms is called intrusive igneous rock. Intrusive igneous rock usually has a coarse-grained texture because it is well insulated by surrounding rock and cools very slowly. The minerals that form are large, visible crystals. Masses of intrusive igneous rock are named for their size and shape. Common intrusive shapes are shown in Figure 4. Plutons are large, irregular-shaped intrusive bodies. The largest of all igneous intrusions are batholiths. Stocks are intrusive bodies that are exposed over smaller areas than batholiths. Sheetlike intrusions that cut across previous rock units are called dikes, whereas sills are sheetlike intrusions that are oriented parallel to previous rock units. intrusive igneous rock rock formed from the cooling and solidification of magma beneath the Earth’s surface Quiz ---------------------------------------------------------------------g 1. Name five types of bodies of intrusive igneous rock. (batholiths, stocks, dikes, sills, and volcanic necks) 2. What is a fissure? (A fissure is Figure 4 Igneous intrusive bodies have different shapes and sizes. a long crack in the Earth’s crust through which lava erupts and flows.) Volcanic neck Dike Alternative Assessment ---------------------------g Modeling Igneous Rock Bodies Have students create a model cross section that shows the formation of both intrusive and extrusive igneous rock. Students can use Figure 4 as a basis for their models. Supply students with several different colors of clay so that they can color-code different bodies, such as the magma source, dikes, sills, plutons, and the lava that forms extrusive rock. l Kinesthetic/Visual Dike Stock Sill e Batholith h -----------------------------a Volcanic Necks A volcanic neck is the hardened core of a volcano that is left behind after the volcano erodes away. Ship Rock, a volcanic neck on a Navajo reservation in New Mexico, soars 518 m above the desert. Devils Tower National Monument, in Wyoming, rises 386 m. Have students research one of these formations to learn how it formed. Writing l Logical 100 Chapter 4 • Rocks: Mineral Mixtures Batholith Extrusive Igneous Rock Answers to Section Review Igneous rock that forms from magma that erupts, or extrudes, onto the Earth’s surface is called extrusive igneous rock. Extrusive rock is common around volcanoes. It cools quickly on the surface and contains very small crystals or no crystals. When lava erupts from a volcano, a lava flow forms. Figure 5 shows an active lava flow. Lava does not always flow from volcanoes. Sometimes lava erupts and flows from long cracks in the Earth’s crust called fissures. Lava flows from fissures on the ocean floor at places where tension is causing the ocean floor to be pulled apart. This lava cools to form new ocean floor. When a large amount of lava flows out of fissures onto land, the lava can cover a large area and form a plain called a lava plateau. Pre-existing landforms are often buried by these lava flows. 1. Sample answer: Intrusive igneous rock forms from magma that solidifies underground. Extrusive igneous rock forms from magma that solidifies after it has reached the surface. 2. c 3. Temperature, pressure, and a change in the composition of a rock can cause magma to form. A rise in temperature can cause minerals in a rock to melt, forming magma. When pressure in a rock that is hot is released, the minerals in that rock can melt, forming magma. When fluids such as water combine with rock, the composition of the rock changes. This change in composition lowers the melting point of the rock enough to melt it, forming magma. 4. When magma cools slowly, crystals have a longer time to grow, so the igneous rock that forms is coarse grained. When magma cools quickly, crystals have a short time to grow, so the igneous rock that forms is fine grained. 5. 1,825 ft 3.28 ft/m ! 556.4 ft/m 6. A sill intrudes rock parallel to the surrounding rock layers. A dike cuts across the surrounding rock layers. 7. Because the rock formed from slowly cooling magma deep inside the Earth, the crystals had more time to grow. Therefore, the texture of the rock would most likely be coarse grained. ✓Reading Check How does new ocean floor form? Review Summary • • • Igneous rock forms when magma cools and hardens. The texture of igneous rock is determined by the rate at which the rock cools. Igneous rock that solidifies at Earth’s surface is extrusive. Igneous rock that solidifies within Earth’s surface is intrusive. of common • Shapes igneous intrusive bodies include batholiths, stocks, sills, and dikes. Figure 5 An active lava flow is shown in this photo. When exposed to Earth’s surface conditions, lava quickly cools and solidifies to form a fine-grained igneous rock. extrusive igneous rock rock that forms as a result of volcanic activity at or near the Earth’s surface Using Key Terms Critical Thinking 1. In your own words, write a definition for each of the following terms: intrusive igneous rock and extrusive igneous rock. Understanding Key Ideas 2. 7. Predicting Consequences An igneous rock forms from slowcooling magma deep beneath the surface of the Earth. What type of texture is this rock most likely to have? Explain. is an example of a coarsegrained, felsic, igneous rock. a. b. c. d. 6. Making Comparisons Dikes and sills are both types of igneous intrusive bodies. What is the difference between a dike and a sill? Basalt Gabbro Granite Rhyolite 3. Explain three ways in which magma can form. 4. What determines the texture of igneous rocks? Developed and maintained by the National Science Teachers Association For a variety of links related to this chapter, go to www.scilinks.org Math Skills 5. The summit of a granite batholith has an elevation of 1,825 ft. What is the height of the batholith in meters? Answer to Reading Check New sea floor forms when lava that flows from fissures on the ocean floor cools and hardens. Topic: Igneous Rock SciLinks code: HSM0783 CHAPTER RESOURCES Chapter Resource File CRF • Section Quiz g • Section Review g • Vocabulary and Section Summary g Technology Transparencies • Intrusive Igneous Rock Bodies Section 2 • Igneous Rock 101