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HSES_1eTE_C09.qxd 5/16/04 10:10 AM Page 254 Section 9.2 9.2 Plate Tectonics 1 FOCUS Section Objectives 9.4 9.5 9.6 Explain the theory of plate tectonics. Describe lithospheric plates. Identify the three types of plate boundaries. Key Concepts ◆ What are lithospheric plates? ◆ What are the three types of plate boundaries? Reading Focus Build Vocabulary ◆ Comparing and Contrasting Copy the table. After you read, compare the three types of plate boundaries by completing the table. Boundary Type Relative Plate Motion convergent a. ? divergent b. ? transform fault c. ? According to the plate tectonics theory, the uppermost mantle, along with the overlying crust, behaves as a strong, rigid layer. This layer is known as the lithosphere. The outer shell lies over a weaker region in the mantle known as the asthenosphere. The lithosphere is divided into segments called plates, which move and continually change shape and size. Figure 8 on pages 256-257 shows the seven major plates. The largest is the Pacific plate, covering most of the Pacific Ocean. Notice that several of the large plates include an entire continent plus a large area of the seafloor. This is a major departure from Wegener’s continental drift hypothesis, which proposed that the continents moved through the ocean floor, not with it. Note also that none of the plates is defined entirely by the margins of a continent. The lithospheric plates move relative to each other at a very slow but continuous rate that averages about 5 centimeters per year—about as fast as your fingernails grow. This movement is driven by the unequal distribution of heat within Earth. Hot material found deep in the mantle moves slowly upward as part of Earth’s internal convection system. At the same time, cooler, denser slabs of oceanic lithosphere descend into the mantle, setting Earth’s rigid outer shell into motion. The grinding movements of Earth’s lithospheric plates generate earthquakes, create volcanoes, and deform large masses of rock into mountains. L2 2 INSTRUCT Earth’s Major Plates L2 Using Analogies Crack the shell of a hard-boiled egg. Ask students if the egg reminds them of anything. The egg can be seen as a tiny model of Earth. The thin eggshell is analogous to Earth’s crust, divided into plates. Within the shell is the firm mantle. Have students move the pieces of shell around. They should notice how the shell buckles in some places and exposes “mantle” in other places. This movement is analogous to the movement of Earth’s crust. However, Earth’s movement results in the formation of mountains, earthquakes, and new ocean floor. Kinesthetic, Visual 254 Chapter 9 ◆ Reading Strategy Earth’s Major Plates a. plates move together b. plates move apart c. plates slide past each other Build Science Skills ◆ plate tectonics plate divergent boundary convergent boundary transform fault boundary L2 Concept Map Have students make a concept map using the term plate tectonics as the starting point. All the vocabulary terms in this section should be used. Reading Strategy Vocabulary What is the theory of plate tectonics? What is plate tectonics? 254 Chapter 9 HSES_1eTE_C09.qxd 5/16/04 10:10 AM Page 255 Types of Plate Boundaries Figure 7 Three Types of Plate Boundaries All major interactions among individual plates occur along their boundaries. The three main types of boundaries are convergent, divergent, and transform fault boundaries. A Build Reading Literacy Convergent boundaries Convergent boundaries form where Transform fault boundaries Transform fault boundaries are margins where two plates grind past each other without the production or destruction of lithosphere, as shown in Figure 7C. The San Andreas Fault zone in California is an example of a transform fault boundary. Each plate contains a combination of these three types of boundaries. Although the total surface area of Earth does not change, plates may shrink or grow in area. This shrinking or growing depends on the locations of convergent and divergent boundaries. The Antarctic plate is growing larger. The Philippine plate is descending into the mantle along its margins and is becoming smaller. New plate boundaries can be created because of changes in the forces acting on these rigid slabs. Visualize Have students read the section on types of plate boundaries and then try to form a mental picture of each type of boundary. Ask: In which type of boundary do the plates move without changing the lithosphere? (transform fault boundary) Visual, Verbal Divergent boundary B two plates move together. This process results in oceanic lithosphere plunging beneath an overriding plate, and descending into the mantle, as shown in Figure 7B. At other locations, plates carrying continental crust are presently moving toward each other. Eventually, these continents may collide and merge. Thus, the boundary that once separated two plates disappears as the plates become one. L1 Refer to p. 502D in Chapter 18, which provides the guidelines for this visualizing strategy. Divergent boundaries Divergent boundaries (also called spreading centers) occur when two plates move apart. This process results in upwelling of material from the mantle to create new seafloor, as shown in Figure 7A. A relatively new divergent boundary is located in Africa, in a region known as the East African Rift valley. Types of Plate Boundaries 3 ASSESS Convergent boundary Evaluate Understanding C L2 To assess students’ knowledge of section content, have them draw diagrams of the three types of plate boundaries. Each diagram should have a caption describing the movement of the plates. Transform fault boundary L1 Reteach Have students use any materials they wish, such as blocks of wood, to illustrate the plate movements in the three types of boundaries. Section 9.2 Assessment Reviewing Concepts 1. 2. 3. Define the term lithospheric plate. List the three types of plate boundaries. What theory proposes that Earth’s outer shell consist of a number of rigid slabs? In Wegener’s hypothesis, the continents moved through the ocean floor. The ocean floor did not move and was not part of the block of continental crust. The boundaries of the continents were defined by either the shorelines or continental shelves. In plate tectonics, the plates are divided by boundaries along which different types of motion and deformation occur. The ocean floors are part of the plates and move along with the continents. 5. Drawing Conclusions What is the major difference in the role of the ocean floor between the continental drift hypothesis and the theory of plate tectonics? Critical Thinking 4. Comparing and Contrasting Compare the plate motions in the three types of boundaries. Plate Boundaries Use what you have learned about plate tectonics to compare Wegener’s continental drift hypothesis to the theory of plate tectonics. Plate Tectonics Section 9.2 Assessment 1. a section of the crust and upper mantle (the lithosphere) that moves as a unit 2. convergent, divergent, and transform fault boundaries 3. the theory of plate tectonics 4. In convergent boundaries, the two plates move together. In divergent boundaries, the two plates move apart. In transform fault boundaries, the two plates grind past each other. 255 5. In the continental drift hypothesis, the continents plowed through the ocean floors; in the plate tectonics theory, the ocean floors are an integral part of the lithospheric plates and move with the continents. Answer to . . . A theory that states that Earth’s rigid outer shell is broken into plates made up of the crust and upper mantle, also known as the lithosphere. A plate moves as a unit with respect to the surrounding plates. Plate Tectonics 255 HSES_1eTE_C09.qxd 5/16/04 10:11 AM Page 256 Section 9.2 (continued) Earth’s Tectonic Plates North American plate Answer Locate South American plate. Divergent: Caribbean plate, Mid-Atlantic Ridge; Southeast Indian Ridge, Antarctic plate. Convergent: Nazca plate, South American plate; Australian-Indian plate, Eurasian plate. Transform fault: Antarctic plate, Pacific plate; Caribbean plate, North American plate Use Visuals Urals Aleutian Arc Eurasian plate Baikal Rift Japan Arc Himalayas Arabian plate L1 M id African plate st we uth So Pacific plate ian Ridge Figure 8 Have students examine the figure. They may need help from an atlas or a globe to locate the features in the questions. Ask: How do you think the Andes Mountains were formed? (The Nazca plate collided with the South American plate.) How do you think the Red Sea was formed? (The African plate and the Arabian plate moved apart, forming a rift that became the Red Sea.) Which plate is the largest? Where is it located? (The Pacific plate; it is mostly within the Pacific Ocean.) Visual d -In East African Rift Philippine plate India Em per or-H awa iian Cha in Mariana Arc In an di g Rid Australian-Indian plate e So uth e as t I nd ian Ridge Tonga Arc Kermadec Arc Alpine Fault Antarctic plate Figure 8 Location None of the A Convergent Model L2 Purpose Students will observe what happens when two plates collide in a model of a convergent boundary. Materials two slabs of modeling clay, wax paper Procedure Place the slabs of clay on the wax paper on a table so they will slide easily. Push the two slabs of clay together to model a collision of two plates. Expected Outcome The clay slabs will buckle up to create folds and breaks that resemble mountains. Kinesthetic, Visual plates are defined entirely by the margins of a continent. Over a dozen smaller plates have been identified but are not shown. Locate Find a major plate that includes an entire continent plus a large area of seafloor. Name two other examples of a divergent boundary, a convergent boundary, and a transform fault boundary. Seven Major Plates Intermediate Plates North American Eurasian Caribbean South American Australian-Indian Nazca Cocos Pacific Antarctic Philippine Scotia African 256 Chapter 9 Customize for English Language Learners Students who are learning disabled will benefit by having globes in the classroom. You might want to mark the major plates on a globe so 256 Chapter 9 students can better visualize how Earth’s lithosphere is divided into tectonic plates. Arabian HSES_1eTE_C09.qxd 5/16/04 10:12 AM Page 257 L2 Students may think that the movement of lithospheric plates causes Earth’s surface area to become either larger or smaller (depending on which way the plates move.) Explain that individual plates can become larger or smaller, but Earth’s total surface area cannot change. Give students a basketball or soccer ball and ask them to imagine what will happen to the interior of the ball if the surface area changed. (If surface area decreased, the interior would become compressed and internal pressure would increase. If surface area increased, either the interior would expand or the lithosphere would separate from the mantle.) Visual, Logical North American plate Iceland Eurasian plate Canadian Shield c Ro ky ou M Juan de Fuca plate ins nta Basin and Range lac pa Ap San Andreas Fault nM hia Alps ts. Caribbean plate African plate dMi Cocos plate Pacific plate tic R idge South American An plate de s ountains Nazca plate lan Antilles Arc M East Pacific Rise Galapagos Ridge At Use Community Resources L2 Invite a geologist or physicist in your community to talk to the class about global positioning systems (GPS), how they work, and what they are used for. Encourage students to think about and ask questions about how GPS can be used to measure the movements of landmasses. Verbal, Interpersonal Chile Ridge Scotia plate Antarctic plate Plate Tectonics 257 Facts and Figures The continents are still moving, and eventually they will probably collide to form a single landmass again. Earth scientists predict that the continents will probably merge again somewhere in the Pacific Ocean. When will this happen? Research suggests that, based on the current rate of plate movements, a single landmass is formed about once every 500 million years. Since it has been about 200 million years since Pangaea broke up, the next supercontinent may form in a few hundred million years. Plate Tectonics 257