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8. Earth's Moving Plates Many of the important discoveries about the structure of the earth were made by scientists who found evidence in the oceanic crust for regular patterns of crustal movement. As we will see, these movements are thought to be related to convection currents in the mantle. An example of a convection current in liquids is shown in Fig. 8-1. In the beaker, hot water rises at the point where heat is applied, goes to the surface, then spreads out and cools. Cooler liquid sinks to the bottom at places where there is less heat. The rising up, spreading out, and sinking of gas, liquid, or molten material produces convection currents. Fig. 8-1. liquid 62 Convection currents in a beaker of Cracks in the Earth's Crust The solid crust acts as a heat insulator for the hot interior of the earth. Below the crust, in the mantle, is the molten material called magma. Tremendous heat and pressure within the earth cause the hot magma to flow in convection currents. Periodically it rises to the surface, cracks the crust, erupts as lava, steam, or ash, and releases heat. The lava cools to form new rock on the oceanic or continental crust. A worldwide system of great cracks in the oceanic and continental crust has been mapped. These cracks divide the crust into rigid plates, huge sections of the earth that move relative to each other. See Fig. 8-2. Movements of the Earth's Plates There are four major kinds of movements of the plates in the earth's crust: 1. subduction, when one plate plunges beneath another 2. collision, when two continental plates are shoved together 3. seafloor spreading, when two plates are pushed apart 4. transform faulting (also called transverse fracturing), when two plates slide past each other Subduction is the downward movement of an oceanic plate into the mantle. Locate the Nazca and South American plates (Fig. 8-2), then follow the process of subduction shown in Fig. 8-3. The Nazca Plate plunges under the South American Plate at a rate of 2 to 3 em per year. As the • • 1 Rift valley system ~ Ridge WI.WJ. Subduction zones A Principal vo lcanoes .~ J:J ANTARCTIC PLATE f;j Fig. B-2. The earth's plate system pl e UNIT 1. EARTH AND OCEAN BASINS Fig. 8-3. Subduction of the Nazca Plate below the South American Plate forming composite volcanoes e oceanic crust enters the mantle, pressure breaks the crustal rock, heat from friction melts it, and a pool of magma develops. This thick magma, called andesite lava, consists of a mixture of basalt from oceanic crust and granite from continental crust. Forced by tremendous pressure, it eventually flows along weaker crustal channels toward the surface. The magma periodically breaks through the crust to form great, violently explosive composite volcanoes, steep-sided, cone-shaped mountains like those in the Andes at the margin of the South American Plate. Ocean trenches form at the regions where one plate moves downward beneath another. These trenches are deep (up to 10.8 km), narrow (about 100 krn), and long (from 800 to 5,900 km) , with very steep sides. Places where subduction occurs are also sites of deep earthquakes caused by rocks slipping over other rocks deep in the mantle. 64 Subduction pulls the seafloor steadily downward , as if it were a giant conveyor belt. Continental collision occurs if two plates carrying continents collide and the subduction is interrupted. Because continental crusts are composed of low-density material, they do not sink. So when the continents collide, the crust moves upward , and the crustal material folds, buckles, and breaks. Many of the great mountain ranges were formed by the collision of continents. Fig. 8-4 illustrates the continuous process of plate movement over very long periods of time. (A) shows how a subduction zone forms when oceanic crust slides under continental crust. (B) shows how the collision of two continental crusts interrupts the subduction process and forms a new mountain chain. (C) shows how the oceanic crust continues sliding under the continental crust, forming a new subduction zone and a new submarine trench, and how two continental crusts begin to fuse. A subduction zone (ocean trench) c ocean new subduction zone (ocean trench) ocean Fig. 8-4. Subduction and continental collision 65 UNIT 1. EARTH AND OCEAN BASINS Seafloor Spreading and Transform Faults While one boundary of an oceanic plate is being pushed down into the mantle by subduction, new material is coming up at the opposite boundary by a process called seafloor spreading. See Fig. 8-5. Great cracks develop in the ocean floor where hot magma rises to the crustal surface and forms midocean ridges or rises. As the lava cools, it forms new seafloor features such as rift valleys, seamounts (volcanic peaks that extend more than I km above the seafloor) , and abyssal hills (volcanic peaks that extend less than I km above the seafloor). The outpouring of material continues as the oceanic plates pull apart. Crustal movement related to this spreading causes frequent shallow earthquakes with distinct seismic wave patterns. In fact, earthquake data were used to help map active oceanic ridge systems. Today we know that midocean ridges and rises are the largest continuous features on the earth. They are tens of thousands of kilometers long, running through and connecting most of the ocean basins. Oceanographic data reveal that seafloor spreading is slowly widening the Atlantic Ocean, the Red Sea, and the Gulf of California. Similar spreading on land is also causing a large rift valley in Africa that may eventually split that continent. As we might expect, spreading does not occur evenly along a ridge. Perpendicular breaks or fracture zones, called transform faults, occur when the sections of the plates slip by each other and displace segments of the midocean ridges. See Fig. 8-5. Great tension can build up before slippage occurs and causes shallow earthquakes. People living near the San Andreas Fault, a transform fault in California, regularly experience such quakes. ' I rift valley abyssaI hII s '" Fig. 8-5. Seafloor spreading and the formation of transform faults 66 Topic 8. Earth's Moving Plates ACTIVITY Interpret information about the earth's moving plates from the map in Fig. 8-2. MATERIALS • pencil • crayons or colored pencils (red, green, yellow, orange) • world map 5. Follow the areas of seafloor spreading around the map. What do you notice about the relationship between them and how far around the earth they extend? 6. Why are the lines representing the midocean ridges jagged rather 'than smooth? What type of movement explains this irregularity? PROCEDURE 1. On the map in Fig. 8-2, put a star on the plate where you are now. 2. Locate ridges (the areas of seafloor spreading) and color them green. 7. Locate the areas of seafloor spreading at the boundaries of the Pacific Plate. In what direction does spreading seem to be driving the plate? 8. Mount St. Helens, a volcano in Wash3. Locate areas of subduction and color them red. 4. Locate the areas where composite volcanoes are occurring and color them yellow. 5. Find Iceland and the region where the Mid-Atlantic Ridge comes to the surface and pushes the Eurasian and North American plates apart. In this area volcanic activity and earthquakes happen at the surface. Color this region orange. ington state, erupted violently a few years ago. What might account for the eruption of this mountain? How was the volcano probably formed? 9. Locate the island arcs that form the Aleutian Islands, Japan, and the Philippines. What processes might have formed them? Explain. 10. In what direction does the Indo-Australian Plate seem to be moving? Describe what is happening at the northern boundary of this plate. QUESTIONS 1. How many plates are there? 2. Which plate are you on? 3. Which plates, if any, carry no continents? 4. What kinds of structures are found at the boundaries of plates? 11. Inspect Fig. 8-2 to find other locations where continents have collided. What mountain ranges were formed? 12. San Francisco is approximately 10,000 km from Tokyo. The Japan Trench is gobbling up crust at the rate of 3 em a year. How many years will it take until San Francisco collides with Tokyo? 67 UNIT 1. EARTH AND OCEAN BASINS 13. Scientists use the term plate tectonics to refer to the theory that major structural features on the earth's surface form as crustal plates move. How could convection currents of materials within the earth be related to the theory of plate tectonics? Hot Spots Some volcanoes form over stationary hot spots in the middle of plates. See Fig. 8-6. Magma flowing from these volcanoes is called basalt Java. It is high in iron and magnesium. This lava flows like hot, thick syrup, gradually forming shield volcanoes shaped like domes with gently sloping sides. These volcanoes are much less explosive than the composite volcanoes formed at regions of subduction. Some shield volcanoes begin on the ocean floor and, with repeated eruptions, grow slowly until they reach the surface of the water and Fig. 8-6. Formation of volcanic islands 68 form islands. Peaks of some of these islands reach 3.6 km above sea level in regions where the ocean is 7 kID deep, making them the largest mountains on earth as measured from their bases on the seafloor to their summits. Almost all ofthe mid-Pacific and mid-Atlantic islands formed in this way. Over time, as the plate moves or twists, a volcano that was over the hot spot moves away, ceases to erupt, and becomes extinct. New islands in the island chain (archipelago) form as other parts of the oceanic crust move over the hot spot. See Fig . 8-6. Erosion (wearing away) and subsidence (sinking of the earth 's crust) eventually cause older islands to sink below sea level. Atolls, groups of islands that ring a shallow lagoon, form when coral reefs grow on top of submerged volcanoes. Found even deeper are guyots, which are extinct, flattopped, eroded volcanoes 1,800 to 3,000 m below the ocean surface.