Final PDF to printer CHAPTER 10 Plate Tectonics and Regional Features THE SURFACE OF EARTH IS NOTABLE for its dramatic mountains, beautiful valleys, and intricate coastlines. Beneath the sea are unexpected features, such as undersea mountain ranges, deep ocean trenches, and thousands of submarine mountains. In this chapter, we examine the distribution of these features, along with the locations of earth- quakes and volcanoes, to explore the theory of plate tectonics. These images of the world show large topographic features on the land, colored using satellite data that show areas of vegetation in green and areas of rocks and sand in tan. Colors on the seafloor indicate depths below sea level, ranging from light blue for seafloor that is at relatively shallow depths to dark blue for seafloor that is deep. 10.00.a1 The seafloor west of North America displays a long, fairly straight fracture, named the Mendocino Fracture Zone, that trends east-west and ends abruptly at the coastline. North of this fracture, a ridge called the Juan de Fuca Ridge zigzags across the seafloor. What are these features on the seafloor and how did they form? South America has two very different sides. The mountainous Andes parallel the western coast, but a wide expanse of lowlands, including the Amazon Basin, makes up much of the rest of the continent. The western edge of the continent drops steeply into the Pacific Ocean and is flanked by a deep trench. The eastern edge of the continent continues well beyond the shoreline and forms a broad bench covered by shallow waters (shown in light blue). Why are the two sides of the continent so different? A huge mountain range, longer than any on land, is hidden beneath the waters of the Atlantic Ocean. The part of the range shown here is halfway between South America and Africa. The ridge zigzags across the seafloor, mimicking the shape of the two continents. What is this underwater mountain range, and why is it almost exactly in the middle of the ocean? 306 rey42432_ch10_306-343.indd 306 10/11/16 01:38 PM Final PDF to printer Plate Tectonics and Regional Features 307 TOPICS IN THIS CHAPTER 1 0.1 What Is Inside Earth? 308 10 .8 What Happens Along Transform Boundaries? 322 10 .2 What Are the Major Features of Earth? 31 0 10 .9 Why and How Do Plates Move? 324 10 .3 Why Do Some Continents Have 10 .10 How Is Paleomagnetism Used to Determine Matching Shapes? 31 2 Rates of Seafloor Spreading? 326 10 .4 What Is the Distribution of Earthquakes, 1 0.1 1 What Features Form at Oceanic Hot Spots? 328 Volcanoes, and Mountain Belts? 31 4 1 0.1 2 What Features Form at Continental Hot Spots? 330 10 .5 What Causes Tectonic Activity to Occur 1 0.1 3 What Are Continents and How Do They Form? 332 in Belts? 31 6 1 0.1 4 How Did the Continents Join and Split Apart? 334 10 .6 What Happens at Divergent Boundaries? 31 8 1 0.1 5 Where Do Mountain Belts and High 1 0.7 What Happens at Convergent Boundaries? 320 Regions Form? 336 1 0.1 6 How Do Internal and External Processes Interact to Form Landscapes? 338 1 0.1 7 CONNECTIONS: Why Is South America Lopsided? 340 1 0.1 8 INVESTIGATION: What Is the Plate Tectonics of This Place? 342 The Tibetan Plateau of southern Asia rises four or five kilometers above the lowlands of India and Bangladesh to the south. The Himalaya mountain range with Mount Everest, the highest mountain on Earth, is perched on the southern edge of this plateau. Why does this region have such a high elevation, and to what processes does it owe its existence? Japan lies along the intersection of large, curving ridges mostly submerged beneath the ocean. Each ridge is flanked to the east by a deep trench in the seafloor. This area is well known for its destructive earthquakes and for Japan’s picturesque volcano, Mount Fuji. Do submarine ridges and trenches play a role in earthquake and volcanic activity? The Arabian Peninsula provides much of the world’s oil. East of the peninsula, the Persian Gulf has a shallow and smooth seafloor and is flanked by the world’s largest oil fields. West of the peninsula, the Red 10 .0 Sea has a well-defined fissure-like feature down its center. How did the Red Sea form, and what processes are causing its seafloor to be disrupted? rey42432_ch10_306-343.indd 307 10/11/16 01:38 PM Final PDF to printer 308 1 0.1 What Is Inside Earth? UNDERSTANDING EARTH’S SURFACE requires knowing what is inside the Earth. You can directly observe the surface of Earth, but what is down below, in the subsurface? Earth consists of concentric layers that have different compositions. The outermost layer is the crust, which includes continental crust and oceanic crust. Beneath the crust is the mantle, Earth’s most voluminous layer. The molten outer core and the solid inner core are at Earth’s center. How Does Earth Change with Depth? 10.01.a1 10.01.a2 1. Continental crust has an average composition similar to this granite (⊳ ). Continental crust, the thin, light-gray layer on the figure to the right, averages 35 to 40 km (20 to 25 mi) in thickness. 2. Oceanic crust exists beneath the deep oceans and has an average composition that is the same as basalt, a common dark lava rock (▼). Oceanic crust has an average thickness of about 7 km (4 mi), which is much thinner than can be shown here (the barely visible dark-gray layer). 10.01.a3 Grants, NM 3. The mantle extends from the base of the crust down to a depth of 2,900 km (1,800 mi). Much of the upper mantle is composed of the green mineral olivine, as exposed in the center of this rock (▼) brought to the surface in a volcano. 4. The lower mantle has a composition similar to the upper 10.01.a4 Durango, Mexico mantle, but it contains minerals formed at very high pressures. Nearly all of the mantle is solid, not molten. High tempera- tures cause some parts to be partially molten, while other parts flow because they are weak solids. 5. Based on studies of earthquakes, observations of meteorites, and models for the density of Earth, geoscientists interpret the core to consist of metallic iron and nickel, such as that observed in iron-nickel meteorites (⊳ ). The outer core is molten, but the inner core is solid. 10.01.a5 rey42432_ch10_306-343.indd 308 10/11/16 01:38 PM Final PDF to printer Plate Tectonics and Regional Features 309 Are Some Layers Stronger than Others? In addition to layers with different compositions, Earth has layers that are defined by strength and by how easily the material in the layers fractures or flows when subjected to forces. 1. The uppermost mantle is relatively strong and solidly attached to the overlying crust. The crust and uppermost mantle together form a rigid upper rigid layer called the lithosphere (lithos means “stone” in Greek), which averages about 100 km (about 60 mi) in thickness. The part of the uppermost mantle that is in the lithosphere is the lithospheric mantle. 2. The mantle directly beneath the lithosphere is mostly solid, but it is hotter than the rock above and can flow under pressure. This part of the upper mantle, called the astheno- sphere, functions as a soft, weak zone under which the lithosphere moves. The word asthenosphere is from a Greek term for “not strong.” In most regions, the asthenosphere 10.01.b1 is approximately 80 km to more than 150 km thick, so it can be deeper than 250 km. Why Do Some Regions Have High Elevations? Why is the Gulf Coast of Texas near sea level, while the Colorado mountains are 3 to 5 km (2 to 3 mi) above sea level? Why are the continents mostly above sea level, but the ocean floor is below sea level? The primary factor controlling the elevation of a region is the thickness of the underlying crust. 1. The crust is 2. The thickness of continental crust ranges from less than 25 km 3. The crust beneath low-elevation regions like the less dense than (16 mi) to more than 60 km (37 mi). Regions that have high elevation Gulf Coast of Texas is thinner. If the crust is thinner the underlying generally have thick crust. The crust beneath the Rocky Mountains of than 30 to 35 km (18 to 20 mi), the area will mantle, and so it Colorado is commonly more than 45 km (28 mi) thick. probably be below sea level, but it can still be part rests, or floats, on of the continent. top of the mantle. 4. Most islands are volcanic mountains built on The underlying oceanic crust, but some are small pieces of litho spheric continental crust. mantle is mostly solid, not liquid. 5. Oceanic crust is thinner than continental crust and consists of denser rock than continental crust. 10.01.c1 As a result, regions underlain only by oceanic crust are well below sea level. Density and Isostasy he relationship between regional eleva- crustal roots. As in the case of the floating If the lithosphere in some region is heated, it tion and crustal thickness is similar to that wooden blocks, most of the change in crustal expands, becoming less dense, and so the region of wooden blocks of different thicknesses thickness occurs at depth and less occurs near the rises in elevation.