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Plate Tectonics: Evolution of the Ocean Floor

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Plate Tectonics: Evolution of the Ocean Floor CHAPTER 4 Plate Tectonics: Evolution of the Ocean Floor CRITICAL CONCEPTS USED IN THIS CHAPTER CC1 Density and Layering in Fluids CC2 Isostasy, Eustasy, and Sea Level CC3 Convection and Convection Cells CC7 Radioactivity and Age Dating CC11 Chaos CC14 Photosynthesis, Light, and Nutrients An eruption of Pu‘u O‘o, the active vent of the Kilauea Volcano in Hawaii. This hot-spot volcano erupts relatively smoothly, but it was ejecting cin- ders and ash into the atmosphere that were “raining” on the airplane windshield like a hailstorm when this photograph was taken. The shiny surface below the vent mouth is a crust of solidified lava that conceals a rapidly flowing molten river of lava.The river of lava extended several kilometers down the slopes of Kilauea, where it destroyed several homes shortly after this photograph was taken. The ocean floor has irregular and complextopographic experience. The global-scale processes that continuously reshape features—including mountain ranges, plains, depressions, and the face of the planet are, as yet, far from fully understood. How- plateaus—that resemble topographic features on land (Fig. 3-3). ever, we do know that these processes, called plate tectonics, During the period of human history, those features have remained originate deep within the Earth. essentially unchanged aside from local modifications due toero - Layered Structure of the Earth sion, sedimentation, volcanic eruptions, and coral reef forma- tion. However, the human species has existed for only a very The Earth consists of a spherical central core surrounded by brief period of the planet’s history (Fig. 1-5). In the billions of several concentric layers of different materials (Fig. 4-1). The years before humans appeared, the face of the Earth was reshaped layers are arranged by density, with the highest-density material a number of times. at the Earth’s center and the lowest-density material forming the outer layer, the Earth’s crust. This arrangement came about early STRUCTURE OF THE EARTH in the Earth’s history, when the planet was much hotter than it is The continents and ocean basins are not permanent. Instead, today and almost entirely fluid. The densest elements sank toward the location, shape, and size of these features are continuously the Earth’s center, and lighter elements rose to the surface (CC1). changing, albeit imperceptibly slowly on the timescale of human The Earth’s center is still much hotter than its surface. Heat is 60 CHAPTER 4: Plate Tectonics: Evolution of the Ocean Floor Lithosphere FIGURE 4-1 A cross section of the Earth 5–90 km showing its layers. Note that the thickness Asthenosphere 100 km of the lithosphere has been greatly exag- gerated in this diagram. If it were drawn 250 km to the correct scale, the lithosphere would 2900 km appear as just a thin line at the Earth’s Lower 5150 km surface. mantle 6380 km Sea level Ocean Continental Oceanic crust re crust Upper mantle Lithosphe Average density of upper mantle, asthenosphere, Average density and lower mantle 4.5 g . cm-3 of inner core Average density of crust Average density -3 of outer core 13 g • cm 2.8 g • cm-3 9 –10 g • cm-3 generated continuously within the Earth, primarily by the decay outermost layer of the Earth, which varies from just a few kilo- of radioisotopes (CC7). meters in thickness at the oceanic ridges to 100 to 150 km in the The core, which is about 7000 km in diameter, is composed older parts of the ocean basins and up to 250 to 300 km under primarily of iron and nickel, and is very dense. It consists of the continents. The lithosphere consists of the mostly rigid outer a solid inner core and a liquid outer core. The mantle, which shell of the mantle plus the solid crust that lies on the mantle. surrounds the core, is composed of material that is about half The lithosphere is less dense than the asthenosphere and essen- as dense as the core. The upper mantle, known as the asthe- tially floats on top of the plastic asthenosphere. The oceans and nosphere, is thought to consist of material that is very close to atmosphere lie on top of the lithosphere. Pieces of lithosphere are its melting point and “plastic,” so it is capable of flowing very rigid, but they move across the Earth’s surface and in relation to slowly without fracturing. The best examples of such materials in each other as they float on the asthenosphere. They can be many everyday life are glass and glacial ice. Although glass appears to thousands of kilometers wide, but they are generally less than be a solid, examination of glass windowpanes that have been in 200 km thick. Hence, being platelike, they are called lithospheric place for centuries reveals that the glass is thicker at the bottom plates (or “tectonic plates” or just “plates”). Processes that occur than at the top. The glass is flowing slowly downward in response where the plates collide, where they move apart, or where plates to gravity. Similarly, ice within glaciers also flows slowly. Much “slide” past each other, are the principal processes that create the of the deeper mantle that generally appears solid, is also thought mountains and other surface features of the continents and the to be capable of flowing very slowly. The ability to flow is a ocean floor. critically important factor in the tectonic processes shaping the Lithosphere, Hydrosphere, and Atmosphere Earth’s surface. Relative to the Earth, the lithosphere is thin, rather like the The asthenosphere is surrounded by the lithosphere, the skin on an apple (Fig. 4-1). At the top of this thin layer are the Continental Mountain Sediment margin belt FIGURE 4-2 Structure of the lithosphere. Sea level Continental crust is typically 35 to 40 km 0 thick, whereas oceanic crust is typically Water only 6 to 7 km thick. Continental crust has a lower density than oceanic crust, but 10 Oceanic crust both continental and oceanic crust have Continental crust Average density = (2.9 g • cm–3) a considerably lower density than upper Average density = (~2.8 g • cm–3) mantle material. 20 Moho Upper mantle –3 30 Average density = (~3.3 g • cm ) Depth below sea level (km) 40 CHAPTER 4: Plate Tectonics: Evolution of the Ocean Floor 61 130 260 390 520 FIGURE 4-3 Relative +10 +10 heights of parts of the Mt. Everest Highest mountain 8848 m +8 +8 Earth’s surface. The average (8.84 km) Average elevation of elevation of the continents +6 exposed land +6 is 841 m above sea level, 29% 841 m Land area and the average depth of the +4 land +4 139 million km2 Total area of Earth surface oceans is 3865 m. Almost Elevation (km) 500 million km2 30% of the Earth’s surface is +2 +2 above sea level. Much of the Sea 0 0 surface of the continental level Two Average elevation of the crust is below sea level. Earth surface – 2070 m –2 –2 –4 71% –4 ocean Area of oceans and marginal –6 –6 2 Average depth seas 360 million km Depth (km) –8 Mariana of oceans –8 Ocean trenches Trench 3865 m –10 (11.03 km) –10 Greatest ocean depth 11,033 m 0 130 260 390 520 0102030 Surface area of the Earth in millions of square kilometers Earth's surface (%) mountains, ocean basins, and other features of the Earth’s sur- terized by low, rolling abyssal hills. face. From the deepest point in the ocean to the top of the highest A layer of sediment lies on top of the oceanic crustal rocks, mountain is a vertical distance of approximately 20 km. This 20- constituting part of the crust. The thickness of the sediment is km range is very small compared to the Earth’s radius, which is highly variable, for reasons discussed in this chapter and Chapter more than 6000 km. Consequently, the planet is an almost smooth 6. sphere (from space it looks smoother than the skin of an orange) The hydrosphere consists of all water in the lithosphere that on which the mountain ranges and ocean depths are barely per- is not combined in rocks and minerals— primarily the oceans and ceptible. the much smaller volume of freshwater (Table 5-1). The oceans There are two types of crust—oceanic and continental—both cover all the oceanic crust and large areas of continental crust of which have a substantially lower density than the upper mantle around the edges of the continents—all of which total more than material on which they lie. Oceanic crust has a higher density two-thirds of the Earth’s surface area. than continental crust (Fig. 4-2). According to the principle of The atmosphere is the envelope of gases surrounding the isostasy (CC2), lithospheric plates float on the asthenosphere at lithosphere and hydrosphere and is composed primarily of nitro- levels determined by their density. Consequently, if the continen- gen (78%) and oxygen (21%). Although these gases have much tal and oceanic crusts were of equal thickness, the ocean floor lower densities than liquids or solids, they are dense enough to be would be lower than the surface of the continents. However, oce- anic crust is much thinner (typically 6–7 km thick) than continen- Continental Shelf Continental shelf break tal crust (typically 35–40 km), which causes an additional height slope difference between the surface of the continents and the ocean floor (Fig. 4-3). The density difference between continental and oceanic crust is due to differences in their composition.
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