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Multimedia Manager A Microsoft® PowerPoint® Link Tool for Oceanography An Invitation to Marine Science 6th Edition by Tom Garrison http://oceanography.brookscole.com/garrison6e Chapter 3 Earth Structure and Plate Tectonics Look For The Following Key Ideas In Chapter 3 Some seismic waves–energy associated with earthquakes–can pass through Earth. Analysis of how these waves are changed, and the time required for their passage, has told researchers much about conditions inside Earth. Earth is composed of concentric spherical layers, with the least dense layer on the outside and the most dense as the core. The lithosphere, the outermost solid shell that includes the crust, floats on the hot, deformable asthenosphere. The mantle is the largest of the layers. Large regions of Earth’s continents are held above sea level by isostatic equilibrium, a process analogous to a ship floating in water. Plate motion is driven by slow convection (heat-generated) currents flowing in the mantle. Most of the heat that drives the plates is generated by the decay of radioactive elements within Earth. Plate tectonics theory suggests that Earth’s surface is not a static arrangement of continents and ocean, but a dynamic mosaic of jostling segments called lithospheric plates. The plates have collided, moved apart, and slipped past one another since Earth’s crust first solidified. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Key Ideas Continued… The confirmation of plate tectonics rests on diverse scientific studies from many disciplines. Among the most convincing is the study of paleomagnetism, the orientation of Earth’s magnetic field frozen into rock as it solidifies. Most of the large-scale features seen at Earth’s surface may be explained by the interactions of plate tectonics. Plate tectonics also explains why our ancient planet has surprisingly young seafloors, the oldest of which is only as old as the dinosaurs – that is, about 1/23 of the age of Earth. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Earth’s Interior Is Layered Inside Density is a key concept for understanding the structure of Earth. Density measures the mass per unit volume of a substance. Density = _Mass_ Volume Density is expressed as grams per cubic centimeter. Water has a density of 1 g/cm3 © 2006 Brooks/Cole, a division of Thomson Learning, Inc. The Study of Earthquakes Provides Evidence for Layering What evidence supports the idea that Earth has layers? The behavior of seismic waves generated by earthquakes give scientists some of the best evidence about the structure of Earth. Low frequency pulses of energy generated by the forces that cause earthquakes can spread rapidly through Earth in all directions and then return to the surface. (a) Earthquake waves passing through a homogenous planet would not be reflected or refracted (bent). The waves would follow linear paths (arrows). (b) In a planet that becomes gradually denser and more rigid with depth, the waves would bend along evenly curved paths. (c) P Waves (compressional waves) can penetrate the liquid outer core but are bent in transit. A P-wave shadow zone forms between 103 and 143 from an earthquake’s source. (d) Our earth has a liquid outer core through which the sideto-side S waves cannot penetrate, creating a large “shadow zone” between 103 and 180 from an earthquake’s source. Very sensitive seismographs can sometimes detect weak Pwave signals reflecting off the solid inner core. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Earth’s Inner Structure Was Gradually Revealed A cross section through Earth showing the internal layers. Note: in the expanded section the relationship between lithosphere and asthenosphere, and between crust and mantle. The thin oceanic crust is primarily basalt, a heavy dark-colored rock. The thicker continental crust is granite, a speckled rock. The mantle is thought to consist mainly of oxygen, iron, magnesium, and silicon. The outer and inner core consist mainly of iron and nickel. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Earth’s Layers May Also Be Classified by Physical Properties Layer Chemical Properties Continental Crust Composed primarily of granite Density = 2.7 g/cm3 Oceanic Crust Composed primarily of basalt Density = 2.9 g/cm3 Mantle Composed of silicon, oxygen, iron, and magnesium Density = 4.5 g/cm3 Core Composed primarily of iron Density = 13 g/cm3 Note that Earth is density stratified, that is, each deeper layer is denser than the layer above. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Earth’s Layers May Be Classified by Composition Layer Physical Properties Lithosphere Cool, rigid, outer layer Asthenosphere Hot, partially melted layer which flows slowly Mantle Denser and more slowly flowing than the asthenosphere Outer Core Dense, viscous liquid layer, extremely hot Inner Core Solid, very dense and extremely hot When we examine the chemical and physical properties of Earth’s layers, we see that a cool, rigid, less dense layer (the lithosphere) floats on a hot, slowlyflowing, more dense layer (the asthenosphere). © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Isostatic Equilibrium Supports Continents above Sea Level Think for a moment about the lithosphere. Why doesn’t it sink into the asthenosphere? How are features such as mountains supported? Earth’s continental crust and lithosphere are supported on the on the denser asthenosphere. Instead of buoyancy, the term isostatic equilibrium describes the way the lithosphere is supported on the asthenosphere. (Right) The concept of buoyancy is illustrated by a ship on the ocean. The ship sinks until it displaces a volume of water equal to the weight of the ship and its cargo. Icebergs sink into water so that the same proportion of their volume (about 90%) is submerged. The more massive the iceberg, the greater this volume is. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Isostatic Equilibrium Supports Continents above Sea Level Erosion and isostatic readjustment can cause continental crust to become thinner in mountainous regions. (a) A mountain range soon after its formation. (b) As mountains are eroded over time, isostatic uplift causes their roots to rise. The same thing happens when a shi is unloaded or an iceberg melts. (c) Further erosion exposes rocks that were once embedded deep within the peaks. Deposition of sediments away from the mountains often causes nearby crust to sink. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. The Age of Earth Was Controversial and Not Easily Determined The age of Earth has been subject to debate. Scientists now use an age of 4.6 billion years. The principle of uniformitarianism was introduced in 1788. This principle sates that the forces which shaped Earth are identical to forces working today. Catastrophism is the thought that Earth is very young, and events described in the Bible are responsible for the appearance of Earth’s features. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. The Fit between the Edges of Continents Suggested That They Might Have Drifted The fit of all the continents around the Atlantic at a water depth of about 137 meters (450 feet), as calculated by Sir Edward Bullard at the University of Cambridge in the early 1960s. This widely reproduced graphic was an effective stimulus to the tectonic revolution. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. The Fit between the Edges of Continents Suggested That They Might Have Drifted Alfred Wegener’s theory of continental drift was out of favor with the scientific community until new technology provided evidence to support his ideas. • Seismographs revealed a pattern of volcanoes and earthquakes. • Radiometric dating of rocks revealed a surprisingly young oceanic crust. • Echo sounders revealed the shape of the Mid-Atlantic Ridge © 2006 Brooks/Cole, a division of Thomson Learning, Inc. A Synthesis of Continental Drift and Seafloor Spreading Produced the Theory of Plate Tectonics The ideas of continental drift and seafloor spreading were tied together in the theory of plate tectonics. Main points of the theory include: • Earth’s outer layer is divided into lithospheric plates • Earth’s plates float on the asthenosphere • Plate movement is powered by convection currents in the asthenosphere seafloor spreading, and the downward pull of a descending plate’s leading edge. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. A Synthesis of Continental Drift and Seafloor Spreading Produced the Theory of Plate Tectonics Where does the heat within Earth’s layers come from? Heat from within Earth keeps the asthenosphere flowing. This allows the lithosphere to keep moving. Most of the heat that drives the plates is generated by radioactive decay, given off when nuclei of unstable elements break apart. (Left) The tectonic system is powered by heat. Some parts of the mantle are warmer than others, and convection currents form when warm mantle material rises and cool material falls. Above the mantle floats the cool, rigid, lithosphere, which is fragmented into plates. Plate movement is powered by gravity: The plates slide down the ridges at the places of their formation; their dense, cool leading edges are pulled back into the mantle. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Most Tectonic Activity Occurs at Plate Boundaries Seismic events worldwide, January 1977 through December 1986. The locations of about 10,000 earthquakes are colored red, green, and blue to represent event depths of 0-70 kilometers, 70-300 kilometers, and below 300 kilometers. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Most Tectonic Activity Occurs at Plate Boundaries The major lithospheric plates, showing their directions of relative movement and the location of the principal hot spots. Most of the million or so earthquakes and volcanic events each year occur along plate boundaries. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Most Tectonic Activity Occurs at Plate Boundaries (Above) Plate boundaries in action. As Plate A moves to the left (west), a gap forms behind it (1) and an overlap with Plate B forms in front of (2). Sliding occurs along the top and bottom sides (3). The margins of Plate A experience the three types of interactions: At (1), extension characteristic of divergent boundaries; at (2), compression characteristic of convergent boundaries; and at (3), the shear characteristic of transform plate boundaries. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Most Tectonic Activity Occurs at Plate Boundaries The lithospheric plates interact with the neighboring plates in several ways. (1) Divergent, (2) Convergent, (3) Transform. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Most Tectonic Activity Occurs at Plate Boundaries The lithospheric plates interact with the neighboring plates in several ways. Divergent plate boundaries – Boundaries between plates moving apart, further classified as: • • Divergent oceanic crust – for example, the Mid-Atlantic Ridge Divergent continental crust - for example, the Rift Valley of East Africa. (Right) Extension of divergent boundaries causes splitting and rifting. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Most Tectonic Activity Occurs at Plate Boundaries Convergent Plate Boundaries - Regions where plates are pushing together. (Below) Compression at convergent boundaries produces buckling and shortening. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Most Tectonic Activity Occurs at Plate Boundaries Transform plate boundaries - locations where crustal plates move past one another, for example, the San Andreas fault. (above) Translation at transform boundaries causes shear. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Ocean Basins Are Formed at Divergent Plate Boundaries Divergent plate boundaries – Boundaries between plates moving apart, further classified as: Divergent oceanic crust – for example, the Mid-Atlantic Ridge Divergent continental crust – for example, the Rift Valley of East Africa. (Figure to the Right) (a) As the lithosphere began to crack, a rift formed beneath the continent, and molten basalt from the asthenosphere began to rise. (b) As the rift continued to open, the two new continents were separated by a growing ocean basin. Volcanoes and earthquakes occur along the active rift area, which is the mid-ocean ridge. The East African Rift Valley currently resembles this stage. (c) A new ocean basin (shown in green) of the Atlantic. resembles this stage. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Warping, stretching of East Africa Continental crust Formation of rift valley Red Sea Stepped Art Fig. 3-17a-c, p. 76 Ocean Basins Are Formed at Divergent Plate Boundaries Seafloor spreading was an idea proposed in 1960 to explain the features of the ocean floor. It explained the development of the seafloor at the Mid-Atlantic Ridge. Convection currents in the mantle were proposed as the force that caused the ocean to grow and the continents to move. (Right) The Mid-Atlantic Ridge showing its conformance to the coastlines of the adjacent continents. The first inset shows a detail of the ridge generated from side-scan sonar data – the central rift is clearly visible. Red and orange colors indicate the crest of the ridge; dark blue, the deeper seabed on either side. The second insert shows the location of the ridge and associated valley in the Atlantic. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Ocean Basins Are Formed at Divergent Plate Boundaries The breakup of Pangaea shown in five stages beginning about 225 million years ago. (Note: Ma = mega-annum, indicating millions of years ago). Inferred motion of lithospheric plates is indicated by arrows. Spreading centers (midocean ridges) are shown in red. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Island Arcs Form, Continents Collide, and Crust Recycles at Convergent Plate Boundaries Convergent Plate Boundaries Regions where plates are pushing together can be further classified as: Oceanic crust toward continental crust - for example, the west coast of South America. Oceanic crust toward oceanic crust - occurring in the northern Pacific. Continental crust toward continental crust – one example is the Himalayas. (Above Right) A cross section through the west coast of South America, showing the convergence of a continental plate and an oceanic plate. The subducting oceanic plate becomes more dense as it descends, its downward slide propelled by gravity. At a depth of about 80 kilometers (50 miles), heat drives water and other volatile components from the subducted sediments into the overlying mantle, lowering its melting point. Masses of the melted material, rich in water and carbon dioxide, rise to power Andean volcanoes. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Island Arcs Form, Continents Collide, and Crust Recycles at Convergent Plate Boundaries (Above) The formation of an island arc along a trench as two oceanic plates converge. The volcanic islands form as masses of magma reach the seafloor. The Japanese islands were formed in this way. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Island Arcs Form, Continents Collide, and Crust Recycles at Convergent Plate Boundaries The distribution of shallow, intermediate, and deep earthquakes for part of the Pacific Ring of Fire in the vicinity of the Japan trench. Note that earthquakes occur only on one side of the trench, the side on which the plate subducts. The great Indonesian tsunami of 2005 was caused by these forces; the site of the catastrophic 1995 Kobe subduction earthquake is marked. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Island Arcs Form, Continents Collide, and Crust Recycles at Convergent Plate Boundaries (Above) A cross section through southern China, showing the convergence of two continental plates. Neither plate is dense enough to subduct; instead, their compression and folding uplift the plate edges to form the Himalayas. Notice the massive supporting “root” beneath the emergent mountain needed for isostatic equilibrium. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Crust Fractures and Slides at Transform Plate Boundaries (a) Transform faults (orange) form because the axis of seafloor spreding on the surface of a sphere cannot follow a smoothly curving line. The motion of two diverging lithospheric plates (arrows) rotates about an imaginary axis extending through Earth. (b) A long transform plate boundary, which includes California’s San Andreas Fault. Notethe offset plate boundaries caused by divergence on a sphere. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. A History of Plate Movement Has Been Captured in Residual Magnetic Fields Paleomagnetism: strips of alternating magnetic polarity at spreading regions. The patterns of paleomagnetism support plate tectonic theory. The molten rocks at the spreading center take on the polarity of the planet while they are cooling. When Earth’s polarity reverses, the polarity of newly formed rock changes. (a) When scientists conducted a magnetic survey of a spreading center, the Mid-Atlantic Ridge, they found bands of weaker and stronger magnetic fields frozen in the rocks. (b) The molten rocks forming at the spreading center take on the polarity of the planet when they are cooling and then move slowly in both directions from the center. When Earth’s magnetic field reverses, the polarity of new-formed rocks changes, creating symmetrical bands of opposite polarity © 2006 Brooks/Cole, a division of Thomson Learning, Inc. A History of Plate Movement Has Been Captured in Residual Magnetic Fields The age of the ocean floors. The colors seen here represent an expression of seafloor spreading over the last 200 million years as revealed by paleomagnetic patterns. Note, especially the relative symmetry of the Atlantic basin in contrast with the asymmetrical Pacific, where the spreading center is located close to the eastern margin and intersects the coast of California. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. A History of Plate Movement Has Been Captured in Residual Magnetic Fields Apparent Polar wandering: plate movement causes the apparent position of the magnetic poles to have shifted. (a) The magnetic properties of rocks in North America, South America, Europe, and Africa suggest that the north magnetic pole apparently migrated to its present position from much farther south, possibly from a point in the Pacific Ocean. (b) The problem can be solved if the pole remained fixed but the continents have moved. For example, if Europe and North America were previously joined, the paleomagnetic fields in the rocks would indicate a single pole until the continents drift apart. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Plate Movement above Mantle Plumes and Hot Spots Provides Evidence of Plate Tectonics (above) Formation of a volcanic island chain as an oceanic plate moves over a stationary mantle plume and hot spot. In this example, showing the formation of the Hawai’ian Islands, Loihi is such a newly forming island. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Sediment Age and Distribution, Oceanic Ridges, and Terranes Are Explained by Plate Tectonics Age and distribution of ocean sediments: The sediment in the ocean is thinner and younger than the age of the ocean indicates it should be The Oceanic ridges: Oceanic ridges are clear indicators of past events. Terranes: Oceanic plateaus that form by uplifting and mountain building as they strike a continent. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Sediment Age and Distribution, Oceanic Ridges, and Terranes Are Explained by Plate Tectonics Oceanic plateaus usually composed of relatively lowdensity rock are not subducted into the trench with the oceanic plate. Instead, they are “scraped off,” causing uplifting and mountain building as they strike a continent (a-d). Though rare, assemblages of subducting oceanic lithosphere can also be scraped off (obducted) onto the edges of continents. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Scientists Still Have Much to Learn about the Tectonic Process In case you think that geophysicists have all the answers, consider just a few of the theory’s unsolved problems: Why should long lines of asthenosphere be any warmer than adjacent areas? Is the increasing density of the leading edge of a subducting plate more important than the plate’s sliding off the swollen mid-ocean ridge in making the plate move? Why do mantle plumes form? What causes a superplume? How long do they last? How far do most plates descend? Recent evidence suggests that much of the material spans the entire mantle, reaching the edge of the outer core. Has seafloor spreading always been a feature of Earth’s surface? Has a previously thin crust become thicker with time, permitting plates to function in the ways described here? There is evidence of tectonic movement prior to the breakup of Pangaea. Will the process continue indefinitely, or are there cycles within cycles? © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Scientists Still Have Much to Learn about the Tectonic Process The Wilson cycle, named in honor of John Tuzo Wilson’s synthesis of plate tectonics. Over great spans of time, ocean floors form and are destroyed. Mountains erode, sediments subduct, and continents rebuild. Seawater moves from basin to basin. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Chapter in Perspective In this chapter you learned that Earth is composed of concentric spherical layers, with the least dense layer on the outside and the most dense at the core. The layers may be classified by chemical composition into crust, mantle, and core; or by physical properties into lithospere, asthenosphere, mantle, and core. Geologists have confirmed the existence and basic properties of the layers by analysis of seismic waves, which are generated by the forces that cause large earthquakes. The theory of plate tectonics explains the nonrandom distribution of earthquake locations, the curious jigsaw-puzzle fit of the continents, and the patterns of magnetism in surface rocks. Plate tectonics theory suggests that Earth’s surface is not a static arrangement of continents and ocean but a dynamic mosaic of jostling lithospheric plates. The plates have converged, diverged, and slipped past one another since Earth’s crust first solidified and cooled, driven by slow, heat-generated currents flowing in the asthenosphere. Continental and oceanic crusts are generated by tectonic forces, and most major continental and seafloor features are shaped by plate movement. Plate tectonics explains why our ancient planet has surprisingly young seafloors, the oldest of which is only as old as the dinosaurs, that is, about 1/23 the age of Earth. In the next chapter you will learn about seabed features, which the slow process of plate movement has made and remade on our planet. The continents are old; the ocean floors, young. The seabed bears the marks of travels and forces only now being understood. Hidden from our view until recent times, the contours and materials of the seabed have their own stories to tell. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. End of Chapter 3