CHAPTER 2 The Way the Earth Works: Plate Tectonics Learning Objectives 1. Students should be aware of Wegener’s amassed evidence for continental drift. The fit of coastal outlines and the distribution of rocks, fossils, and ancient climatic belts all strongly suggest that the continents were once aligned to form a supercontinent named Pangaea. Wegener’s ideas had few supporters during his lifetime at least in part because he could not provide a workable mechanism through which continents could move with respect to one another. 2. During the twentieth century, paleomagnetic data showed that continents must have drifted, because the rocks of isolated continents produce unequal apparent polar-wander paths for the north magnetic pole. 3. Continents are passively pushed by the activity of sea-floor spreading, in which molten rock rises at mid-ocean ridges, cools to form new oceanic crust, and spreads laterally. Simultaneously, crust (and mantle lithosphere) is pulled downward and engulfed at deepocean trenches. 4. Sea-floor spreading was proven in the late 1960s by examination of marine magnetic anomalies, which are symmetric about the mid-ocean ridges. Combined with age dating, these patterns clearly show that oceanic crust is created at the ridges and spreads outward, with crustal age increasing away from the ridge axis in either direction. Areas of positive anomaly (including the ridges themselves) arise from rocks that crystallized and cooled during times when Earth’s magnetic polarity was normal (the same as it is today); rocks producing negative anomalies cooled during times of reversed polarity. 5. Together, the evidence for sea-floor spreading and continental drift have been combined with geophysical observations to form the basis of our modern understanding of plate tectonics—the unifying theory of geology that explains the distribution of earthquakes, volcanism, and mountain building, and the links among these phenomena. 6. The “plates” of plate tectonics are discrete slabs of lithosphere (crust and rigid portion of the mantle) that move with respect to each other. They glide over a ductile layer of the mantle termed the asthenosphere. Boundaries between plates are convergent (where 1 plates move toward each other), divergent (where plates are pushed apart at a mid-ocean ridge), or transform (where plates slide past each other). Relative plate motions are on the order of a few centimeters a year (a common analogy is that these rates approximate the rate of human fingernail growth). 7. Plate motion at all three boundary types triggers earthquakes. Plate boundaries are delineated by belts of high historical earthquake frequency. 8. Volcanism is associated with both convergent (volcanic arcs) and divergent (mid-ocean ridges) boundaries but not transform boundaries. 9. Only oceanic lithosphere is dense enough to be subducted at convergent boundaries. When continental lithosphere is pushed (by a ridge) and pulled (by a leading edge of subducting oceanic lithosphere) into another continent, a mountainous collision zone is formed, and the two plates involved become sutured together. Conversely, a single large plate can become rifted apart when its lithosphere is stretched, thinned, and broken apart by a developing ridge. Summary from the Text Alfred Wegener proposed that continents had once been joined together to form a single huge supercontinent (Pangaea) and had subsequently drifted apart. This idea is the continental drift hypothesis. Wegener drew from several different sources of data to support his hypothesis: (1) the correlation of coastlines; (2) the distribution of late Paleozoic glaciers; (3) the distribution of late Paleozoic climatic belts; (4) the distribution of fossil species; and (5) correlation of distinctive rock assemblages now on opposite sides of the ocean. Rocks retain a record of the Earth’s magnetic field that existed at the time the rocks formed. This record is called paleomagnetism. By measuring paleomagnetism in successively older rocks, geologists discovered apparent polar-wander paths. Apparent polar-wander paths are different for different continents. Continents move with respect to one another, while the Earth’s magnetic poles remain roughly fixed. Around 1960, Harry Hess proposed the hypothesis of sea-floor spreading. According to this hypothesis, new sea floor forms at mid-ocean ridges, above a band of upwelling mantle, then spreads symmetrically away from the ridge axis. Eventually, the ocean floor sinks back into the mantle at deep-ocean trenches. Magnetometer surveys of the sea floor revealed marine magnetic anomalies. Positive anomalies, where the magnetic field strength is greater than expected, and negative anomalies, where the magnetic field strength is less than expected, are arranged in alternating stripes. Geologists documented that the Earth’s magnetic field reverses polarity every now and then. The record of reversals, dated by isotopic techniques, is called the magnetic-reversal chronology. A proof of sea-floor spreading came from the association of marine magnetic anomalies with the reversal chronology and from drilling studies proving that the sea floor gets progressively older away from a mid-ocean ridge. The lithosphere, the rigid outer layer of Earth, is broken into discrete plates that move relative to each other. Plates consist of the crust and the uppermost (cooler) mantle. Lithosphere plates effectively float on the underlying soft asthenosphere. Continental drift and sea-floor spreading are manifestations of plate movement. Most earthquakes and volcanoes occur along plate boundaries; the interiors of plates remain relatively rigid and intact. There are three types of plate boundaries—divergent, convergent, and transform— distinguished from one another by the movement the plate on one side of the boundary makes relative to the plate on the other side. Divergent boundaries are marked by mid-ocean ridges. At divergent boundaries, seafloor spreading produces new oceanic lithosphere. Convergent boundaries are marked by deep-ocean trenches and volcanic arcs. At convergent boundaries, oceanic lithosphere of the downgoing plate subducts beneath an overriding plate. Transform boundaries are marked by large faults at which one plate slides sideways past another. No new plate forms and no old plate is consumed at a transform boundary. Triple junctions are points where three plate boundaries intersect. Hot spots are places where volcanism occurs at an isolated volcano. As a plate moves over the hot spot, the volcano moves off and dies, and a new volcano forms over the hot spot. Hot spots may be caused by mantle plumes. A large continent can split into two smaller ones through the process of rifting. During rifting, continental lithosphere stretches and thins. If it finally breaks apart, a new mid-ocean ridge forms, and sea-floor spreading begins. Convergent plate boundaries cease to exist when a buoyant piece of crust (a continent or an island arc) moves into the subduction zone. When this happens, collision occurs. Ridge-push force and slab-pull force contribute to driving plate motions. Plates move at rates of about 1 to 15 cm per year. Modern satellite measurements can detect these motions. Answers to Review Questions 1. What was Wegener’s continental drift hypothesis? What was his evidence? Why didn’t other geologists all agree? ANS: Wegener stated that the continents had once been contiguous, forming a supercontinent (which he termed Pangaea), which later broke apart to form the modern continents and then drifted to their present configuration. Wegener’s evidence included the fit of continental coastlines and the distributions of late Paleozoic fossils, rocks, and climatic belts, all of which were explained well by the presence of Pangaea and none of which could be readily explained by fixed continents. Most geologists did not accept the continental drift hypothesis primarily because Wegener had no working mechanism to explain what would cause continents to move. 2. How do apparent polar-wander paths show that the continents, rather than the poles, have moved? ANS: Apparent polar-wander paths for different continents do not agree with one another concerning the position of the ancient magnetic north pole. Magnetic poles can potentially wander, but as Earth’s magnetic field is largely a dipole field, there is only one possible position at any one time for an ancient north pole. 3. Describe the hypothesis of sea-floor spreading. ANS: Sea-floor spreading is the idea that new oceanic basalt is produced at mid-ocean ridges and spreads laterally to either side. It is in part the push of oceanic basalts that causes the continents to drift over the surface of Earth. 4. Describe the pattern of marine magnetic anomalies across a mid-ocean ridge. How is this pattern explained? ANS: Over the ridge crest, Earth’s magnetic field is anomalously strong (a positive anomaly). This elongate belt of positive anomaly is flanked on either side by belts of anomalously weak magnetic field strength (negative anomaly). The alternating sequence of positive and negative anomalies continues in either direction outward from the ridge, forming a pattern that possesses mirror-image symmetry about the ridge axis. The explanation for the pattern is that magnetic fields of crystals formed in the most recent past have remnant magnetism in concert with today’s global magnetic field (and are said to have normal polarity). Extra field strength derives from the alignment of all these mini-magnets reinforcing the modern dipole. The same is true for all positive anomalies, representing crystallization and cooling that took place during times when Earth’s magnetic polarity was the same as it is today. Negative anomalies are derived from bodies of rock that crystallized and cooled during times when Earth’s magnetic field had a polarity opposite to that of today. The magnetic fields of these rocks destructively interfere with the modern dipole, weakening the observed magnetic strength. 5. How did drilling into the sea floor contribute further proof of sea-floor spreading? How did the seafloor-spreading hypothesis explain variations in ocean floor heat flow? ANS: Sediments atop oceanic basalts become thicker away from mid-ocean ridges, and the lowermost (oldest) layers become progressively older with increasing distance from the ridge. Heat flow is greatest near the mid-ocean ridges, where hot mantle rock is rising and partially melting to yield new basalts. With increasing distance away from the ridges, rates of heat flow decrease. 6. What are the characteristics of a lithosphere plate? Can a single plate include both continental and oceanic lithosphere? ANS: The lithosphere is the rocky portion of Earth, relatively cool and rigid as compared to underlying mantle material (the hotter, weaker asthenosphere). The lithosphere is composed of the crust and the uppermost portion of the mantle. A plate may be topped by a combination of continental and oceanic crust. 7. How does oceanic lithosphere differ from continental lithosphere in thickness, composition, and density? ANS: Oceanic lithosphere is thinner and denser, with a more mafic crust (largely basaltic, whereas continental crust is granitic). 8. How do we identify a plate boundary? ANS: Plate boundaries are marked by linear or arc-like segments of relatively high earthquake frequency (earthquake belts). 9. Describe the three types of plate boundaries. ANS: Divergent plate boundaries exist where lithosphere on either side moves away from the boundary. At convergent plate boundaries, lithosphere on either side comes together, bringing either subduction (if oceanic lithosphere is involved) or collision (of two continental plates). At transform plate boundaries, plates slide past each other. 10. How does crust form along a mid-ocean ridge? What is a black smoker in a ridge? ANS: The high-heat flux at the ridge melts mantle material to form magma, which is relatively light and rises to the surface. Some of the magma crystallizes beneath the surface (as gabbro or in thin basaltic dikes), and some erupts to form volcanic lava, which flows and ultimately solidifies to form pillow basalt. A black smoker is a vent of hot seawater that erupts from a chimney of basalt; the water issued from the chimney is black due to minerals that come out of solution as the superheated water cools on contact with surrounding colder water. 11. Why is the oldest oceanic lithosphere found on the Earth today less than 200 million years old? ANS: Oceanic lithosphere greater than 200 million years old has been subducted down trenches. 12. Describe the major features of a convergent boundary. ANS: If the convergent boundary is a subduction zone, it is marked by a deep trench where the subducting oceanic plate bends downward in opposition to the horizontal overriding plate. Sediments scrape off the subducting plate to form an accretionary prism at the edge of the overriding plate. Behind the prism, melting associated with the subducting plate produces either a volcanic continental arc or a volcanic island arc. If a collisional boundary is present instead of a subduction zone, then a broad, thick, nonvolcanic mountain range will be present. 13. Why are transform plate boundaries required on an Earth with spreading and subducting plate boundaries? ANS: Transform plate motion is a necessary transition between convergence and divergence. 14. What is a triple junction? ANS: A triple junction is a point at which three plate boundaries meet. 15. How is a hot-spot track produced, and how can hot-spot tracks be used to track the past motions of a plate? Why is only the volcano at the end of a hot-spot track active? ANS: A large volume of very hot rock from within the mantle rises at the hot spot and produces abundant magma, yielding copious volcanism to form the hot spot. Hot spots are relatively stable points, whereas the plates that overlie them, and which bear the associated volcanoes, are moving. Over periods of millions of years, as a plate slides over the hot spot, extinct volcanoes are ferried in the direction of plate motion, while new volcanoes are formed at the hot spot. Only the volcano directly over a hot spot will be active; all those dragged off the hot spot lack a source of magma. 16. Describe the characteristics of a continental rift, and give examples of where this process is occurring today. ANS: Continental rifts appear as elongate valleys bounded on either side by faults. Volcanism occurs along the rift as asthenosphere rises to accommodate the thinning lithosphere and melts. Rifts can be found in East Africa and in the Great Basin of the western United States. 17. Describe the process of continental collision, and give examples of where this process has occurred. Why can’t continental crust subduct? ANS: At a collisional belt, a colliding plate sutures itself to its adjacent plate, deforming the rocks in the zone of collision, and thickening the crust locally to form a nonvolcanic mountain range. Continental rock is not dense enough to subduct into the asthenosphere. 18. Discuss the major forces that move lithosphere plates. ANS: Plates are pushed by mid-ocean ridges as elevated lithosphere directly over a ridge pushes downward on less-elevated lithosphere to either side. Plates are pulled by descending slabs at subduction zones, because old oceanic lithosphere is generally more dense than the asthenosphere into which the slabs sink. 19. Explain the difference between relative plate velocity and absolute plate velocity. ANS: Relative plate velocity describes rates of motion calculated for the movement of material on one plate with respect to material from an adjacent plate (or with respect to a plate boundary). Absolute plate velocity is calculated using age and distance data from the material on a plate, with the distance calculated from a hot spot or other fixed point of reference on Earth’s surface. On Further Thought 20. Why are the marine magnetic anomalies bordering the East Pacific Rise in the southeastern Pacific Ocean wider than those bordering the Mid-Atlantic Ridge in the South Atlantic Ocean? ANS: The East Pacific Rise is spreading faster, so it produces a greater width of basalt in the time intervals between polarity reversals. 21. The North Atlantic Ocean is about 3600 km wide. Sea-floor spreading along the MidAtlantic Ridge occurs at 2 cm per year. When did rifting break North America away from Africa so that the Atlantic Ocean floor started to form? ANS: Assuming a constant spreading rate equal to today’s 2 cm per year, 180 million years ago. 22. The Pacific Plate moves north relative to the North American Plate at a rate of 6 cm per year. How long will it take Los Angeles (a city on the Pacific Plate) to move northward by 480 km, the present distance between Los Angeles and San Francisco? ANS: Assuming no future change in plate velocity, 8 million years.