Chapter 7 Plate Tectonics Underlies All Earth History

CHAPTER 7—PLATE TECTONICS UNDERLIES ALL EARTH HISTORY

CHAPTER OVERVIEW

This chapter addresses the dynamic, physical processes and internal energy that drives the Earth. These processes are best understood through the study of seismic waves which has led to the current interpretation of the Earth’s internal structures: the core, mantle, and crust. Major crustal structures such as faults, folds, anticlines and synclines are also discussed in order to show some of the associations that extend to plate tectonics. This conceptual view of how continents grow, where mountains come from, and how and why volcanoes and earthquakes occur offers a comprehensive view of a dynamic Earth. These explanations give a clearer approach of applying the principles of structural deformation and regional geology to plate tectonics.

LEARNING OBJECTIVES

By reading and completing information within this chapter, you should gain an understanding of the following concepts:

 Explain the differences between the three types of seismic waves and the characteristics of primary (P), secondary (S) and surface waves that are generated during an earthquake.  Discuss the formation of the Earth’s three major zones (core, mantle, crust) based on density, composition, and discontinuities.  Describe how the wave characteristics validate the composition of the Earth’s major zones.  Explain how the following structural features are formed: faults, folds, antisynclines, synclines, domes and basins.  Describe today’s theory of plate tectonics including an explanation of how the movement of lithospheric plates is driven by convection in the underlying mantle.  Discuss how the theory of continental drift contributes to today’s theory of plate tectonics.  Discuss paleomagnetism and its use in present day magnetism and remnant magnetism.  Discuss some of the concept of seafloor spreading as it relates to plate tectonics.  Describe what occurs at plate margins.

CHAPTER OUTLINE

I. Earthquake, Waves Reveal Earth’s Mysterious Interior A. Primary Waves or P-Waves (Compressional) B. Secondary Waves or S-Waves (Shear) C. Body Waves D. Surface Waves

II. Earth’s Internal Zones A. Mohorovicic Discontinuity (Moho) B. Gutenberg Discontinuity C. Earth’s Liquid/Solid Core D. The Mantle

III. Earth’s Two Types of Crust A. Oceanic Crust (More Dense) B. Continental Crust (Less Dense) C. Isostacy

IV. Broken, Squeezed, or Stretched Rocks Produce Geologic Structures Chapter 7—Plate Tectonics Underlies All Earth History

A. Faults B. Folds

V. Plate Tectonics Theory Ties It All Together

VI. Drifting Continents A. Early Hypotheses B. Alfred Wegener

VII. Evidence for Continental Drift A. Clues From Global Geography B. Clues From Paleoclimatology C. Clues From Fossils D. Clues From Rock Sequence

VIII. Paleomagnetism: Ancient Magnetism Locked Into Rocks A. How Is Earth’s Magnetic Field Recorded in Rocks? B. Earth’s Wandering Magnetic Poles

IX. Today’s Plate Tectonics Theory A. Seafloor Spreading (Divergent Boundaries) B. Transform Boundaries C. Convergent Boundaries

X. What Happens at Plate Margins A. Continental-Crust Convergence B. Oceanic-Oceanic Crust Convergence C. Continental-Oceanic Crust Convergence D. Wilson Cycles: Closings and Openings of Oceanic Basins

XI. What Drives Plate Tectonics A. Convection Cells in the Mantle B. Ridge-Push and Slab-Pull Model C. Thermal Plumes

XII. Verifying Plate Tectonics Theory A. Further Paleomagnetic Evidence 1. Determining Seafloor Age 2. Calculating Rates of Seafloor Spreading B. Oceanic Sediment Evidence C. Satellite Evidence D. Seismic Evidence E. Gravity Evidence

XIII. Thermal Plumes, Hotspots, and Hawaii

XIV. Exotic Terranes A. Exotic Terranes—Continental Crust B. Exotic Terranes—Oceanic Crust C. Exotic Terranes and Earth History Chapter 7—Plate Tectonics Underlies All Earth History

KEY TERMS (Pages 161–205) accretionary prism (188): The contorted and metamorphosed body of rock compressed onto the margin of a continent. anticline (170): A geologic structure in which strata are bent into an upfold or arch. The oldest rocks are at the center and the youngest are on the flanks. apparent polar wandering path (180): Lines on a map connecting ancient pole positions relative to a specific continent for various times during the geologic past. asthenosphere (166): A zone between 50 and 250 kilometers below the surface of the Earth where shock waves of earthquakes travel at much reduced speeds, perhaps because of less rigidity. May be a zone where convective flow of material occurs. basin (171): A depressed area that serves as a catchment area for sediments (basin of deposition). A structural basin is an area in which strata slope inward toward a central location. Has an elliptical to roughly circular outcrop pattern in which beds dip from all sides toward the center of the structure. blue schist (189): A distinctive kind of metamorphic rock containing blue amphiboles. Formed at high pressure, but relatively low temperatures. These conditions are characteristic of subduction zones where the relatively cool oceanic plate plunges rapidly into deep zones of high pressure. body seismic wave (162): Term used to describe waves that are able to penetrate deep into the interior or body of the planet. Body waves travel faster in rocks of greater elasticity, and their speeds therefore increase steadily as they move downward into more elastic zones of the Earth’s interior and then decrease as they begin to make their ascent toward the Earth’s surface. Primary and secondary waves are considered body waves. continental crust (167): That portion of the Earth’s crust which lies beneath the Earth’s continents. Thickness averages 35 kilometers. It is thicker and less dense than oceanic crust. The continents “float” higher on the denser mantle than the adjacent oceanic crustal segments. convection cell (189): As mantel material heats, it expands consequently becoming less dense and slowly rises. This displaces cooler material which sinks. convergent plate boundary (185): Develop when two plates move toward one another and collide. Characterized by a high frequency of earthquakes and are thought to be the zones along which folded mountain ranges or deep-sea trenches may develop. dip (170): The angle of inclination of the tilted layer also measured from the horizontal plane. discontinuity (seismic) (164): Boundaries where seismic waves experience an abrupt change in velocity or direction. divergent plate boundary (181): Develop when two plates move away from each other. May manifest themselves as mid-oceanic ridges complete with tensional (pull-apart) geologic structures. The rending of the crust is accompanied by earthquakes and enormous outpourings of volcanic materials that are piled high to produce the ridges itself. dome (171): An upfold in rocks having the general configuration of an inverted bowl. Strata in a dome dip outward and downward in all directions away from a central area. exotic terrane (201): Small patches of the crust that may become incorporated into the crumpled margin of larger continent. fault (169): A fracture in the Earth’s crust along which rocks on one side have been displaced relative to rocks on the other side. Chapter 7—Plate Tectonics Underlies All Earth History fold (170): Bends in rock strata that are evidence of crustal movement. Metamorphic action due to the compressional force. Tend to occur like a series of petrified wave crests and troughs. footwall (169): In normal faults, the mass of rock that lies below the shear plane. It appears to move upward relative to the opposite side or hanging wall. In reverse faults, the footwall appears to move downward relative to the hanging wall.

Glossopteris flora (177): An assemblage of fossil plants found in rocks of Late Paleozoic and early Triassic age in South Africa, India, Australia, and South America. The flora takes its name from the seed fern Glossopteris.

Gondwana (=Gondwanaland) (175): The great Permo-Carboniferous Southern Hemisphere continent, comprising the assembled present continents of South Africa, India, Australia, Africa-Arabia, and Antarctica. gravity anomaly (194): The difference between the observed value of gravity at any point on the Earth and the calculated theoretic value.

Gutenberg discontinuity (164): The boundary separating the mantle of the Earth from the core below. The Gutenberg discontinuity lies about 2900 kilometers below the surface. guyot (184): Submerged mountains with flat, rather than conical, summits. hanging wall (169): In normal faults, the mass of rock that lies above the shear plane. It appears to move downward relative to the opposite side or footwall. In reverse faults, the hanging wall has moved up relative to the footwall. hotspot (196): Are formed when the upwelling of mantel rock of lava works its way to the surface to erupt as a volcano on the seafloor. As the seafloor moves over the volcano, a series of islands are formed, i.e., Hawaiian Islands. isostasy (167): The condition of balance that exists among segments of the Earth’s crust as they come into flotational equilibrium with denser mantle material. lateral fault (strike-slip fault) (169): A fault in which the movement is largely horizontal and in the direction of the trend of the fault plane. Sometimes called a strike-slip fault.

Laurasia (177): A hypothetical supercontinent composed of what is now Europe, Asia, Greenland, and North America. lithosphere (181): The outer shell of the Earth, lying above the asthenosphere and comprising the crust and upper mantle. mantle (164): A thick, homogeneous layer surrounding the core composed of several concentric layers. Believed to have a stony, rather than metallic, composition. Oxygen and silicon probably predominate and are accompanied by iron and magnesium as the most abundant metallic ions. Probably composed of peridotite, an iron- and magnesium-rich rock. mélange (188): A body of intricately folded, faulted, and severely metamorphosed rocks, examples of which can be seen in the Franciscan rocks of California. microcontinent (201): Term used to designate bits of continental crust that are surrounded by oceanic crust. They are recognized by their granitic composition, by the velocity with which compressional seismic waves traverse them, by their general elevation above the oceanic crust, and by their comparatively quiet seismic nature.

Mohorovičić discontinuity (164): A plane that marks the boundary separating the crust of the Earth from the underlying mantle. The “moho,” as it is sometimes called, is at a depth of about 70 kilometers below the surface of the continents and 6 to 14 km below the floor of the ocean. moncline (170): A simple bend or flexure in otherwise horizontal or uniformly dipping rock layers. Chapter 7—Plate Tectonics Underlies All Earth History normal fault (169): A fault in which the hanging wall appears to have moved downward relative to the footwall; normally occurring in areas of crustal tension (forces that tend to stretch the crust). oceanic crust (168): That part of the crust which lies beneath the ocean floors. Approximately 5 to 12 kilometers thick. Consists of three layers: the upper surface is a thin layer of unconsolidated sediment that rests on the irregular surface of the igneous basement layer; the second layer consists of basalts that have been extruded under water; the nature of the deepest layer is not clear. ophiolite suite (188): Splinters of the oceanic plate that were scraped off the upper part of the descending plant and inserted into the crushed forward edge of the continent. Ophiolites mark the zone of contact between colliding continental and oceanic plates. paleomagnetism (178): The Earth’s magnetic field and magnetic properties in the geologic past. Studies of paleomagnetism are helpful in determining position of continents and magnetic poles.

Pangea (175): In Alfred Wegener’s theory of continental drift, the supercontinent that included all present major continental masses.

Panthalassa (175): The great universal ocean that surrounded the supercontinent Pangea prior to its breakup. passive continental margin (trailing edge) (183): The void created at divergent plate boundaries by the separating plates is filled with molten rock which rises from below the lithosphere and solidifies in the fissure. New crust is added to the trailing edge of each separating plate as it moves slowly away from the mid-oceanic ridge. Trailing edges are the actual edges of the plates as they move apart. plate tectonics (171): The theory that explains the tectonic behavior of the crust of the Earth in terms of several moving plates that are formed by volcanic activity at oceanic ridges and destroyed along great ocean trenches. primary seismic wave (P-wave) (162): Seismic waves that are propagated through solid rock as a train of compressions and dilations. Direction of vibration is parallel to direction of propagation. Are able to pass through solids, liquids, and gases. reverse fault (170): A fault formed by compression in which the hanging wall appears to move up relative to the footwall. ridge-push, slab-pull model (189): Spreading centers, as mid-ocean ridges, stand high on the ocean floor. Their elevation above adjacent regions of the ocean floor results in a tendency for the ridge material to slide down slope, thereby transmitting a push to the tectonic plate. seafloor spreading (183): The process by which new seafloor crust is produced along mid-oceanic ridges (divergent zones) and slowly conveyed away from the ridges. secondary seismic wave (S-wave) (162): A seismic wave in which the direction of vibration of wave energy is at right angles to the direction the wave travels. seismic wave (162): A term used for elastic waves that are produced by earthquakes or explosions that permit scientists to determine the location, thickness, and properties of the Earth’s interior. seismogram (162): The record made by a seismograph that would record an earthquake or explosion. seismograph (162): An instrument used to record all three types of waves generated by the Earth. shadow zone (164): Area in which seismic waves from earthquakes do not appear. The outer core is a barrier to secondary waves and causes a shadow zone on the side of the Earth opposite an earthquake. This shadow zone occurs at 105 degrees from the earthquake focus. The primary wave shadow zone extends from about 105 to 140 degrees from the earthquake focus. These shadow zones are the basis for the theory of a liquid outer core. Chapter 7—Plate Tectonics Underlies All Earth History spreading center (181): An area where two plates would separate as in divergent plate boundaries, or along mid-oceanic boundaries. strike (170): The compass direction of the line produced by intersection of an inclined stadium (or other feature such as fault plane) with a horizontal plane. subduction zone (185): An inclined planar zone, defined by high frequency of earthquakes, that is thought to locate the descending leading edge of a moving oceanic plate. surface seismic wave (163): Seismic or earthquake waves that move only about the surface of the Earth. suture zone (187): The zone of convergence between two plates, recognized by the severity of folding, faulting, and intrusive activity. syncline (170): A geologic structure in which strata are bent into a downfold. The youngest beds are in the center and the oldest rocks are on the flanks.

Tethys Sea (187): A sea which existed for extensive periods of geologic time between the northern and southern continents of the Eastern Hemisphere. thermal plume (191): A “hot spot” in the upper mantle believed to exist where a huge column of upwelling magma lies in a fixed position under the lithosphere. Thermal plumes are thought to cause volcanism in the overlying lithosphere. thrust fault (170): A low-angle reverse fault, with inclination of fault plane generally less than 45 degrees. Caused by compressional forces. transform plate boundary (185): A plate boundary where two plates move sideways past each other. Movement is compared to a strike-slip movement, i.e. California’s San Andreas Fault. transform fault (189): A strike-slip fault bounded at each end by an area of crustal spreading that tends to be more or less perpendicular to the trace of the fault.

Wadati-Benioff seismic zone (194): An inclined zone along which frequent earthquake activity occurs and that marks the location of the plunging, forward edge of the lithospheric plate during subduction.

Wilson Cycle (189): The opening of a new ocean basin along divergent zones, the expansion of the basin as seafloor spreading continues, and the ultimate closure of the basin as plates converge. Chapter 7—Plate Tectonics Underlies All Earth History

MULTIPLE-CHOICE QUESTIONS

1. Seismic waves where the rock segments vibrate at right angles to the travel directions of energy and cannot pass through liquids or gases. a. primary waves c. longitudinal waves b. secondary waves d. density waves

2. Which wave reaches a seismographic station first when compared with others? a. secondary c. surface b. primary d. longitudinal

3. Boundaries where seismic waves experience an abrupt change in velocity or direction are called a. discontinuities. c. anticlines. b. shadow zones. d. synclines.

4. The feature located nearly halfway to the center of the Earth, at a depth of 2900 kilometers whose location is marked by an abrupt decrease in P-wave velocities and the disappearance of S-waves is the a. Mohorovicic discontinuity. c. lower mantle. b. Gutenberg discontinuity. d. upper mantle.

5. This zone on the side of the Earth opposite the Earth focus (rupture point of an earthquake) which begins 105 degrees from the earthquake’s location. a. shadow zone c. P-wave shadow zone b. mantle zone d. L zone

6. Which of the following is an iron and magnesium rich rock? a. granite c. hornfel rhyolite b. limestone andesite d. peridotite

7. The Earth’s mantle is composed primarily of a. oxygen, silicon, iron, and magnesium. c. silicon, potassium, and sedimentary rocks. b. iron, magnesium, and calcium. d. iron and nickel.

8. The outer shell of the Earth lying above the asthenosphere and comprising the crust and upper mantle is called a. low velocity zone. d. lithosphere. b. monocline. e. surface zone. c. Laurasia.

9. Breaks in crustal rocks along which there has been a displacement of one side relative to the other are a. folds. d. domes. b. faults. e. strata. c. synclines.

10. In normal faults, the mass of rock that lies above the shear plane is called the a. fault scarp. c. thrust fault. b. foot wall. d. hanging wall.

11. A fault in which the hanging wall moves downward in relation to the footwall is called a a. lateral fault. c. reverse fault. b. normal fault. d. strike-slip fault.

12. Reverse faults in which the shear zone is inclined only a few degrees from horizontal are termed a. normal faults. c. reverse lateral faults. b. lateral faults. d. thrust faults. Chapter 7—Plate Tectonics Underlies All Earth History

13. Downwardly folded rocks that have the youngest beds in the center and the oldest rocks on the flanks are called a. anticlines. c. folds. b. synclines. d. basins.

14. The characteristic of anticlines and domes in relation to the instability of folds is a. a down-folded strata with the basin being more or less circular. b. strata that dips for an indefinite length in one direction and returns to its former horizontal attitude. c. an up-arched strata with beds dipping more or less equally away from a central point. d. a downward hanging wall relative to a foot wall.

15. Gondwanaland included which three present day continents? a. Africa, Asia, Australia c. India, South America, Southern Africa, Australia b. North America, India, Europe d. Australia, Africa, India

16. Deposits of poorly sorted clay, sand, cobbles, and boulders that are associated with glaciers are called a. suturites. c. drifts. b. tillites. d. magnalites.

17. What is a small aquatic reptile whose fossil is found in the Southern Hemisphere? a. Glossopteris c. Panthalassa b. Gondwanaland d. Mesosaurus

18. The study of the Earth’s magnetic field and magnetic properties in the geologic past is known as a. magnetology. c. gravitational anomalies. b. paleomagnetism. d. magnetizationism.

19. When basalt’s temperature falls below 580 degrees Celsius, the crystals become magnetized in the Earth’s magnetic field. This point is called a. magnetic point. c. Curie temperature. b. peak temperature. d. magnetization.

20. The type of boundary that occurs when plates move sideways past one another, such as the San Andreas Fault in California, is called a. convergent boundary. c. divergent plate boundary. b. shear (transform) boundaries. d. trailing edge. Chapter 7—Plate Tectonics Underlies All Earth History

FILL IN THE BLANK

1. A compressional seismic body wave is called .

2. The seismic body wave that does not pass through fluids is called .

3. The discontinuity that marks the boundary between the mantle and the outer core is the ______.

4. The mantle is composed predominantly of iron and magnesium silicate minerals represented by the igneous rock .

5. Shear stress will often result in the formation of .

6. The temperature above which a substance is no longer magnetic is called the ______.

7. The type of fault in which the relative motion of the hanging wall with respect to the foot wall is down is called a .

8. Splinters of the oceanic plate that are scraped off the upper part of a descending oceanic plate and welded onto the forward edge of the overriding continental plate is called ______.

9. Plate movement is on a weak, partially molten region of the upper mantle called the . . 10. In Alfred Wegener’s theory of continental drift, the supercontinent included in all of present day major continental masses is called .

11. That portion of the supercontinent that separated and formed North America and Eurasia came to be called .

12. Large-motion waves that travel through the outer crust of the Earth are called ______.

13. ______is a condition of vertical balance or “floating depth.”

14. ______are uparched rocks (think of McDonald’s Golden Arches).

15. Plates move apart at ______which may manifest as mid-oceanic ridges complete with tensional geologic structures. Chapter 7—Plate Tectonics Underlies All Earth History

TRUE/FALSE

1. Primary and secondary waves (body waves) pass deep within the Earth and are responsible for most of the destructive effects.

2. The seismic boundary that separates the mantle from the overlying crust is the Mohorovicic discontinuity.

3. Sudden changes in earthquake wave velocities are termed unconformities.

4. The Wadati Benioff Seismic Zone is associated with frequent earthquake activity and marks the location of the plunging forward edge of the lithosphere.

5. Below the Curie temperature, magnetic characteristics of the mineral are altered.

6. The approximate age of the oldest sediments on the seafloor are approximately 500 million years old.

7. The underlying surface of an inclined fault plane is called the hanging wall.

8. In general the rocks of the ocean floor are younger and less complex than those found on the continents.

9. The great Permo-Carboniferous Southern Hemisphere Continent Gondwanaland comprised the assembled continents of South America, India, Australia, Africa, Arabia and Antarctica.

10. The velocity of plate movement is uniform round the world. Chapter 7—Plate Tectonics Underlies All Earth History

ANSWER KEY

Multiple Choice Fill Ins True/False

1. a 1. P-wave 1. F 2. b 2. S-wave 2. T 3. a 3. Gutenburg discontinuity 3. F 4. b 4. peridotite 4. T 5. a 5. strike-slip-faults 5. F 6. d 6. Curie point 6. F 7. a 7. normal fault 7. F 8. d 8. ophiolite suites 8. T 9. b 9. asthenosphere 9. F 10. d 10. Pangea 10. F 11. b 11. Laurasia 12. d 12. surface waves 13. b 13. Isostasy 14. c 14. Anticlines 15. c 15. divergent plate 16. b boundaries 17. d 18. b 19. c 20. b Chapter 7—Plate Tectonics Underlies All Earth History

RESPONSES TO QUESTIONS ACCOMPANYING SELECTED FIGURES

FIGURE 7–1 (p. 162) Surface waves are most likely to cause the most damage.

FIGURE 7–5 (p. 165) The Gutenberg discontinuity marks the boundary between the outer core and the mantle. The average density of the layers of the Earth increases from the crust to the mantle, to the outer core to the inner core.

FIGURE 7–6 (p. 165) Seismograph stations located in Buenos Aires would not receive secondary waves from an earthquake with an epicenter at the North Pole.

FIGURE 7–7 (p. 166) Meteorites are easier to collect in Antarctica than in warmer parts of the Earth because the dark-colored meteorites are easier to see against the white background of snow and ice.

FIGURE 7–13 (p. 169) Normal faults are not the result of compressional forces.

FIGURE 7–14 (p. 170) Movement across the fault line on the opposite block shows that the movement has been to the right.

FIGURE 7–15 (p. 170) It is a normal fault. Note the yellow bed marker has moved down.

FIGURE 7–19 (p. 172) A structural dome (the Ozark Dome) underlies southern Missouri. Other nearby uparched structures (colored purple) include the Cincinnati Arch to the northeast of Missouri and the Nashville Dome centered in Tennessee.

FIGURE 7–21 (p. 160) From the outermost limbs of the anticline to the axial plane, strata would be increasingly younger in age.

FIGURE 7–22 (p. 174) Streams are confined to the beds of less resistant rock (probably shales) areas exposed between ridges composed of resistant strata.

FIGURE 7–23 (p. 174) Because of seafloor spreading, volcanoes far from the ridge have been transported from their place of origin and subsided as they were being conveyed away. The oceanic crust itself slowly subsides as it is conveyed away from the spreading center. Because they are older than volcanoes currently forming at the spreading center, volcanoes far from the ridge have experienced more erosional loss.

FIGURE 7–42 (p. 186) The San Andreas Fault is a right lateral fault. It can also be termed a transform fault.

FIGURE 7-49 (p. 192) The southward-trending arm known as the East African rift is the aulacogen in the Afar Triangle.

FIGURE 7–50 (p. 192) The oldest magnetically reversed bands are the outermost bands on either side.

FIGURE 7–54 (p. 195) The ocean floor was conveyed about 580 kilometers along line A-A’ between 81 and 53 million years ago.

FIGURE 7–55 (p. 195) The Pacific Ocean region generally has faster moving plates.