Chapter 2: Plate Tectonics
PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) Chapter 2: Plate Tectonics Chapter Summary: Plate tectonics is a theory that suggests Earth's surface is divided into several large plates that change position and size. Intense geologic activity occurs at plate boundaries.
Plate tectonics combines the concepts of sea-floor spreading and continental drift.
Alfred Wegener proposed continental drift in the early 1900s. His evidence included coastline fit, similar fossils and rocks in now-separated continents, and paleoclimatic evidence for apparent polar wandering. Wegener proposed that all continents were once joined together in the supercontinent Pangea.
Wegener's ideas were not widely accepted until the 1950s, when work in paleomagnetism revived interest in polar wandering.
Evidence for continental drift includes careful fits of continental edges and detailed rock matches between now-separated continents. The positions of continents during the past 200 million years have been mapped.
Hess's hypothesis of sea-floor spreading suggests that the sea floor moves away from the ridge crest and toward trenches as a result of mantle convection.
According to the concept of sea-floor spreading, the high heat flow and volcanism of the ridge crest are caused by hot mantle rock rising beneath the ridge. Divergent convection currents in the mantle cause the rift valley and earthquakes on the ridge crest, which is a spreading axis (or centre). New sea floor near the rift valley has not yet accumulated pelagic sediment.
Sea-floor spreading explains trenches as sites of sea-floor subduction, which causes low heat flow and negative-gravity anomalies. Benioff zones and andesitic volcanism are caused by interaction between the subducting sea floor and the rocks above.
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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) Sea-floor spreading also explains the young age of the rock of the sea floor as caused by the loss of old sea floor through subduction into the mantle.
Plates are composed of blocks of lithosphere riding on the plastic upper mantle or asthenosphere. Plates move away from spreading axes, which add new sea floor to the trailing edges of the plates.
An apparent confirmation of plate motion came in the 1960s with the correlation of marine magnetic anomalies to magnetic reversals by Morley, Vine, and Matthews. The origin of magnetic anomalies at sea apparently is due to the recording of normal and reverse magnetization by dikes that intrude the crest of the mid-oceanic ridge, then split and move sideways to give anomaly patterns a mirror symmetry.
The Morley-Vine-Matthews hypothesis gives the rate of plate motion (generally 1 to 6 cm/year) and can predict the age of the sea floor before it is sampled.
Deep-sea drilling has apparently verified plate motions and the age predictions made from magnetic anomalies.
Earthquake distribution and first-motion studies on transform faults on fracture zones also verify plate motions.
Divergent plate boundaries are marked by rift valleys, shallow-focus earthquakes, high heat flow, and basaltic volcanism.
Transform boundaries between plates sliding past one another are marked by strike-slip (transform) faults and shallow-focus earthquakes.
Convergent plate boundaries can cause subduction or continental collision. Subducting plate
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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) boundaries are marked by trenches, low heat flow, Benioff zones, andesitic volcanism, and young mountain belts or island arcs.
Continental-collision boundaries have shallow-focus earthquakes and form young mountain belts in continental interiors.
The distribution and origin of most volcanoes, earthquakes, young mountain belts, and major sea-floor features can be explained by plate tectonics.
Plate motion was once thought to be caused by mantle convection, but is now attributed to the cold, dense, leading edge of a subducting plate pulling the rest of the plate along with it (slabpull). Plates near mid-oceanic ridges also slide down the sloping lithosphere-asthenosphere boundary at the ridge (ridge-push). Trench-suction may help continents diverge.
Mantle plumes are narrow columns of hot, rising mantle rock. They cause flood basalts and may split continents, causing plate divergence.
An aseismic ridge may form as an oceanic plate moves over a mantle plume acting as an eruptive centre (hot spot).
Questions: 1. The southern supercontinent was called: (Page 23) A. Laurentia B. Glossopteris C. Gondwana D. Pangea 2. The sliding of seafloor beneath a continent or island arc is known as: (Page 43-46) A. Obfuscation B. Obduction C. Subduction
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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) D. Obliteration 3. The tectonic plates of Earth are part of Earth's: (Page 32) A. Crust B. Mantle C. Lithosphere D. Asthenosphere 4. The Morley-Vine-Matthews hypothesis explains the origin of: (Page 33-34) A. polar wandering B. seafloor magnetic anomalies C. continental drift D. mid-ocean ridges 5. The San Andreas Fault in California is a: (Page 42) A. convergent plate boundary B. divergent plate boundary C. transform plate boundary D. none of these 6. Which of the following features would you expect to find at an ocean-ocean convergent boundary? (Page 43-44) A. volcanic island arc B. ocean trench C. earthquakes D. all of these 7. Which of the following features would you expect to find at an ocean-continent convergent boundary? (Page 44) A. deep ocean trench B. volcanic mountain chain C. earthquakes D. all of these 8. Which of the following would you expect to find at a continent-continent convergent boundary? (Page 45-47)
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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) A. Deep ocean trench B. suture zone C. volcanic mountain chain D. all of these 9. Passive continental margins are created at: (Page 37) A. divergent plate boundaries B. convergent plate boundaries C. transform plate boundaries D. all of these 10. The Hawaiian Islands are the result of: (Page 57-60) A. subduction B. seafloor spreading C. a mantle plume D. none of these 11. Metallic ores are created at divergent plate boundaries: (Page 40-41) A. through hydrothermal processes B. in lava flows C. in sedimentary deposits D. all of these 12. A large supercontinent that existed 225 million years ago was: (Page 23-25) A. Gondwanaland B. Laurasia C. Glossopteris D. Pangea 13. Alfred Wegener was a: (Page 22) A. geologist B. geophysicist C. astronomer D. meteorologist 14. Which of the following was not used by Wegener as evidence of continental drift? (Page 23-
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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) 27) A. Fossils that were common to many continents. B. Evidence of glaciation on widely separated continents. C. The geometric fit of the continents. D. Magnetic anomalies on the seafloor. 15. Continental drift and the reorganization of this hypothesis to Plate Tectonics was made possible by advances in: (Page 25-29) A. Understanding of Earth's magnetic field. B. Understanding of Earth's fossil record. C. Development of radiometric dating methods. D. None of these. 16. An early hypothesis that eventually led to the Theory of Plate Tectonics was: (Page 29) A. The Elastic Rebound Theory of G.K. Gilbert in the early 1900's. B. "Theory of the Earth" by James Hutton, 1788. C. Seafloor Spreading by Harry Hess, 1962. D. All of these. 17. The age of the seafloor is relatively young because: (Page 31-32) A. It is continuously generated at mid-ocean ridges. B. It is continuously destroyed at subduction zones. C. Both of the above. D. None of the above. 18. If a plate moves 20 mm per year, how far will it move in 1 million years? (Box 2.1) A. 20 meters B. 20 kilometers C. 20 centimeters D. 2,000 kilometers 19. The East African Rift is an example of: (Page 59) A. A continent-continent convergent boundary. B. An ocean-continent convergent boundary. C. An ocean-ocean convergent boundary.
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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) D. A divergent plate boundary. 20. The Wilson Cycle is: (Box 2.5, Page 54) A. The formation of hot spots B. an ocean current C. the melting of rocks D. the opening and closing of an ocean 21. The oldest rocks on Earth are found in ocean basins. (Page 31-32) True False 22. Glaciers covered parts of Africa, Australia, South America, India, and Antarctica about 230 million years ago. (Page 24) True False 23. Glossopteris is a type of plant fossil found on a number of continents. (Page 23) True False 24. Wegener's evidence for continental drift was readily accepted by the scientific community. (Page 25) True False 25. Benioff Zones are associated with continent-continent convergent margins. (Page 43, 46) True False 26. Ocean-ocean convergent boundaries are also sites of subduction. (Page 43) True False 27. Explosive volcanic eruptions are associated with mid-ocean ridge volcanism. (Page 38-39) True False 28. All plates are presently moving in the same direction. (Ch.2)
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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) True False 29. The direction of motion of a plate cannot change over time. (Ch.2) True False 30. Most active geologic processes (earthquakes, volcanoes) are associated with plate boundaries. (Ch.2) True False 31. Apparent polar wander was really caused by the movement of plates. (Ch.2) True False 32. Africa and South America were once joined together. (Page 29) True False 33. There have been a number of supercontinents during Earth's history. (Ch.2) True False 34. Divergent plate boundaries form where there is upwelling from the asthenosphere. (Ch.2) True False 35. Active continental margins form near mid-ocean ridges. (Ch.2) True False 36. Large magnitude earthquakes emanate from subduction zones. (Ch.2) True False 37. Polar wandering refers to the great Arctic explorers who discovered the North magnetic pole. (Page 28) True
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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) False 38. Plate tectonic processes are responsible for development of large mountain belts. (Page 51) True False 39. Plate tectonic processes are influenced by processes deep inside Earth. (Page 60) True False 40. Plate tectonics theory unifies much of the information presented in your textbook. (Ch.2) True False
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Plate tectonics combines the concepts of sea-floor spreading and continental drift.
Alfred Wegener proposed continental drift in the early 1900s. His evidence included coastline fit, similar fossils and rocks in now-separated continents, and paleoclimatic evidence for apparent polar wandering. Wegener proposed that all continents were once joined together in the supercontinent Pangea.
Wegener's ideas were not widely accepted until the 1950s, when work in paleomagnetism revived interest in polar wandering.
Evidence for continental drift includes careful fits of continental edges and detailed rock matches between now-separated continents. The positions of continents during the past 200 million years have been mapped.
Hess's hypothesis of sea-floor spreading suggests that the sea floor moves away from the ridge crest and toward trenches as a result of mantle convection.
According to the concept of sea-floor spreading, the high heat flow and volcanism of the ridge crest are caused by hot mantle rock rising beneath the ridge. Divergent convection currents in the mantle cause the rift valley and earthquakes on the ridge crest, which is a spreading axis (or centre). New sea floor near the rift valley has not yet accumulated pelagic sediment.
Sea-floor spreading explains trenches as sites of sea-floor subduction, which causes low heat flow and negative-gravity anomalies. Benioff zones and andesitic volcanism are caused by interaction between the subducting sea floor and the rocks above.
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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) Sea-floor spreading also explains the young age of the rock of the sea floor as caused by the loss of old sea floor through subduction into the mantle.
Plates are composed of blocks of lithosphere riding on the plastic upper mantle or asthenosphere. Plates move away from spreading axes, which add new sea floor to the trailing edges of the plates.
An apparent confirmation of plate motion came in the 1960s with the correlation of marine magnetic anomalies to magnetic reversals by Morley, Vine, and Matthews. The origin of magnetic anomalies at sea apparently is due to the recording of normal and reverse magnetization by dikes that intrude the crest of the mid-oceanic ridge, then split and move sideways to give anomaly patterns a mirror symmetry.
The Morley-Vine-Matthews hypothesis gives the rate of plate motion (generally 1 to 6 cm/year) and can predict the age of the sea floor before it is sampled.
Deep-sea drilling has apparently verified plate motions and the age predictions made from magnetic anomalies.
Earthquake distribution and first-motion studies on transform faults on fracture zones also verify plate motions.
Divergent plate boundaries are marked by rift valleys, shallow-focus earthquakes, high heat flow, and basaltic volcanism.
Transform boundaries between plates sliding past one another are marked by strike-slip (transform) faults and shallow-focus earthquakes.
Convergent plate boundaries can cause subduction or continental collision. Subducting plate
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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) boundaries are marked by trenches, low heat flow, Benioff zones, andesitic volcanism, and young mountain belts or island arcs.
Continental-collision boundaries have shallow-focus earthquakes and form young mountain belts in continental interiors.
The distribution and origin of most volcanoes, earthquakes, young mountain belts, and major sea-floor features can be explained by plate tectonics.
Plate motion was once thought to be caused by mantle convection, but is now attributed to the cold, dense, leading edge of a subducting plate pulling the rest of the plate along with it (slabpull). Plates near mid-oceanic ridges also slide down the sloping lithosphere-asthenosphere boundary at the ridge (ridge-push). Trench-suction may help continents diverge.
Mantle plumes are narrow columns of hot, rising mantle rock. They cause flood basalts and may split continents, causing plate divergence.
An aseismic ridge may form as an oceanic plate moves over a mantle plume acting as an eruptive centre (hot spot).
Questions: 1. The southern supercontinent was called: (Page 23) A. Laurentia B. Glossopteris C. Gondwana D. Pangea 2. The sliding of seafloor beneath a continent or island arc is known as: (Page 43-46) A. Obfuscation B. Obduction C. Subduction
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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) D. Obliteration 3. The tectonic plates of Earth are part of Earth's: (Page 32) A. Crust B. Mantle C. Lithosphere D. Asthenosphere 4. The Morley-Vine-Matthews hypothesis explains the origin of: (Page 33-34) A. polar wandering B. seafloor magnetic anomalies C. continental drift D. mid-ocean ridges 5. The San Andreas Fault in California is a: (Page 42) A. convergent plate boundary B. divergent plate boundary C. transform plate boundary D. none of these 6. Which of the following features would you expect to find at an ocean-ocean convergent boundary? (Page 43-44) A. volcanic island arc B. ocean trench C. earthquakes D. all of these 7. Which of the following features would you expect to find at an ocean-continent convergent boundary? (Page 44) A. deep ocean trench B. volcanic mountain chain C. earthquakes D. all of these 8. Which of the following would you expect to find at a continent-continent convergent boundary? (Page 45-47)
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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) A. Deep ocean trench B. suture zone C. volcanic mountain chain D. all of these 9. Passive continental margins are created at: (Page 37) A. divergent plate boundaries B. convergent plate boundaries C. transform plate boundaries D. all of these 10. The Hawaiian Islands are the result of: (Page 57-60) A. subduction B. seafloor spreading C. a mantle plume D. none of these 11. Metallic ores are created at divergent plate boundaries: (Page 40-41) A. through hydrothermal processes B. in lava flows C. in sedimentary deposits D. all of these 12. A large supercontinent that existed 225 million years ago was: (Page 23-25) A. Gondwanaland B. Laurasia C. Glossopteris D. Pangea 13. Alfred Wegener was a: (Page 22) A. geologist B. geophysicist C. astronomer D. meteorologist 14. Which of the following was not used by Wegener as evidence of continental drift? (Page 23-
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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) 27) A. Fossils that were common to many continents. B. Evidence of glaciation on widely separated continents. C. The geometric fit of the continents. D. Magnetic anomalies on the seafloor. 15. Continental drift and the reorganization of this hypothesis to Plate Tectonics was made possible by advances in: (Page 25-29) A. Understanding of Earth's magnetic field. B. Understanding of Earth's fossil record. C. Development of radiometric dating methods. D. None of these. 16. An early hypothesis that eventually led to the Theory of Plate Tectonics was: (Page 29) A. The Elastic Rebound Theory of G.K. Gilbert in the early 1900's. B. "Theory of the Earth" by James Hutton, 1788. C. Seafloor Spreading by Harry Hess, 1962. D. All of these. 17. The age of the seafloor is relatively young because: (Page 31-32) A. It is continuously generated at mid-ocean ridges. B. It is continuously destroyed at subduction zones. C. Both of the above. D. None of the above. 18. If a plate moves 20 mm per year, how far will it move in 1 million years? (Box 2.1) A. 20 meters B. 20 kilometers C. 20 centimeters D. 2,000 kilometers 19. The East African Rift is an example of: (Page 59) A. A continent-continent convergent boundary. B. An ocean-continent convergent boundary. C. An ocean-ocean convergent boundary.
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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) D. A divergent plate boundary. 20. The Wilson Cycle is: (Box 2.5, Page 54) A. The formation of hot spots B. an ocean current C. the melting of rocks D. the opening and closing of an ocean 21. The oldest rocks on Earth are found in ocean basins. (Page 31-32) True False 22. Glaciers covered parts of Africa, Australia, South America, India, and Antarctica about 230 million years ago. (Page 24) True False 23. Glossopteris is a type of plant fossil found on a number of continents. (Page 23) True False 24. Wegener's evidence for continental drift was readily accepted by the scientific community. (Page 25) True False 25. Benioff Zones are associated with continent-continent convergent margins. (Page 43, 46) True False 26. Ocean-ocean convergent boundaries are also sites of subduction. (Page 43) True False 27. Explosive volcanic eruptions are associated with mid-ocean ridge volcanism. (Page 38-39) True False 28. All plates are presently moving in the same direction. (Ch.2)
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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) True False 29. The direction of motion of a plate cannot change over time. (Ch.2) True False 30. Most active geologic processes (earthquakes, volcanoes) are associated with plate boundaries. (Ch.2) True False 31. Apparent polar wander was really caused by the movement of plates. (Ch.2) True False 32. Africa and South America were once joined together. (Page 29) True False 33. There have been a number of supercontinents during Earth's history. (Ch.2) True False 34. Divergent plate boundaries form where there is upwelling from the asthenosphere. (Ch.2) True False 35. Active continental margins form near mid-ocean ridges. (Ch.2) True False 36. Large magnitude earthquakes emanate from subduction zones. (Ch.2) True False 37. Polar wandering refers to the great Arctic explorers who discovered the North magnetic pole. (Page 28) True
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PHYSICAL GEOLOGY AND THE ENVIRONMENT (2ND CANADIAN EDITION) False 38. Plate tectonic processes are responsible for development of large mountain belts. (Page 51) True False 39. Plate tectonic processes are influenced by processes deep inside Earth. (Page 60) True False 40. Plate tectonics theory unifies much of the information presented in your textbook. (Ch.2) True False
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