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Geology Ch. 9 Notes Objectives: 1. Describe the hypothesis of continental drift 2. Evaluate the evidence in support of continental drift 3. Identify the main objections to Wegner’s hypothesis of continental drift 4. Explain the theory of plate tectonics 5. Describe lithospheric plates 6. Identify the three types of plate boundaries 7. Explain how seafloor spreading and continental rifting cause formation of new lithosphere 8. Describe the process of lithosphere destruction that takes place at subduction zones 9. Differentiate among subduction at oceanic-continental, oceanic-oceanic, and continental-continental convergent boundaries 10. Describe the action of plates at a transform fault boundary 11. Explain how Paleomagnetism and magnetic reversals provide evidence that supports the theory of plate tectonics 12. Evaluate how earthquakes, ocean drilling, and hot spots provide evidence that supports the theory of plate tectonics 9.1 Continental Drift An Idea Before Its Time Wegener’s continental drift hypothesis stated that the continents had once been joined to form a single ____________________________ • Wegener proposed that the supercontinent, __________, began to break apart 200 million years ago and form the present landmasses. Evidence • The Continental Puzzle • ______________________________ - Fossil evidence for continental drift includes several fossil organisms found on different landmasses. • Rock Types and __________________________ - Rock evidence for continental exists in the form of several ________________that end at one coastline, only to reappear on a landmass across the ocean. • Ancient ___________________________________ Rejecting the _______________________________ A New Theory Emerges • Wegener could not provide an explanation of exactly what made the continents move. News technology lead to findings which then lead to a new theory called__________________________. 9.2 Plate Tectonics Earth’s Major Roles According to the plate tectonics theory, the________________, along with the overlying crust, behaves as a strong, rigid layer. This layer is known as the lithosphere. • A_________________ is one of numerous rigid sections of the lithosphere that move as a unit over the material of the asthenosphere. Types of Plate Boundaries Divergent boundaries (also called____________________) are the place where two plates move apart. Convergent boundaries form where two plates move _____________. Transform fault boundaries are margins where two plates grind past each other without the production or destruction of the ___________________ 9.3 Actions at Plate Boundaries Divergent Boundaries _____________________and Seafloor Spreading • Oceanic ridges are continuous elevated zones on the floor of all major ocean basins. The____________ at the crest of ridges represent divergent plate boundaries. • __________________are deep faulted structures found along the axes of divergent plate boundaries. They can develop on the seafloor or on land. • ____________________produces new oceanic lithosphere. Continental Rifts • When spreading centers develop within a continent, the landmass may split into two or more smaller segments, forming_________________. Convergent Boundaries A _____________________occurs when one oceanic plate is forced down into the mantle beneath a second plate. Oceanic-Continental • Denser oceanic slab sinks into the ______________________ • Pockets of magma develop and rise. • Continental ______________________form in part by volcanic activity caused by the subduction of oceanic lithosphere beneath a continent • Examples include the Andes, Cascades, and the _________________________________________ Oceanic-Oceanic • Two oceanic slabs converge and one descends beneath the other. • This kind of boundary often forms ______________on the ocean floor. • Volcanic ______________________form as volcanoes emerge from the sea. • Examples include the Aleutian, __________________, and Tonga islands. Continental-Continental • When subducting plates contain continental material, two continents collide. • This kind of boundary can produce new mountain ranges, such as the __________________________ Transform Fault Boundaries At a transform fault boundary, plates _________________each other without destroying the lithosphere. Transform faults • Most join two segments of a_________________________. • At the time of formation, they roughly ____________the direction of plate movement • They aid the movement of_______________ crustal material. 9.4 Testing Plate Tectonics Evidence for Plate Tectonics __________________is the natural remnant magnetism in rock bodies; this permanent magnetization acquired by rock can be used to determine the location of the magnetic poles at the time the rock became magnetized. • __________________—when rocks show the same magnetism as the present magnetism field • ___________________—when rocks show the opposite magnetism as the present magnetism field The discovery of strips of alternating polarity, which lie as mirror images across the ocean ridges, is among the strongest evidence of________________________. Earthquake Patterns • found a close link between deep-focus earthquakes and_______________. • The absence of deep-focus earthquakes along the oceanic ridge system was shown to be consistent with the new theory. Ocean Drilling • The data on the_____________ of seafloor sediment confirmed what the seafloor spreading hypothesis predicted. The youngest oceanic crust is at the ridge crest, and the oldest oceanic crust is at the____________________________________. Hot Spots • A _____________is a concentration of heat in the mantle capable of producing magma, which rises to Earth’s surface; The Pacific plate moves over a hot spot, producing the______________________. • Hot spot evidence supports that the plates move over the Earth’s surface. 9.5 Mechanisms of Plate Motion Causes of Plate Motion Scientists generally agree that convection occurring in the mantle is the basic driving force for plate movement • ___________________is the motion of matter resulting from changes in temperature Slab-Pull and Ridge-Push • _________________is a mechanism that contributes to plate motion in which cool, dense oceanic crust sinks into the mantle and “pulls” the trailing lithosphere along. It is thought to be the primary downward arm of convective flow in the mantle. • ________________causes oceanic lithosphere to slide down the sides of the oceanic ridge under the pull of gravity. It may contribute to plate motion. Mantle Convection • ________________are masses of hotter-than-normal mantle material that ascend toward the surface, where they may lead to igneous activity. • The _____________________distribution of heat within Earth causes the thermal convection in the mantle that ultimately drives plate motion. Definitions: 1. Continental drift: a hypothesis that originally proposed that continents had once been joined to form a single supercontinent broken into pieces that drifted into their present positions 2. Pangaea: the proposed supercontinent that 200 hundred million years ago broke apart and formed the present day continents 3. plate tectonics: the theory that proposes that earth’s outer shell consists of individual plates that interact in various ways and thereby produce earthquakes, volcanoes, mountains and the crust itself 4. plate: one of numerous rigid sections of the lithosphere that moves as a unit over the material of the asthenosphere 5. divergent boundary: a region where the rigid plates are moving apart, typified by oceanic ridges 6. convergent boundary: a boundary in which two plates move together 7. transform fault boundary: a boundary in which two plates slide past each other without creating or destroying lithosphere 8. oceanic ridge: a continuous elevated zone on the floor of all major ocean basins and varying in width from 1,000 to 4,000km; the rifts at the crests of the ridges represent divergent plate boundaries 9. rift valley: deep faulted structure found along the axes of divergent plate boundaries, rift valleys can develop on the seafloor or on the land 10. seafloor spreading: the process by which plate tectonics produces new oceanic lithosphere at ocean ridges 11. subduction zone: a destructive plate margin where oceanic crust is being pushed down into the mantle beneath a second plate. 12. trench: a surface feature in the seafloor produced by the descending plate during subduction 13. continental volcanic arc: mountains formed in part by volcanic activity caused by subduction of oceanic lithosphere beneath a continent 14. volcanic island arc: a chain of volcanic islands generally located a few hundred kilometers from a trench where subduction of one oceanic slab beneath another is occurring 15. Paleomagnetism: the study of changes in the earth’s magnetic field as shown by evidence of magnetism in rocks that have formed over time 16. normal polarity: a magnetic field that is the same as that which exists during the present day 17. reverse polarity: a magnetic field opposite to that which exist today 18. hot spot: a concentration of heat in the mantle capable of producing magma which rises to the earth’s surface. The Pacific plate moves over a hot spot producing the Hawaiian islands 19. convective flow: the motion of matter resulting from changes in temperature; the convective flow within the mantle is due to earth’s unequal heating and causes the tectonic plates to move 20. slab-pull: the mechanism that contributes to plate motion in which cool dense oceanic crust sinks into the mantle and pulls the trailing lithosphere along 21. ridge-push: mechanism that may contribute to plate motion, it involves the oceanic lithosphere sliding down the oceanic ridge under the pull of gravity 22. mantle plume: a mass of hotter than normal mantle material that ascends to the surface where it may lead to igneous activity