Chapter 2 - Plate Tectonics

Chapter 2 - Plate Tectonics - Grotzinger • Jordan...

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Unformatted text preview: Grotzinger • Jordan Understanding Earth Sixth Edition Chapter 2: PLATE TECTONICS: The Unifying Theory © 2011 by W. H. Freeman and Company Chapter 2: Chapter Plate Plate Tectonics: Tectonics: The Unifying The Theory Theory About Plate Tectonics About • It is the movement of plates and the It forces acting on them. forces • It explains the distribution of volcanoes, It earthquakes, folded mountain chains, rock assemblages, and seafloor structures. • The forces that drive plate motions arise The from the mantle convection system. from Lecture Outline Lecture 1. The discovery of plate tectonics 2. Plates and their boundaries 3. Rates and history of plate movements 4. The grand reconstruction 5. Mantle convection: the engine of plate 5. tectonics tectonics Lecture Outline Lecture 6. Theory of plate tectonics and the 6. scientific method scientific 1. Evolution of the Theory 1. Continental drift: “jjig-saw ig-saw puzzle” fit of continents 1. Evolution of the Theory 1. Continental Continental drift: drift: similarity of rock similarity assemblages and ages across oceans oceans 1. Evolution of the Theory 1. Continental Continental drift: drift: distribution of distribution certain fossils 1. Evolution of the Theory 1. Seafloor Seafloor spreading: spreading: geological geological activity in midactivity ocean ridges ocean 1. Evolution of the Theory 1. Seafloor Seafloor spreading: spreading: new crust new formed there formed Thought questions for this chapter Thought What mistakes did Wegener make in formulating his What theory of continental drift? Do you think the geologists of his era were justified in rejecting his theory? his 2. The Mosaic of Earth’s Crustal Plates 2. ● mosaic of rigid plates mosaic 2. The Mosaic of Earth’s Crustal Plates 2. ● three types of boundaries three 2. The Mosaic of Earth’s Crustal Plates 2. ● divergent, convergent, transform divergent, 2. The Mosaic of Earth’s Crustal Plates 2. ● next: a detailed look at the above next: 1. Divergent Boundaries 1. Divergent Boundaries (a) Oceanic plate separation rifting, volcanoes, and earthquakes rican Ame h Nort Plate Mid­ Atlantic Ridge Eura sian Plat e 1. Divergent Boundaries 1. Divergent Boundaries (b) Continental plate separation rift valleys, volcanoes, and earthquakes East African Rift Valley te n Pla frica A Soma li Sub plate 2. Convergent Boundaries 2. Convergent Boundaries (a) Ocean­ocean convergence oceanic trench, volcanic island arc, and deep earthquakes Mariana Islands e ppin Phili Plate Marianas Trench Pacifi c Plat e 2. Convergent Boundaries 2. Convergent Boundaries (b) Ocean­continent convergence volcanic mountain chain, folded mountains, and deep earthquakes Andes Mountains Peru­Chile Trench te azca Pla N South American Plate 2. Convergent Boundaries 2. Convergent Boundaries (c) Continent­continent convergence Himalaya Mountains crustal thickening, folded mountains, and earthquakes tralian Plate Indian­Aus Tibetan Plateau subduction Eurasian Plate 3. Transform­Fault Boundaries 3. Transform­Fault Boundaries (a) Mid­ocean ridge transform fault lateral (transform) faults and earthquakes Eurasi an Pla te e ican Plat er North Am 3. Transform­Fault Boundaries 3. Transform­Fault Boundaries (b) Continental transform fault lateral (transform) fault and earthquakes te ific Pla Pac North A merica n Plate Thought questions for this chapter Thought Why are there active volcanoes along the Pacific coast in Why Washington and Oregon but not along the east coast of the United States? the How do the differences between continental and oceanic How crust affect the way lithospheric plates interact? crust 3. Rates and History of Plate Motion 3. Ship towing a Ship sensitive magnetometer sensitive Mid-Atlantic Ridge high intensity low intensity lo Magnetic Magnetic anomalies: anomalies: seafloor areas seafloor of high and low magnetic values 3. Rates and History of Plate Motion 3. Iceland Mid-Atlantic Ridge Mid-Atlantic Ridge high intensity low intensity ● seafloor as a magnetic tape recorder seafloor 3. Rates and History of Plate Motion ● magnetic time scale developed magnetic 3. Rates and History of Plate Motion 3. Figure 2.15 ● magnetic isochrons on the seafloor magnetic 3. Rates and History of Plate Motion 3. ● velocity of seafloor spreading = d / t velocity 3. Rates and History of Plate Motion 3. ● example area: mid-ocean ridge, south of Iceland south 3. Rates and History of Plate Motion 3. ● Velocity = 60 km / 3.3 mil. yr. = 18 km / mil. yr. (or 18 mm / yr) 18 3. Rates and History of Plate Motion 3. Example relative plate velocities: East Pacific Rise (Pacific and Nazca plates) – East 138 to 150 mm/yr 138 South Atlantic (Mid-Atlantic Ridge) – South 34 to 35 mm/yr 34 Southern Ocean, south of Australia – Southern 70 to 75 mm/yr 70 Southern Ocean, south of Africa – Southern 14 mm/yr 14 Thought questions for this chapter Thought In Figure 2.15, the isochrons are symmetrically distributed In in the Atlantic Ocean, but not in the Pacific Ocean. For example, seafloor as much as 180 million years old (in darkest blue) is found in the western Pacific, but not in the eastern Pacific. Why? eastern 4. The Grand Reconstruction 4. Reconstructing the history of plate motions: 1. Assembly and breakup of the supercontinent Rodinia supercontinent Rodinia 1. Assembly and breakup of the supercontinent Pangaea supercontinent Pangaea 4. The Grand Reconstruction 4. The Earth’s geography one billion years The ago. Let’s see continental motion! ago. ASSEMBLY OF RODINIA Late Proterozoic (750 Ma) Formed about 1.1 billion years ago; began to break up about 750 million years ago ASSEMBLY OF PANGAEA Late Proterozoic (650 Ma) The distribution of continents and oceans between Rodinia and the assembly of Pangaea ASSEMBLY OF PANGAEA Middle Ordovician (458 Ma) The distribution of continents and oceans about 458 million years ago ASSEMBLY OF PANGAEA Early Devonian (390 Ma) The distribution of continents and oceans about 390 million years ago ASSEMBLY OF PANGAEA Early Triassic (237 Ma) The distribution of continents and oceans about 237 million years ago; Pangaea is formed BREAKUP OF PANGAEA Early Jurassic (195 Ma) The breakup of the supercontinent about 195 million years ago; Pangaea is being rifted BREAKUP OF PANGAEA Late Jurassic (152 Ma) The distribution of continents and oceans about 152 million years ago BREAKUP OF PANGAEA Late Cretaceous­Early Tertiary (66 Ma) The distribution of continents and oceans about 66 million years ago; much like today in some ways PRESENT DAY The distribution of continents and oceans as we know them today 5. Mantle Convection: The Engine 5. of Plate Tectonics of Upper Upper mantle Theory 1: Theory whole mantle convection convection Plate recycling extends to the core­ mantle boundary. 700 km Lower mantle 2900 km Outer core 5. Mantle Convection: The Engine 5. of Plate Tectonics of Theory 2: Theory stratified convection convection The lower mantle convects more sluggishly than the upper mantle. Boundary near Boundary near 700 km separates the two different convection systems. 5. Mantle Convection: The Engine of Plate Tectonics of spreading spreading centers and hot spots spots 6. Theory of Plate Tectonics and the Scientific Method Scientific • Plate tectonics is not a dogma, but a confirmed theory Plate whose strength lies in its simplicity, its generality, and its consistency with many types of observations. its • This theory has survived so many attempts to prove it This wrong and has been so important in explaining and predicting so many phenomena that geologists treat the theory as fact. the • Reasons why proof and acceptance took so long: very Reasons cautious approach of many scientists studying this issue; global scale of the problem; and specialized technology required to gain data took time to develop. technology Thought questions for this chapter Thought Would you characterize plate tectonics as a hypothesis, a Would theory, or a fact? Why? theory, The theory of plate tectonics was not widely accepted The until the banded patterns of magnetization on the ocean floor were discovered. In light of earlier observations – the jigsaw-puzzle fit of the continents, the occurrence of fossils of the same life-forms on both sides of the Atlantic, and the reconstruction of ancient climate conditions – why are these banded patterns of magnetism such key pieces of evidence? pieces Key terms and concepts Continental drift Continental Covergent boundary Divergent boundary Geodesy Island arc Isochron Magnetic anomaly Magnetic time scale Mantle plume Mid-ocean ridge Pangaea Plate tectonics Relative plate velocity Rodinia Key terms and concepts Seafloor spreading Seafloor Spreading center Subduction Transform fault ...
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