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10/9/2008 Understanding Earth Fifth Edition Grotzinger • Jordan • Press • Siever Chapter 2: PLATE TECTONICS Lecturer: H Mohammadzadeh Assistant professors, Department of Geology, FUM Copyright © 2007 by W. H. Freeman and Company Chapter 2 Plate Tectonics 1 10/9/2008 About Plate Tectonics • It is the movement of plates and the forces acting between them. • It explains the distribution of volcanoes, earthquakes, folded mountain chains, rock assemblages, and seafloor structures. • The forces that drive plate motions arise from the mantle convection system. Lecture Outline 1. Evolution of the theory of plate tectonics 2. Mosaic of the Earth’s crustal plates 3. Rates and history of plate motion 4. Mantle convection: the engine of plate tectonics 2 10/9/2008 1. Evolution of the theory Continental drift • fit of continents “jigsaw puzzle” • similarity of rock assemblages and ages across oceans 1. Evolution of the theory Continental drift distribution of certain fossils 3 10/9/2008 1. Evolution of the theory Seafloor spreading geological activity in midmidocean ridges 1. Evolution of the theory Seafloor spreading new crust formed there 4 10/9/2008 2. The mosaic of Earth’s crustal plates • mosaic of rigid plates • three types of boundaries (divergent,, convergent, transform) (divergent 2. The mosaic of Earth’s crustal plates ● next: a detailed look at the above 5 10/9/2008 1. Divergent Boundaries (a) Oceanic plate separation rifting, volcanoes, and earthquakes MidMidAtlantic Ridge 1. Divergent Boundaries (b) Continental plate separation rift valleys, volcanoes, and earthquakes East African Rift Valley 6 10/9/2008 2. Convergent Boundaries (a) OceanOcean-ocean convergence oceanic trench, volcanic island arc, and deep earthquakes Mariana Islands Marianas Trench 2. Convergent Boundaries (b) OceanOcean-continent convergence volcanic mountain chain, folded mountains, and deep earthquakes Andes Mountains Peru--Chile Trench Peru South American Plate 7 10/9/2008 2. Convergent Boundaries (c) ContinentContinent-continent convergence crustal thickening, folded mountains, and earthquakes Himalayan Mountains Tibetan Plateau subduction Eurasian Plate 3. TransformTransform-Fault Boundaries (a) MidMid-ocean ridge transform fault lateral (transform) faults and earthquakes 8 10/9/2008 3. TransformTransform-Fault Boundaries (b) Continental transform fault lateral (transform) fault and earthquakes 3. Rates and history of plate motion Magnetic anomalies seafloor areas of high and low magnetic values 9 10/9/2008 3. Rates and history of plate motion ● sea floor as a magnetic tape recorder 3. Rates and history of plate motion 5.0 Ma 4.0 Gilbert reversed chron 3.0 Gauss normal chron 2.0 1.0 Matuyama reversed chron Present Brunhes normal chron ● magnetic time scale developed 10 10/9/2008 3. Rates and history of plate motion ● magnetic isochrons on the sea floor 3. Rates and history of plate motion ● velocity of seafloor spreading = d / t 11 10/9/2008 3. Rates and history of plate motion ● example area: midmid-ocean ridge, south of Iceland Velocity = 60 km / 3.3 mil. yr. = 18 km / mil. yr. (or 18 mm / yr) 3. Rates and history of plate motion Example relative plate velocities: East Pacific Rise (Pacific and Nazca plates) – 138 to 150 mm/yr South Atlantic (Mid(Mid-Atlantic Ridge) – 34 to 35 mm/yr Southern Ocean, south of Australia – 70 to 75 mm/yr Southern Ocean, south of Africa – 14 mm/yr 12 10/9/2008 3. Rates and history of plate motion Reconstructing the history of plate motions: 1. Assembly and breakup of the supercontinent Rodinia 2. Assembly and breakup of the supercontinent Pangaea 3. Rates and history of plate motion The Earth’s geography one billion years ago. Let’s see continental motion! 13 10/9/2008 ASSEMBLY OF RODINIA Late Proterozoic Formed about 1.1 billion years ago; began to break up about 750 million years ago ASSEMBLY OF PANGAEA Late Proterozoic The distribution of continents and oceans between Rodinia and the assembly of Pangaea Middle Ordovician The distribution of continents and oceans about 458 million years ago 14 10/9/2008 ASSEMBLY OF PANGAEA Early Devonian The distribution of continents and oceans about 390 million years ago Early Triassic The distribution of continents and oceans about 237 million years ago; Pangaea is formed BREAKUP OF PANGAEA Early Jurassic The breakup of the supercontinent about 195 million years ago; Pangaea is being rifted Late Jurassic The distribution of continents and oceans about 152 million years ago 15 10/9/2008 BREAKUP OF PANGAEA Late CretaceousEarly Tertiary The distribution of continents and oceans about 65 million years ago; much like today in some ways PRESENT DAY The distribution of continents and oceans as we know them today 4. Mantle convection: the engine of plate tectonics Upper mantle ● theory 1: whole mantle convection 700 km Lower mantle 2900 km Outer core 16 10/9/2008 4. Mantle convection: the engine of plate tectonics ● theory 2: stratified convection Boundary near 700 km separates the two different convection systems 4. Mantle convection: the engine of plate tectonics spreading centers and hot spots 17 10/9/2008 Thought questions for this chapter 1. What is the theory of plate tectonics? 2. What are some of the geologic characteristics of plate boundaries? 3. How can the age of the seafloor be determined? 4. What is the engine that drives plate tectonics? Key terms and concepts Continental drift Geodesy Island arc Isochron Magnetic anomaly Magnetic time scale Pangaea Plate tectonics Relative plate velocity Rodinia Seafloor spreading Spreading center Subduction Transform fault 18