Plate Tectonics PDF

The Good Earth Introduction to Earth Science Chapter 4 Plate Tectonics © McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reprodu...

The Good Earth Introduction to Earth Science Chapter 4 Plate Tectonics © McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written consent of McGraw-Hill Education.  Outline 1. Continental Drift 2. Evidence from the Seafloor 3. Plate Tectonics 4. Plate Boundaries © reproduction Copyright © McGraw Hill LLC. All rights reserved. No McGraw-Hill Education. or distribution without the prior written consent of McGraw Hill LLC. 4-2  Continental Drift The geographer Ortelius noted the matching shapes of coastlines He pointed out that the pieces almost seemed fitting together as a jigsaw puzzle All matching patterns were considered little more than coincidence © McGraw-Hill Education. 4-3  Continental Drift In the 17th and18th centuries: Earth had a hot interior and that it was losing heat through its surface Earth was slowly cooling and contracting - some features on Earth`s surface as a result of contraction-known as the contracting Earth model! © McGraw-Hill Education. 4-4  Pre-Plate Tectonics Paradigm Contracting Earth: Planet is slowly cooling and contracting as heat of formation is lost Mountains represent “wrinkles” formed by the contraction of the surface Collapse of surface formed ocean basins Continents, oceans effectively fixed in place Vertical crustal movements dominate © reproduction Copyright © McGraw Hill LLC. All rights reserved. No McGraw-Hill Education. or distribution without the prior written consent of McGraw Hill LLC. 4-5  Paradigm Shift Wegener The first person to present a well-reasoned alternative explanation for the origin of continents and oceans Beginning in 1912, proposed the continental drift theory What is that? © McGraw-Hill Education. 4-6  Wegner`s Continental Drift Continents occupied different locations on Earth’s surface in the geologic past HOW? 200-250 million years ago the continents were all together in a “supercontinent”, Pangaea “drifted” to their present locations © McGraw-Hill Education. 4-7  Continental Drift Pangaea, 250 Mya Laurasia and Gondwana, 210 Mya Most modern continents had formed by 65 Mya © McGraw-Hill Education. 4-8 HOW? Wegener’s Observations: Matching features Geologic features on different continents would line up if the continents were reassembled as Pangea Distribution of plant and animal fossils matched between continents Plant and animal fossils formed linked patterns, if the continents reassembled a. Fossil distribution a: Adapted from USGS © McGraw-Hill Education. 4-9  Continental Drift b. Match of mountain belts among North America, Europe, and Greenland b: Adapted from USGS © McGraw-Hill Education. 4-10 Fit of the Continents Opposing edges of continents fit together A classic fit between the southwest coast of Africa and the east coast of S. America c. Fit of continents, matching rock units c: Adapted from USGS © McGraw-Hill Education. 4-11 Paleoclimates Documented ancient glacial deposits formed ~ 300 million years ago d. Glacial features. Arrows illustrate direction of ice movement © McGraw-Hill Education. 4-12 CONTINENTAL DRIFT Wegener’s Continental Drift hypothesis was not widely accepted! How the continents had moved? Wegener was unable to propose an acceptable mechanism…. Assumed that the continents had pushed through the rocks of ocean floor There was no obvious explanation © McGraw-Hill Education. 4-14  Acceptance of Wegener’s Ideas Wegener’s Continental Drift hypothesis was not widely accepted because: Wegener was considered an “outsider” among geologists, and his use of deductive reasoning was considered unusual for the time Wegener could not explain how the continents moved Supporters of the contracting Earth hypothesis came up with alternative explanations for some of Wegener’s observations e.g., land bridges allowed fossil organisms to move between continents © reproduction Copyright © McGraw Hill LLC. All rights reserved. No McGraw-Hill Education. or distribution without the prior written consent of McGraw Hill LLC. 4-15 Continental drift? A compelling explanation for the distribution of common features…. Continental drift theory - have to wait another 50 years………. WHY? Some of its components would return in the theory of plate tectonics © McGraw-Hill Education. 4-16  The amount of information about Earth`s surface greatly increased after World War II WHY? The features of the seafloor were mapped to allow submarines to move around the oceans without being detected © McGraw-Hill Education. 4-17  Evidence from the Seafloor In the decades following Wegener’s research, key observations about the seafloor contributed to a new understanding of Earth processes What are they? Seafloor topography Age of the seafloor Heat flow Volcanoes Earthquakes © McGraw-Hill Education. 4-18  Evidence from the Seafloor As a result? Sufficient evidence accumulated to provide a mechanism for Wegener`s continental drift theory - known as the seafloor spreading!!!! © McGraw-Hill Education. 4-19  The Seafloor Spreading Theory Evidence from the Seafloor? Seafloor Topography; Ocean floor gradually deepens from the coast, moving seaward across the shallow continental shelf. Key features: Continental shelf Abyssal plain Oceanic ridge Oceanic trench © McGraw-Hill Education. 4-20  Evidence from the Seafloor Continental shelf Narrow, shallow ocean surrounding continents Abyssal plain Relatively level seafloor, often with volcanoes Oceanic ridge Submarine mountain range that is a source of volcanic activity Oceanic trench Narrow, deepest portion of ocean floor © McGraw-Hill Education. 4-21  Oceanic ridges and trenches are important. WHY? – Play key roles in the process of plate tectonics OCEANIC RIDGE - occupies much of the OCEANIC TRENCHES - adjacent to some seafloor in all the world’s ocean basins continents or island chains, and along the Often found toward center of oceans margins of oceans Can be followed continuously through the major Most common around Pacific Ocean oceans – like a giant zipper © McGraw-Hill Education. 4-22  Age of the Ocean Floor Age of seafloor rocks varies Rocks of the seafloor are young compared to most rocks on the continents Recycling system? © McGraw-Hill Education. 4-23 Heat Flow, Volcanoes, and Earthquakes HEAT FLOW - greatest along oceanic ridge systems WHY? Magma forces its way into the oceanic crust – erupting onto seafloor The Ring of Fire? - the region around the Pacific Ocean Most active volcanoes are located around the Ring of Fire © McGraw-Hill Education. 4-24  SEAFLOOR SPREADING THEORY - A better theory Harry Hess - the seafloor spreading hypothesis, using data he collected during World War II. The seafloor is not permanent – moves away from oceanic ridges IMPORTANT? Provided a potential mechanism to explain the motion of the continents Wegener – Continental drift theory © McGraw-Hill Education. 4-25  Earth’s Magnetic Field Additional observations about the magnetic properties of seafloor rocks supported the seafloor spreading hypothesis Earth has a magnetic field because it has: 1. Fluid, iron-rich outer core 2. Energy to move the fluid (heat) 3. Rotation to mix the fluid © reproduction Copyright © McGraw Hill LLC. All rights reserved. No McGraw-Hill Education. or distribution without the prior written consent of McGraw Hill LLC. 4-26  PALEOMAGNETISM Evidence related to magnetization of ocean floor – paleomagnetism Magnetic Field Reversals HOW? © McGraw-Hill Education. 4-27  Paleomagnetism Preserves ancient magnetic field – paleomagnetism Atoms in magnetic minerals aligned parallel to the magnetic field when magma cooled to form seafloor rocks © reproduction Copyright © McGraw Hill LLC. All rights reserved. No McGraw-Hill Education. or distribution without the prior written consent of McGraw Hill LLC. 4-28  Paleomagnetism Magnetic Field Reversals Each period of normal or reversed polarity averages 250,000 years Longest = 10’s of millions of years Shortest = 10’s of thousands of years Few thousand years to change polarity (normal  reverse or reverse  normal) © reproduction Copyright © McGraw Hill LLC. All rights reserved. No McGraw-Hill Education. or distribution without the prior written consent of McGraw Hill LLC. 4-29 HOW PALEOMAGNETISM SUPPORTS THE SEAFLOOR SPREADING THEORY? Preserves ancient magnetic field = Paleomagnetism Symmetrical pattern on either side of ridge “Stripes” of similar width and polarity - areas of normal and reverse polarity HOW? Related to the formation of seafloor HOW? Atoms in magnetic minerals align parallel to the magnetic field when magma cooled to form seafloor rocks © McGraw-Hill Education. 4-30 Paleomagnetism and Seafloor Spreading Symmetrical pattern on either side of ridge “Stripes” of similar width and polarity © reproduction Copyright © McGraw Hill LLC. All rights reserved. No McGraw-Hill Education. or distribution without the prior written consent of McGraw Hill LLC. 4-31  Theory of PLATE TECTONICS Continental drift and Seafloor spreading imply that both the continents and ocean floors are in motion However, a little idea – how their motions related! SO? Theory of PLATE TECTONICS – many different land and ocean - based observations. Such as? © McGraw-Hill Education. 4-32  Key observations about the seafloor contributed to a new understanding of Earth processes o Seafloor topography o Age of the seafloor o Heat flow o Volcanoes o Earthquakes © reproduction Copyright © McGraw Hill LLC. All rights reserved. No McGraw-Hill Education. or distribution without the prior written consent of McGraw Hill LLC. 4-33  PLATE TECTONICS Lithosphere: Broken into a series of rigid, mobile tectonic plates Examples: N. American, S. American, Pacific, Nazca, Eurasian, African, Antarctic, and Indian-Australian Plates are composed of what? continental, oceanic lithosphere or both? Oceanic ridges and trenches - the principal boundaries © McGraw-Hill Education. 4-34  Plates of the World Lithosphere divided into mobile tectonic plates © McGraw-Hill Education. 4-35  Continental Margins Most continental margins are not plate boundaries Passive margins - free of volcanism and earthquake activity Continental margins that represent plate boundaries - active margins - characterized by volcanoes and earthquakes © McGraw-Hill Education. 4-36  Rate of Plate Movements Different plates move at different rate. Ultraslow spreading rates: less than 1 centimeter per year – at the oceanic ridge in the Arctic Ocean Slow spreading rates: 1-2 centimeters per year – the N. American and Eurasian plates - the ocean ridge in the northern Atlantic Ocean Rapid spreading rates: more than 15 centimeters per year – the Pacific and Nazca plates along the East Pacific Rise The Pacific Ocean is much wider than Atlantic Ocean because of more rapid plate motion © McGraw-Hill Education. 4-37 ENERGY FOR PLATE MOVEMENTS? The heat flow from Earth`s interior HOW? Remaining from Earth’s early formation Decay of radioactive elements The gradual cooling of Earth HOW? Heat slowly escapes from the interior through the surface © McGraw-Hill Education. 4-39  Over millions of years, the movements of plates have opened and closed oceans Have formed or broken apart continents Plate tectonics is an ongoing process. Plates and plate boundaries change over time. Movement may stop at some locations, new boundaries will form, and plates will look different in the future How Earth Will Look In 250 million Years? https://www.youtube.com/watch? v=hos7w8xrcEs © McGraw-Hill Education. 4-40  Plate Boundaries Classified into 3 categories HOW? based on their relative plate motions a. Divergent b. Convergent (e.g. oceanic ridges) (e.g. subduction zones) c. Transform (e.g. San Andreas fault, CA) © McGraw-Hill Education. 4-41 © McGraw-Hill Education. 4-42  Divergent Plate Boundaries Motion: Spreading Effect: Constructive (oceanic lithosphere created) Topography: Ridge/Rift valley Volcanic Activity: Yes a: Adapted from Charles (Carlos) Plummer, Physical Geology 11e © 2007, reproduced with permission of McGraw-Hill Education © McGraw-Hill Education. 4-43  Convergent Plate Boundaries Motion: Subduction Effect: Destructive (oceanic lithosphere destroyed) Topography: Trench Volcanic Activity: Yes © McGraw-Hill Education. 4-44 Oceanic–continental convergence Denser oceanic slab sinks into the asthenosphere Continental volcanic arc forms Oceanic–oceanic convergence Two oceanic slabs converge, and one descends beneath the other Often forms volcanoes on the ocean floor Continental– continental convergence When subducting plates contain continental material, © McGraw-Hill Education. 4-45  Transform plate boundaries Motion: Lateral sliding Effect: Conservative (oceanic lithosphere neither created or destroyed) Topography: No major effect - large faults may form Volcanic Activity: No Adapted from Charles (Carlos) Plummer, Physical Geology 11e © 2007, reproduced with permission of McGraw-Hill Education © McGraw-Hill Education. 4-46