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Chapter 5 Lecture Outline Foundations of Earth Science Seventh Edition Plate Tectonics: A Scientific Revolution Unfolds Natalie Bursztyn Utah State University © 2014 Pearson Education, Inc. From Continental Drift to Plate Tectonics • Before 1960 geologists saw the positions of ocean basins and continents as fixed – Continental drift suggested but not agreeable • A new model of tectonic processes – A scientific revolution – Tectonic processes deform crust and create major structural features © 2014 Pearson Education, Inc. Focus Question 5.2 • What is the evidence in support of Wegener’s continental drift hypothesis? © 2014 Pearson Education, Inc. Continental Drift: An Idea Before Its Time • World maps in the 1600s suggested that South America and Africa fit together • In 1915, Alfred Wegener outlined the hypothesis of continental drift – Single supercontinent of all of Earth’s land: Pangaea – Fragmented ~200 mya and smaller landmasses drifted to their present positions © 2014 Pearson Education, Inc. Continental Drift: An Idea Before Its Time • Similarity between coastlines on opposite sides of the Atlantic • Opponents argued that coastlines are modified through time by erosion and deposition • Continental shelf is a better approximation of the boundary of a continent © 2014 Pearson Education, Inc. Continental Drift: An Idea Before Its Time • Identical fossils found in South America and Africa • Paleontologists agree: land connection necessary to explain fossil record © 2014 Pearson Education, Inc. Continental Drift: An Idea Before Its Time • Mesosaurus – Small Permian aquatic freshwater reptile – Found in eastern S. America and western Africa • Glossopteris – Seed fern – Africa, Australia, India, S. America, and Antarctica • Opponents explain fossil patterns by rafting, oceanic land bridges, and island stepping stones © 2014 Pearson Education, Inc. Continental Drift: An Idea Before Its Time • Rock types and geologic features match up – 2.2 billion-year-old igneous rocks in Brazil and Africa – Mountain belts end at coastlines and reappear across oceans © 2014 Pearson Education, Inc. Continental Drift: An Idea Before Its Time • Evidence for glaciation on continents now at tropical latitudes – Can be explained by supercontinent located near the South Pole © 2014 Pearson Education, Inc. Focus Question 5.2 • What is the evidence in support of Wegener’s continental drift hypothesis? – – – – Jigsaw-puzzle fit of the continents Fossils Rock types and geologic structures Evidence for glaciation © 2014 Pearson Education, Inc. Focus Question 5.3 • What are the main objections to the continental drift hypothesis? © 2014 Pearson Education, Inc. The Great Debate • Wegener’s hypothesis of continental drift was met with criticism • Objections were based on lack of mechanism for continental drift – Wegener proposed that tidal forces moved continents and that sturdy continents broke through thin oceanic crust © 2014 Pearson Education, Inc. Focus Question 5.3 • What are the main objections to the continental drift hypothesis? – No mechanism for moving the continents © 2014 Pearson Education, Inc. Focus Question 5.4 • What are the major differences between the lithosphere and asthenosphere and how are they each important to plate tectonics? © 2014 Pearson Education, Inc. The Theory of Plate Tectonics • Oceanographic exploration increased dramatically following World War II – Discovery of global oceanic ridge system – Earthquakes at great depths in western Pacific ocean trenches – No oceanic crust older than 180 million years – Thin sediment accumulations in deep-ocean basins © 2014 Pearson Education, Inc. The Theory of Plate Tectonics • Lithosphere is the crust and uppermost (coolest) mantle – Oceanic lithosphere varies in thickness • Thin at ridges, up to 100 km thick • Mafic composition • More dense than continental lithosphere – Continental lithosphere 150–200 km thick • Felsic composition – Responds to forces by bending or breaking © 2014 Pearson Education, Inc. The Theory of Plate Tectonics • Asthenosphere is the hotter, weaker mantle below the lithosphere – Rocks are nearly melted at this temperature and pressure – Responds to forces by flowing – Moves independently from lithosphere © 2014 Pearson Education, Inc. The Theory of Plate Tectonics © 2014 Pearson Education, Inc. The Theory of Plate Tectonics • Lithosphere is broken into irregular plates – Plates move as rigid units relative to other plates – 7 major plates make up 94% of Earth © 2014 Pearson Education, Inc. The Theory of Plate Tectonics • Interaction between plates at plate boundaries – Divergent boundaries (constructive margins) • Two plates move apart • Upwelling of hot material from mantle creates new seafloor – Convergent boundaries (destructive margins) • Two plates move together • Oceanic lithosphere descends and is reabsorbed into mantle • Two continental blocks create a mountain belt – Transform plate boundaries (conservative margins) • Two plates slide past each other • No lithosphere is created or destroyed © 2014 Pearson Education, Inc. The Theory of Plate Tectonics © 2014 Pearson Education, Inc. Focus Question 5.4 • What are the major differences between the lithosphere and asthenosphere and how are they each important to plate tectonics? – Lithosphere is brittle and rigid – Asthenosphere flows – Lithosphere is broken up into plates which move around relative to one another © 2014 Pearson Education, Inc. Focus Question 5.5 • How is lithosphere formed at a divergent plate boundary? © 2014 Pearson Education, Inc. Divergent Plate Boundaries and Seafloor Spreading • Most divergent plate boundaries are along the crests of oceanic ridges • New ocean floor is generated when mantle fills narrow fractures in oceanic crust © 2014 Pearson Education, Inc. Divergent Plate Boundaries and Seafloor Spreading © 2014 Pearson Education, Inc. Divergent Plate Boundaries and Seafloor Spreading • Most divergent plate boundaries are associated with oceanic ridges – Elevated seafloor with high heat flow and volcanism – Longest topographic feature on Earth’s surface (covers 20% of surface) – Crest is 2 to 3 km higher than adjacent basin and can be 1000 to 4000 km wide • Rift valley is a deep canyon along the crest of a ridge resulting from tensional forces © 2014 Pearson Education, Inc. Divergent Plate Boundaries and Seafloor Spreading • Seafloor spreading is the process by which new seafloor is created along the ocean ridge system – Average spreading rate is ~5 cm/year • Up to 15 cm/year or as slow as 2 cm/year – New lithosphere is hot (less dense) but cools and subsides with age and distance from the ridge system © 2014 Pearson Education, Inc. Divergent Plate Boundaries and Seafloor Spreading • Continental rifting occurs when divergent boundaries develop within a continent – Tensional forces stretch and thin the lithosphere – Brittle crust breaks into large blocks – Eventually become ocean basins © 2014 Pearson Education, Inc. Divergent Plate Boundaries and Seafloor Spreading © 2014 Pearson Education, Inc. Focus Question 5.5 • How is lithosphere formed at a divergent plate boundary? – Adjacent plates move apart and create space where hot mantle rocks upwell © 2014 Pearson Education, Inc. Focus Question 5.6 • What are the characteristics of each of the three types of convergent boundaries? © 2014 Pearson Education, Inc. Convergent Plate Boundaries and Subduction • Convergent plate boundaries occur when two plates move toward each other – Convergence rate is equal to seafloor spreading – Characteristics vary depending on subducting crust – Subduction zones • Lithosphere descends into the mantle – Old oceanic crust is ~2% denser than asthenosphere – Continental crust less dense than asthenosphere © 2014 Pearson Education, Inc. Convergent Plate Boundaries and Subduction • Deep ocean trenches – Long, linear depressions – Result of subduction • Angle of subduction varies – Nearly flat to nearly vertical – Depends on density of crust • Older crust is cooler and denser © 2014 Pearson Education, Inc. Convergent Plate Boundaries and Subduction • Characteristics of convergent plate boundaries vary depending on type of crust being subducted – Oceanic + continental – Oceanic + oceanic – Continental + continental © 2014 Pearson Education, Inc. Convergent Plate Boundaries and Subduction • Oceanic lithosphere + continental lithosphere = subduction of oceanic lithosphere – Continental lithosphere is less dense – Water from descending oceanic crust triggers partial melting of asthenosphere at ~100 km – Molten material is less dense and rises • Continental volcanic arcs © 2014 Pearson Education, Inc. Convergent Plate Boundaries and Subduction © 2014 Pearson Education, Inc. Convergent Plate Boundaries and Subduction © 2014 Pearson Education, Inc. Convergent Plate Boundaries and Subduction • One slab subducts under another at oceanicoceanic convergent boundaries – Volcanism because of partial melting – Generates volcanic island arcs • Volcanic cones underlain by oceanic crust © 2014 Pearson Education, Inc. Convergent Plate Boundaries and Subduction © 2014 Pearson Education, Inc. Convergent Plate Boundaries and Subduction • Continental crust is buoyant – Neither plate subducts during continent-continent collisions • Folding and deformation of rocks • Mountain building © 2014 Pearson Education, Inc. Focus Question 5.6 • What are the characteristics of each of the three types of convergent boundaries? – Oceanic-continental collisions result in subduction zones, deep ocean trenches, and continental volcanic arcs – Oceanic-oceanic collisions result in volcanic island arcs and deep ocean trenches – Contiental-continental collisions result in deformed sedimentary rocks and mountain belts © 2014 Pearson Education, Inc. Focus Question 5.7 • What is the relative motion along a transform fault? © 2014 Pearson Education, Inc. Transform Plate Boundaries • Transform plate boundaries form when two plates slide horizontally past one another – Transform faults – No lithosphere is produced or destroyed – Connect spreading centers and offsets oceanic ridges • Linear breaks in the seafloor are fracture zones – Fracture zones are inactive – Active faults occur between offset ridge segments © 2014 Pearson Education, Inc. Transform Plate Boundaries © 2014 Pearson Education, Inc. Transform Plate Boundaries • Few transform faults cut through continental crust – San Andreas Fault (California) and Alpine Fault (New Zealand) are exceptions © 2014 Pearson Education, Inc. Focus Question 5.7 • What is the relative motion along a transform fault? – horizontal © 2014 Pearson Education, Inc. Plate Tectonics: A Scientific Revolution Unfolds • What type of plate boundary shapes the landscape nearby you? © 2014 Pearson Education, Inc. Focus Question 5.8 • What is the evidence in support of the plate tectonic theory? © 2014 Pearson Education, Inc. Testing the Plate Tectonics Model • Evidence from Deep Sea Drilling Project – Collect sediment and oceanic crust – Date fossils in sediment • Sediment age increases with distance from ridge • Sediment is thicker with increased distance from the ridge • Oldest seafloor is 180 million years old © 2014 Pearson Education, Inc. Testing the Plate Tectonics Model © 2014 Pearson Education, Inc. Testing the Plate Tectonics Model • Volcanoes in the Hawaiian Island-Emperor Seamount Chain increase in age with distance from Hawaii – A cylinder of upwelling hot rock (mantle plume) is beneath Hawaii – A hot spot is an area of volcanism, high heat flow, and crustal uplift above a mantle plume – A hot-spot track formed as the Pacific Plate moved over the hot spot © 2014 Pearson Education, Inc. Testing the Plate Tectonics Model © 2014 Pearson Education, Inc. Testing the Plate Tectonics Model • Today North and South magnetic poles align approximately with geographic North and South poles • Iron-rich minerals influenced by magnetic pole – Basalt erupts above the curie temperature, so magnetite grains are nonmagnetic – Grains align to magnetic field during cooling – Rocks preserve a record of the direction of magnetic poles at the time of formation • Paleomagnetism or fossil magnetism • Position of paleomagnetic poles appears to change through time because of continental drift © 2014 Pearson Education, Inc. Testing the Plate Tectonics Model • Magnetic field reverses polarity during a magnetic reversal – Rocks with same magnetic field as today have normal polarity – Rocks with opposite magnetism have reverse polarity • Polarity of lava flows with radiometric ages was used to generate a magnetic time scale – Divided into chrons ~1 million years long – Finer-scale reversals within each chron • Vine and Matthews (1963) suggested stripes of normal and reverse polarity are evidence of seafloor spreading © 2014 Pearson Education, Inc. Testing the Plate Tectonics Model © 2014 Pearson Education, Inc. Testing the Plate Tectonics Model © 2014 Pearson Education, Inc. Testing the Plate Tectonics Model © 2014 Pearson Education, Inc. Focus Question 5.8 • What is the evidence in support of the plate tectonic theory? – – – – Age and thickness of seafloor sediment Hot-spot tracks Apparent polar wander Magnetic reversals and seafloor spreading © 2014 Pearson Education, Inc. Focus Questions 5.9 • What is meant by plate-mantle convection? • What are the primary driving forces of plate motion? © 2014 Pearson Education, Inc. What Drives Plate Motions? • Mantle is solid, but hot and weak enough to flow • Convection occurs as hot, less dense material rises and surface material cools and sinks • During slab pull, cold, dense oceanic crust sinks because it is denser than the asthenosphere • During ridge push, gravity causes lithospheric slabs to slide down the ridge • Drag in the mantle also affects plate motion © 2014 Pearson Education, Inc. What Drives Plate Motions? © 2014 Pearson Education, Inc. What Drives Plate Motions? • Mantle convective flow drives plate motion • Subducting plates drive downward component of convection • Upwelling of hot rock at oceanic ridges drives upward component of convection • Convective flow is the heat transfer mechanism from Earth’s interior © 2014 Pearson Education, Inc. What Drives Plate Motions? • Multiple models for convective flow: – Whole-mantle convection • Cold oceanic lithosphere sinks and stirs entire mantle • Subducting slabs sink to core-mantle boundary • Balanced by buoyant mantle plumes – Layer cake model • Subducting slabs do not sink past 1000 km © 2014 Pearson Education, Inc. What Drives Plate Motions? © 2014 Pearson Education, Inc. Focus Questions 5.9 • What is meant by plate-mantle convection? – Cool, dense oceanic lithosphere sinks and warm mantle upwells at mid-ocean ridges and mantle plumes • What are the primary driving forces of plate motion? – Slab pull and ridge push © 2014 Pearson Education, Inc. Focus Question 5.10 • Why are some plates getting larger while others are getting smaller? © 2014 Pearson Education, Inc. How Do Plates and Plate Boundaries Change? • Total surface area of Earth is constant • Size and shape of individual plates changes – African and Antarctic Plates are growing • Surrounded by divergent boundaries – Pacific Plate is being consumed • Surrounded by convergent boundaries) • Plate boundaries move and change through time © 2014 Pearson Education, Inc. How Do Plates and Plate Boundaries Change? • New ocean basins were created during the breakup of Pangaea © 2014 Pearson Education, Inc. How Do Plates and Plate Boundaries Change? • Present plate motions can be used to predict future continental positions © 2014 Pearson Education, Inc. Focus Question 5.10 • Why are some plates getting larger while others are getting smaller? – Earth’s surface area does not increase – As new crust forms, old crust is destroyed © 2014 Pearson Education, Inc.