Chapter 5 Lecture Outline Foundations of Earth Science Seventh Edition Plate Tectonics: A Scientific Revolution Unfolds Natalie Bursztyn Utah State University
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
Focus Question 5.2 What is the evidence in support of Wegener s continental drift hypothesis?
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
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
Continental Drift: An Idea Before Its Time Identical fossils found in South America and Africa Paleontologists agree: land connection necessary to explain fossil record
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
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
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
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
Focus Question 5.3 What are the main objections to the continental drift hypothesis?
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
Focus Question 5.3 What are the main objections to the continental drift hypothesis? No mechanism for moving the continents
Focus Question 5.4 What are the major differences between the lithosphere and asthenosphere and how are they each important to plate tectonics?
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
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
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
The Theory of Plate Tectonics
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
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
The Theory of Plate Tectonics
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
Focus Question 5.5 How is lithosphere formed at a divergent plate boundary?
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
Divergent Plate Boundaries and Seafloor Spreading
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
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
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
Divergent Plate Boundaries and Seafloor Spreading
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
Focus Question 5.6 What are the characteristics of each of the three types of convergent boundaries?
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
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
Convergent Plate Boundaries and Subduction Characteristics of convergent plate boundaries vary depending on type of crust being subducted Oceanic + continental Oceanic + oceanic Continental + continental
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
Convergent Plate Boundaries and Subduction
Convergent Plate Boundaries and Subduction
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
Convergent Plate Boundaries and Subduction
Convergent Plate Boundaries and Subduction Continental crust is buoyant Neither plate subducts during continent-continent collisions Folding and deformation of rocks Mountain building
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
Focus Question 5.7 What is the relative motion along a transform fault?
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
Transform Plate Boundaries
Transform Plate Boundaries Few transform faults cut through continental crust San Andreas Fault (California) and Alpine Fault (New Zealand) are exceptions
Focus Question 5.7 What is the relative motion along a transform fault? horizontal
Plate Tectonics: A Scientific Revolution Unfolds What type of plate boundary shapes the landscape nearby you?
Focus Question 5.8 What is the evidence in support of the plate tectonic theory?
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
Testing the Plate Tectonics Model
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
Testing the Plate Tectonics Model
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
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
Testing the Plate Tectonics Model
Testing the Plate Tectonics Model
Testing the Plate Tectonics Model
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
Focus Questions 5.9 What is meant by plate-mantle convection? What are the primary driving forces of plate motion?
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
What Drives Plate Motions?
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
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
What Drives Plate Motions?
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
Focus Question 5.10 Why are some plates getting larger while others are getting smaller?
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
How Do Plates and Plate Boundaries Change? New ocean basins were created during the breakup of Pangaea
How Do Plates and Plate Boundaries Change? Present plate motions can be used to predict future continental positions
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