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Chapter Tests You are only required to take and submit for grading the Chapter Tests To receive credit, you must register for the course on www.mygeoscienceplace.com Not required to take the Concept Quiz, GEODe Quiz or Interactive Animation Quiz Access code for website came with your textbook or can be purchased separately Use class ID = cm284806. If you have any questions contact the Pearson UH representative Jessica Elbern ([email protected]) Plate Tectonics Fundamental Concept and Unifying Theory in Earth Science Idea is > 100 yrs old Acceptance only within the past 30 yrs 1 Drilling into the seafloor provided more evidence supporting sea-floor spreading Micropaleontology of sediments Dating of the underlying lavas Drilling ship Glomar Challenger Age of Seafloor Crust Realizing that the ocean basins are very young was key to acceptance of Plate Tectonics Theory 2 Ocean crust records magnetic reversals 3 Seafloor Spreading Hypothesis New seafloor forms by upwelling at the center of MOR and moves laterally Earth is not expanding so that must mean that older crust is destroyed in the subduction zones at the trenches Seafloor is younger than 200 MY Solved Continental Drift problem Plate Tectonics: The New Paradigm Earth’s major plates Associated with Earth's strong, rigid outer layer: Known as the lithosphere Consists of uppermost mantle and overlying crust Overlies a weaker region in the mantle called the asthenosphere 4 Inside the Earth Fig. 1.12 Plate Tectonics: The New Paradigm Earth’s major plates Seven major lithospheric plates Plates are in motion and are continually changing in shape and size. The largest plate is the Pacific plate. Several plates include an entire continent plus a large area of seafloor. 5 Earth’s Tectonic Plates Mosaic of Earth’s Plates 6 Rates of plate motion Mostly obtained from magnetic anomalies on seafloor Fast spreading: 10 cm/year Up to 20 cm/year Slow spreading: 2 cm/year Plate Tectonics: The New Paradigm Earth’s major plates Plates move relative to each other at a very slow but continuous rate. About 5 centimeters (2 inches) per year Cooler, Cooler, denser slabs of oceanic lithosphere descend into the mantle. 7 Plate Tectonics: The New Paradigm Plate boundaries Interactions among individual plates occur along their boundaries. Types of plate boundaries: Divergent plate boundaries (constructive margins) Convergent plate boundaries (destructive margins) Transform fault boundaries (conservative margins) Magnetic Anomalies in the Atlantic 8 We can map spreading centers with swath bathymetry… Divergent Plate Boundary 9 We can look at new “sea floor” on land Thingvellir, Thingvellir, Iceland: America to the left, Eurasia to the right, a rift down the middle. Iceland is being pulled apart as it sits astride the Mid-Atlantic Ridge. Gudmundur E. Sigvaldason, Nordic Volcanological Institute 10 Nothing beats going down there in person, or at least observing from a remotely-operated vehicle Mir (sub) Alvin (sub) Jason II (ROV) Ropos (ROV) UH/HURL Submersibles Pisces IV Pisces V 11 At spreading centers, you can look at the ocean crust in cross section Pillow lava Feeder dikes At fracture zones, you can look at the ocean crust in cross section Surface flows 12 Sometimes, sea floor gets thrust up on land to form what is called an ophiolite Oman, on the Persian Gulf Pillow lava Sheeted dikes How we think the oceanic crust forms Hot rock rises and partially melts The melt is erupted to form a layer of basalt lava flows and pillows 13 How we think the oceanic crust forms The feeders to the flows are vertical sheets called dikes Below the dikes, massive gabbro (like basalt, but with larger crystals) solidifies from the melt. How do we figure out the deep structure? Remote Sensing from the surface: Echo sounding using high-frequency sound (several KHz = thousand cycles per second But high-frequencies bounce off the hard rocks We need low frequency sound (5-100 Hz) to penetrate through the rock beneath the seafloor – this is called “seismic” prospecting 14 How do we figure out the deep structure? Then: explosives Now: Air Guns Maurice Ewing Seismic streamer Multichannel seismics - measure structure in two and even three dimensions 15 Sea floor structure What do we find right at the spreading axis? Black smoker: Hydrogen sulfide Giant tube worms and clams live on the Black smokers 16 What do we find right at the spreading axis? A single spot on the East Pacific Rise (pictures about a year apart) How does it work? Cold sea water circulates down through cracks Water heats up as it passes through hot rock Water interacts with rock -- dissolves minerals and becomes laden with dissolved sulfides 17 How does it work? Sulfides precipitate on exposure to cold water (black smoker) Bacteria oxidize the sulfides (chemosynthesis) Worms have a symbiotic relationship with the bacteria in their guts How does it work? Crabs live on dead worms, bacterial mats, and “snow” A complete complex food web is established 18 What are those offsets in the spreading center, and what are those parallel ridges? The problem was solved by the Canadian Geophysicist J. Tuzo Wilson 19 Transform Fault Boundaries Plates slide past one another and no new lithosphere is created or destroyed. Transform faults Most join two segments of a mid-ocean ridge along breaks in the oceanic crust known as fracture zones. A few (the San Andreas Fault and the Alpine Fault of New Zealand) cut through continental crust. Transform Faults offset Spreading Centers 20 Transform Fault Boundaries Notice how Wilson’s theory explain depth offsets across fracture zones 21 The San Andreas Fault: A transform fault separating the Pacific and North American Plates Earthquakes in mid-ocean areas 22 23 Divergent Plate Boundaries Continental rifting Splits landmasses into two or more smaller segments along a continental rift Examples include: East African Rift Valleys Rhine Valley in Northern Europe Produced by extensional forces Continental Rifting 24 Anatomy of a Plate Convergent Plate Boundaries Older portions of oceanic plates are returned to the mantle at these destructive plate margins. Surface expression of the descending plate is an ocean trench. Also called subduction zones Average angle of subduction = 45 degrees. 25 Convergent Plate Boundaries Types of convergent boundaries: Oceanic–continental convergence The denser oceanic slab sinks into the asthenosphere. asthenosphere. Along the descending plate, partial melting of mantle rock generates magma. The resulting volcanic mountain chain is called a continental volcanic arc. arc. (The Andes and the Cascades are examples.) Oceanic–Continental Convergence 26 Convergent Plate Boundaries Types of convergent boundaries: Oceanic–oceanic convergence When two oceanic slabs converge, one descends beneath the other. Often forms volcanoes on the ocean floor If the volcanoes emerge as islands, a volcanic island arc is formed. (Japan, the Aleutian islands, and the Tonga islands are examples.) Oceanic–Oceanic Convergence 27 Convergent Plate Boundaries Types of convergent boundaries: Continental–continental convergence Continued subduction can bring two continents together. Less dense, buoyant continental lithosphere does not subduct. subduct. The resulting collision produces mountains. (The Himalayas, the Alps, and the Appalachians are examples.) Continental–Continental Convergence 28 Continental Collision forms Mountains Types of Plate Margins 29 Tuzo Wilson’s other great contribution was the concept of a stationary hot spot Testing the Plate Tectonics Model Hot spots and mantle plumes Caused by rising plumes of mantle material Volcanoes can form over them (Hawaiian Island chain). Mantle plumes LongLong-lived structures Some originate at great depth. 30 Hawaiian-Emperor chain 31 Long-lived Global Hot Spots What Drives Plate Motions? Researchers agree that convective flow in the mantle is the basic driving force of plate tectonics. Forces that drive plate motion: Slab-pull Ridge push 32 Three possible mechanisms for the movement of lithosphere over the asthenosphere Forces Driving Plate Motions 33 What Drives Plate Motions? Models of plate–mantle convection Any model must be consistent with observed physical and chemical properties of the mantle. Models: Layering at 660 kilometers WholeWhole-mantle convection Convection in a Pot 34 Convection in the Mantle Increased Heat with Depth 35 Convection in the Mantle Upper Mantle Convection as a Possible Mechanism for Plate Tectonics 36 Importance of Plate Tectonics The theory provides explanations for: Earth’s major surface processes Distribution of earthquakes, volcanoes, and mountains Distribution of ancient organisms, rock types, geologic structures and mineral deposits 37