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Lecture Outlines Physical Geology, 14/e Plummer, Carlson & Hammersley Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Plate Tectonics: The Unifying Theory Physical Geology 14/e, Chapter 19 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Plate Tectonics Plate tectonics – Earth’s surface is composed of a few large, thick plates that move slowly and change in size •combination of continental drift and seafloor spreading hypotheses from the late 1960s Plate boundaries – plates move away, toward, or past each other •intense geologic activity Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Early Case for Continental Drift Puzzle-piece fit of coastlines of Africa and South America has long been known Alfred Wegener – noted South America, Africa, India, Antarctica, and Australia have almost identical late Paleozoic rocks and fossils in early 1900s • Glossopteris (plant), Lystrosaurus and Cynognathus (animals) fossils found on all five continents • Mesosaurus (reptile) fossils found in Brazil and South Africa only Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Early Case for Continental Drift Pangaea – supercontinent proposed by Wegener • Laurasia - northern supercontinent containing North America and Asia (excluding India) • Gondwanaland - southern supercontinent containing South America, Africa, India, Antarctica, and Australia Late Paleozoic glaciation patterns – on southern continents best explained by their reconstruction into Gondwanaland Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Early Case for Continental Drift Coal beds – from North America and Europe support reconstruction into Laurasia Reconstructed paleoclimate belts – suggested polar wandering, potential evidence for continental drift Continental drift hypothesis initially rejected • Wegener could not come up with viable driving force • continents should not be able to plow through sea floor rocks while crumpling themselves but not the sea floor Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Paleomagnetism & Continental Drift Revived Rock magnetism studies – allowed determination of magnetic pole locations through time Paleomagnetism – uses mineral magnetic properties to determine direction and distance to the magnetic pole when rocks formed – steeper dip angles indicate rocks formed closer to the magnetic poles – rocks with increasing age point to pole locations increasingly far from present magnetic pole positions Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Paleomegnetism & Continental Drift Revived Apparent polar wander curves for different continents suggest real movement relative to one another Reconstruction of supercontinents using paleomagnetic information fits Africa and South America like puzzle pieces • improved fit results in rock units (and glacial ice flow directions) precisely matching up across continent margins Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Seafloor Spreading Seafloor spreading – proposed in 1962 by Harry Hess • seafloor moves away from the mid-oceanic ridge due to hot mantle rock rising by convection Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Seafloor Spreading Increasing distance from ridge – seafloor, and mantle rocks beneath it, cools and become more dense Subduction zones – locations where cool and dense rock sink back into the mantle, giving rise to oceanic trenches Overall young age for sea floor rocks (everywhere <200 million years) is explained by this model Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Plates & Plate Motion Tectonic plates – composed of the relatively rigid lithosphere • lithospheric thickness and age of seafloor increase with distance from mid-oceanic ridge • float upon ductile asthenosphere • interact at their boundaries o divergent boundaries – plates move apart o convergent boundaries – plates move together o transform boundaries – plates slide past one another Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Marine magnetic Anaomalies Marine magnetic anomalies – bands of stronger and weaker than average magnetic field strength that parallel mid-oceanic ridges • symmetric bar-code anomaly pattern reflects plate motion away from ridge coupled with magnetic field reversals • matches pattern of reversals seen in continental rocks (Vine and Matthews) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Evidence of Plate Motion Seafloor age increases with distance from mid-oceanic ridge • rate of plate motion – equals distance from ridge divided by age of rocks • symmetric age pattern reflects plate motion away from ridge Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Evidence of Plate Motion Mid-oceanic ridges – offset along fracture zones • transform fault – fracture zone segment between offset ridge crests • relative motion along fault is result of seafloor spreading from adjacent ridges Plate motion can be measured using satellites, radar, lasers and global positioning systems Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Divergent Plate Boundaries Divergent plate boundaries – plates move away from each other • can occur in the middle of the ocean or within a continent • marked by rifting, basaltic volcanism, and eventual ridge uplift • eventually creates a new ocean basin Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Transform Plate Boundaries Transform plate boundaries– plates slide horizontally past one another • marked by transform faults • transform faults may connect: o two offset segments of mid-oceanic ridge o a mid-oceanic ridge and a trench o two trenches • transform offsets of mid-oceanic ridges allow series of straight-line segments to approximate curved boundaries required by spheroidal Earth Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Convergent Plate Boundaries Convergent plate boundaries – plates move toward one another – ocean-ocean plate convergence – marked by ocean trench, Benioff zone, and volcanic island arc – ocean-continent plate convergence – marked by ocean trench, Benioff zone, volcanic arc, and mountain belts – continent-continent plate convergence – marked by mountain belts and thrust faults Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Movement of Plate Boundaries Plate boundaries – can move over time • mid-oceanic ridge crests can migrate toward or away from subduction zones or abruptly jump to new positions • convergent boundaries can migrate if subduction angle steepens or overlying plate has a trenchward motion of its own • transform boundaries can shift as slivers of plate shear off Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. What Causes Plate Motions? Any proposed mechanism must explain why: • mid-oceanic ridges are hot and elevated, while trenches are cold and deep • ridge crests have tensional cracks • the leading edges of some plates are subducting sea floor, while others are continents (which cannot subduct) Mantle convection – may be the cause or an effect of circulation set up by ridgepush and/or slab-pull Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Mantle Plumes & Hot Spots Mantle plumes – narrow columns of hot mantle rock rise through the mantle • stationary with respect to moving plates • large mantle plumes may spread out and tear apart the overlying plate o flood basalt eruptions o rifting apart of continental land masses • new divergent boundaries may form Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Mantle Plumes & Hot Spots Mantle plume hot spots in the interior of a plate produce volcanic chains • orientation of the volcanic chain shows direction of plate motion over time • age of volcanic rocks can be used to determine rate of plate movement • Hawaiian islands are a good example Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Plate Tectonics & Ore Deposits Metallic ore deposits –often located near plate boundaries • commonly associated with igneous activity • divergent plate boundaries often marked by mineral-rich hot springs (black smokers) on sea floor • hydrothermal circulation near island arcs can produce metal-rich magmatic fluids Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. End of Chapter 19 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.