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Transcript
Plate Tectonics and Continental Drift There are numerous ‘seams’ on the surface of the Earth Questions and Topics 1. What are the theories of Plate Tectonics and Continental Drift? 2. What is the evidence that Continents move? 3. What are the forces that drive plate tectonics? 4. What happens at the boundaries between plates? 5. How do the different types of plate boundaries impact the regional geology and geomorphology? 6. How has continental drift affected the positions of the continents over time? Answers 1. Large crustal plates at the Earth’s surface move about, colliding with one another. 2. There is geographic, geomagnetic, paleontologic and other evidence that this occurs 3. Convection in the mantle is the main driver of plate movement 4. Neighboring plates move relative to one another, causing earthquakes and volcanic eruptions 5. Active plate boundaries produce mountains and trenches 6. Continents have changed position Plate Tectonics • Tectonics – Movement of Earth’s crust • Plate tectonics – Movement of discrete segments of Earth’s crust in relation to one another 5 Continental Drift • Alfred Wegener (1880-1930) – Proposed that all of the continents were once part of a large supercontinent Pangaea – Based on: • Similarities in shorelines • Distinctive rock and fossil groups found in Africa & South America Continental drift maps by Wegner (1915) Wegener’s Pangea Modern reconstruction of Pangea Continental drift maps by Wegner (1915) Continental Drift • Wegner mechanism for drift was not credible – Less dense silicic rocks (the continents) plowed through more dense ocean floor – Earth’s rotation was driving force • Other scientists didn’t buy it What is the evidence for Continental Drift? Evidence for Continental Drift • Paleontological – Similarity of fossils on opposite sides of the Atlantic Ocean • Plants and land dwelling animals • No mechanism to transport across ocean • Glossopteris on all southern continents • Divergence of species following break-up Paleontological evidence Evidence for Continental Drift • Rock type & structures – Distinct and similar rock types and geologic structures on both sides of the Atlantic Ocean • Cape fold belt and equivalent – S.Africa & Argentina • Appalachian Mtns and equivalent – U.S., Canada, Scotland & Norway • Only occur in rocks > 145 mya!!!!!!!!! Rock type & structure evidence Evidence for Continental Drift • Glaciation – Late Paleozoic glaciation • Covered large portions of the southern continents • Distinct glacial deposit • No evidence for glaciation on northern continents at this time Geologic Time Scale Eon Era Period Quaternary C e n o z o i c Next homework is to recreate this figure P h a n e r o z o i c 0.01 1.8 Neogene 5.3 23.8 Tertiary 33.6 Paleocene 54.8 65 M e s o z o i c Cretaceous 144 Jurassic Triassic Permian P a l e o z o i c Pennsylvanian Mississippian Devonian Silurian Ordivician Cambrian P r e c a m b r i a n Age (Myrs) 206 248 290 323 354 417 443 490 543 Proterozoic 2500 Archean 3800 Hadean Age of the Earth 4600 Myrs (4.6 Byrs) Source: Geological Society of America (1999) Epoch Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene Reconstruction from glacial deposits Evidence for Continental Drift • Paleoclimate – Evidence of extreme changes in climate as compared to the present • Coal deposits in Antarctica • Evidence from evaporite deposits, eolian deposits & coral reefs • Paleoclimate reconstruction shows strange patterns unless continents are moved Fig. 17.6. Paleoclimate evidence Paleomagnetism • Magnetization of ancient rocks at the time of their formation • Declination – Angle that a compass needle makes with the line running to the geographic north pole • Rocks lock in this orientation at formation 20 Reconstruction from paleomagnetic data Geology of the Ocean Floor • Paleomagnetism – Fe rich rocks are weakly magnetized by the Earth’s magnetic field as minerals form – Orientation of magnetic field is preserved – Magnetic field orientation varies with position on Earth’s surface Modern Plate Tectonic Theory • Original evidence for continental drift was from continental rocks • Technological advances in the 1950’s and 1960’s allowed investigation of the sea floor • Geophysics & paleomagnetism provided new data Geology of the Ocean Floor • Topography of the ocean basins – Basins are divided by a large ridge system, which is continuous around the entire globe – Central rift valley within the ridge Geology of the Ocean Floor • Physical properties – Composed of basalt – Younger in age than most continental rocks – Oceanic crust is thinner than continental – No evidence of crustal deformation – folded mountains Crustal Properties Crust Density continental ~2.8 g/cm3 oceanic ~3.2 g/cm3 Composition Thickness Felsic Thick: 20-70 km Mafic Thin: 2-10 km Age Old: up to 4 Byrs Young: <200 Mys Geology of the Ocean Floor • Seafloor spreading proposed by Hess (1960) – Considered new data on ocean floor – Proposed mechanisms of: • Mantle convection • Rifting and volcanism along ridge system • Continents pushed along w/ spreading seafloor • Recycling of oceanic crust by subduction Fig. 17.21. Models of plate tectonic motion Geology of the Ocean Floor • Vine & Matthews (1963) tested Hess’s hypothesis using magnetism – Magnetic polarity reversals recorded in ocean floor basalt • Magma cools forming new crust • Polarity at time of cooling preserved • Old crust pushed aside Geology of the Ocean Floor • Magnetic polarity stripes in ocean crust parallel ridges – Symmetrical on either side of the ridge – Polarity chrons give age of seafloor • Increases away from ridge • Rates of plate motion may be calculated Fig. 17.10. Patterns of magnetic reversals Age of the sea floor Geology of the Ocean Floor • Seafloor sediments support plate tectonic theory – Youngest sediments resting directly on basalt near the ridge – Sediment just above the basalt gets older moving away from the ridge – Accumulation rates of ~3 mm/1000 yr Plate Geography • Lithosphere is divided into individual plates – Boundaries based on structural features, not land and ocean – Plates are outlined by ridges, trenches and young mountain belts – Plates are not permanent features Major tectonic boundaries Divergent Plate Margins • Oceanic-Oceanic Crust • Mid-oceanic ridge with central rift valley • Shallow earthquakes, less than 100km • Basaltic lavas The Mid Atlantic Ridge Line of fissures and volcanoes in Iceland Rates of Seafloor Spreading FAST SLOW (East Pacific Rise) (Mid Atlantic Ridge) ~10-20 cm/year ~1-2 cm/year Life of a person 100 years 10 meters 1-2 meters Civilization 10,000 years 1 km 100-200 m Modern Humans 100,000 years 10 km 1-2 km Stone tools 1,000,000 years 100 km 10-20 km Width of the Pacific Ocean ~ on the order of 10,000 km (16,000 miles) wide. How long would it take to create this much ocean crust. East Pacific Rise Fast spreading Ridges subside and are covered with sediment Composition of the Ocean Crust • Seismic surveys suggest oceanic crust is ~7 km thick and comprised of three layers – First layer is marine sediment of various composition and thickness (extensively sampled) – Second layer is pillow basalt overlying basaltic dikes (extensively sampled) – Third layer is thought to be composed of sill-like gabbro intrusions (not directly sampled) • Ophiolites are rock sequences in mountain chains on land that are thought to represent slivers of ocean crust and uppermost mantle – Composed of layers 1-3 overlying ultramafic rock Ju an d eF uc aS mo k er. av i Divergent Plate Margins • Continental-Continental Crust – Rift Valley – Shallow earthquakes, less than 100km – Basaltic and Rhyolitic volcanism • New material rising from the mantle produces basaltic lavas • Thinning continental crust melts to produce rhyolitic lavas & instrusions • Example: East African Rift Valley East Africa • East African system extends 3000 km from Ethiopia to Mozambique – Red Sea rift extends from Ethiopia to the Sinai and Dead Sea – Complex volcanism throughout – Oceanic crust present in SE Red Sea – Lakes form in isolated down-dropped blocks – Several areas below sea level Fig. 19.33. The Red Sea A transform fault in the Gulf of Aqaba The parting of the Red Sea The Red Sea Fig. 17.15. Divergent plate margins Convergent Plate Margins • Oceanic-Oceanic – Seafloor Trench – Shallow and deep earthquakes, 0-700 km deep – Andesitic volcanoes in an island arc – Example: Japan Ocean-Ocean convergence The Aleutian Island Chain Seismic activity in the Aleutian Islands The MarianaTrench http://www.ngdc.noaa.gov/mgg/image/2minrelief.html Oceanic-Oceanic and Oceanic-Continental Subduction Convergent Plate Margins • Oceanic-Continental – Subduction Zone – Shallow and deep earthquakes, 0-700 km deep – Andesitic volcanoes in a continental arc – Example: Cascade range Ocean-Continent convergence Major tectonic boundaries Mt. Vesuvius The people of Pompeii; mummified in 5-8 meters of hot ash in A.D. 79 The smoldering city of Pierre, Martinique Convergent Plate Margins • Continental-Continental – Intensely folded and thrust faulted mountain belts – Metamorphic rocks dominate • Sediments accumulated along continental margin are squeezed – Igneous rocks commonly included • Granitic magmas • Example: The Himalayas Continent-Continent Collision Convergent plate boundaries Transform Fault Margins • Transform faults are large vertical fractures or faults in the crust – Movement along faults is side to side – May extend for long distances – Example: San Andreas fault and transform faults in the ocean Fig. 19.29. Basin & Range Province Juan de Fuca plate Fig. 20.12b. Landforms along the San Andreas Rates of Plate Motion • Two ways to look at plate motion – Relative velocity – the movement of one plate relative to another • Age of seafloor / distance from ridge – Absolute velocity – compares plate movement to a fixed position • Use hotspots as fixed points of reference • Rates vary from 1 to 20 cm/yr Fig. 17.20. Rates of plate motion around the world Where do we see deep earthquakes? What is happening there? Tectonic Mechanisms • Convection of heat from the core and mantle drives tectonics – Convection cells bring new material to the surface – Old crust is pushed away from ridges – Subduction carries cool crust back into the mantle Fig. 17.21. Models of plate tectonic motion