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Plate Tectonics and Continental Drift Vearth4.mpg 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? Plate Tectonics • Tectonics – Movement of Earth’s crust • Plate tectonics – Movement of discrete segments of Earth’s crust in relation to one another 4 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) Continental drift maps by Wegner (1915) Continental Drift • Wegner proposed a mechanism for drift – Less dense silicic rocks plowed through more dense ocean floor – Earth’s rotation was driving force • Although supporting evidence existed, the theory was not widely accepted 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 Continental Drift • DuToit’s evidence – Expanded Wegener’s ideas – Mesosaurus fossils • Found on Gondwana continents • Freshwater – Could not swim across Atlantic 11 Continental Drift • Glossopteris flora present only in southern hemisphere continents 12 Additional Evidence • Lystrosaurus – Heavyset herbivore – Found in Antarctica • 1969 13 Evidence for Continental Drift • Rock type & structures – Distinct rock type 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 Continental Drift • Geologic Similarities – Brazil and South Africa have nearly identical geologic sequences • Similar in Antarctica and India – Glacial sediments – Coal 15 Rock type & structure evidence Evidence for Continental Drift • Glaciation – Late Paleozoic glaciation • Covered large portions of the southern continents • Distinct glacial deposit • Glacial striations indicate direction of movement • No evidence for glaciation on northern continents at this time 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 • Apparent Polar Wander – First studies indicated poles had moved – Instead, plates had moved – North American and European paths met 23 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 – Ridge system 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 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 30 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 Geology of the Ocean Floor • Polar wandering – Earth’s north magnetic pole was shown to have moved through time • Systematic change in position – Polar wandering paths varied by continent – Multiple magnetic poles are not possible Reconstruction from paleomagnetic data 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 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 The Mid Atlantic Ridge 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 Fig. 17.15. Divergent plate margins Passive Continental Margin Size comparison of various volcanic features 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 • East African Rift Valley Convergent Plate Margins • Oceanic-Oceanic – Seafloor Trench – Shallow and deep earthquakes, 0-700 km deep – Andesitic volcanoes in an island arc – Japan The Aleutian Island Chain Seismic activity in the Aleutian Islands 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 – Cascade range 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 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 – In oceanic crust, deep valleys are formed – Transform faults may extend onto continents – San Andreas fault Juan de Fuca plate 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. 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 Tectonic Mechanisms • Plates are active participants in the convection process – Slab pull – dense ocean crust descends under its own weight – Ridge push – gravity pulls lithosphere down & away from ridge – Friction – resistance to movement from various sources More evidence…. Mantle Plumes and Hot Spots • Mantle plumes may form “hot spots” of active volcanism at Earth’s surface – Approximately 45 known hotspots • 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 The World’s Hot Spots Plate Motion • GPS – Global Positioning System – Earth-orbiting satellites identify motion • Transmitter on satellite • Ground-based receiver • Average rate – 5 cm/year 81 Tectonic setting Lavas and pyroclastics Rock/sediment type Felsic Granites and Rhyolite Turbidites, clays, silts, sands Marine sediments (cherts, limestones, red clays) Basalts (Ophiolites) Mafic 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 Smoker.mov http://www.scotese.com/earth.htm http://www.scotese.com/earth.htm http://www.scotese.com/earth.htm http://www.scotese.com/earth.htm http://www.scotese.com/earth.htm http://www.scotese.com/earth.htm http://www.scotese.com/earth.htm http://www.scotese.com/earth.htm http://www.scotese.com/earth.htm http://www.scotese.com/earth.htm http://www.scotese.com/earth.htm http://www.scotese.com/earth.htm http://www.scotese.com/earth.htm http://www.scotese.com/earth.htm http://www.scotese.com/earth.htm http://www.scotese.com/earth.htm http://www.scotese.com/earth.htm http://www.scotese.com/earth.htm