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Classroom presentations to accompany Understanding Earth, 3rd edition prepared by Peter Copeland and William Dupré University of Houston Chapter 20 Plate Tectonics: The Unifying Theory Plate Tectonics: The Unifying Theory Peter W. Sloss, NOAA-NESDIS-NGDC Plate Tectonics • Fundamental concept of geoscience • Integrates from many branches • First suggested based on geology and paleontology • Fully embraced after evidence from geophysics Mosaic of Earth’s Plates Peter W. Sloss, NOAA-NESDIS-NGDC Fig. 20.3 Plates • Group of rocks all moving in the same direction • Can have both oceanic and continental crust or just one kind. Types of plate boundaries • divergent: mid-ocean ridges • convergent: collision zones volcanic arcs • strike-slip: San Andreas fault Alpine fault, N.Z. Divergent plate boundaries Usually start within continents— grow to become ocean basin Features of Mid Ocean Ridges • Central rift valley (width is inversely proportional to the rate of spreading) • Shallow-focus earthquakes • Almost exclusively basalt Continental Rifts • East Africa, Rio Grande rift • Beginning of ocean formation (may not get that far) • Rifting often begins at a triple junction (two spreading centers get together to form ocean basin, one left behind). • Rock types: basalt and sandstone Rifting and Seafloor Spreading Fig. 20.4a Rifting and Seafloor Spreading Along the Mid-Atlantic Ridge Peter W. Sloss, NOAA-NESDIS-NGDC Fig. 20.4a Inception of Rifting Within a Continent Fig. 20.4b Inception of Rifting Along the East African Rift System Peter W. Sloss, NOAA-NESDIS-NGDC Fig. 20.4b Nile Delta Gulf of Suez Gulf of ‘Aqaba Red Sea Earth Satellite Corp. Fig. 20.5a The Gulf of California Formed by Rifting of Baja California from Mainland Mexico Worldsat International/Photo Researchers Fig. 20.5b “Fit” of the Continents Fig. 20.1 Anomalous Distribution of Fossils Fig. 20.2 Convergent boundaries • New crust created at MOR—old crust destroyed (recycled) at subduction zones (i.e., the Earth is not expanding) • Relative important densities: continental crust ≈ 2.8 g/cm3 oceanic crust ≈ 3.2 g/cm3 asthenosphere ≈ 3.3 g/cm3 Convergent boundaries Three types: ocean–ocean Philippines ocean–continent Andes continent–continent Himalaya Ocean–Ocean Island arcs: • Tectonic belts of high seismic ????? • High heat flow arc of active volcanoes (andesitic) • Bordered by a submarine trench Ocean–Ocean Subduction Zone Fig. 20.6b Ocean–Continent Continental arcs: • Active volcanoes (andesite to rhyolite) • Often accompanied by compression of upper crust Ocean-Continent Subduction Zone Fig. 20.6a Continent–Continent • In ocean–continent boundaries convergence, collision convergence is taken up by subduction (± thrusting). • Continent–continent boundaries, convergence is accommodated by • Folding (shortening and thickening) • Strike-slip faulting Continent-Continent Collision Fig. 20.6c Himalayas and Tibetan Plateau • Product of the collision between India and Asia. • Collision began about 45 M yr. ago, continues today. • Before collision, southern Asia looked something like the Andes do today. Himalayas and Tibetan Plateau Models • Underthrusting • Distributed shortening •Strike-slip faulting Spreading Centers Offset by Transform Boundary Fig. 20.7 Wilson cycle Plate tectonics repeats itself: rifting, seafloor spreading, subduction, collision, rifting, … Plate tectonics (or something like it) seems to have been active since the beginning of Earth’s history. Examples of Plate Boundaries O-C convergent O-O convergent O-O divergent C-C divergent O-O divergent O-O divergent O-C convergent Fig. 20.8a,b Ocean–Continent Convergent Boundaries Fig. 20.8c Continent–Continent Convergent Boundary Fig. 20.d Rates of plate motion Mostly obtained from magnetic anomalies on seafloor Fast spreading: 10 cm/year Slow spreading: 3 cm/year Magnetic Anomalies Fig. 20.9 Formation of Magnetic Anomalies Fig. 20.10 Age of Seafloor Crust R. Dietmar Muller, 1997 Fig. 20.11 Relative Velocity and Direction of Plate Movement Data from C. Demets, R.G> Gordon, D.F. Argus, and S. Sten, Model Nuvel-1, 1990 Fig. 20.12 Opening of the Atlantic by Plate Motion After Phillips & Forsyth, 1972 Fig. 20.13 Rock assemblages and plate tectonics • Each plate tectonic environment produces a distinctive group of rocks. • By studying the rock record of an area, we can understand the tectonic history of the region. Idealized Ophiolite Suite Deep-sea sediments Pillow basalt Gabbro Peridotite Fig. 20.14 Model for Forming Oceanic Crust at Mid-ocean Ridges Fig. 20.15 Precambrian Ophiolite Suite Pillow basalt M. St. Onge/Geological Survey of Canada Fig. 20.16 Volcanic and Nonmarine sediments are deposited in rift valleys Fig. 20.17a Cooling and subsidence of rifted margin allows sediments to be deposited Fig. 20.17b Carbonate platform develops Fig. 20.17c Continental margin continues to grow supplied from erosion of the continent Fig. 20.17d Parts of an Ocean–Ocean Convergent Plate Boundary Fig. 20.18 Parts of an Ocean–Continent Convergent Plate Boundary Fig. 20.19 Continued Subduction Fig. 20.20a Continent– Continent Collision Fig. 20.20b Approaching Arc or Microcontinent Fig. 20.21a Collision Fig. 20.21b Accreted Microplate Terrane Fig. 20.21c Microplate terranes Added to Western North America Over the Past 200 Million Years After Hutchinson, 1992-1993 Fig. 20.22 After Hutchinson, 1992-1993 Fig. 20.22 Tectonic reconstructions A variety of evidence traces the motion of continents over time: • Paleomagnetism • Deformational structures • Environments of deposition • Fossils • Distribution of volcanoes Assembly of Pangaea I.W.D. Dalziel, 1995 Fig. 20.23 Breakup of Pangaea 200 million years ago After Dietz & Holden, 1970 Fig. 20.24a Breakup of Pangaea 140 million years ago After Dietz & Holden, 1970 Fig. 20.24b Breakup of Pangaea 65 million years ago After Dietz & Holden, 1970 Fig. 20.24c Breakup of Pangaea Today After Dietz & Holden, 1970 Fig. 20.24d Driving mechanism of plate tectonics • Thought to be convection of the mantle. • Friction at base of the lithosphere transfers energy from the asthenosphere to the lithosphere. • Convection may have overturned asthenosphere 4–6 times. Other factors • Trench pull • Ridge push Fig. 20.25a Fig. 20.25b Fig. 20.25c Fig. 20.25d Cross Section of Western Canada What tectonics theory explains • Distribution of earthquakes and volcanoes • Relationship of age and height of mountain belts • Age distribution of oceanic crust • Magnetic information in rocks Questions about plate tectonics • What do we really know about convection cells in the mantle? • Why are some continents completely surrounded by spreading centers? • Why are tectonics in continental crust and oceanic crust so different? Examining Deep-sea Drill Cores Texas A&M University Age of the Ocean Basins After map by Sclater & Meinke