105_02-platetectonics_part_1 - Plate Tectonics Plate...

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Unformatted text preview: Plate Tectonics Plate Tectonics Lithosphere Lithosphere consists of rigid plates Plates Plates are created and destroyed at plate boundaries The The center of plates tend to be stable Dominant Dominant process affecting the evolution of our planet Chapter 2 Plate Tectonics Explain why mountains, earthquakes, and volcanoes occur where they do. World map, 1440 World map, 1520 World map, 1570 1 Early Tectonic Theories Continental Continental Fit – 1620 - Sir Francis Bacon New New global maps just produced First First to notice jigsaw-puzzle style continents jigsaw- Early Tectonic Theories Continental Drift – continents move freely over the Earth’s surface The The crust was floating on fluid interior – 1782 - Benjamin Franklin “The crust of the Earth must be a shell floating on a fluid interior. Thus the surface of the globe would be capable of being broken and disordered by the violent movements of the fluids on which it rested”. In 1858, geographer Antonio Snider-Pellegrini made Sniderthese two maps showing his version of how the American and African continents may once have fit together, then later separated. Early Tectonic Theories Edward Edward Suess: 1885 Glossopteris Glossopteris flora found in – India, Australia, South Africa, South America Early Tectonic Theories Supercontinent Supercontinent Gondwana composed of all southern continents – Gondwana: province in India with Glossopteris Suess Suess thought continents were connected by land bridges Glossopteris Upper Permian 250 Million years South Africa Early Tectonic Theories Frank Frank Taylor (1910) – Continental drift theory – Mountains originate as result of lateral movement of continents – Large polar continents broke apart and moved to equator to form present-day continents present– Mechanism: gigantic tidal forces (generated when Earth captured the moon) that slowed Earth’s rotation – MAR marks the site where continent broke apart Australia 2 History of Plate Tectonics Continental Continental Drift – Alfred Wegener (1880-1930) (1880- History of Plate Tectonics Continental Continental Drift: Pangaea and Panthalassa – Reconstruction of a supercontinent Origin of Continents and Oceans (1915) History of Plate Tectonics Continental Continental Drift: Pangaea and Panthalassa – Reconstruction of a supercontinent The Evidence Continental Continental fit – Not expected to be perfect because of erosion and deposition – Bullard (1965): best fit at depth of 2000 m Precise matching by computer analysis The Evidence Rocks Rocks The Evidence Mountain Mountain Ranges glaciers coal sandstone basalt 3 The Evidence Mountain Mountain Ranges Climate Climate The Evidence – Coal in Arctic and Antarctic – Glacial striations Rock Rock fragments scrape and polish bedrock: striations (scratches) Direction Direction of ice movement Striations, Blue Mounts, Minnesota Striations, Lake Superior, Ontario The Evidence 280 million year old glacial deposits Glacial Striations, South Africa If continents did not move in the past: Ice moved from the oceans onto land - unlikely Ice formed in tropical to subtropical climate - impossible 4 The Evidence • Reconstruction of Gondwana: • South Africa at the south pole • Glacial movement makes sense • Glacier is located in polar climate and moves radially outward Fossils Fossils – Glossopteris flora: glossopteris fern and other land plants – Seeds too large to be transported across the oceans – Climate of South America, Africa, India, Antarctica, and Australia much too diverse to support the glossopteris flora The Evidence Mesosaurus: Mesosaurus: – freshwater reptile The Evidence Home Home of the mesosaurus The Evidence Mesosaurus: Mesosaurus: – freshwater reptile – Found in Brazil and South Africa and nowhere else in the world – Freshwater reptile cannot swim across the ocean and find a freshwater environment identical to its former home – If it could swim the oceans, it would be widely found, not just in two locations Land reptile Land reptile 5 Evidence for Pangaea – Continental Fit – Related Rocks and Mountains – Climate – Animal habitats Hypothesis REJECTED!!! ….but, Wegener had no MECHANISM to move continents! Argument against Continental Drift, 1912-1963 1912Physically Physically impossible to drag continents around the globe: too much friction, no force strong enough. Continental drift? Despite Despite overwhelming evidence not accepted until…. New New evidence from – Magnetic field (1950s) – Ocean basins Topography Topography by sonar Ocean Ocean drilling Nevin, 1949 Earth’s Earth’s magnetic field Paleomagnetism: Paleomagnetism: study of the remnant magnetism of ancient rocks Earth’s magnetic field Lines Lines of force like a bar magnet 6 Earth’s magnetic field Lines Lines of force gradually change orientation from equator to pole Lines Lines of force can be used to determine latitude Earth’s magnetic field Most Most rocks contain magnetic minerals (magnetite, Fe3O4) Magnetic Magnetic field orientation and strength is recorded in rocks when they form rock magnetite Earth’s magnetic field Earth’s magnetic field Paleomagnetism Paleomagnetism in the 1950s showed many surprises: – Magnetism of recent rocks was consistent with current magnetic field – Magnetism of ancient rocks indicated the N-pole apparently moved around Measure Measure magnetization of rock and determine at which latitude it was formed or where the pole was at the time Polar wandering Curves 3 interpretations: 1. Magnetic north pole moved 2. Continent moved 3. Both moved Magnetic Reversals Discovery Discovery of periodic reversals of the earth’s magnetic field: north and south poles switch places Current Current field ‘normal’ ‘normal’ Reversal timeReversal time-scale established 7 Sea Floor Spreading 1962 1962 - Harry Hess oceans rifting apart at the midmid-ocean ridges, new sea floor forms and continents move apart Magnetic reversals: Can be used to determine age mantle mantle convection proposed as driving mechanism Magnetic anomalies Magnetic Stripes 1961 1961 marine magnetic surveys showed variation in STRENGTH of earth’s magnetic field in a pattern of stripes. Magnetic Stripes Magnetic Stripes 8 ...
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