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THE THEORY OF PLATE TECTONICS

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THE THEORY OF PLATE TECTONICS

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  1. THE THEORY OF PLATE TECTONICS

  2. INTRODUCTION • Tectonics- large scale deformational features of the crust • Plate tectonics • Earth’s outer shell divided into plates • Plates move & change in size thru time • Activity at plate boundaries • Combines: • Continental drift • Sea-floor spreading • Paleomagnetism

  3. INTRODUCTION • Ideas • Continental drift- Alfred Wegener • Sea-floor spreading • Paleomagnetism

  4. Early Case for Continental Drift-Alfred Wegener (F.B. Taylor & H.H. Baker) • Continental coastlines fit together • 1620 Sir Francis Bacon: Africa and S. America • Rocks & structures indicated that continents joined • Pangea- supercontinent of the late Paleozoic • Separated into Laurasia & Gondwanaland • Fossil evidence- Glossopteris & Mesosaurus • Late Paleozoic glaciation • Skepticism about Continental Drift • Problem of driving mechanism

  5. Continental Drift • Alfred Wegener 1912 • Pangaea (ALL EARTH) • Evidence: • Continents FIT together like the pieces of a puzzle • Fossils • Rocks & structures • Paleoclimate

  6. Continental Drift • Alfred Wegener 1912 • Pangaea (ALL EARTH) • Evidence: • Continents FIT together like the pieces of a puzzle • Fossils • Rocks and structures • Paleoclimate

  7. Continental Drift • Alfred Wegener 1912 • Pangaea (ALL EARTH) • Evidence: • Continents FIT together like the pieces of a puzzle • Fossils • Rocks and structures • Paleoclimate

  8. Evidence for Continental Drift- Revisited • Fitting continents at continental slope rather than shoreline- 1000m depth • Refined matches of rocks between continents • Isotopic ages support matches • Glacial evidence • Matches between Africa and South America are particularly convincing

  9. INTRODUCTION • Tectonics- large scale deformational features of the crust • Plate tectonics • Earth’s outer shell divided into plates • Plates move & change in size • Activity at plate boundaries • Combined: • Paleomagnetism • Sea-floor spreading

  10. INTRODUCTION • Tectonics- large scale deformational features of the crust • Plate tectonics • Earth’s outer shell divided into plates • Plates move & change in size • Activity at plate boundaries • Combined: • Paleomagnetism • Sea-floor spreading

  11. Paleomagnetism • Iron becomes magnetized below the Curie Point (600oC) • Magnetite and hematite aligns on existing magnetic field • Dip indicates old magnetic pole position • Apparent motion of north magnetic pole through time • Split in path • indicates continents split apart

  12. Paleomagnetism • Magnetite aligns on existing magnetic field • Dip indicates old magnetic pole position • Apparent motion of north magnetic pole through time • Split in path • indicates continents split apart

  13. INTRODUCTION • Tectonics- large scale deformational features of the crust • Plate tectonics • Earth’s outer shell divided into plates • Plates move & change in size • Activity at plate boundaries • Combined: • Paleomagnetism • Sea-floor spreading

  14. SEA-FLOOR SPREADING • Magnetic anomalies • 1950’s detection of 10-50km wide strips symmetrical about ocean ridges • Vine and Matthews: magnetic reversals • Sea-floor moves away from mid-oceanic ridge • Plunges beneath continent or island arc- subduction (earthquake define zone, Benioff zone) • Plate movement rate of 1 to 20 cm/year, 5 cm/yr average • Driving force • Mantle convection • Ridge Push- Slab Pull forces

  15. SEA-FLOOR SPREADING • Explanations • Mid-oceanic ridge • Hot mantle rock beneath ridge • High heat flow • Basalt eruptions • Rift valley • Shallow-focus earthquakes

  16. SEA-FLOOR SPREADING • Explanations • Oceanic trenches • Low heat flow • Negative gravity anomalies • Benioff zone earthquakes • Andesitic volcanism • Age of sea floor • Young age of sea floor rocks (oldest 160 my) • Implies youngest should be at ridges, oldest at trenches • Explains pattern of pelagic sediment

  17. How do we know that plates move? • Marine magnetic anomalies • Vine-Matthews Hypothesis • Anomalies • Reversals • Normal and reverse polarity • Positive and negative anomalies • Measuring the rate of sea floor spreading • Predicting sea floor age

  18. Plates and Plate Motion • Plate • Entirely sea floor or • continental and oceanic • Lithosphere • Crust & uppermost mantle • Thickness increases away from ridge • Asthenosphere • Low seismic velocity zone • behaves plastically

  19. Plates and Plate Motion • Plate • Entirely sea floor or • continental and oceanic • Lithosphere • Crust & uppermost mantle • Thickness increases away from ridge • Asthenosphere • Low seismic velocity zone • behaves plastically

  20. History of Continental Positions • Pangea split up 200 m.y. • Continents in motion for at least 2 billion years

  21. How do we know that plates move? • Fracture Zones & Transform Faults • Pattern of earthquakes at ridges and fracture zones • Transform fault • Measuring plate motion directly • Use of satellites

  22. Plates and Plate Motion • Interior of plates relatively inactive- Cratons • Activity along boundaries • Trenches (zone of subduction), melanges (complex of shear rock), accretionary prism (sedimentary and volcanic wedges separated by high angle faults) • e.g., earthquakes, volcanoes, young mountain belts • Plate tectonics a unifying theory for geology • Boundaries • Divergent • Convergent • Transform

  23. DIVERGENT BOUNDARIES • Continuing divergence • Widening sea • Mid-oceanic ridge system • 70,000 km in length; 30-35 km wide; 1-2 km deep • New crust formed at mid-oceanic ridge • Ophiolite Sequence (FROM THE TOP DOWN) • Pillow basalt • sheeted dikes • Layered Gabbro • Peridotites: layered ultramafic rocks

  24. DIVERGENT BOUNDARIES • During break up of a continent • Rifting, basaltic eruptions (Flood Basalts), uplifting • Extension- normal faults, rift valley (graben) forms • Shallow focus earthquakes • Continental crust separates • Fault blocks along edges • Oceanic crust created • Rock salt may develop in rift

  25. East African Rift System • early stages of rifting • continental rifting

  26. Red Sea Rift • Red Sea • Gulf of Eilat • Dead Sea • Linear Seas

  27. DIVERGENT BOUNDARIES • Continuing divergence • Widening sea • Mid-oceanic ridge • New crust formed at mid-oceanic ridge • Ophiolite Sequence (FROM THE TOP DOWN) • Pillow basalt • sheeted dikes • Layered Gabbro • Peridotites: layered ultramafic rocks • Marine sediment covers continental edges • Passive continental margin

  28. TRANSFORM BOUNDARIES • Two plates slide past each other • Usually between mid-oceanic ridge segments • Can also connect ridge and trench • Or trench to trench • Origin of offset of ridges

  29. CONVERGENT BOUNDARIES • Plates move toward each other • One plate overrides the other • Subduction zone