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Chapter Twelve Plate Tectonics: Creating Oceans and Continents

Chapter Twelve Plate Tectonics: Creating Oceans and Continents. Key elements of Plate Tectonics. Earth’s lithosphere consists of rigid plates Plates move relative to one another by divergence, convergence, or transform motion

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Chapter Twelve Plate Tectonics: Creating Oceans and Continents

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  1. Chapter TwelvePlate Tectonics: Creating Oceans and Continents

  2. Key elements of Plate Tectonics • Earth’s lithosphere consists of rigid plates • Plates move relative to one another by divergence, convergence, or transform motion • Oceanic lithosphere forms at divergent plate boundaries and are consumed at subduction zones • Most earthquakes, volcanism, faulting and mountain building take place at plate boundaries. • Plate centers tend to be geologically stableKey

  3. Basic Assumptions of Plate Tectonic Theory • Lithosphere consists of rigid plates (100 km average; 70 km for ocean & 150 km for continents) • Plates move relative to one another by Divergence, Convergence, or Transform motion • Formation of Oceanic lithosphere at divergent plate boundaries and is consumed at subduction zone • Most earthquake activity, volcanism, faulting, and mountain building take place at plate boundaries • Centers of plates are stable

  4. Plate velocity - determination • Satellite based studies- plate motion and geomagnetic research • Velocity of plates measures directly by ground-based laser which beams off reflectors on selected satellites • Hot Spots- can serve as fixed reference point to measure absolute plate velocities • Structures formed as plates move over hot spots also reveal speed and direction of plates • Marine Magnetic Anomalies are used to estimate rates of divergence, and therefore plate motion • Eco-sounder sonar- topography of sea floor and seismic profiling- study underlying layers

  5. Relative Plate velocities

  6. Relative Plate velocities-contd.

  7. Satellite Global system

  8. Mid-Pacific Hot spot

  9. Tracking Magnetic Field Reversals • DISCOVERY OF MARINE MAGNETIC ANOMALIES (WW-II MAGNETOMETER OBSERVATIONS IN THE SHIP) • VINE & MATHEWS’ HYPOTHESIS OF MAGNETIC REVERSALS (EVIDENCE OF SEA-FLOOR SPREADING AT DIVERGENT PLATE BOUNDARIES) • MEASURING PLATE MOVEMENT BY MAGNETIC ANOMALIES (ANOMALY’S DISTANCE FROM SPREADING RIDGE) • PLATE SPEED: 1 TO 10 CM PER YEAR

  10. Marine magnetic anomalies

  11. Marine magnetic anomaly-contd.

  12. Marine magnetic anomaly-contd.

  13. Directions and Rates of Plate movement

  14. Nature and Origin of the Ocean Floor • Rifting • rising mantle over hot spot- 3 radiating valleys. One fails to open and later becomes filled with sediments- aulacogen. When rifting stops- the rift edge becomes inactive tectonically and therefore have passive continental margin • Divergent Plate Boundaries • as divergence continues, full seaway forms and new oceanic lithosphere forms at the mid-oceanic ridge as up-welling ultramafic melt produce basaltic magma • Transform Boundaries/offset mid-oceanic ridges • transform fault becomes divided into short offset segments by ocean • Information about the Ocean Floor – Echo-Sounding sonar, Seismic Profiling, Deep-Sea Drilling Project & Submersible vessels

  15. Active Rifting

  16. Rifting and Origin of Ocean Basins • Active Arms marked by: • High heat flow • Normal faulting • Frequent shallow earthquakes • Widespread basaltic volcanism Rift valley widening leads to the development of new seaways and evaporite belts In Ocean, it leads to the formation of sediment deposition and development of Passive continental margins

  17. The East Africa Rift Zone

  18. The growth of oceanic basin

  19. Growth of ocean basin – contd.

  20. Growth of ocean basin – contd.

  21. Nature and Origin of the Ocean floor – contd. • Oceanic trench • forms where dense oceanic plate plunges (subducts) under less dense plates forming a depression in the earth’s surface • Melange • mixture of oceanic sediments and ophiolite rocks- form massive accretionary wedge that may be attached to the edge of the overriding plate • Convergent • collision of two plates resulting in suture zones • Volcanic arc • chain of volcanoes formed from subduction

  22. Structure of Oceanic Lithosphere • Upper surface – 200 M: sediment of siliceous or carbonate ooze and/or reddish clay • 200 m- 2 km: Oceanic basalt with top layer of pillow structures • 3-6 km: Gabbro • Below Gabbro is Peridodite • Serpentinite formed by alteration of rocks by water – Entire sequence of ocean-floor rock may be altered its faults & fissures

  23. Structure of Oceanic Lithosphere-contd. • H2O + Pyroxene (in basalt, gabbro) – chlorite • H2O +Mg olivine (Umafic peridotite)- Serpentine (Mg-Si mineral)

  24. Layer of Ophiolite suite

  25. Transform Boundaries & Offset Mid-Ocean Ridges • Occur where plates slide past one another in opposite direction • 15% total length of plate margins • Faulted blocks move in opposite directions --- produce stress --- leads to earthquakes

  26. Direction of motion of a plate

  27. Subduction-zone feature

  28. Breadth of arc-trench gap

  29. Breadth of arc-trench gap-contd

  30. Anatomy of a continent

  31. Continental shield

  32. Origin of a Supercontinent

  33. Origin of the supercontinent Pangaea

  34. Earth’s plates before Pangaea • Gondwana: ~500 million yrs BP: Near South Pole; 4 Northern landmasses • Laurasia: 3 Landmasses in the Northern Hemisphere; North America, Northern Europe, Southern Europe + Parts of Africa & Siberia • Pangaea: Laurasia and Gondwana colloided to form Pangaea

  35. Future Events • 50-100 Million yrs from now: Australia will collide with Southeast Asia • Western part of California will separate from North America along the San Andreas fault and become a separate microcontinent • Mediterranean will close as the African and Eurasian Plates collide • Subduction may occur along the East coast of North America • All of the Earth’s landmasses may reunite into another Pangaea-like supercontinent

  36. Driving Forces of Plate Motion • Uncertainty on what drives Plate Motion • Slow Asthenosphere convection currents • Evidence against plate pushing by rising magma wedges at divergent boundaries • Evidence in favor of plate pulling by descending slabs at subduction zones • Gravity forces plates away from uplifted mid-ocean ridge

  37. Convection Cells Hypothesis • Deep convection cell hypothesis: Lower mantle to the surface of the earth • Shallow convection cell hypothesis: Cells are driven within the Asthenosphere

  38. Three factors that may drive plate tectonics

  39. Convection cell hypothesis

  40. Convection cell hypothesis-contd.

  41. Configuration cell hypothesis

  42. Seismic Topography

  43. Seismic topography-contd.

  44. Thermal plumes

  45. Two patterns of Marine anomalies

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