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Plate Tectonics: The Unifying Theory

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Plate Tectonics: The Unifying Theory

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  1. Plate Tectonics:The Unifying Theory Peter W. Sloss, NOAA-NESDIS-NGDC

  2. Fundamental concept of geoscience Integrates from many branches First suggested based on geology and paleontology Fully embraced after evidence from geophysics Plate Tectonics

  3. • Distribution of earthquakes and volcanoes • Relationship of age and height of mountain belts • Age distribution of oceanic crust • Magnetic information in rocks What tectonics theory explains

  4. Earth’s upper mantle (lithosphere) is broken into rigid plates which move with respect to each other Plates rest on and move in the asthenosphere There are 3 kinds of plate margins that are marked by earthquakes and volcanoes Divergent Convergent Transform The movement is driven by uneven distribution of heat within Earth and the mechanism that drives plate movement is not well known What is Plate Tectonics Theory?

  5. Mosaic of Earth’s Plates Peter W. Sloss, NOAA-NESDIS-NGDC

  6. Theory evolved from 2 concepts: • Continental Drift - by Alfred Wegener • Fit of continents - Pangaea • Ancient climatic evidence - glacial deposits • Fossil evidence - Glossopteris & Mesosaurus • Matched mineral zones & mountain chains • (Problem - no acceptable method of motion)

  7. “Fit” of the Continents

  8. Anomalous Distribution of Fossils(Mesosaurus)

  9. ANDPALEOMAGNETIC EVIDENCE • Polar Wandering - either • the North Pole or the continents had moved • both Europe and North America had apparently moved as a single continent for several hundred million years

  10. Apparent polar wandering curves

  11. Seafloor Spreading • “Geopoetry” of Harry Hess & Robert Dietz • New seafloor forms by upwelling at the center of MOR and moves laterally • Older crust is destroyed in the subduction zones at the trenches • Seafloor is younger than 200 MY • Solved Continental Drift problem

  12. Modern Proof of Seafloor Spreading • Deep Sea Drilling Project - sampled sea floor sediment & crust • Age & thickness increased with distance from MOR • Ages were symmetrical about MOR • Magnetic Anomalies - found weak & strong signal • Alternating normal & reversed polarization • Stripes || to & symmetrical about MOR

  13. Magnetic Anomalies

  14. Ocean crust records magnetic reversals

  15. Age of Seafloor Crust Fig. 20.11 R. Dietmar Muller, 1997

  16. More Proofs • Hot Spots - deep, long-lived, stationary mantle magma sources • Expressed at the surface by: • linear chain of volcanoes • aged with distance from hot spot • Over 100 identified • Used as fixed points against which plate motion is measured

  17. Hawaiian-Emperor chain

  18. Long-lived global hot spots

  19. PUTTING IT ALL TOGETHER

  20. Group of rocks all moving in the same direction Can have both oceanic and continental crust or just one kind Plate interior are relatively quiet Most activity is located at plate margins I. Plates

  21. • divergent: mid-ocean ridges • convergent: collision zones volcanic arcs • strike-slip: San Andreas Fault Alpine Fault, N.Z. II. Types of plate boundaries

  22. Usually begin within continents as hot spot burns thru crust and eventually grows to become ocean basin Two kinds of divergent margins Mid Ocean Ridges (MOR) Continental Rift Valleys A. Divergent plate boundaries

  23. • Central rift valley (width is inversely proportional to the rate of spreading) • Region of high heat flow Shallow-focus earthquakes • Almost exclusively basalt volcanism Features of Mid Ocean Ridges

  24. Earthquakes Associated with Divergent Margins

  25. Rifting and Seafloor Spreading Along the Mid-Atlantic Ridge Peter W. Sloss, NOAA-NESDIS-NGDC

  26. Rifting and Seafloor Spreading

  27. • East Africa, Rio Grande rift • Beginning of ocean formation (may not get that far) • Rifting begins at a triple junction (two spreading centers get together to form ocean basin, one left behind). • Rock types: basalt and sandstone Features of Continental Rifts

  28. Hot Spot induced rifting • Hot spot burns thru crust -> 3 branched rift • Divergence begins • 2 branches are active • 1 branch is not active • Linear sea develops

  29. Inception of Rifting Along theEast African Rift System Peter W. Sloss, NOAA-NESDIS-NGDC

  30. Inception of Rifting Within a Continent

  31. Nile Delta Gulf of ‘Aqaba Gulf of Suez Red Sea Earth Satellite Corp.

  32. The Gulf of California Formed by Rifting of Baja California from Mainland Mexico Worldsat International/Photo Researchers

  33. New crust created at MOR—old crust destroyed (recycled) at subduction zones 2 kinds: subduction & collision Relative important densities: continental crust ≈ 2.8 g/cm3 oceanic crust ≈ 3.2 g/cm3 asthenosphere ≈ 3.3 g/cm3 B. Convergent boundaries

  34. Three types: ocean–ocean Philippines ocean–continent Andes continent–continent Himalaya Convergent boundaries

  35. Island arcs: Chain of volcanic islands • Highly seismic tectonic belt of shallow to deep earthquakes • High heat flow arc of active andesitic volcanoes • Bordered by a submarine trench Ocean–Ocean

  36. Ocean–Ocean Subduction Zone

  37. Continental arcs: • Magmatic belt of active volcanoes (andesite to rhyolite) • Often accompanied by compression of upper crust which builds mountains Bordered by a submarine trench Ocean–Continent

  38. Ocean-ContinentSubduction Zone

  39. Continent–continent boundaries, convergence is accommodated by • Folding (shortening and thickening) • Strike-slip faulting • Underthrusting (intracontinental subduction) Continent–Continent

  40. Continent–Continent Convergent Boundary

  41. Continent-Continent Collision

  42. • 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

  43. Stages in the collision of India with Asia

  44. C. Transform Boundary Offsetting Spreading Centers

  45. Mostly obtained from magnetic anomalies on seafloor Slow spreading: 3 cm/year Fast spreading: 10 cm/year Very fast spreading: 17cm/year III. Rates of plate motion

  46. Relative Velocity and Direction of Plate Movement Data from C. Demets, R.G> Gordon, D.F. Argus, and S. Sten, Model Nuvel-1, 1990

  47. 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. IV. Rock assemblages and plate tectonics