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Plate Tectonics

Plate Tectonics. A Unifying Theory. http://vulcan.wr.usgs.gov/. Mt. St. Helens 10/12/04. Discovering Plate Boundaries. Classified boundaries based on OBSERVATIONS of PATTERNS : Topographic expression Seismicity - distribution and depth of earthquakes

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Plate Tectonics

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  1. Plate Tectonics A Unifying Theory http://vulcan.wr.usgs.gov/ Mt. St. Helens 10/12/04

  2. Discovering Plate Boundaries • Classified boundaries based on OBSERVATIONS of PATTERNS: • Topographic expression • Seismicity - distribution and depth of earthquakes • Volcanology - distribution of volcanoes • Age of the sea floor • terra.rice.edu/plateboundary/

  3. Boundary Observations: • Ocean floor boundaries with positive topography • Shallow earthquakes • Volcanism, basalt (low silica) • Symmetric age patterns; young age • ? Anomalies Divergent - Plates Moving Apart http://earthview.sdsu.edu/trees/tecqest.html

  4. Boundary Observations: • Deep ocean trench near high positive relief • Shallow, intermediate, and deep earthquakes • Volcanism, andesite (intermediate silica) • Asymmetric age patterns; often older ages • ? Anomalies Convergent - Plates Moving Together http://earthview.sdsu.edu/trees/tecqest.html

  5. Boundary Observations: • High positive relief; continental crust • Shallow and intermediate earthquakes • Volcanism, andesite (intermediate silica) • No age information • ? Anomalies Convergent - Plates Moving Together http://earthview.sdsu.edu/trees/tecqest.html

  6. Boundary Observations: • No topographic markers • Shallow earthquakes • No volcanism • Asymmetric age patterns • ? Anomalies Transform - Plates Moving Past Each Other http://earthview.sdsu.edu/trees/tecqest.html

  7. Observations Explained in the Theory of Plate Tectonics • The upper mechanical layer of Earth (lithosphere) is divided into rigid plates that move away, toward, and along each other • Most (!) deformation of Earth’s crust occurs at plate boundaries

  8. Continent-Continent Collision

  9. Transform Boundaries

  10. 3 Main Boundary Types • Convergent • Ocean-continent • Ocean-ocean • Continent-continent • Oceanic crust subducts – cold, dense • Divergent • New oceanic crust created • Transform • Often difficult to recognize

  11. Plate Tectonics ... • Recent • Unifying theme of solid earth sciences • Framework on which we hang (test!) observations about Earth’s geology and geophysics • Is a THEORY What makes the plates?

  12. Earth: three main layers defined by composition: • Crust - Outer • Mantle - Middle • Core - Center

  13. Composition - How Do We Know?Best Guess! Whole Earth • Meteorites - Fe, Ni (same age as Earth) • Information from velocities of seismic waves indicate material Crust (5-40 Km) • Samples (mountain building helps!) Mantle (5/40 to 2885 Km) • Kimberlite pipes - intrusive igneous rock from the mantle • Lava / volcanic rock • Mountain building Core (2885 to 6371 Km) • Inference • Earth’s mean density = 5.5 g/cm3 • Crust 2.5 to 3 g/cm3; mantle 3.3 g/cm3 to 5.5 g/cm3 • Density of core at least 10 to 11 g/cm3 (iron and nickel)

  14. Crust • Two types of crust: • Continental • 30% of crust • Granites and Diorites - rich in silicates and feldspars (lighter materials) • 40 Km thick • Oldest is 3.8 billion years (90% solar system age; missing ~700 m.y.) • Oceanic crust • Basalt - Mg, Fe (heavier materials) • 5-10 Km thick • 200 Ma oldest; 100 Ma average Our deepest hole: 9 Kilometers ….. we have a long way to go!

  15. Mantle • MOHO - Mohorovicic Discontinuity • Core mantle boundary - change in mineralogy • Density - getting heavier • 3.3 - 5.5 g/cm3 • Probably material such as peridotite (lots of heavy olivine - Fe, Mg) • Samples from kimberlites, xenoliths in volcanic eruptions, basalt composition; lab experiments

  16. Core • Outer core • Molten, near solid point (does not transmit certain seismic waves) • Density of pure iron or nickel/iron • Includes ~ half of diameter of Earth • 2x density of mantle • Inner core • Solid (higher pressure than outer core) • Density of pure iron or nickel/iron • ~ Size of moon

  17. Crust • Mantle • Core • (Composition) • Earth: three main layers defined by mechanical properties (strength): • Lithosphere • Asthenosphere • Mesosphere

  18. Lithosphere • PLATES in Plate Tectonics • Upper 100 km • Crust and upper mantle • Rigid • Asthenosphere • 100 km to ~700 Km • Upper mantle • Near melting point; little strength; ductile - NOT A LIQUID! • Plates moving on this • Magma generation • Mesosphere • Extends to core • Also hot; strong due to pressure

  19. Why Do the Plates Move?

  20. Loss of original heat of formation (geothermal / core is cooling) Radioactive decay of elements in Earth’s materials The Sun - external; not important to plate tectonics Got Heat? Earth - 3 Heat Sources:

  21. Convection: Driving Force of Plate Tectonics • Interior of Earth has sluggish convection in some regions • Heat from core rises, creates convection cells in the mantle NOT LIQUID

  22. Rising hot material at mid-ocean ridges and mid-ocean volcanic islands • Descending cooler material at trenches • Lithospheric plates “carried” with the convection cells

  23. Plate Tectonics as the Unifying Concept of Earth Science Accumulation of Observations - Evidence • Patterns of continents • Paleontology • Geology • Patterns of sea floor ages • Patterns of seafloor depth • Patterns of volcanoes • Patterns of earthquakes

  24. 1912 Continental Drift • Observations • Fit of Continents • Geology • Paleontology • Climate belts • Pangaea 200 Ma • Breakup 180 Ma • Rigid bodies moving through • yielding seafloor • No mechanism of movement Alfred Wegener

  25. Jigsaw-Puzzle Fit of Continents • Matching glacial evidence • Matching mountain ranges

  26. Matching Fossils

  27. Mechanism for Plate Movement! • Author Holmes (Early 20’s) • Interior of Earth has sluggish convection (transport of heat from core) • New ocean crust injected into ocean floor (where?)

  28. Crust moves with convection currents New ocean crust at MOR’s Ocean crust dragged down at trenches; mountains form here Continental crust too light; remains at surface Earthquakes occur where crust descends Harry Hess and Seafloor Spreading • “It explains everything….”

  29. Mapping Ocean Basins • Ocean mapping revealed • a ridge system 65,000 km long • the most extensive mountain range in the world • The Mid-Atlantic Ridge • is the best known • divides Atlantic Ocean basin in two nearly equal parts

  30. Atlantic Ocean Basin Mid-Atlantic Ridge

  31. Seafloor Spreading - Observations • Fit of continents - new material pushes them apart • Topography of ocean floors - hot ridges, trenches • Volcanism at ridge axes - hot mantle material • Seismic zones near margins - descending plates

  32. Magnetism – The Final Piece • Earth has magnetic field • Similar to a giant dipole magnet • magnetic poles essentially coincide with the geographic poles • may result from different rotation of outer core and mantle

  33. Magnetic Reversals • Earth’s present magnetic field is called normal • magnetic north near the north geographic pole • magnetic south near the south geographic pole • At various times in the past, Earth’s magnetic field has completely reversed • magnetic south near the north geographic pole • magnetic north near the south geographic pole

  34. When magma cools, takes on signature of Earth’s prevailing magnetic field • magnetic iron-bearing minerals align with Earth’s magnetic field

  35. Confirmation of Hess’s Hypothesis

  36. Confirmation of Hess’s Hypothesis • The magnetic anomalies were discovered to be striped, parallel to the oceanic ridges and symmetrical with the ridges

  37. How Do Magnetic Reversals Relate to Seafloor Spreading?

  38. Oceanic Crust Is Young • Seafloor spreading theory indicates that • oceanic crust is geologically young • forms during spreading • destroyed during subduction • Radiometric dating confirms young age • youngest oceanic crust occurs at mid-ocean ridges • and the oldest oceanic crust is less than 180 million years old • oldest continental crust is 3.96 billion yeas old

  39. Age of Ocean Basins

  40. Building Continental Material: Observable Trends?

  41. How Fast Do Plates Move?What Absolute Direction?How Do We Know?

  42. Intra-Plate Volcanism • Provides absolute rates and motions • Fixed “hot spot” in the mantle; deep, long-lived magma chamber • Plume? • If hot spot is fixed then plates are moving

  43. Hot Spots

  44. Hawaiian Ridge-Emperor Seamounts chain - 6,000-km-long • Bend at 43 Ma indicates motion of Pacific Plate abruptly changed from north to west (??related to collision of India and Eurasia??)

  45. Theory of Plate Tectonics Fit of continents Patterns of heat flow Ocean floor topography/Sediment patterns Age patterns of seafloor Volcanism at ridge axes / hot spots Magnetic stripes “Polar” wander Seismic zones Patterns of mountains

  46. How Does Plate Tectonics Affect the Distribution of Life? • Present distribution of plants and animals • climate • geographic barriers • Barriers create biotic provinces • distinctive assemblage of plants and animals • Plate movements largely control barriers • when continents break up, new provinces form • when continents come together, fewer provinces result • as continents move north or south they move across temperature barriers

  47. How Does Plate Tectonics Affect the Distribution of Life? • Example: Isthmus of Panama creates a barrier to marine organisms • Physical barriers caused by plate movements include • intraplate volcanoes • island arcs • mid-ocean ridges • mountain ranges • subduction zones Pacific Caribbean

  48. Summary • Plate tectonic theory • widely accepted by the 1970s • overwhelming evidence supporting it • It helps explain • volcanism • earthquake activity • mountain building • global climate changes • distribution of biota and resources

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