Plate tectonics
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Plate tectonics






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Plate tectonics Plate tectonics Presentation Transcript

  • Plate Tectonics Continental Drift
  • 4-1 Continental Drift – objectives…
    • Explain Wegener’s hypothesis of continental drift.
    • List evidence for Wegener’s hypothesis of continental drift.
    • Describe seafloor spreading.
  • Continental Drift
    • Began with observations made over 400 years ago.
    • The first reliable maps were studied and it was noticed that the continents fit together like a puzzle.
  • Hypothesis on Continental Drift.
    • The continents were once part of the same landmass – Pangea
    • Surrounding Pangea was a great ocean – Panthalassa
    • About 200 mya, Pangea began breaking apart.
    • This motion caused crumpling which led to today’s land features.
  • Alfred Wegener
    • A german scientist.
    • Proposed his hypothesis on continental drift in 1912.
  • Observations led to questions…
    • Were the continents once part of the same landmass?
    • If so, what caused this landmass to break apart?
    • What caused the continents to move to their present locations?
  • Evidence
    • Coastline similarities
  • Evidence
    • Fossils
      • Mesosaurus
      • Cynognatyhus
      • Lystrosaurus
      • Glossopteris
  • Geologic Evidence
    • Age and types of rocks
    • Mountains
      • Applachian mountains fit continuously with a band of mountains in Greenland.
  • Climatic Evidence
    • Layers of glaciers in southern Africa and South America
    • Coal deposits in the Eastern US, Europe, and Sibera
    • These climatic similarities are easy to explain if the continents were once joined.
  • Climatic Evidence - glaciation
    • Evidence of ancient glaciation indicated that parts of southern Africa, India, Australia, and South America were covered by a large ice sheet. Arrows indicate direction of ice movement.
  • There was little support for Wegner’s hypothesis….until.two decades after his death…
  • Seafloor Spreading
    • 1947, mapping the Mid-Atlantic Ridge
      • Discovered the ocean floor is much younger than continental rocks.
        • Oceanic rocks – 150 million years old
        • Continental rocks – 4 billion years old
    • Harry Hess suggested a hypothesis
      • Seafloor spreading
        • Rocks in/near the rift were younger as a result of the upwelling and solidification of magma.
  • Seafloor Spreading
    • Seafloor spreading provided the support Wegner needed for a mechanism for continental drift.
    • Still no proof…
  • Paleomagnetism
    • Magnetism resulting from the cooling of magma mirrors the existing magnetism of the earth.
    • Magnetic orientations were discovered that appeared normal then reversed…normal then reversed…
    • This supported seafloor spreading…and thus further anchored Wegner’s Hypothesis.
  • Paleomagnetism
  • Review of 4-1…
    • What observation first led to Wegner’s hypothesis of continental drift?
    • What types of evidence support Wegener’s hypothesis?
    • Describe the process of seafloor spreading.
    • Explain how scientists know that the earth’s magnetic poles have reversed themselves many times during earth’s history.
  • 4-2 – Plate Tectonics - objectives
    • Summarize the theory of Plate Tectonics.
    • Compare the characteristic geologic activities that occur along the three types of plate boundaries.
    • Explain the possible role convection currents in plate movement.
    • Summarize the theory of suspect terranes.
  • A theory emerges…
    • 1960’s…
    • A combination of continental drift and seafloor spreading…
    • Describes continental movement but also proposes a possible explanation of why and how continents move.
  • The Earth’s Layers
    • Two types of crust:
      • Oceanic – ocean floor
      • Continental – dry land
    • Both types of crust plus the rigid upper mantle make up the Lithosphere.
    Himilayian Crust
  • The Earth’s Layers
    • Lithosphere – the thin outer layer
      • Lithospheric plates
    • Asthenosphere – a layer of “plastic” rock
      • Denser than the lithosphere.
  • Earth’s Layers
  • Lithospheric Plate Boundaries
  • Lithospheric Plate Boundaries
    • Continent margins don’t necessarily follow plate boundaries.
    • 30 plates have been identified at some speed of constant motion.
  • Types of Plate Boundaries
  • Divergent Boundaries
    • spreading centers where two lithospheric plates move away from each other
  • Divergent Boundaries
  • Convergent Boundaries
    • A converging boundary forms when two plates move toward each other
    • Two Types
      • Collision
      • Subduction
  • Convergent Boundary - Collision
    • Oceanic crust – Oceanic crust
  • Convergent Boundary - Collision
    • Oceanic crust – continental crust
  • Convergent Boundaries
    • Continental crust – continental crust
  • Transform Fault Boundaries
    • San Andreas fault, California
  • Causes of Plate Motion
    • Many scientists think that the movement of the lithospheric plates is due to convection currents.
  • Convection Currents