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Continental Drift and Seafloor Spreading: The Dance of the Continents

This section explains Alfred Wegener's hypothesis of continental drift and provides evidence for it. It also discusses seafloor spreading and the theory of plate tectonics, including the three types of plate boundaries and the role of convection currents in plate movement.

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Continental Drift and Seafloor Spreading: The Dance of the Continents

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  1. Plate Tectonics Chapter 4

  2. Section 4.1 Objectives 1. Explain Wegener’s hypothesis of continental drift 2. List evidence for Wegener’s hypothesis of continental drift 3. Describe seafloor spreading

  3. 4.1 Continental Drift • In 1912, Alfred Wegener proposed his continental drift hypothesis. • It stated that the continents once formed part of a single landmass, which he named Pangaea, which means all lands. • Wegener thought that Pangaea began breaking up into smaller continents about 200 million years ago, and drifted to their present locations.

  4. EVIDENCE: • Fit of the shapes of the continents • Fossils of plants and animals found on different continents that are separated by oceans

  5. Evidence 1: Geologic Fit of the continents 1. Age and type of rocks match in West Africa and Brazil 2. The Appalachian mountains match up with the mountain range that runs through Scotland and North Europe.

  6. Mesosaurus fossils date back to 270 million years ago in South America and West Africa. Did it swim that far? There is not evidence of a land bridge.

  7. EVIDENCE: 3. Glacial grooves show that glaciers looked like they moved from sea to land

  8. EVIDENCE: 4. Climatic patterns shown by rock layers: Some rock types only form in certain climates, for example coal, which forms in warm, very wet (rainy) environments. If coal is found in a place that is not warm and rainy, then either the climate has changed or the rock has moved.

  9. The Dance of the Continents… GO>

  10. The evidence that Alfred was looking for was found …On the Ocean floor!

  11. Seafloor Spreading Hypothesis • In 1947, a group of scientists set out to map the Mid-Atlantic Ridge. This is part of an 80,000 km.-long system of mid-ocean ridges. • The oldest rocks found on the sea floor were less than 175 million years old. The oldest continental rocks are about 4 billion years old. WHY IS THIS INFORMATION SURPRISING?

  12. Seafloor Spreading Hypothesis • The valley at the center of the MOR is a rift (a long fracture in the crust) • Magma is coming up from inside the earth into the rift. • This magma can find space in the crust because the ocean floor is moving away from both sides of the MOR

  13. PALEOMAGNETIc REVERSALS • As magma solidifies to form rock, the magnetic fields of iron-rich minerals align with the earth’s magnetic field, just like a compass. • Scientists have discovered that this is not always the case. they have found minerals that point south.

  14. PALEOMAGNETIc REVERSALS • Throughout the earth’s history, the magnetic field has reversed itself many times. • Such reversals have come at irregular intervals,averaging about every 300,000 years; the last one was 780,000 years ago. Are we overdue for another? No one knows

  15. PALEOMAGNETISM of THE OCEAN FLOOR • Scientists discovered magnetic patterns locked into the rocks of the ocean floor • These patterns showed alternating bands of normal and reversed magnetism • As molten rock rises from the rift in an MOR, it quickly cools and hardens and its magnetic orientation becomes fixed.

  16. Paleomagnetism of the ocean floor

  17. Sea Floor Spreading

  18. Section 4.2 Objectives 1. Summarize the theory of plate tectonics. 2. Compare the characteristic geologic activities that occur along the three types of plate boundaries. 3. Explain the possible role of convection currents in plate movement. 4. Summarize the theory of microplate terranes.

  19. ooo Plate Tectonic Theory: Summary • Earth’s outer layer is broken into about 30 sections called plates. • The plates are composed of uppermost mantle and either continental crust or oceanic crust (lithosphere). • The plates ride on the asthenosphere. • The plates move because of convectioncurrents flowing in the mantle below the plates. .

  20. Earth’s Tectonic Plates

  21. Types of Crust 1. oceanic crust 2. continental crust

  22. Lithosphere/Asthenosphere • The oceanic and continental crust and the rigid upper mantle make up the lithosphere. • The lithosphere forms a thin outer shell that lies above the plastic rock of the asthenosphere.

  23. Divergent Boundary • The Red Sea occupies an area that contains a divergent boundary. • Seafloor is pushing the African and Arabian plates away from each other. • A rift valley runs down the center of the sea. Arabian Plate African Plate

  24. Arabian Plate African Plate Red Sea Boundary Birth Ocean Divergent Boundary

  25. ooo Convergent Boundary • convergent boundary: the direct collision of one plate with another. • Three types of convergent boundaries: • ocean to ocean • continent to ocean • continent to continent

  26. Watch Video Convergent Boundary 3. Ocean To Ocean: oceanic plate subducting under another oceanic plate

  27. Convergent Boundary OCEAN TO OCEAN

  28. Convergent Boundary Watch Video • Continent To Ocean: • when the oceanic crust slides beneath continental crust. • this is a process called subduction. • subduction zones create deep trenches… as the plate is subducted (tucked under) heat is released, the mantle partially melts, and magma rises to the surface: VOLCANOES

  29. Convergent Boundary Watch Video 1. Continent to Continent Neither plate is subducted because they have the same density. Instead the land is crumpled and uplifted creating MOUNTAIN RANGES!

  30. Transform Boundary • Transform boundary: two plates are grinding side-by-side past each other

  31. Transform Boundary These boundaries have sudden spurts of activity and then periods of no motion

  32. Plate Boundaries How many plates? Convergent boundaries? Divergent boundaries? Transform boundary? 4 2 1 3

  33. Watch Video Causes Of Plate Motion • Scientists think that the movement of lithospheric plates is due toconvection.

  34. Microplate Terranes • Theory of Microplate Terranes: continents are actually a patchwork of crustal blocks, called terranes. Each block has its own distinct geological history. • Terranes are regions that are bounded by faults and have rocks of different character (age, type, fossils) than in surrounding regions. • Terranes form in another part of the world and are moved by plate motion to their present locations

  35. Microplate Terranes of Virginia

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