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PLATE TECTONICS

Learn about Alfred Wegener's Continental Drift Theory, plate tectonics, evidence supporting it, and interactions at plate boundaries. Discover the movement of lithospheric plates and the Earth's changing surface features. Dive into the geological processes shaping our planet.

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PLATE TECTONICS

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  1. PLATE TECTONICS • PLATES Large, rigid sections of the lithosphere Move independently of each other • TECTONICS Refers to all forms of bending & breaking of the Earth’s crust

  2. Plate Tectonic Theory • 6 large plates (e.g., Pacific Plate) • 7-10 small plates (e.g., Arabian Plate)

  3. 1960’s Alfred Wegener Theory of Continental Drift

  4. CONTINENTAL DRIFT Alfred Wegener in the early 1900’s proposed that the continents were once joined together in a single large land mass he called Pangea (meaning “all land” in Greek). He proposed that Pangea had split apart and the continents had moved gradually to their present positions - a process that became known as continental drift.

  5. Pangaea about 200 million years ago, before it began breaking up. Wegener named the southern portion of Pangaea Gondwana, and the northern portion Laurasia.

  6. KEY TECTONIC CONCEPTS • Lithospheric plates are denser than asthensospheric mantle = tend to sink • Are they pulled down by their own Weight? • Are they pushed from forces behind them? • Slow convective currents in the mantle are linked to tectonics • Movements are caused by differences in density

  7. 1 – 10 cm per year

  8. Wegner’s Evidence for Continental Drift Continents fit together like a puzzle….e.g. the Atlantic coastlines of Africa and South America. The Best fit includes the continental shelves (the continental edges under water.)

  9. Wegner’s Evidence for Continental Drift Fossils of plants and animals of the same species found on different continents.

  10. Wegner’s Evidence for Continental Drift • Glacial sediment deposits found in places where glaciers do not exist today. • Glacial Scratches (scratches on rock caused by glacial movement) line up like a jigsaw puzzle when continents are reassembled. • Both show that the land masses were all joined and partially covered by a single large ice cap over the ancient south pole!

  11. Evidence of Plate Tectonic • Coastline fit (fits like a puzzle) • Geologic fit (Landforms & rock ages Africa/S. America) • Paleoclimatology (Coal formed in tropical climates. Why is it found at the poles today?) • Paleoglaciation

  12. Ocean chemistry & circulation has changed • Fossil records show mass extinction of species & explosions of diversity of species

  13. Paleomagnetism Evidence for seafloor spreading comes from magnetic field reversals recorded in rocks as they form along spreading centers.

  14. Paleomagnetism Throughout earth’s time the magnetic north and south have switched RANDOMLY and sporadically every 1000-10,000 years. When magnetism switches, its called a REVERSAL, where lines of magnetism run North to South.

  15. Paleomagnetism Rocks record the direction of the earth’s magnetic field at the time the rocks form. Small magnetite (Fe) crystals in cooling magma act like compass needles that record the direction of the earth’s magnetic field when the magma solidifies. Once the minerals solidify, the magnetism they posses will remain.

  16. Other Supportive Data A hot spot is a persistent volcanic center thought to be located directly above a rising plume of hot mantle rock. Its magma rises through the lithosphere to erupt and form a volcano or volcanic island. Hot spot mantle plumes remain stationary while the lithosphere moves over it. This process forms a chain of volcanic islands. The chain of islands formed indicates the direction of plate movement over the hot spot.

  17. Other Supportive Data Interactions at Plate Boundaries Plate boundaries are associated with active margins. The distribution of earthquakes and volcanic eruptions indicate that these phenomena are concentrated in belts or linear chains at the boundaries of the lithospheric plates.

  18. Plate Tectonic Theory • 6 large plates (e.g., Pacific Plate) • 7-10 small plates (e.g., Arabian Plate)

  19. Plate Boundaries: What you need to be able to do!!! • Sketch a diagram (side view) depicting plate movement and dynamics involved. - Label the plates involved (Sima/Sial) - Include and label features that are created as a result of the process - Identify/locate locations of the boundary types (World Map) • Identify landforms the earth’s surface (photo) and discuss the processes involved in their formation • State the pro’s/con’s of each boundary type in regards to human interaction (hazards, benefits) • Tectonic Boundaries • Ring of Fire (Magma Type)

  20. “CONSTRUCTIVE” “Tension”

  21. Divergent Boundary

  22. Continental crust bends upward over hotspot - magma fills cracks Crust splits/breaks Blocks of crust fall down – form Rift Valleys and Block Mountains Rift Valley deepens as continents split apart Magma rises & forms new crust (oceanic) between continents 1 2 3 OCEAN BASIN FORMATION

  23. M.O.R. : long ridge of mtns crossing earth’s major oceans. 80,000 km long +1000 km wide

  24. Harry Hess and Sea Floor Spreading 1960’s Harry Hess concluded in 1960 that new sea floor was being created at mid-ocean ridges (MOR) by volcanic activity. But the earth is not getting larger. Therefore he concluded that an equal amount of oceanic crust is probably being lost at trenches. The driving force is convection currents in the mantle caused by heat from earth’s formation, radioactive decay and gravity. Convection currents carry the rigid crust away from the mid-ocean ridge like a conveyor belt and drive it into the mantle at trenches.

  25. Harry Hess and Sea Floor Spreading 1960’s Locations of spreading ridges (upwelling) and trenches (downwelling) are determined by the convection cells.

  26. Driving force is not just convection.Ridge Push and Slab Pull

  27. Where is sediment thickest? Why?

  28. Passive Plate Boundary

  29. Where is sediment thickest? Why?

  30. Mid Ocean Ridge is not a continuous line – due to Earth’s curvature Broken into short “step like” sections Transform Faults Volcanoes & Earthquakes

  31. Submerged flat topped mountain: Seamount Low Viscosity

  32. Seamount “flat topped mountains”

  33. Why would the following be different for locations A and B? a)Ages b)Magnetic Records c)Thickness of sediment d)Depth below sea level A B

  34. Fissures Gentle eruptions Little presssure Few gases Basaltic lava – Low viscosity

  35. What is this? Explain the processes involved in its formation?

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