1. Plate Tectonics Chapter 17

2. Early Observations 500s, Abraham Ortelius: noticed the apparent fit of continents on either side of the Atlantic Ocean. Drifting Continents 1800s, Eduard Sues: hypothesized that the present southern continents had once been joined as a single landmass, Gondwanaland. 1912, Alfred Wegener: moving continents was proposed as a serious scientific hypothesis

3. Continental Drift continental drift : Earth’s continents had once been joined as a single landmass. Drifting Continents Pangaea began to break apart about 200 million years ago continents continued to slowly move to their present positions. Pangaea: Greek word that means “all the earth,” refers to the combined landmass. Why Pangea Broke up

4. Evidence Drifting Continents –similar groups of rocks were observed in the United States, Greenland, and Europe –fossils of several different animals and plants have been found on widely separated continents. –ages predated Wegener’s time frame for the breakup of Pangaea. –Fossils of Glossopteris, a seed fern that resembled low shrubs, have been found on many continents: – indicating that the areas had a single climate that was close to the equator.

5. Continental Drift Evidence from Rock Formations Drifting Continents

6. Continental Drift Evidence from Fossils Drifting Continents

7. Continental Drift Ancient Climatic Evidence Drifting Continents –Coal deposits in Antarctica suggested that it must have been closer to the equator. –Glacial deposits found in Africa, India, Australia, and South America suggested that these areas had once been covered by thick ice caps. –Wegener proposed that they once were located near the south pole before Pangaea began to fracture.

8. Continental Drift Ancient Climatic Evidence Drifting Continents

9. A Rejected Hypothesis most scientists rejected Wegener’s hypothesis Drifting Continents What forces could move continents? How could continents move without shattering?

10. What do we remember about the topography of the ocean floor? How do scientists map the ocean floor? Magnetometer: device that can detect small changes in magnetic fields construct magnetic maps of the seafloor. Seafloor Spreading

11. Ocean Rocks and Sediments important discoveries. Seafloor Spreading ages of the rocks that make up the seafloor vary in different places age of oceanic crust consistently increases with distance from a ridge. –oldest part of the seafloor is 180 million years old (how old is the earth?). thickness of ocean-floor sediment was much less than expected thickness of the sediments increases with distance from an ocean ridge. GRAPH???

12. Ocean Rocks and Sediments Seafloor Spreading

13. Magnetism Rocks (especially BASALT) containing iron-bearing minerals provide a record of Earth’s magnetic field. Seafloor Spreading Paleomagnetism : study of Earth’s magnetic record.

14. A magnetic field that is the same as the present has normal polarity. A magnetic field that is opposite to the present has reversed polarity. Magnetism The Geomagnetic Time Scale Seafloor Spreading –a pattern of magnetic reversals over geologic time. – magnetic reversal: change in Earth’s magnetic field.

15. magnetic readings of the ocean floor matched Earth’s present field, a stronger-than-normal reading (+) was recorded. magnetic data were reversed in relation to Earth’s present magnetic field, a lower-than-normal reading (–) was recorded. Magnetism The Geomagnetic Time Scale Seafloor Spreading –Towing magnetometers behind ships

16. Magnetism The Geomagnetic Time Scale Seafloor Spreading

17. Magnetism Magnetic Symmetry Seafloor Spreading –positive and negative areas form a series of stripes parallel to ocean ridges. –magnetic pattern on one side of the ridge is a mirror image of the pattern on the other side

18. Magnetism Magnetic Symmetry Seafloor Spreading –Scientists compared magnetic data from ocean floor to basalt flows on land –determined the age of the ocean floor from a magnetic recording –created isochron maps of the ocean floor. –Isochron: line on a map that connects points that have the same age.

19. Magnetism Seafloor Spreading

20. Harry Hess proposed the theory of seafloor spreading. Seafloor Spreading –Magma fills the gap that is created. –Bill Nye Sea Floor SpreadingBill Nye Sea Floor Spreading Seafloor spreading : new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches.

21. Seafloor Spreading –Each cycle of spreading and the intrusion of magma results in the formation of another small section of ocean floor, which slowly moves away from the ridge. – When the magma hardens, a small amount of new ocean floor is added to Earth’s surface.

22. Seafloor Spreading The Missing Link Seafloor Spreading –Seafloor spreading was the missing link !!! TO WHAT?

23. Theory of Plate Tectonics Earth’s crust and rigid upper mantle are broken into enormous moving slabs called plates. Theory of Plate Tectonics There are a dozen or so major plates and several smaller ones. move in different directions and at different rates over Earth’s surface.

24. Theory of Plate Tectonics

25. Plate Boundaries Tectonic plates interact at places called plate boundaries. 3 types Theory of Plate Tectonics

26. Plate Boundaries 1. Divergent Boundaries Theory of Plate Tectonics –places where two tectonic plates are moving apart. –in rifts, or fault-bounded valleys, which form along the axis of an ocean ridge. –rift valley narrow depression, is created when a divergent boundary forms on a continent.

27. Plate Boundaries 2. Convergent Boundaries Theory of Plate Tectonics –places where two tectonic plates are moving toward each other. –three types of convergent boundaries: 1.Oceanic crust converging with oceanic crust 2.Oceanic crust converging with continental crust 3.Continental crust converging and colliding with continental crust.

28. Plate Boundaries Convergent Boundaries Theory of Plate Tectonics

29. Plate Boundaries Convergent Boundaries Theory of Plate Tectonics –Subduction one of the two converging plates descends beneath the other. –one oceanic plate, which has become denser as a result of cooling, descends below another plate creating a deep-sea trench. –subducted plate descends into the mantle and melts. –Some of the magma forms new oceanic crust at the ridge or is forced back to the surface, forming an arc of volcanic islands that parallel the trench.

30. Plate Boundaries Convergent Boundaries Theory of Plate Tectonics –When an oceanic plate converges with a continental plate, the denser oceanic plate is subducted. –Oceanic-continental convergence produces a trench and a series of volcanoes along the edge of the continental plate. –Two continental plates collide when an ocean basin between converging oceanic and continental plates is entirely subducted. –Because continental rocks are too buoyant to be forced into the mantle, the colliding edges of the continents are crumpled and uplifted to form a mountain range.

31. Plate Boundaries 3. Transform Boundaries Theory of Plate Tectonics –place where two plates slide horizontally past each other, deforming or fracturing the crust. –characterized by long faults –The San Andreas Fault is an exception to the fact that transform boundaries rarely occur on continents.

32. Causes of Plate Motions What actually causes the plates to move is not well understood. One of the leading hypotheses proposes that large-scale motion in the mantle is the mechanism that drives the movement of tectonic plates.

33. Mantle Convection Convection is the transfer of thermal energy by the movement of heated matter. Causes of Plate Motions Convection currents in the mantle are thought to be the driving mechanism of plate movements, set in motion by the transfer of energy between Earth’s hot interior and its cooler exterior.

34. Mantle Convection rising part of a convection current spreads out with both upward and lateral forces. Causes of Plate Motions forces lift and split the lithosphere at divergent plate boundaries. downward part of a convection current occurs where a sinking force pulls tectonic plates downward at convergent boundaries.

35. Mantle Convection Push and Pull Causes of Plate Motions –During the formation of an ocean ridge, forces in the mantle cause the asthenosphere to rise. –ridge push, the weight of the uplifted ridge is thought to push an oceanic plate toward the trench formed at the subduction zone.

36. Mantle Convection Push and Pull Causes of Plate Motions –A sinking region of a mantle convection current could suck an oceanic plate downward into a subduction zone. –slab pull, the weight of a subducting plate helps pull the trailing lithosphere into the subduction zone.