Plate Tectonics

1. Plate Tectonics Text ref. Ch.17 (pg. 442)

2. Contents • Continental Drift • Seafloor Spreading • Plate Tectonics

3. Continental Drift Text ref. Ch.17 (442-447)

4. Objectives • Describe one piece of early evidence that led people to suggest the Earth’s continents may have once been joined. • Discuss evidence of continental drift. • Explain why continental drift was not accepted when it was first proposed.

5. Drifting Continents • Early observations • Map-makers noticed how continents fit on either side of the Atlantic. • Thought continents had been separated by floods and earthquakes • Edward Suess proposed continents had once been joined together as supercontinent known as Gondwanaland.

7. Continental Drift • Wegener proposes continental drift, calling his supercontinent Pangaea, which means “all Earth” in Greek • Proposes supercontinent began breaking apart 200 million year ago. • Begins collection of scientific evidence to support his theory

8. Evidence from Rock Formations • Wegener hypothesized that the same types of rock formations should exist on both sides of the Atlantic. • Noticed rocks in the Appalachian mountains shared features with rocks in Greenland and Europe. • All rocks were dated older than 200 million years; therefore found together before continental drift began.

10. Evidence from Fossil Records • Fossils of the same animals and plants were can be found on separate continents. • Kannemeyerids and Labyrinthodonts fossils can be found in Africa, North America, South America, Antarctica, Greenland, and Northern Europe. • Glossopteris plants can also be found in many continents, strengthening the hypothesis that these continents shared the same climate.

11. Glossopteris Kannemeyerid

13. Ancient Climatic Evidence • Wegener studied sedimentary rocks and found evidence of widespread climate change. • Coal deposits in Antarctica suggest that plants once lived in Antarctica; therefore Antarctica was once closer to the equator.

15. Glacial Evidence • Glacier deposits can be found in Africa, India, Australia, and South America. This suggests that these areas were once cold enough for glaciers to form.

17. Opposition to the Hypothesis • Wegener could not explain why the continents had moved throughout geologic time. • Scientists doubted that such forces could exist on Earth to move entire continents. • Wegener could explain how continents could move through the ocean floor to new locations on Earth. • Wegener continued to collect evidence until his death in 1930.

18. Seafloor Spreading Text ref. Ch.17.2 (pg.448-455)

19. Objectives • Summarize the evidence that led to the discovery of seafloor spreading. • Explain the significance of magnetic patterns on the seafloor. • Explain the process of seafloor spreading.

20. Technological Advance • People before the 1900’s assumed the seafloor was essentially flat. • Advances in technology in the 1940’s and ’50s led to new ideas. • Sonar can map surfaces by bouncing high frequency sound waves in order to calculate distances (elevation)

22. Ocean Floor Topography • Use of magnetometer to detect variations in magnetic fields allow scientists to map the seafloor. • Images revealed underwater mountain ranges, called ocean ridges. • Ridges form the largest continuous mountain range on Earth.

27. Sonar revealed ridges had corresponding trenches • Mariana Trench is over 11 km deep.

28. Questions • What could have formed the ridges and trenches found on the seafloor? • What is the source of volcanism associated with these locations? • What forces could be at work to move the entire seafloor and create trenches 6 times as deep as the Grand Canyon?

29. Ocean Rocks and Sediments • Scientists found predictable patterns in the ages of rock on the seafloor. • Rock ages with distance from ocean ridges • Ocean floor is dated at its oldest around 180 million years. • Continental rocks are dated at 3.8 billion years. • Why is the seafloor so young in comparison?

31. Ocean Rocks and Sediments cont… • Thickness of ocean sediments is much thinner than expected. • Typically a few hundred meters thick. • Continents may have sediments up to 20 km thick. • Why are the sediments so thin? • Why isn’t seafloor as thick as continental crust? • Sediment is found to be thicker the further it is from ridges.

32. Magnetism • Study of the Earth’s magnetic field in iron-bearing minerals is known as paleomagnetism. • Seafloor is composed of iron-rich basalt, in which the iron becomes aligned with Earth’s magnetic field as molten rock cools. • When rock hardens, the iron orients itself to Earth’s magnetic field at that time

33. The Geomagnetic Time Scale • Studies in the 1960’s revealed pattern of magnetic reversals over time. • Magnetic reversals are changes in the Earth’s magnetic field. • Data from these reversals allowed scientists to construct a geomagnetic time scale.

35. The Investigation • Scientists towed magnetometers behind ships to measure the magnetic field of the ocean floor. • Scientists noticed alternating + and – values for the magnetic field readings.

37. Magnetic Symmetry • Scientists noticed that the + and – magnetic bands were symmetrical, originating from the ocean ridges. • Scientists also discovered basaltic lava flows on land matching these magnetic reversals. • Age of the ocean floor was ascertained from this data.

40. Isochronic Mapping • Isochron is a line on a map that connects points that have the same age. • Young rock is found at mid-ocean ridges, while older rock is found further away near deep sea trenches.

42. Seafloor Spreading • After all this data, Harry Hess proposes the theory of seafloor spreading. • Theory proposes that new ocean floor is created at the ridges and destroyed at the trenches. • Magma rises to the surface and hardens at the ocean ridges. • Consecutive bands of seafloor are created in this way.

44. Wegener’s Theory Revisited • Wegener’s theory of continental drift did not account for the forces in action. • Seafloor spreading completes the picture and describes the forces that shape the drifting of the continents. • Continents are like groceries on the conveyer belt at the checkout line; they don’t push through the ocean floor, they ride on top of it as the ocean floor moves.

45. Theory of Plate Tectonics Text ref. Ch.17.3 (pg. 455 – 459)

46. Objectives • Explain the theory of plate tectonics. • Compare and contrast the three types of plate boundaries and the features associated with each.

47. The Theory • Why do earthquakes happen? • Why do volcanoes erupt? • Why do mountains exist? • Plate Tectonics is a theory that states the Earth’s crust and rigid upper mantle are broken into enormous slabs called plates. • Each plate moves at a particular rate, and measured using a system of satellites and receivers

49. Plate Boundaries • Tectonic plates interact at places called plate boundaries. • Divergent • Convergent • Transform • Each boundary has certain geologic characteristics.

50. Divergent Boundaries • Places where two tectonic plates are moving apart. • Most are found on the seafloor as ocean ridges. • Formation of new ocean crust accounts for volcanism along these locations. • A rift valley is formed when a divergent plate boundary is located on continental crust. • Ex. East African Rift Valley