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Plate Tectonics

Plate Tectonics

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Plate Tectonics

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  1. Plate Tectonics The Unifying Theory of Geology

  2. The 4 “Big Ideas” in Geology • The Rock Cycle (Friday’s lecture) • By the mid-1700s • Antiquity of Earth - “Deep Time” (Wednesday’s Lecture) • By the early 1800s, millions of years • Faunal Succession (Fossil Record – Core 6) • By mid-1800s • Plate Tectonics • By late 1960s

  3. The Big Ideas  Earth is Old and Dynamic Rather than Young and Static

  4. Recall: The Scientific Method • Observations • Hypothesis (a testable explanation) • Includes testing by prediction • More observations (testing) • If ALL observations fit ---> Theory

  5. The Ancient Greeks • Knew that Earth was round (not discovered by Columbus) by 300 B.C. • In fact Eratosthenes (ca. 250 B.C.) measured its circumference PhotoNot Available

  6. Early Observations • First European explorers started to gather knowledge on world geography • Early 1500s: E.g., Magellan & da Gama

  7. Early Observations • Francis Bacon (1561-1626) first noted how coasts of Africa and South America fit. Yikes!!!… bacon!

  8. Today it’s known: Fit best along their continental shelves

  9. The 1800s • Better maps available • Some people proposed that all continents could fit together • In 1872, British Challenger began mapping ocean floor by soundings • Similarities in rocks of NW Europe and NE America were discovered

  10. About 1910, the Mid-Atlantic Ridge was discovered

  11. Paleontology The study of fossils

  12. Evidence from Paleontology Similar fossils in South America and Africa

  13. Glossopteris, a seedfern whose seeds are too large to be carried far by wind

  14. Mesosaurus, a fresh water reptile that couldn’t swim across the open sea

  15. Paleoclimatology The study of ancient climates

  16. Evidence of glaciers

  17. Evidence of glaciers

  18. in South America, Africa, India, and Australia...At same time the Northern Hemisphere had lush swamps...

  19. and the pieces fit together like a puzzle... “Gondwanaland”

  20. Coal • Deposits found in Antarctica brrrrr…. • Coal requires a warm, lush climate ===> • What’s Antarctica like today? ===>

  21. Paleomagnetism • Magnetic minerals in molten rock align with Earth’s magnetic field

  22. Paleomagnetism • When igneous rock cools, magnetism is “frozen in”, like little compasses...

  23. Polar Wandering = apparent change in position of poles over time

  24. Polar Wandering • Different continents indicate different position of North Pole - very strange... • Only logical explanation is that the CONTINENTS have moved since the rocks were formed

  25. WEGENER and CONTINENTAL DRIFT • In the 1910s Alfred Wegener put all these pieces of evidence together and made the hypothesis of CONTINENTAL DRIFT... • The Origin of Continents and Oceans (1915)

  26. CONTINENTAL DRIFT • When Wegener proposed to fit the continental shelves together, rather than the coastlines, we got a better fit for all modern continents...

  27. All continents have moved to their present positions from one “supercontinent” he called PANGAEA - 200 Ma

  28. Scientific Consensus is that Earth is about 4.5 billion years old (4.5 Ga)

  29. Breakup of Pangaea

  30. 1912 - 1945 • A few geologists looked for more evidence during this period... • Seismologists began studying the deeper layers of the earth and discovered a dense mantleand liquid outer core.

  31. WWII and SONAR • Sonar, developed to find enemy subs, was used in the decade after WWII to map the deep sea floor...

  32. SONAR • Previously, most geologists thought the sea floor was rather flat and featureless • They were wrong... • A diverse topography was discovered

  33. Topography of the Sea Floor

  34. Topography of the Sea Floor • Researchers found an undersea mountain range 40,000 miles long. • And a trenches seven times deeper than the Grand Canyon. • Even more striking were the geophysical findings...

  35. Some Geophysics Thousands of drilling samples were taken...

  36. Geophysics • Paleomagnetism in the deep sea floor rocks indicated that many episodes of magnetic reversals had taken place...

  37. These reversals occur in parallel paired bands on opposite sides of a mid-ocean ridge. Radiometric dating showed the rocks get older the farther you get from the ridge.

  38. Sea-Floor Spreading • In 1962, these data were collected into a theory called SEA-FLOOR SPREADING • New crust forms at mid-ocean ridges... • Oceanic crust pushes outwards from the ridge and (perhaps) takes the continents along

  39. Earthquakes and Volcanoes • Seismology is the study of earthquakes

  40. Remember the Tsunami?

  41. Earthquakes and Volcanoes • Data indicate that earthquakes and volcanoes do not occur at random locations, rather...

  42. Earthquakes and volcanoes occur mostly along or near trenches and mid-ocean ridges...

  43. E.g., Pacific “Ring of Fire”

  44. Seismology studies • located a zone of weakness where seismic waves travel more slowly • It was called the asthenosphere. • The rigid lithosphere sits on top of this

  45. Finally, a unifying theory... • In 1968, seismologists at Columbia put all the evidence together and came up with the theory of PLATE TECTONICS... • This combined the sub-theories of CONTINENTAL DRIFT and SEA-FLOOR SPREADING

  46. PLATE TECTONICS The rigid upper 35 miles or so of the earth (lithosphere) is broken up into a dozen or so plates, which can slide around on the zone of weakness.