Plate Tectonics Part1


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Plate Tectonics Part1

  1. 1. PLATE TECTONICS Geology 105
  2. 2. SCIENTIFIC REVOLUTION <ul><li>According to Thomas Kuhn (1962), science does not proceed by slowly accumulating knowledge toward ultimate truth </li></ul><ul><li>Instead, it is marked by stable “paradigms” of accepted assumptions about the world, which are modified but rarely challenged by scientific research </li></ul>
  3. 3. SCIENTIFIC REVOLUTION <ul><li>Eventually, enough inconsistent observations accumulate that cannot be explained by the paradigm </li></ul><ul><li>Then a revolutionary new model is proposed which explains the old data as well as the new, and a scientific revolution is born </li></ul><ul><li>Since scientists are human, it takes almost a generation or more for acceptance. The “old guard” resist the new idea, but the young accept it and build their careers with it </li></ul>
  4. 4. SCIENTIFIC REVOLUTION Kuhn’s (1962) ideas have been controversial, but do in fact apply to most of the sciences
  5. 5. SCIENTIFIC REVOLUTION <ul><li>For example, Copernican astronomy completely replaced Ptolemaic astronomy, because it explained planetary motions better </li></ul><ul><li>Newtonian physics replaced Aristotelian ideas, and in turn was replaced by Einsteinian physics for extreme conditions </li></ul>
  6. 6. SCIENTIFIC REVOLUTION <ul><li>Darwinian evolution completely revolutionized biology, replaced old “natural theology” </li></ul><ul><li>Chemistry has not had a comparable revolution as total as these other examples </li></ul><ul><li>Geology’s scientific revolution: plate tectonics </li></ul>
  7. 7. CONTINENTAL DRIFT <ul><li>Old school of of thought assumed fixed, stable continents, but had a lot of unsolved problems </li></ul><ul><li>First ideas proposed in 1915 by Wegener, but were not accepted </li></ul><ul><li>In the 1950s to 1963, enough new kinds of data pushed the hypothesis from crazy to widely accepted </li></ul><ul><li>Some of the “old guard” never accepted it to the day they died; others embraced it; but the major “players” in plate tectonics were all young scientists at the time </li></ul>
  8. 8. ALFRED WEGENER <ul><li>German meteorologist, gifted with a diverse range of interests and wide imagination </li></ul><ul><li>BUT not a conventional geologist by training, so dismissed by professional geologists </li></ul>
  9. 9. ALFRED WEGENER Led one of the first major meteorological expeditions to Greenland in 1930
  10. 10. ALFRED WEGENER Died on Oct. 30, 1930, returning from a supply drop in bad weather
  11. 11. ALFRED WEGENER Published The Origin of Continents and Oceans in 1915, but his arguments were scoffed at by geologists, because he was not formally trained in geology, and because he provided no mechanism for how continents could move through oceanic crust
  12. 12. WEGENER’S EVIDENCE FOR CONTINENTAL DRIFT <ul><li>Fit of the continents </li></ul><ul><li>Permian Pangea glaciations </li></ul><ul><li>Permian climatic belts </li></ul><ul><li>Identical Permian deposits </li></ul><ul><li>Matching bedrock across the Atlantic </li></ul><ul><li>Distinctive fossils on the Permian continents </li></ul>
  13. 13. 1. FIT OF THE CONTINENTS Originally noticed in the 1600s when the first accurate maps of the Atlantic suggested that Africa and South America fit together, but Wegener expanded on the idea, and suggested that all the continents once formed Pangea
  14. 14. 1.FIT OF THE CONTINENTS In the 1950s, geophysicist Sir Edward Bullard did a more rigorous fit, and showed the coastline match is no accident (orange overlaps due to later growth after ripping apart)
  15. 15. 2. PERMIAN PANGEA GLACIATIONS Glacial deposits of Permian (250-290 million years old) age only make sense if southern continents once were joined to form Gondwanaland. Modern distribution of those ice sheets is otherwise nonsense
  16. 16. 2. PERMIAN PANGEA GLACIATIONS Even some of the ancient glacial scratches appear to line up as if they crossed the Atlantic
  17. 17. 3. PERMIAN CLIMATIC BELTS Distribution of Permian tropical coal deposits, subtropical desert deposits, etc., only make sense in the Pangea configuration (impressive to a meteorologist like Wegener, but not other geologists)
  18. 18. 4. IDENTICAL PERMIAN DEPOSITS Identical sequence of Permian glacial deposits and redbeds with lavas on most Gondwana continents
  19. 19. 5. MATCHING BEDROCK Even the ancient bedrock trends match across the Atlantic
  20. 20. 6. DISTINCTIVE PERMIAN FOSSILS <ul><li>Seed fern Glossopteris </li></ul><ul><li>Primitive aquatic reptile Mesosaurus </li></ul><ul><li>Small herbivorous synapsid (“mammal-like reptile”) Lystrosaurus </li></ul><ul><li>Large predatory synapsid Cynognathus </li></ul><ul><li>NONE of these could have swum or floated across the modern Atlantic </li></ul>
  21. 21. 6. DISTINCTIVE PERMIAN FOSSILS Tongue-shaped leaves of the extinct seed fern Glossopteris , found on all the Permian Gondwana continents
  22. 22. 6. DISTINCTIVE PERMIAN FOSSILS Small aquatic reptile Mesosaurus , found in lake beds in Brazil and South Africa, but too small to have swum across the modern Atlantic
  23. 23. 5. DISTINCTIVE PERMIAN FOSSILS Small herbivorous synapsid (formerly known as “mammal-like reptiles”) Lystrosaurus
  24. 24. 5. DISTINCTIVE PERMIAN FOSSILS Bear-sized predatory synapsid Cynognathus
  26. 26. WEGENER’S MODEL Visualized continents as rafts or icebergs, floating on the mantle, drifting apart and colliding. But opponents could not imagine continents plowing through oceanic crust (where are the deformed crustal rocks?) and also what could drive them
  27. 27. ARTHUR HOLMES (1928) Groundbreaking Scottish geologist 1915: published the first radiometric dates that established the age of the earth 1928: suggested a remarkably modern-looking idea of continents driven by mantle convection
  28. 28. ARTHUR HOLMES Arthur Holmes (1890-1965) “ Father of the Geological Time Scale” Originator of the idea of mantle convection driving continental drift Received the Vetlesen Prize in Geology, 1964
  29. 29. ALEXANDER L. DUTOIT Continental drift was always more popular in the Southern Hemisphere, where the evidence is abundant, but most geologists lived in the north and never saw or thought about this evidence South African geologist A.L. Dutoit’s 1937 map from Our Wandering Continents
  30. 30. CONTINENTAL DRIFT REJECTED <ul><li>By the 1950s, most Northern Hemisphere geologists dismissed continental drift as a crackpot, fringe idea </li></ul><ul><li>They could not imagine how continents could plow through oceanic crust without leaving huge amounts of crumpled rock as evidence </li></ul><ul><li>Major 1949 (published in 1952) symposium dismissed the evidence of fossils, and argued that they could be explained by rafting or dispersal </li></ul>
  31. 31. CONTINENTAL DRIFT REBORN <ul><li>Ironically, at the same time land-based geologists continued to dismiss continental drift, new data was being gathered in the sea </li></ul><ul><li>Post-WWII, major effort to understand the deep sea through seismics, magnetics (using old antisubmarine technology) and deep sea cores of sediment </li></ul>
  32. 32. PLATE TECTONICS: NEW EVIDENCE <ul><li>Paleomagnetism : APW curves and magnetic reversal time scale </li></ul><ul><li>Oceanic surveying : mapped seafloor for first time, discovered mid-ocean ridges, fracture zones, and deep trenches </li></ul><ul><li>Seismology : Benioff zones beneath trenches </li></ul><ul><li>Gravity : Evidence that trenches have less gravity than they should </li></ul><ul><li>Sea floor magnetic anomalies </li></ul>
  33. 33. 1. PALEOMAGNETISM In 1956, Cambridge paleomagnetists Keith Runcorn and Ted Irving both showed that apparent polar wander curves were better explained by movement of continents
  34. 34. 1. PALEOMAGNETISM Starting in 1959, but especially 1963-1969, Cox, Doell and Dalrymple (USGS) and Ian McDougall (Australian National University) established the magnetic reversal time scale, the Rosetta Stone for sea floor spreading
  35. 35. COX, DOELL, AND DALRYMPLE Allan Cox drilling paleomagnetic cores Brent Dalrymple (Oxy’59) doing K-Ar dates Cox, Doell, and Runcorn receive Vetlesen Prize
  36. 36. 2. OCEANIC SURVEYING By the 1950s, detailed maps of the ocean floor were produced for the first time, showing the gigantic chain of mid-ocean ridges, fracture zones, and trenches
  37. 37. Mid-ocean ridges: discovered in the 1950s, longest chain of mountains in world, with Grand-Canyon-sized rift valley down middle Heezen and Tharp maps
  38. 38. 3. SEISMOLOGY In the 1930s, Wadati and Benioff used seismology to show that a deep crustal slab must lie under oceanic trenches. The 1964 Alaska earthquake showed the power of subduction
  39. 39. 4. GRAVITY As early as 1938, Harry Hess noticed that the gravity near trenches was much less than expected, suggesting lighter crustal material (not mantle) at depth. Heat flow was also higher than expected
  40. 40. “GEOPOETRY” In 1962, Harry Hess proposed the essentially modern model of plate tectonics, using all the data that had been gathered so far. However, without the evidence of seafloor spreading, he called in “an exercise in geopoetry.” A year later, that evidence was discovered. . . Harry Hess
  41. 41. 5. SEAFLOOR MAGNETICS Starting in the late 1940s, most oceanographic cruises routinely towed a proton-precession magnetometer (originally developed in WWII to detect submarines) behind the ship to survey the details of seafloor magnetism over a wide area. An immense amount of data had to be collected before a pattern began to emerge
  42. 42. 5. SEAFLOOR MAGNETICS To their surprise, the seafloor had a pattern of “zebra stripes” of seafloor that was anomalously stronger than present-day earth’s field ( positive anomaly ) or less than the field ( negative anomaly ). First profile across the Pacific was complex, confusing
  43. 43. 5. SEAFLOOR MAGNETICS Then in 1963, Fred Vine and Drummond Matthews were working on a much simpler, more symmetrical pattern in the North Atlantic, and realized it could be explained by seafloor spreading recording the flips of the earth’s magnetic field
  44. 44. 5. SEAFLOOR MAGNETICS <ul><li>Symmetrical “stripes” of positive and negative gravity anomalies originally mysterious </li></ul><ul><li>Vine and Matthews (1963) realized that positive anomalies result when oceanic crust is normally magnetized, and adds to modern magnetic field of earth felt by magnetometer; negative anomalies occur when submarine crust is reversely magnetized, and partially cancels the earth’s field felt by magnetometer (giving lower than average magnetic readings) </li></ul>
  45. 45. VINE & MATTHEWS At Cambridge in 1963, Fred Vine (left) and Drummond Matthews (right) first provided evidence for seafloor spreading
  46. 46. 5. SEAFLOOR MAGNETICS The symmetrical “zebra stripes” of positive and negative magnetic anomalies can be matched with the Cox, Doell, and Dalrymple reversal timescale
  47. 47. 5. SEAFLOOR MAGNETICS Anomalies are mirror-image symmetrical over center One profile Same profile flipped
  48. 48. 5. SEAFLOOR MAGNETICS <ul><li>Vine and Matthews were vindicated in 1970, when the Deep Sea Drilling Project drilled both sides of the mid-Atlantic ridge, and found that the rocks got older away from the ridge crest </li></ul><ul><li>Combining many different oceanic spreading records, and calibrating it with the dated magnetic sections on land, Heirtzler et al. (1968) generated a magnetic time scale for the past 100 million years </li></ul><ul><li>This in turn could be used to date the age of the seafloor using magnetic anomalies </li></ul>
  49. 49. 5. SEAFLOOR MAGNETICS The surprise is that the seafloor is very young (no older than 150 m.y.), a fraction of the age of the older continental rocks--so seafloor recycles rapidly
  50. 50. HERESY TO PARADIGM <ul><li>By 1970, plate tectonics was well established among younger geologists, and second-generation studies were applying it to new fields </li></ul><ul><li>Older generation (especially oil geologists) continued to resist and publish embarrassing reactionary papers; most died off or were eventually converted </li></ul><ul><li>Most of the giants of plate tectonics were young when they made their discoveries and are still alive today--the most recent scientific revolution of all </li></ul>
  51. 51. 3 PLATE MARGIN TYPES <ul><li>Divergent = spreading = passive </li></ul><ul><li>Convergent = subduction = active </li></ul><ul><li>Transform: neither subducting nor spreading, but sliding on huge strike-slip faults </li></ul>
  53. 53. CRUSTAL CONSERVATION Unless earth is expanding, the rate of production of new crust must balance the rate of destruction of old crust
  54. 54. TRIPLE JUNCTIONS All plate boundaries must come to an end at some other plate boundary. In some places, three plate edges come together to form a triple junction
  55. 55. TRIPLE JUNCTIONS <ul><li>Rocks include: </li></ul><ul><li>Deep-sea turbidites, shales and cherts scraped off top of oceanic plate </li></ul><ul><li>Slices of ophiolite from downgoing slab </li></ul><ul><li>Blueschists recycled from deeper in trench </li></ul><ul><li>All are intensely sheared and deformed into melange </li></ul>Ridge-rift-transform triple junction in the Afar triangle