The document summarizes the scientific revolution of plate tectonics theory. It describes how early evidence from matching continents, glacial patterns, and fossils was initially dismissed. By the 1950s-1960s, accumulating seafloor mapping data and discoveries like magnetic striping and Benioff zones provided strong evidence that was widely accepted by the late 1960s, establishing plate tectonics as the new geological paradigm.

1. PLATE TECTONICS Geology 105

4. SCIENTIFIC REVOLUTION Kuhn’s (1962) ideas have been controversial, but do in fact apply to most of the sciences

9. ALFRED WEGENER Led one of the first major meteorological expeditions to Greenland in 1930

10. ALFRED WEGENER Died on Oct. 30, 1930, returning from a supply drop in bad weather

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

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. 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. 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. 2. PERMIAN PANGEA GLACIATIONS Even some of the ancient glacial scratches appear to line up as if they crossed the Atlantic

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. 4. IDENTICAL PERMIAN DEPOSITS Identical sequence of Permian glacial deposits and redbeds with lavas on most Gondwana continents

19. 5. MATCHING BEDROCK Even the ancient bedrock trends match across the Atlantic

21. 6. DISTINCTIVE PERMIAN FOSSILS Tongue-shaped leaves of the extinct seed fern Glossopteris , found on all the Permian Gondwana continents

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. 5. DISTINCTIVE PERMIAN FOSSILS Small herbivorous synapsid (formerly known as “mammal-like reptiles”) Lystrosaurus

24. 5. DISTINCTIVE PERMIAN FOSSILS Bear-sized predatory synapsid Cynognathus

25. 6. DISTINCTIVE PERMIAN FOSSILS

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. 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. 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. 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

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. 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. 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. 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. 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. 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. 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. “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. 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. 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. 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

45. VINE & MATTHEWS At Cambridge in 1963, Fred Vine (left) and Drummond Matthews (right) first provided evidence for seafloor spreading

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. 5. SEAFLOOR MAGNETICS Anomalies are mirror-image symmetrical over center One profile Same profile flipped

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

52. 3 PLATE MARGIN TYPES

53. CRUSTAL CONSERVATION Unless earth is expanding, the rate of production of new crust must balance the rate of destruction of old crust

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