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Earth History GEOL 2110. Theory of Plate Tectonics Part 1: Toward the Development of the Grand Paradigm. Major Concepts.
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Earth History GEOL 2110 Theory of Plate Tectonics Part 1: Toward the Development of the Grand Paradigm
Major Concepts • Most early models attempting to explain the Earth’s topographic features (mountains, cratons, ocean basins, volanic arcs, …) assumed the immobility of the continents and appealed to a cooling, contracting earth. • Several lines of evidence that the continents had drifted apart were presented by Alfred Wegener in 1915, but was largely dismissed because it lacked a mechanism. • Paleomagnetic pole studies in the 1950’s followed by bathymetry and magnetic polarity mapping of the sea-floor brought on the theory of sea-floor spreading • With the recognition that the new crust is being made at mid-oceanic rift zones, the final piece of the puzzle was realized and the Theory of Plate Tectonics was born.
Mountain-Building and the Geosyncline Theory Cross-section through the Appalachians (1842)
Mountain-Building and the Geosyncline Theory In seeking an explanation for the deformed nature of strata in mountain belts, Hall noted that the stratigraphic units thickened considerably from the cratons to the mountain belts. He suggested that the weight of the over-thickened trough of sediments caused the crust to depress and eventually crumple. James Hall (1811-1898) New York State Geologist Paleontologist and Geologist
Mountain-Building and the Geosyncline Theory Had a similar idea to Hall’s, but suggested that the over-thickened trough of sediment, which he termed a “geosyncline”, was the result, not the cause of downwarping of the continent at their margins. He believed the warping resulted from cooling and contraction of the earth. JD Dana (1813-1895) Mineralogist, Volcanologist and Geologist
The Role of Magmatism in Mountain Belts • Although granite batholiths and volcanic rocks were known to be integral parts of mountain belts, the role of magmatism was largely ignored in the Geosyncline Theory. • Early on (1790’s) Hutton had suggested intrusions of granites played a major role in uplift of mountains
Continental Drift and Mountains The discovery of radioactive heating of the earth in the early 1900’s not only put Kelvin’s age of the earth into question, but is also made mountain building by horizontal compression due to a contracting Earth impossible. Scientists started to speculate that maybe, just maybe, the continents could move horizontally – DRIFT. 1908 – American F.B. Taylor suggested that drifting of the continents was responsible for the great Cenozoic mountain belts. Speculated that capture of the moon was responsible.
Pre-Drift Continental Reconstruction 1912 – Another American H. Baker postulate the first reconstruction of what we now call the Pangea supercontinent. He speculated that the moon was ripped from Earth to form the Pacific basin prompting the continents to drift toward the void.
Alfred Wegener Father of the Theory of Continental Drift 1915 – The Origin of Continents and Oceans (3rd edition, 1922) Meteorologist by training Introduced the hypothesis of Continental Drift Postulated the existence of the supercontinent he named Pangea Evidence: Fit of the continents Fossil record Similar geologic structures Similar paleoclimates Alfred Wegener1880-1930
Fit of the Continents ~250 Ma Wegener’s 1915 Reconstructions (brown areas are submerged areas of the continents)
Similar Geologic Structures Wegener’s Sketch (1931)
Paleoclimate Evidence ~300 Ma
General Dismissal of Wegner’s Theory • an amateur (not even a geologist!) • inadequate driving mechanism for horizontal motion of continents (tidal energy, earth’s rotation) • uncertainty about the seafloor and earth’s mantle • some supporters: DuToit (S Africa), Holmes (England), but many more detractors especially in the US (“utter, damn rot!” W.B. Scott, American Phil. Society) Continental Collapse Continental Drift (Wegner) Continental Drift and Seafloor Spreading from J. Holden
Holmes Suggests a MechanismMantle Convection Radiochronologist Arthur Holmes suggested in 1928 that radioactivity heating of the mantle may cause convection which could be the driving force behind drift Sial: felsic upper crust Siam: mafic lower crust
Paleomagnetism:New Evidence for Continental Drift in the 1950’s Declination – azimuth direction to the magnetic pole Inclination – angle that magnetic field is tilted into the earth
Investigations of the SeafloorThe Final Pieces of the Puzzle
Earthquakes, Volcanoes, Ocean Ridges, and Ocean Trenches Ocean Ridges Earthquakes Volcanoes Ocean Trenches
Hot Spot and the Aging of Volcanic Islands The evolution of active volcanic islands to atolls and seamounts implied that hot spot (mantle plumes) are fixed relative to a moving ocean crust. Radiometric dating verified this aging.
Hypothesis of Seafloor Spreading An “Essay in Geopoetry” by Harry Hess (1961)
Magnetic Striping of the SeafloorConfirmation of the Seafloor Spreading Hypothesis Vine and Matthews (1963)
Continental Drift + Seafloor Spreading Theory of Plate Tectonics Emerges and Becomes Accepted 1965 - J. TuzoWilson who had been a promoter of the sea floor spreading hypothesis and continental drift from the very beginningadds the concept of transform faults to the model. A symposium on continental drift was held at the Royal Society of London in 1965 is regarded as the official start of the acceptance of plate tectonics by the scientific community. 1966 - Wilson published the paper that referred to previous plate tectonic reconstructions, introducing the concept of what is now known as the "Wilson Cycle". 1967, at the American Geophysical Union's meeting, a session was planned to discuss the pros and cons of the new Theory of Plate Tectonics. By the time the meeting was convened all those scheduled to speak about the arguments against Plate Tectonics backed out!
Next Lecture Theory of Plate Tectonics Part 2: Elements of Plate Tectonics Quiz : Chapters 6 & 7
