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The Plate Tectonics Revolution A paradigm shift in the Earth Sciences.

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1 The Plate Tectonics Revolution A paradigm shift in the Earth Sciences

2 Topics: ! Wegener’s Theory of continental drift ! Seafloor spreading: the Vine-Matthews- Morley hypothesis ! The plate-tectonics revolution ! What is a plate? How many are there? ! J. Tuzo Wilson and his cycles ! Triple junctions

3 Alfred Wegener (1880-1930) ! Promoted continental drift with publication of The Origin of Continents and Oceans in 1915 ! Alternative to land bridges as theory to explain similar fossils on different continents

4 Alfred Wegener ! Supporting evidence: Appalachian mountains match Scottish Highlands; similar rocks in South Africa and Brazil ! Coined term Pangea (Greek, meaning “all the Earth”) Appalachians (eastern N.A.) Scottish Highlands

5 Harry Hess In 1962, U.S. Navy marine geologist Harry Hess proposed that ocean crust is much younger than continental crust, and forms by “seafloor spreading”

6 Raff, A. D. and R. G. Mason. 1961. Magnetic survey off the west coast of North America, 40º N. latitude to 52º N. latitude. Geological Society of America Bulletin 72:1267-1270. ! Magnetic surveys revealed a distinctive striped pattern in the oceans

7 In the meantime, scientists in U.S.A. and Australia discovered that the Earth’s magnetic field has reversed many times

8 Vine, F. J. and D. H. Matthews. 1963. Magnetic anomalies over oceanic ridges. Nature 199:947-949. The Vine-Matthews Hypothesis Magnetic Stripes = Seafloor Spreading + Magnetic Field Reversals ! Basalt extruded at mid-ocean ridges is magnetized in the direction of the Earth’s magnetic field

9 Larry Morley … but did they discover this first? ! several months before Vines and Matthews, Larry Morley (Geological Survey of Canada) had submitted a paper to Nature with the same conclusion ! His paper was rejected as “too speculative” ! Revisionist history: Vines-Matthews hypothesis is now known as the Vines-Matthews- Morley hypothesis


11 The new global tectonics of the 1970’s Earth’s ultimate recycling program ! New oceanic lithosphere is created at mid-ocean ridges ! Oceanic lithosphere is destroyed at deep- sea trenches (subduction zones) ! This process largely explains the global distribution of earthquakes and volcanoes

12 What is the lithosphere? ! Outer part of the Earth, 100-200 km thick ! Includes the crust and part of the upper mantle ! Behaves rigidly ! Forms plates

13 Large plates (6+1): North American, South American, Eurasian, African, Australian, Antarctic Pacific Small plates (2 + 6): Arabian, Indian Scotia, Caribbean, Nazca, Cocos, Juan de Fuca, Philippine Sea Tectonic plates Contain continental lithosphere Purely oceanic

14 J. Tuzo Wilson Proposed hotspot hypothesis in 1963, in support of Hess’ seafloor spreading model Had difficulty getting this work published!

15 J. Tuzo Wilson Proposed transform plate boundary hypothesis in 1965 to explain linear fracture zones in oceans Realized that motion on an oceanic transform fault is opposite to apparent offset of ridges

16 The Wilson Cycle 1. Continental breakup (rifting) 2. Opening of ocean basin 3. Subduction zones form 4. Ocean basin closes 5. Continents collide and form high mountains 6. Crust thins by erosion

17 Types of plate boundaries

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