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Plate Tectonics • It’s a pretty zany idea • What brought it about? • Geologic observations made by Wegener and first suggested in 1915 • Why is it important? Types of Evidence for Continental Drift Geographic Evidence Fossil Evidence Types of Evidence for Continental Drift Evidence from Glacial Deposits Evidence from Coal Deposits Magnetic Evidence The Earth’s magnetic field can be: Normal: Compass points North Reversed: Compass points South The direction of the magnetic field is recorded by rocks as they form Patterns of reversal are nearly mirror image Animation From Smith and Pun Sea Floor Spreading and Sea Floor Magnetism skip Whole Ocean Magnetics How Plates Behave (Bean pot analogy) Smith and Pun Plate Motions Through Time The Plates The plates are essentially RIGID pieces of the Earth’s crust They move due to convection of the mantle They can contain both OCEANIC and CONTINENTAL crust Passive Margin Active Margin (Plate Boundary) Isostacy Thick Pine Thin Pine Thin Oak H T D D=kT H=(1-k)T k= rB/rW Depth below waterline Height above waterline Fraction of block below water line Where does continental crust come from? Continental Crust Three Basic Types of Plate Boundaries • Divergent - plates moving away from one another (Tension) • Convergent - plates moving toward one another (Compression) • Transform - plates moving past one another (Shear) ! Most plates include con/divergent and transform components ! Smith and Pun Correlating Processes and Plate Boundaries Smith and Pun Sea Floor Spreading and Sea Floor Magnetization Motion at Divergent Boundaries skip Formation of Oceanic Crust Sediments Pillow lavas Sheeted dikes Gabbro Peridotite (Mantle rock) Life at Midocean Ridges midocean RIDGES Cooler More Dense Lower Hotter Less Dense Higher Cooler More Dense Lower Smith and Pun Forming a Divergent Boundary ANIMATION 20 - Plate Divergence Smith and Pun Motion at Convergent Boundaries Convergent Boundaries • Oceanic crust is denser and “rides lower” than continental crust • Expect different things to happen Trenches – Ocean-Continent Collisions Mountains– Continent Continent Collisions Animations • Subduction – Animation 20- Ocean-Continent – Animation 20- Continent-Continent Transform Boundaries • San Andreas Fault in California – Animation 20-Transform Boundaries • Transform Faults in Midocean Ridges – Motion at a transform plate boundary The Complex Nature of Plate Boundaries Hotspots – Smith and Pun: Hotspots and Plumes What drives the plates? What drives the plates, really?
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