How do we explain Wegener s observations? With the Theory of Seafloor Spreading. So, in which way was Wegener correct?

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1 So, in which way was Wegener correct? A. His hypothesis of Continental Drift was correct: continents move through the ocean basins. B. He was correct that continents move. C. He was correct that the continents slide toward the equator. D. All of the above How do we explain Wegener s observations? With the Theory of Seafloor Spreading See what Wegener was missing? Ocean Bathymetry! Post WWII sonar mapping of oceans Revealed mid-ocean ridge system circling globe This, along with many other lines of evidence led Hess to propose the Theory of seafloor spreading. The Theory of Seafloor Spreading: Lithosphere ripped apart at Mid-Ocean Ridge Mechanism? - hold your horses Asthenosphere rises beneath the thinned Lithosphere expands thermally and lifts lithosphere forming the ridge The Asthenosphere melts (Let s not discuss how) to produce volcanoes Volcanic system on the MOR creates new crust & Lith. New Lithosphere rafted away as spreading continues 1

2 Vine & Mathews Idea: Observation: Magnetic orientation of seafloor shows (1) pattern of normal and reversed polarity; (2) pattern is symmetrical about the ridge axis. Interpretation: Volcanic rocks cool on the surface and acquire magnetism Spreading continues and more volcanic crust is created Reversals of magnetic field (independent of SFS) produce the observed pattern Animation Age of Oceanic Crust: Symmetry of age distribution pattern (from magnetic data) results from seafloor spreading! Where is the oldest crust? Complexities? Consider for a moment Given that lithosphere is created at Mid-Ocean Ridges, what else must be happening to lithosphere somewhere on the Earth? 2

3 Given the Theory of Seafloor Spreading, what do you know about the age of the oceanic crust? A. B. C. D. The crust becomes older with distance from the mid-ocean ridge The crust at the mid-ocean ridge axis is modern (very young). Given the age of the crust at some location and its distance from the mid-ocean ridge, you could calculate the spreading rate (how fast seafloor is created). All of the above Seafloor Spreading = 1/2 Plate Tectonics If new Lithosphere is created at mid-ocean ridges, and the surface area of the Earth is not increasing What happens to the older lithosphere go? Answer = other 1/2 of P.T. Theory Look at Bathymetry: Why are MOR shallow? Why does Ocean lithosphere sink with age? 3

4 Recall the Lithosphere-Asthenosphere boundary:? What defines the boundary? Notice the lithosphere becomes thicker w/ age, and sinks by isostacy deeper into the asthenosphere. Compare age & depth of the seafloor Oldest lithosphere is also the deepest: Mariana Trough 11,000 mbsl These deep places are called Trenches. What happens near trenches? Most terrestrial volcanoes occur at Trenches Most large earthquakes too Notice EQ depths At Trenches, EQ depth increases under the trench 4

5 Observation: Wadati-Benioff Zones at Trenches Wadati-Benioff Zone = observed dipping plane of EQ Bending: Tension at top, compression bellow (and toe) These Wadati-Benioff zones are found at ALL trenches! Subduction Zones: Interpretation of Wadati-Benioff Zone Old, dense thick lithosphere sinks into asthenosphere = consumed Melting of the asthenosphere above the subducting lithosphere How? Let s not discuss it. Result in volcanoes Subduction zones consume lithosphere. This is the other 1/2 of Plate Tectonic Theory animation 5

6 Theory of Plate Tectonics: Combined Seafloor Spreading and Subduction Lithosphere created at spreading ridges and consumed at subduction zones Continents are along for the ride! Animation Plate Motions: ~0 to ~ 20 cm/yr How can we understand plate motions: their velocities and paths over time? Hot-spots provide absolute plate velocities What are hot-spots? 6

7 Hot-Spots (or plumes) and Plate Velocities Anomalously hot mantle (from core-mantle boundary) Rises Buoyantly (plume) Melts on assent (adiabatic decompressive) Feeds volcanoes on Lithosphere These volcanoes produce tracks as plates move over the hot-spot. Given the age of the volcanoes and the length of the track, we can calculate velocity of the plate!!! Animation - good more cartoons! Create hot-spot track See the hot-spot tracks? Samoa, Hawaii-Emperor, Ninety-East Ridge, Tristan/Walvus Ridge, etc. (they are everywhere!) Should plate velocities from hot-spot tracks agree with spreading rates? A. Yes B. No 2.4 Ma 0.7 Ma 7

8 Absolute vs. Relative plate vectors Do they always agree? Blue vs. red vectors Not always, spreading rate is plate motion relative to divergent boundary. Hot-spot track/ absolute velocity is velocity of plate relative to fixed reference Review Questions Describe how Wegener, Dietz and Hess each contributed to Plate Tectonic Theory. How does magnetism of ocean rocks prove seafloor spreading operates? Why are the deepest places in the oceans not in the center of the ocean basins? Why are mid-ocean ridges shallow? How do you know lithosphere sinks into the asthenosphere at subduction zones? How do hotspot tracks indicate plate velocities? Why don t spreading or convergence rates agree with absolute plate velocities? Under what circumstances might a mid-ocean ridge or a subduction zone migrate? 8