Continental Drift & Seafloor Spreading Question: Continental Drift is an accepted theory in science A. True B. False Question: Plate Tectonics and Continental drift are not the same concept A. True B. False
Alfred Wegener German meteorologist Wounded in WWI Researched during recovery 1915 Published Origins of Continents and Oceans Example of multi disciplinary science Died 1930 Greenland expedition. Wegener s Hypothesis: ~ 1912 (Taylor too) Continents drift over geologic time through the ocean basins. Mountains form at leading edge of continents by plowing through ocean rocks. Mechanism proposed: Polar-Fleeing Force: oval Earth causes continents to slide toward the equator. Oh, really? NONSENSE/preposterous: the physics works, but the force is too small to propel continents. Prior to this hypothesis, Earth was considered Static Evidence for Drift Model: Evidence for Continental Drift: Fit of the continental margins Previously noted by: Bacon 1500s, Buffon 1700s, Humboldt 1800s, Suess 1885
Wegener first to propose Pangaea More evidence: Rock types and Mountain belts fit together after continents reassembled. Glacial Deposits Gondwanaland Age of glaciation is same Flow direction is odd - unless cont. reassembled More Evidence for Cont. Drift: So, Gondwanaland (and thus Pangaea) must be over the (South) pole.
More evidence: Fossil distribution - divergence of fossils younger that 150 ma suggest break-up of Pangaea More Evidence: Environment Distribution But wait! Problems exists Problems: No good mechanism! - Proposed forces not great enough. Failed Prediction: should see plowed up ocean basins. Geologists demonstrated ocean crust is far stronger than continental crust. Thus, continents would crumble and not plow through ocean basins. For these reasons the hypothesis is rejected. BUT NOT THE EVIDENCE SUPORTING IT! Now we wait
Earth s magnetic field & evidence for moving continents Convection of earth s outer core creates a magnetic field! Magnets follow the field flux lines (point N) From this we can determine location on Earth Cell phone and Brunton Rocks as Magnets Some volcanic rocks have magnetic minerals in them. As these rocks cool, the magnetic minerals align their dipoles with the Earth s magnetic field! So, rocks can be used as a sort of magnet that records its location at the time it cooled! (Paleomagnetism) Show with wehrlite and brunton Combining paleomagnetism and absolute dating Consider volcanic rocks from a continent Paleomagnetism of different age rocks give different locations of North Poll Interpreted in 1950s as evidence of Polar Wander Concept was that continents were stationary and the north pole moved. However
Comparing Continents: Wander paths of continents don t agree. Unless the continents were once joined! (Pangaea) So, Wegener was right or was he? His problems still exist, so we can t accept his Hypothesis. We need a new one So, Wegener was right or was he? A. Yes, Wegener was completely correct - continents drift through ocean basins B. Wegener was partly correct - continents move over Earth s surface, but not the way he suggested. C. No, he was completely wrong - continents are stationary, and only the north pole moves. 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.
Bathymetry 1st Bathymetry data from depth soundings: 1800s laying of transatlantic cable found Telegraph Plateau 1874 H.M.S. Faraday Recovered BASALT from seafloor 1872 H.M.S. Challenger Charted 140mi 2 Found Mariana Trough 20th Century: Sonar! Now capable of mapping large areas of seafloor with incredible precision 21st century: sonar still the only means of obtaining precise bathymetry Satellite altimetry data - works well on continents, improving in oceans, but not precise (pixel size >100s m) First good global bathymetry map: 1977 Heezen & Tharp Mid-ocean ridge system circles globe: Ewing & Heezen, 1956. Compiled new bathymetry data to propose a new hypothesis Evidence: Median ridges (next slide) Thin sediment cover on ridge (following slides) Guyots (following slides) Basalt from seafloor (video below) Theory of Seafloor Spreading Basically/simply: ocean basins open by spreading of lithosphere at midocean ridges According to Hess, driven by convection in the asthenosphere Harry Hess, 1962
Evidence: Median Ridges: By 1962 it was known that a ridge existed in the middle of the Atlantic. WHY/HOW? Other oceans too, but somewhat more complex. Evidence: Thin sediment cover on ridge: Ewing s early seismic data What does this say about the age of Ridge? Evidence: Guyots (flat topped seamounts) Eroded seamounts well beneath sea-surface At one time, they must have been exposed above sea-surface Animation
Modern Theory of Seafloor Spreading: Lithosphere ripped apart (thins) at ridge Mechanism? - hold your horses Asthenosphere Rises & expands - uplifting the ridge Melts (how - not for us) Volcanic system creates new crust New crust and lithosphere rafted away as spreading continues Alvin video volcanic seafloor New Evidence: Sediment 1. Sediment thickens away from ridge Old Ewing seismic data (1950s) 2. Age of sediment increases with depth 3. Age of basal sediment increases with distance from axis. (how do we know 2 and 3?)
Drilling of Sediment We can date fossils in sediment using Fossil Succession & correlations! New Evidence: Heat flow Heat flow is greatest at ridge axis Decreases away from ridge (as a function of square-root of distance from axis) Consistent with formation of hot crust at axis, cooling with age. Nail in the coffin: Magnetic Anomalies Remember Earth s magnetic Field? Did you know that it reverses itself over time?
New Evidence: Seafloor Magnetic Anomalies Raff & Mason, 1961 Observed pattern of reversed and normal polarity symmetrical about the ridge axis Vine & Mathews, 1963 Explained in terms of Seafloor Spreading PROOF OF SEAFLOOR SPREADING! Observation: Magnetic orientation of seafloor shows (1) pattern of normal and reversed polarity; (2) pattern is symmetrical about the ridge Interpretation: Volcanic rocks cool on the surface and acquire magnetism Spreading continues and more crust is created Reversals of magnetic field (independent of SFS) produce the observed pattern Better animation Animation Vine & Mathews Idea: Age of Oceanic Crust: Paleomagnetic Time Scale Magnetic time scale Magnetic patterns correlate between oceans Dating reversals allows us to date the age of the seafloor Date reversals on continents Absolute age of lavas & correlation across continents using fossil succession Spreading Rate Rate of seafloor spreading Calculated from age of reversal and distance from ridge Magnitude of spreading is 10-100 km/my or 10-100 mm/yr 0.7 Ma 2.4 Ma
Age of Oceanic Crust: Result of Seafloor Spreading Symmetry results from seafloor spreading! Oldest Crust? Complexities? A. Yes, they plow through the ocean basins B. Yes, as part of larger lithospheric plates C. No, oceans open and close while continents remain stationary So, do continents move? Review Questions What are the flaws/problems with the Continental Drift Hypothesis? What important contributions did Wegener make with his proposed Continental Drift hypothesis? Describe how Wegener, Dietz and Hess each contributed to Plate Tectonic Theory. How does magnetism of rocks prove continents move? 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? Describe in a paragraph how Seafloor Spreading works to open oceans and move continents. How are spreading rates calculated?