Alfred Wegner. Continental Drift & Seafloor Spreading. German meteorologist Wounded in WWI. Died 1930 Greenland expedition.

Transcription

1 Continental Drift & Seafloor Spreading Alfred Wegner German meteorologist Wounded in WWI Researched during recovery 1915 Published Origins of Continents and Oceans Died 1930 Greenland expedition. 1

2 Wegner 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

3 Wegner first to propose Pangaea More evidence: Rock types and Mountain belts fit together after continents reassembled. 3

4 More Evidence for Cont. Drift: Glacial Deposits Gondwanaland Age of glaciation is same Flow direction is odd - unless cont. reassembled 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 4

5 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 5

6 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 Show magnetic field with Brunton and overhead 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) 6

7 Combining paleomagnetism and absolute dating Consider volcanic rocks from some continent Paleomagnetism of different age rocks give different locations Interpreted in 1950s as evidence of Polar Wander 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 7

8 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. 1st Bathymetry data from depth soundings: 1800s 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 entire seafloor with incredible precision 21st Century: Satellite altimetry data - works well on continents, improving in oceans Bathymetery 8

9 First good global bathymetry map: 1977 Heezen & Tharp Mid-ocean ridge system circles globe: Ewing & Heezen, Compiled new bathymetry data to propose a new hypothesis Evidence: Median ridges (next slide) Thin sediment cover on ridge (next slide) Guyots (next slide) Basalt from seafloor Theory of Seafloor Spreading Harry Hess, 1962 basically/simply: ocean basins form by spreading at mid-ocean ridges According to Hess, driven by convection 9

10 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? 10

11 Evidence: Guyots (flat topped seamounts) Eroded seamounts well beneath sea-surface At one time, they must have been exposed above seasurface Modern Theory of Seafloor Spreading: Lithosphere ripped apart at ridge Mechanism? - hold your horses Asthenosphere Rises & expands - forming ridge Melts (how - not for us) Volcanic system creates new crust New crust rafted away as spreading continues 11

12 New Evidence: Sediment 1. Sediment thickens away from ridge Old (1950s) Ewing seismic data 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 relative dating & correlations! 12

13 New Evidence: Heat flow Heat flow is greatest at ridge axis Decreases away from ridge (square-root of distance) 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? 13

14 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 NAIL IN COFFIN! Vine & Mathews Idea: 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 Animation 14

15 Age of Oceanic Crust: Paleomagnetic Time Scale Magnetic time scale Patterns correlate over oceans Date reversals Absolute & correlation of fossils Spreading Rate (calculated age) Age of Oceanic Crust: Result of Seafloor Spreading Symmetry results form seafloor spreading! Oldest Crust? Complexities? 15

16 Cool view: 16