208 lect05a cont drift - classes.geology.uiuc.educlasses.geology.uiuc.edu/10sprgclass/geo208/208...
TRANSCRIPT
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1. Wegener &Continental Drift
2. Evidence for andagainst C. Drift
3. Proof of motion:Apparent PolarWander
Alfred Wegener• German meteorologist• Wounded in WWI
– Researched duringrecovery
– 1915 PublishedOrigins of Continentsand Oceans
• Died 1930 Greenlandexpedition.
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 slidetoward 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:…
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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
Rock types and Mountain belts‘fit together’ after continentsreassembled.
Evidence from geology
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More Evidence: Facies Distribution
Glacial Evidence:• Glacial Deposits
– Gondwanaland– Age of glaciation
is same– Flow direction is
odd - unless cont.reassembled
So, Gondwanaland (andthus Pangaea) must beover the (South) pole.
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Paleontological Evidence:- distribution and- divergence of fossilsyounger that 150 masuggest break-up ofPangaea
Evidence from Paleontology, geology, andenvironments combined
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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 notplow 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 movingcontinents
• Convection of earth’souter core creates amagnetic field!
• Magnets follow the fieldflux lines (point N)– From this we can
determine location onEarth
• The Magnetic Fieldreverse itself over time!– Save this detail for later
inclination
Rocks as Magnets(discussed this DRM, TRM)
• Some volcanic rocks havemagnetic minerals inthem.
• As these rocks cool belowtheir Curie Temperature,the magnetic mineralsalign their dipoles with theEarth’s magnetic field!
• So, rocks can be used as asort of magnet that recordsits location at the time itcooled! (Paleomagnetism)
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Combining paleomagnetism and absolute dating
• Consider volcanic rocksfrom some continent
• Paleomagnetism ofdifferent age rocks givedifferent 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. Weneed a new one…