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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.

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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:…

Evidence for Continental Drift:• Fit of the continental margins

Previously noted by: Bacon 1500s, Buffon 1700s, Humboldt 1800s, Suess 1885

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Wegener first to propose Pangaea

More evidence:

Rock types and Mountain belts‘fit together’ after continentsreassembled.

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More Evidence for Drift:• 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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More evidence: Fossildistribution- divergence of fossilsyounger that 150 masuggest break-up ofPangaea

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More Evidence: Environment Distribution

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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

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Rocks as Magnets

• 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)

Combining paleomagnetism and absolute dating

• Consider volcanicrocks from somecontinent

• Paleomagnetism ofdifferent age rocksgive differentlocations– Interpreted in 1950s

as evidence of PolarWander

• However…

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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…