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Continental Drift & Paleomagnetism

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

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Which of the following best describes tectonics on Earth? A.  Continents float in the oceans and

move as independent bodies like boards floating in a pool.

B.  Continents float on the mantle and move as independent bodies like boards floating in a pool.

C.  Continents and ocean floor are connected and constitute larger ‘boards’ that form a rigid framework (like a roof). Motion results when the ‘roof’ collapses under its weight.

D.  Continents and ocean floor are connected and constitute larger ‘boards’ that float on the asthenosphere. Motion results when the board sinks or is pushed beneath another.

Earth’s magnetic field provides evidence of a dynamic Earth surface

•  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 •  Inclination indicates

latitude!

Cell phone and Brunton

Rocks as Magnets

•  Some volcanic rocks have magnetic minerals in them. –  Example: wehrlite

•  Draw how this works: •  So, rocks can be used as a

sort of magnet that records its location at the time it cooled! (Paleomagnetism)

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How rocks become magnetized

Combining paleomagnetism and age dating

•  Consider volcanic rocks from some continent

•  Paleomagnetism of different age rocks give different locations of apparent North –  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) •  This supports Webener’s hypothesis of Continental

Drift

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Alfred Wegener •  German meteorologist •  Wounded in WWI

–  Researched during recovery

–  1915 Published Origins of Continents and Oceans

•  Died 1930 Greenland expedition.

Wegener’s Hypothesis (1912):

•  Continents drift over geologic time through the ocean basins. – Mountains form at leading edge of continents by

plowing through ocean rocks. •  Prior to this hypothesis, Earth’s surface was

considered Static despite observations of ‘fit of continental margins’…

Fit of the continental margins:

Previously noted by: Bacon 1500s, Buffon 1700s, Humboldt 1800s, Suess 1885 Read section 2.2 in your text and understand the lines of evidence supporting continental drift hypothesis: fit of continents, past glaciations, climate belts, fossil distribution, geologic units.

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Wegener proposed Pangaea

He was ahead of his time. Incorrect Hypothesis, but correct observation; Earth’s surface is dynamic and not static!

But wait! Problems exists… •  Problems:

–  No good mechanism! - Proposed forces are 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!

Wegener was wrong, but not completely. •  All his evidence points to the continents once being joined in

the supercontinent Pangaea –  He was correct in this regard, and this is no longer disputed. –  He was incorrect in explaining how continents move and the

relationship between continents and oceans… •  Science waited 30 years before we could understand this

problem (Tectonics).

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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.

Review Questions

•  What are the flaws/problems with the Continental Drift Hypothesis?

•  What important contributions did Wegener make with his proposed Continental Drift hypothesis?

•  How does magnetism of rocks prove continents move? •  What produces Earth’s Magnetic field

Read about this in your text - More evidence:

Rock types and Mountain belts ‘fit together’ after continents reassembled.

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•  Glacial Deposits –  Gondwanaland –  Age of glaciation on

southern continents is the same

–  Flow direction is odd - unless continents are reassembled

So, Gondwanaland (and thus Pangaea) must be over the (South) pole.

Read about this in your text - More evidence:

More evidence: Fossil distribution���- divergence of fossils younger that 150 ma suggest break-up of Pangaea

Read about this in your text - More evidence:

Read about this in your text - More evidence: