Quick Quiz
Evidence for liquid core in the earth:
a) diffraction of P waves forming shadow
zone
b) shadow zone for S waves
c) volcanic eruptions
d) density measurements of earth
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Quick Quiz
Composition of crust/core
a) silicates/iron and nickel
b) oxides/hydrides
c) carbonates/copper
d) silicon/oxygen
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From Continental Drift
To Plate Tectonics
Continental Drift Theory
Evidence
Problems
Sea Floor Spreading
Evidence
Problems
Plate Tectonics
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Introduction to Continental Drift
Sir Francis Bacon in 1620
Benjamin Franklin
Albert Wegner, 1912
Received a degree in astronomy
Taught meteorology, renowned for his
work in climatology and paleoclimatology
First to present theory of “Continental
Drift”
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Albert Wegner
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Evidence of Continental Drift
Continental fit
Continental structure
Paleontological evidence
Paleoclimatic evidence
Paleomagnetic evidence
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Continental Fit
Coastlines fit together
S. Amer. and Africa, N. Amer. and Europe
Computers fit the coastlines in 1960’s
Better than expected results
Wear and tear has occurred at
continental edges
Wegner named the supercontinent
“Pangea”
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Shields and Platforms: Structure
Continents consist of shields, stable
platforms, and fold mountain belts
Some shields and structural trends end
abruptly at coastlines of current
continents
Putting continents together shows
remarkable continuity in geological
structures
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Paleontological Evidence
Common fossil groups found across the
continents
Glossopteris flora
S. Amer, Africa, Australia, and India
Mesosaurus
South Africa, Brazil
Trilobites
N. Amer., Europe
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Mesosaurus
Trilobite
Paleoclimatic Evidence
Continental glaciers cover thousands of
square kilometers
Leave distinctive geological evidence of
their flow directions and location
Glacial evidence inconsistent on present
day continents
Fitting continents together explains
patterns
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Present day orientation
Ancient orientation
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Rejection of Wegner’s Hypothesis
Wegner had evidence, but no
explanation
Sir Harold Jeffries (1891-1989) proved
that continents can’t move through
crust
Continental drift accepted by some,
rejected by most
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Continental Drift Problems
How do continents move?
Motion of continental rock through
ocean basin rock proved impossible
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Seafloor Spreading
Beginning in 1950’s it became possible to
map the ocean floor
Found long mountain range that circled the
globe
Features: central rift valley, followed outline of
continents
Harry Hess proposes that rift is a spreading
center, with edges in deep ocean trenches
Evidence was tenuous
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Mapping the Ocean Floor
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Mid-Atlantic Rift
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Age of the Ocean Floor
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Guyots
100’s of feet
below sea level
Dead coral found on top of guyots.
Coral needs filtered sunlight to survive.
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Hess’s Proposal
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Magnetic Reversals and Seafloor
Striping
Earth has experienced magnetic reversals
Evidence found in continental basalts containing
magnetite (in France)
During Cenozoic era, reversals happened every
half-million years
Ocean floor shows stripes of alternating
magnetic orientation
How did the stripes get there?
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Magnetic
Reversals
Track Spreading
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Explanation of Striping
1963: Vine & Matthews propose that
magnetic striping results from seafloor
spreading
Tests for the theory:
Young rock should be at ridge, old at edge
Thickness of marine sediment should be
thicker farther from ridge center
Oldest fossils should be found in bottom
layers of sediment
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Seafloor Spreading Problems
Where does the material for the rifts
come from?
Where does it go?
Earth is not increasing in diameter
Runs into continents at margins
What drives the process?
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Plate Tectonics
Driven by data on earthquakes
Combined theories of continental drift
and seafloor spreading
Occurred about 1967
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Earthquake Data
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Plates Postulated Where Quakes
Occur
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Plate Tectonics Passes the Tests
Wegner’s continental drift vindicated
Continents don’t move through the earth’s crust
Continents ride on huge plates
Plates are born at spreading centers, die in
deep ocean trenches (Benioff zones)
Movements of the plates detectible
Interferometry, satellites
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Wegner’s Pangea & Plate Tectonics
Pangea was not the initial orientation,
but a time when the continents came
together
Fossil records indicate some
inconsistencies; is the theory wrong?
Need to look for more evidence and
explanation
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A Modern-Day Galileo
(A Comment on the Way Science Works)
Wegner’s hypothesis vs. Jeffries proofs
– who was right?
Hindsight is always 20/20
Science’s insistence on acceptable
methods is what moved continental
drift to plate tectonics
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