History of geological science:
Continental Drift is not plate tectonics.
Jigsaw fit of continents was noted by Alfred Wegener, a
german meteorologist
he proposed “Continental Drift” in 1915 saying the
continents once fit together as one and naming the
supercontinent Pangaea
the idea was considered absurd
->no mechanism for moving continents
Wegener’s evidence
1. fit of continents
2. matching up past glaciations
3. distribution of equatorial climate belts
4. distribution of fossils/ fauna across continents
5. matching up of rocks/geologic units
Criticism: explanation for how it could happen did not make
sense---continental crust can’t plow through the ocean
crust
Wegener-Continental Drift
This animation presents a view of the continents at
the level of map precision available to Alfred
Wegener, who proposed that the continents once
existed as a vast supercontinent, Pangea, that later
fragmented. Test the fit of the continents for yourself
by clicking and dragging a continent to a new
location; then, using the arrow keys on your
keyboard, rotate the continent into position.
PC version
Mac version
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2. Matching up of past glaciations
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3. Equatorial climate belts
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4. Matching up of fauna
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5. Matching up of rocks,
geological units
The story unfolds:
paleomagnetism
As we talked about earlier, Earth has a magnetic field.
It is due to convection of the molten Fe in the outer core
(geodynamo)
magnetic north pole is not the geographic north pole- the
difference for a given location is the declination
the Earth’s magnetic field has a component of direction (NS) and a dip angle (magnetic inclination):
both can be recorded in magnetic minerals
the direction of the field is easily recorded in some
minerals, especially magnetite
just like a compass needles, tiny grains of magnetite (or a
few other Fe bearing minerals) crystallize in rocks at the
surface. The little magnetic domains in these minerals align
with the magnetic field (once they are below the curie
Temperature)
this gives:
1) direction to magnetic N pole
2) the dip of the field which depends on latitude.
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More trouble…
Paleomagnetic analysis along with radiometric dating indicated
the magnetic N pole was moving all over the place with timeworse yet, different continents gave different polar paths
-> this was called apparent polar wander
Wait! If the same data were interpreted to reflect a fixed pole
but wandering continents, everything worked. YIKES!
But still, how do continents drift???
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More info: ocean depth
Hard to know the shape of the ocean floor-not until WWII did
sonar develop to measure bathymetry (shape of seafloor
surface).
Right in the middle of the Atlantic Ocean, there was a big
mountain range!
bathymetry
seafloor
but big mountain
in
ThereSeafloor
was also
highshowed
heat flat
flow,
a particular
typerange
of earthquake
middle of the Atlantic...and there was high heatflow in these
(reflecting
tensional forces) and sediment thickness increased
mountains... and these mountain ranges were locations of particular type
away of
from
the mountain range…
earthquakes....
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One last piece of info:
maps of earth quake locations was not random--rather EQ
found along distinct belts including the mountain range in
the Atlantic
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Harry Hess’s hypothesis
Proposed the idea of sea-floor spreading:
1) Seafloor spreads and cracks
2) Magma ascends and forms new oceanic crust made of
volcanic rock
3) Crust must be consumed somewhere--said it descends in
ocean trenches.
Testable as will be shown
-> got the big picture right but in detail more than just crust that
is created and destroyed
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How to test the hypothesis
Paleo magnetics gets used again:
Geologists working on volcanic rocks on land noticed
that the magnetic field of the Earth flipped directions on an
irregular basis (this has to do with the geodynamo, and yes
it could happen today or tomorrow…)
These are called polarity reversals
Magnetometers towed across the Mid-Atlantic Ridge
revealed stripes of alternating magnetic direction recorded
in the ocean crust--and it was symmetrical across the ridge!
Sea Floor Spreading
This animation shows progressive stages in the
opening of the Atlantic Ocean. The youngest rocks
(in red) clearly outline the mid-ocean ridge system,
complete with transform faults. The oldest ocean
crust (in blue), is confined to offshore regions
adjacent to the United States, Canada and western
Africa. This distribution demonstrates that the North
Atlantic began to open before the South Atlantic.
PC version
Mac version
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Magnetic Reversals
The polarity of Earth's magnetic field reverses with
time. The main figure demonstrates how sea-floor
anomalies, also known as magnetic stripes, develop
during sea-floor spreading. The inset image records
the reversal of Earth's dipole.
PC version
Mac version
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Zoomable Art
PC Mac
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Finally, radiometric dating of the volcanic
rock away from the ridge was the final proof
The rocks got older away from the ridge
confirming that Hess’s basic idea, that ocean
crust was created and destroyed allowing
the continents to move, was indeed correct.
Next: How Plate Tectonics works