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8-1
The Scientific Method in Action:
The Ideas of Alfred Wegener
Plate Tectonics
In the early 20th century Alfred Wegener took these
earlier ideas and made a case that the continents
indeed move.
Early evidence for continental drift:
He gathered evidence from a variety of areas:
The idea that the continents move is not new.
1. He showed how the continents could be put
together to form one massive continent: Pangaea.
In 1620, shortly after explorers first created maps of
the new world, Francis Bacon noticed how nicely
South America and Africa fit together.
In 1858 Antonio Snider showed how these and other
continents fit together like a jigsaw puzzle.
Perhaps the continents were once together and
drifted apart?
2. Common Fossils
He showed that several fossils from the paleozoic
age found on separate continents are quite similar.
e.g. South America, Africa, India, and Australia
Indeed, in Wegener’s reconstruction, these
continents fit close together.
3. Common Rock Formations
If India hasn’t moved what would a glaciated India
suggest about the Earth’s climate at this time?
In addition to fossils, similar rock formations are
found in India, Africa, South America, Australia, and
Antarctica.
Also North America and Europe.
Again, these continents are close together in
Wegener’s reconstruction.
4. Paleoclimates
Glacial features appear on the southern continents:
South America, Africa, India, and Australia.
India?!?
Yet no such evidence on the northern continents.
Indeed, evidence suggests these continents were
quite warm at this time:
Coal is created in tropical swamps.
Coal in Norway?
Despite this evidence, many geologists remained
unconvinced.
Why?
8-2
Why did Geologists Resist
Wegener’s Ideas?
2. Geography:
Much of Wegener’s evidence came from the
southern hemisphere.
1. No one could explain what forced the continents
to move:
Wegener’s original thought was that the continents
somehow plow through the oceanic crust.
Did not seem plausible given what was known about
the strength of rocks.
Most northern geologists had not seen this evidence
firsthand.
Not surprisingly, southern hemisphere geologists
were more impressed with the idea of continental
drift.
Thus, the idea of continental drift (and plate
tectonics) languished for half a century.
The Revival of Continental Drift
Evidence from Paleomagnetism
During the 1940s and ’50s evidence continued to
trickle in.
The Earth generates a magnetic field.
The poles of the magnetic field are near the
geographic poles of rotation.
In particular, evidence along two lines proved
important in the revival of continental drift:
1. Investigations of the sea floor
2. Geophysical observations, particularly of rock
magnetism
(This is why compasses work—they point towards
the magnetic pole which, for most purposes is close
enough to the rotation poles).
While the generation of the magnetic field is not
fully understood, it is thought to be related to the
Earth’s rotation.
Thus, it is likely that the two have always been close.
Interestingly, however, the magnetic poles appear to
reverse periodically.
8-3
Certain minerals (e.g. magnetite) can act like little bar
magnets.
==> Rocks containing these minerals are magnetic
as well.
These rocks contain records of the past magnetic
field of the Earth.
Thus, when a rock is formed from molten magma it
will record the strength and direction of the Earth’s
magnetic field at the time of its formation.
Once solidified this magnetic signature is frozen into
the rock.
This signature can be used to determine where the
magnetic pole was when the rock was formed.
How?
The magnetic crystals will point towards the pole.
If one heats up a magnet to a certain temperature (the
Curie temperature) it loses its magnetism.
==> direction of the pole. What about distance?
If one then cools a magnet (or magnetic rock) below
this temperature it will become magnetized along
whatever magnetic field is present.
The distance from the pole is determined by the dip
of the crystals.
How will they dip near the equator?
Polar Wander
Magnetic rocks within continents indicate that the
pole has changed position in time.
This suggests one of two possibilities:
In the northern hemisphere?
Either the pole has moved in time
or,
In the southern hemisphere?
the continent has moved in time.
However, rocks from different continents suggest
different positions for the pole at the same time:
8-4
Study of the Sea Floor
What about the problem of how continents move?
Sea-floor spreading provided a plausible mechanism
for the motion of the continents.
Nice theory. Is there any evidence for this?
Studies of the sea floor led to a modification of
Wegener’s original hypothesis in this regard.
The discovery of the mid-oceanic ridge led Harry
Hess in 1962 to suggest that it was the sea-floor, not
the continents, which moved.
This proposal became known as sea-floor spreading
because he proposed that the sea floor spread away
from the mid-oceanic ridge.
He proposed mantle convection as the driving force.
Wegener’s continental drift and sea-floor spreading
have been combined into the theory of Plate
Tectonics.
Plate Tectonics
The basic idea behind the theory:
Theory of plate tectonics has revolutionized geology.
Prior to plate tectonics people tended to look for
local explanations for geologic features.
The theory states that rigid lithospheric plates move
with respect to each other on the surface of the
planet.
How might plates move with respect to each other?
Plate tectonics has provided a global, unifying, theme
to geology.
8-5
Divergent Plate Boundaries
Convergent Plate Boundaries
Sea floor spreading centers.
As we have seen, new lithosphere is continually
being created at divergent plate boundaries.
Would you expect volcanism at such boundaries?
The Earth is not getting bigger—thus if surface area
is being added at some places what must be
occurring elsewhere?
Tensional geologic features. (Normal faults).
Lithosphere is consumed at subduction zones.
Numerous earthquakes but typically not large and
generally quite shallow.
Oceanic crust is denser than continental crust, thus
where oceanic crust meets continental crust the
oceanic plate dives under the continent.
Where two oceanic plates collide, one usually dives
under the other.
Associated with substantial volcanism and
earthquakes.
Continent-continent collision
Transform Plate Boundaries
Continental crust is low density.
Regions where plates are sliding by each other.
Thus it cannot be subducted very far.
Examples?
Thus when two continents collide neither plate dives
into the mantle.
Instead, the plates smash into each other greatly
deforming and folding up the rocks near the plate
boundary.
This increases the thickness of the material leading
to a new mountain range.
Examples:
Associated geologic activity?
8-6
Measuring Plate Motion
The theory of plate tectonics gained acceptance in the
1960s even before we could actually measure the
motion of the plates.
However, since that time it has become possible to
directly measure the actual plate motions.
Hot Spots
Hot spots appear to be regions where thermal
plumes from deep within the Earth’s interior
impinge on the base of a lithospheric plate.
Sometimes these plumes can “burn through” the
overriding plate creating volcanoes.
With the use of GPS one can determine ones
location on the surface of the planet to within feet.
If they reach the surface they can become islands.
With GPS the motion of the continents has been
directly measured.
The hawaiian islands are the classic example.
Rather direct proof that plate motion does actually
take place!
If we look at the hawaiian islands we see that their
ages increase to the northwest.
Only the big island currently has active volcanism.
Given this information, which direction is the Pacific
plate currently moving?
A new island is currently forming to the southeast of
Hawaii.
In fact, the hawaiian islands are but part of a larger
chain of islands, seamounts and guyots running to
the northwest and north to the Aleutian trench.
Distance and age difference between the islands can
be used to determine the speed of motion.
For example:
(Midway, the site of a decisive WWII sea battle is
part of this chain).
These have been dated and found to become older to
the northwest.
Islands created as the Pacific plate moved over the
hot spot which periodically burned through the plate.
If we can directly measure the motion of plates, why
measure the speed from hot spots?
8-7
What Causes Plate Motions?
The mechanism responsible for the motion of the
lithospheric plates is currently an unresolved issue.
2. Magma pushing plates apart
If plates are moved by magma forcing the plates
apart what kind of stresses would the rocks in the
plates be put under?
Several mechanisms have been proposed:
1. Mantle Convection (Mantle Drag)
—Hess’s original proposed mechanism
for sea floor spreading.
If this is the case what kind of features would we
expect to see?
2. Magma intruding into and pushing
plates apart (other books call this
ridge push)
3. Plate Sliding (your book calls this
ridge push)
4. Slab-Pull
3. Plate-Sliding
4. Slab-Pull
Because it is warm and thus less dense the oceanic
crust is raised at the midoceanic ridge.
At subduction zones oceanic plate dives down into
the mantle.
Away from the ridge it cools off and sinks.
Plate is cold relative to surrounding mantle --> more
dense.
Further, cooling away from the ridge results in a
thickening of the lithospheric plate.
Gravity thus pulls the descending slab downwards.
Pulls the rest of the plate with it.
For both these reasons the plate at the ridge is
significantly higher than nearer the edge of the
continents.
Thought to be more powerful than plate-sliding.
May explain why plate motions into subduction
zones tends to be faster than at divergent boundaries.
Plate can thus slide downhill much like a child
sliding down a slide.