Download 3. Plate Tectonics I (p. 37-46)

Physical Geology 101
3. Plate Tectonics I
(p. 37-46)
Plates in Motion
The tectonic plates are constantly in motion. Where they collide, the compression can result in the production
of high ______________________. Where they rip apart, __________________ may form.
Despite our observation of such features for a very long time, the theory of plate tectonics is nonetheless a
fairly recent idea.
The beginnings of plate tectonic theory were introduced by Alfred Wegener in the year __________. He called
his new hypothesis ________________________.
This concept implied that the continents have not always been in their present positions, but have drifted and
changed positions over long periods of geologic time.
What observation about the continents led to Wegener’s hypothesis?
______________________________________________________________________
It was hypothesized that all the continents had once been joined together as one large supercontinent called
_________________. At some point in time, this supercontinent broke apart, and the continents drifted off to
their present locations.
Initially, these ideas were highly criticized by geologists, who could think of no mechanism by which the granitic
continents could possibly force their way through solid basaltic ocean crust. It took many decades for
geologists to finally realize what the mechanism is that allows continental drift to actually occur.
In fact, plate tectonics is responsible for the mountain ranges; ocean basins; ocean current circulation patterns;
climatic variations; distributions, evolution and extinctions of plants and animals; volcanoes and earthquakes;
and economic mineral locations. It is a unifying theory for all topics of geology.
Evidence for Plate Tectonics
Continental Fit
One of the most convincing lines of evidence for continental drift and plate tectonics is the way that the
continents seem to fit so well together despite being separated by oceans today.
Example: _____________________________ and __________________________.
It must be more than sheer chance that these coastlines match so well. But, if we look only at the present day
coastlines of these continents, there seem to be large gaps between them when we bring them back together.
But sea level fluctuates through time, so the shape of the present day coastline can be misleading.
What depth below sea level marks the continental edges? _____________
Using the true edges of the continents as a comparison, the largest gap between Africa and South America is
only 90 km (56 mi).
1
Physical Geology 101
Using the true edges of the continents to match up the ancient jigsaw puzzle pieces, the components of all of
Pangea fit together very well.
Matching Geology
Matching coastlines is one line of evidence but taken alone, it is not enough. One of the obvious things to look
for if two continents were once joined together is a similarity in their geological characteristics (i.e., the rocks).
We would expect sedimentary rocks to form on both continents at the same time if they were actually stuck
together.
Do we see similar geology across the continents?
YES
or
NO
What aged rocks match up very well across the join between Africa and South America?
____________________________
So before this time, Africa and South America must have been linked together.
There is also very clear evidence for the continuity of geologic structures like mountain ranges from one
continent to the next.
For example, a mountain belt in eastern North America called the __________________ matches up across
Pangea with similar aged mountains in Ireland, Great Britain, Greenland, and Scandinavia called the
____________________.
What continents of the southern hemisphere show a remarkable similarity between rocks that are
Carboniferous to Jurassic in age?
_____________ ______________ ______________ ______________ ______________
What do we call this combination of joined land masses that existed in the southern hemisphere during this
time period?
______________________
Paleoclimate Evidence
If the continents were once joined together and there was an ice age during that time, we would expect
glacial features to match up across the continents.
We see this in _______________, _______________, _______________ , and ________________,
where there are similar aged thick glacial deposits that match up across the continents when they are put
back together.
What age are these glacial deposits? ____________________________________
We also see consistency between the ice motion directions across the continents that formed Gondwana. Ice
can scratch deep grooves called __________________ into bedrock that provide evidence of the direction the
ice moved. When the Gondwana landmasses are fit back together, the direction of the grooves is consistent
with the ice sheet radiating away from a central high point in southern Africa.
2
Physical Geology 101
The Carboniferous glacial deposits in Gondwana suggest that it was situated near the ___________________
at that time, which was the high point for all this ice.
What were the northern continents of Pangea collectively called? ___________________
Is there is evidence of for Carboniferous glacial deposits in any of the northern hemisphere continents?
YES
or
NO
This suggests that Laurasia must have been near the ________________ during the Carboniferous.
Fossil Evidence
If the continents were once joined together, we might also expect them to have had the same types of plants
and animals during the time that they were linked.
In Gondwana, the plants and animals evolved very similarly, but after Pangea split up, plants and animals
evolved very differently because they were totally cut off from each other.
What type of plant fossil from the Carboniferous period is found in rocks in Africa, South America, India,
Australia, and Antarctica?
_______________________
Could the seeds of this plant have been carried across the oceans by the wind?
YES or NO ?
Why? ______________________________________
The only way we could have had this same plant in all these different continents is if the continents were
all joined together at some point.
There are also types of animals that are similar between the continents, such as a small freshwater
aquatic reptile called _________________________.
Where can fossils of this species be found? _________________ and _____________
This species was far too small to have been able to swim across and entire ocean to get from one
continent to the next. So the continents must have been together.
The Wandering of the Poles
As if the evidence weren't convincing enough already, geophysicists in the 1950s discovered something
else that seemed to point towards the existence of continental drift. They were looking at the record of
ancient magnetism in rocks, called _____________________.
As a rock forms, any magnetic minerals in the rock become aligned parallel to the Earth’s magnetic field
of the time. For example, rocks forming today (like lava rocks that erupt from a volcano) preserve a record
of the magnetic field that exists on Earth today.
3
Physical Geology 101
Earth's magnetic field is horizontal near the equator and vertical at the magnetic poles. The angle
between the magnetic field direction and the horizontal at any point is called the
__________________________.
At any point on Earth today, the magnetic inclination is constant. It only changes as a function of
_________________ (or distance from the equator towards the poles).
If the continents had always been in their current locations, there shouldn't be any change in the magnetic
inclination recorded by magnetic minerals in rocks at any one location as we move back through geologic
time. But it turns out that the magnetic inclination in rocks in many locations HAS changed a lot through
time.
Geophysicists can use the magnetic inclination to determine the ancient locations of the continents with
respect to the magnetic poles. What they found is that the location of the poles seems to have wandered
around a lot through geologic time.
What do we call this phenomenon? ______________________.
Nonetheless, similar aged rocks in different continents seem to indicate completely different locations of
the magnetic north poles.
Examples: the continents of ______________________ and __________________.
It’s as if the rocks formed in completely different magnetic fields, which can’t possibly be true as there is
only one magnetic north pole. What was weird though was that the _________________ of the polar
wander paths through time are very similar on both continents, suggesting some sort of link between the
two.
Geophysicists somewhat reluctantly started to accept that perhaps it was the continents that had been
moving around, rather than the magnetic pole itself, which is unlikely to have moved around so much and
couldn’t possibly have been in two different places at the same time.
If we rotate North America back towards Europe as if the Atlantic Ocean weren't even there (i.e. fitting
Pangea back together), the positions of the magnetic poles are identical through time on both continents!
During what period of time are the polar wander paths identical in Europe and North America?
____________________________________
This is extremely convincing evidence that these continents were once joined together because they
would be expected have an identical magnetic pole location history if they were joined together. Hooray
for plate tectonics!
FINAL QUESTION:
What is the most convincing evidence of all for plate tectonics?
____________________________
4