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The Continental Drift Theory


Examine original sources related to Alfred Wegener's
Continental Drift Theory. Why was this theory rejected by
the majority of earth scientists in the 1920's? Compare and
contrast the major features of the Continental Drift Theory
and the Plate Tectonics Theory.
Almost everyone who has looked at a map of the world has
noticed that the continents of South America and Africa
seem to fit together fairly well. An even better fit is
obtained if continental margins (the underwater edge of the
continent) are used. Other "fits" of this sort can be
assembled as well. It is difficult to believe that this is
purely accidental, yet it is equally difficult to imagine
and support a theory that explains it. It is one thing to
notice a possibility and another to take it seriously
enough to seek evidence to support the theory, but Alfred
Wegener, a German meteorologist, decided to look for such
Wegener carried out several successful expeditions. He
found fossil distribution patterns that were consistant
between continents. He found that the species dispersed
when the continents were connected and later carried to
their present positions as the continents drifted apart.
For example, Glossopteris, a fern, was found on the
continents of South America, Africa, India, and Austrailia.
If the continents are reassembled into the supercontinent
Pangea, the distribution of Glossopteris can be accounted
for over a much smaller contiguous geographic area.
Also, Wegener theorized that if South America and Africa
had been joined at some earlier time, there would be
similar geological formations such as mountain ranges which
would extend from one continent to another across the same
boundry. Rock sequences in South America, Africa, India,
Antartica, and Austailia show remarkable similarities.
Wegener proposed that three distinct layers occur at each
of these localities.
The bottom, also the oldest, layer is called tillite and is
thought to be a glacial deposit. The middle layer is
composed of sandstone, shale, and coal beds. Glossopteris
fossils are in the bottom and middle layers. The top layer
is the youngest and is made up of lava flows. These three
layers occur in the same order in areas now separated by
great distances of ocean. Wegener proposed that these rock
layers were made when all the continents were part of
Pangea. Thus, they formed in a small contiguous area that
was later broken and drifted apart.
Wegener also used the distribution of specific rock types
to determine climactic zones in the geological past. For
example, glacial till and striations (scratches on the
rocks), sand dunes, and coral reefs, indicate polar,
desert, and tropical climates, respectively. He used this
information to find that, unlike the present distribution,
in which zones parallel the equator, the past zones
occupied very different positions. This implies that the
rotational pole was in a different location relative to
today. Wegener offered an alternative explanation. He
believed that the climate zones remained stationary and the
continents drifted to different locations. The drift of the
continents caused the apparent movement of the climate

Wegener's model was not accepted by all geologists. Some
thought that dispersion by winds or ocean currents could
explain the fossil species being traced to where they were
found. Other geologists thought the poles might wander and
the continents remained stationary. Many people thought
that Wegener's evidence was insufficient.
But the greatest shortcoming, at least in the eyes of
American geologists, was the lack of an adequate mechanism
for moving the continents. Wegener proposed that the
Earth's spin caused the continents to move, plowing through
the oceanic plate and producing mountains on their leading
edge. Geologists at that time understood enough about the
strength of rocks to know that this was highly unlikely
which caused many people to find a different solution for
the moving of the continents. 

These shortcomings became the driving force behind the
introduction of the plate techtonics theory. The lack of a
mechanism for moving the large continents was evident in
this new theory. Deep inside the Earth's crust, at
extremely high temperatures, the iron core heats the bottom
of the rocky mantle. The hottest rocks near the bottom of
the mantle become less dence than the cooler rocks above
them, so buoyancy forces try to push the hootest rocks
upward. Although the rocks in the mantle are solid, the
pressures and heat are so great that the rocks can deform
slowly, like hot wax. So the hot rocks begin to move upward
while the cooler rocks begin to sink to the bottom. Most of
the rock in the mantle moves in this broad cyclic flow.
This zone where rocks are soft enough to flow is called the
The movement of heat by convection in the astenosphere
causes the rocks of the mantle to slowly move in large
streams. The solid, but brittle, rocks of the lithosphere
are resting directly on top of the rocks of the
astenosphere. As the rocks of the astenosphere move in
different directions, they carry parts of the lithosphere
along. The lithosphere rocks can not stretch, so they break
into pieces forming the plates. Once the plates form, they
begin to act independantly of the convection flow because
their cold edges tend to sink into the mantle wherever they
happen to be. It is this model of movement that Alfred
Wegener did not initiate that caused scientists to form a
new opinion of the way continents drift.


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