The Continental Drift Information Page
Most information from Prentice Hall's Exploring Earth Science, Maton et.al., 1999., the Volcano World website @ http://volcano.oregonstate.edu/education/vwlessons/lessons/Pangea/Pangea1.html and the University of California, Berkeley, Museum of Paleontology @ http://www.ucmp.berkeley.edu/history/wegener.html
Continental Drift Theory: When viewing a
map of the world, it is easy to notice that many of the continents look like
neighboring pieces of a jigsaw puzzle. The western coast of Africa seems
to fit directly into the eastern coasts of
North and South America. The
Arabian Peninsula and northeastern Africa also appear to fit together. Click on
the thumbnail map to see the "puzzle pieces"... (courtesy of
The General Libraries, University
of Texas at Austin). It was this observation by Alfred Wegener, a German interdisciplinary
scientist trained in meteorology, that formed the foundation to the continental
drift theory he first proposed in 1915 when he published his book, The
Origins of Continents and Oceans. The continental drift
theory states that the continents have moved and are still moving today.
Wegener proposed that the "super continent" Pangaea existed around
250-300 million years
ago. Pangaea was a single landmass made up of the
present day continents. Panthalassa was the one large ocean
that
surrounded Pangaea. According to
Wegener's continental drift theory, Pangaea broke
into two large landmasses, Laurasia to the north (included N.
America, Europe, Asia, and Greenland), and Gondwanaland
to the south (included S. America, Africa, India, Australia, and Antarctica).
Laurasia and Gondwanaland subsequently broke apart, and the landmasses moved on
their plates to their present locations. The continents and plates
continue to drift at rates
varying from 2.5cm to 15 cm per year!!
Click on the thumbnail (from Dr. Chamberlin's Remarkable Ocean World website) to see drawings that recreate the break up of Pangea. Click on this link to the United States Geological Survey to view an animation of the breakup of Pangaea!!
Early evidence for Continental Drift...
Wegener's Fossil Evidence: Alfred Wegener used the scientific method to gather evidence that would either support or reject his hypothesis. While browsing a university library in 1911, Wegener came across a paper that listed fossils of identical plants and animals found on opposite sides of the world. Fossils are the preserved remains of ancient organisms. Prior to Wegener, the explanation for these identical fossils in distant areas were land bridges, now sunken, that connected the continents. Wegener claimed that the land bridges did not exist, but rather the continents were once joined as Pangaea, and as the continents drifted apart the fossils were carried with them.
One such fossil that supports the continental drift theory is the tropical, fern-like
plant,
Glossopteris (illustration found on the
Australian Museum Online webpage). Glossopteris is an
extinct
(no longer living)
plant whose fossils are found in 250
million year old rocks in South Africa,
Australia, India, and Antarctica. The seeds of Glossopteris were
too large to have been dispersed to the distant continents by wind, and too fragile to have survived
the long trip by sea. This suggests that the places in which the plant's
fossils are found must have been closer together at one time. In addition, the presence of tropical Glossopteris
fossils in the frozen conditions of Antarctica suggests that Antarctica had a
different climate millions of years ago, probably the result of an equatorial location!!
Fossils of tropical plants and animals have also been found in cold regions of North America and Greenland, suggesting that these continents too were once in a position closer to the equator than they are today. In fact, the fossils in Cincinnati's limestone rocks are tropical marine organisms, suggesting that Ohio must have been both covered in sea water and closer to the equator to support tropical life.
Wegener's Rock Evidence: To convince people of his continental drift hypothesis, Wegener knew he needed more evidence than fossils, so he looked to scientific disciplines outside of paleontology (the study of prehistoric life using fossils). Wegener next looked to the geology and what he found was amazing!! Wegener learned that the Appalachian Mountains of Eastern North America matched closely with the highlands of Scotland, and the distinctive rock layering of the Karroo System of South Africa was identical to the rock layering of the Santa Catarina System in Brazil!! This suggested that the mountain ranges on opposite continents were formed as one!
Evidence After Wegener: After Wegener, and with better technology, even more evidence was found in
rocks to support the continental drift theory. Scientists today can read
the history of the rock record by studying the age and mineral content of rocks.
One
place that was studied extensively is the Triple Junction,
a three way split in the lithosphere that separated the continents of South America and Africa.
This split caused massive lava flows over hundreds of square miles of Africa and
South America. The rocks that formed from the lava flows have been
found to be identical in age, meaning they were formed at the same time, and
identical in mineral content, meaning they were formed in the same place.
This strongly suggests that South America and Africa were once connected,
lending even more support for the continental drift theory.
(Triple Junction figure from
Volcano World)
Wegener's missing piece of evidence: Despite excellent evidence to support his theory, Wegener was missing a crucial piece of information to justify his hypothesis...HOW THE CONTINENTS WERE ACTUALLY MOVING!!! Most scientists in his time did not believe his idea that continents simply plowed through the ocean floor. This gap in his research led to ridicule from his scientific peers. It was not until the mid-1900s when technology provided new information that helped explain the movement of landmasses. The Plate Tectonic Info Page talks about how this information helped form the concept of Plate Tectonics.