EARTH SCIENCE
THEORY OF CONTINENTAL DRIFT READING
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Background
During the early 1900’s Alfred Wegener surprised the scientific community with a new idea – the
theory of continental drift. Since the mapping of the Atlantic Ocean, people had noticed that the
coastlines of South America and Africa looked as though they would fit like adjacent pieces of a jigsaw
puzzle. Although his formal profession was meteorology, Wegener had always been curious about the
remarkable fit of the coastlines. Was it just a coincidence? He speculated that perhaps the continents had
once been joined together into a single land mass and had later split and drifted apart. Wegener named this
hypothetical super-continent Pangaea from the Greek root words for “all lands.” Although an interesting
idea, as a good scientist, Wegener recognized that additional evidence would be required to support this
idea. Therefore, he began seeking additional evidence in 1910. Wegener received overwhelming criticism
for his ideas from most leading geologists during his lifetime. However, he continued work on the project
until he froze to death in 1930 during an expedition crossing the Greenland ice cap. In addition to the fit of
coastlines, Wegener ultimately examined records of ancient life, ancient climates, large geologic structures,
and distinctive rock types.
Fit of Continental Coastlines
Studying world maps, Wegener noticed that not only did the continents on both sides of the Atlantic
fit together well, but that other continents, when moved properly, also fit like additional pieces of the same
jigsaw puzzle. He constructed a map of the Earth based on the fit of continents. On his map, all the
continents were joined together, forming one supercontinent, which he called Pangaea. The northern part
of Pangaea is commonly called Laurasia and the southern part Gondwanaland.
In 1965, Sir Edward Bullard realized that maybe “today’s coastline” was not the same as it was in
the past. Of course the processes of weathering and erosion can alter coastlines; however, the question is
perhaps better stated as “What depth best represents the edge of the continent?” He decided to do a
computer study on the fit of Africa and South America using different depths of water on the continental
shelf. His study found that at depth of 1000 meters on today’s continental shelf provided a better fit than
today’s coastline.
Evidence from Fossils
Mesosaurus is a sharp-toothed reptile that looks like an alligator. Mesosaurus fossils have been
found in South America and Africa, but nowhere else in the world. It is difficult to imagine how a slow,
wallowing, freshwater swamp dweller could swim thousands of kilometers across the Atlantic Ocean to
populate two different continents. However, Wegener noted that the fossil sites could lie adjacent to one
another by arranging the modern continents into the Pangaea supercontinent.
Wegener also mapped the location of the fossil remains of a fern called Glossopteris across
Antarctica, Africa, Australia, South America, and India. Since the spores (seeds) of most ferns rapidly
decay in salt water, an ocean thousands of miles wide would be a major obstacle. The presence of the same
flora on these far-flung continents is further evidence that the continents were all together in the past. In
addition, it is difficult to explain why the fern would populate areas with climatic extremes from tropical to
polar. Of course, if the continents were at one time joined, they would likely have experienced very similar
climates.
Geologic Evidence
Wegener noticed several instances in which an uncommon rock type or a distinctive sequence of
rocks on one side of the Atlantic Ocean was identical to rocks on the other side. When he plotted the rocks
on a Pangaea map, those on the east side of the Atlantic were continuous with their counterparts on the
west side. For example, the Cape Fold Belt of South Africa consists of a sequence of deformed rocks
similar to rocks found in the Buenos Aires Province of Argentina. It also has been observed that large
geologic structures such as mountain ranges have the same age yet lie on separate continents. Two ranges
that include parts of North America, Greenland, Europe, and Africa include the Caledonians and the
Alleghenians.
Evidence from Paleoclimatology
Paleoclimatology is the study of ancient climates. As both a meteorologist and a geologist,
Wegener knew that certain types of sedimentary rocks form in certain climatic zones of the Earth. Glaciers
and glacial sediment, for example, concentrate in high latitudes. Wegener also reasoned that coal layers
would have been formed in the equatorial zone, where lush swamp forests would have grown. Further salt
beds, gypsum, and petrified sand dunes would likely have formed in subtropical deserts. Thus, sedimentary
rocks reflect the latitudes at which they formed. This suggests that many sedimentary rocks are not located
at the same latitude as when the sediments were deposited.