In 1915, Alfred Wegener, a German Meteorologist first proposed his theory of CONTINENTAL DRIFT. His theory states that the continents have and will continue to change positions of the face of the Earth. He proposed that about 220 million years ago, all of the present-day continents were joined together in a supercontinent called PANGAEA. Pangaea slowly broke apart, first into two smaller continents, one northern and one southern. The great northern continent was called LAURASIA, the southern was called GONDWANALAND. These continents were said to have existed about 135 million years ago. Since then, the continents have continued to drift, ending up in their present day configuration.
JIGSAW CONTINENTS: Simply stated, the continents look like they could fit together like the pieces of a jigsaw puzzle. Look closely at the continents, especially at South America and Africa. Can you see how they might fit together? Granted, this is a simple observation, often made first by elementary school students, but sometimes, the simplest arguments are the most convincing.
FOSSIL EVIDENCE: Wegener found some unusual fossils in his travels. These fossils were unusual because they didn't always seem to belong in the places that he found them.
TROPICAL FERNS IN ANTARCTICA
Alfred found some fossilized tropical
ferns called Glossopteris, that had been extinct on Earth for
millions of years. When they did exist on this planet, they were
only located in warm, tropical areas near the equator. These
fossils that Alfred found were located in the glacial ice pack
of Antarctica! Wegener knew that if Antarctica were to have
always occupied its present position of the globe, near the
south pole, the ferns could never have grown there, since we
know that the poles of a planet are always going to be cooler
areas, since they only receive slanted and indirect rays from
the sun. If Antarctica was once warmer, it had to have
been that Antarctica was once located in a warmer part of the
globe, closer to the equator and the sun's direct rays.
The presence of fossils in what is now such a frigid area is a
pretty clear indicator that at one point in the past, Antarctica
must have occupied a latitude that would be warm enough to allow
these tropical ferns to thrive. He also found fossils of
the Triassic Land reptile known as Lystrosaurus in India,
Madagascar, Antarctica and Africa. These places today are
separated by thousands of miles of ocean. He found fossils
the freshwater reptile known as Mesosaurus in both Africa and
South America. He found fossils of the Triassic land
reptile known as Cynognathus in both South America and
Africa. Fossils of a species of Land Snail was found both
in North America and Europe.
WORLD WIDE ANIMAL AND PLANT DISTRIBUTION
Isolated environments tend to give rise to unique species. A classic example is modern day Australia. The unique, marsupial forms of life there have only been able to achieve dominance because of Australia's relative isolation from the rest of the world. Australian life forms are not normally seen in other parts of the world because of the barrier presented by the ocean surrounding this island-continent. Wegener's reasoning is that if the continents have always been so separate and distant from one another, then each would have developed a collection of fairly unique animals native only to it. What we see, however, is that the animal and plant species in existence on Earth is nowhere near as unique and isolated as we expect it should be. Animals and plants are relatively similar in North America and Europe, for example. How could all those animals and plants cross the vast oceans to populate two continents so far apart? Wegener's answer was that at one time the continents were connected, and the animals and plants spread throughout the land. When the continents split up, some plants and animals ended up on both continents.
GEOLOGIC
EVIDENCE:Wegener noted
similar looking mountain ranges in South America and Africa.
Rock samples from the coastlines confirm that these mountain
ranges have identical mineral and rock composition, and
identical ages. Translation, the ranges were once connected,
only to be split up by the separating continents. Similar
geology was found to exist between Europe and North America, as
well as other locations around the globe. This seems to indicate
that these landmasses were joined together in the past. He also
found glacial deposits in places where glaciers don't exist
today. The only explanation is that these places used to
be located on a part of the globe where glaciers could form.
Wegener's theory of Continental Drift was never very widely accepted by the scientific community of the time. Wegener was unable to come up with a mechanism for his theory. He wasn't able to explain HOW something as large as a continent would move, or what force was capable of moving it. In fact, many of Wegener's fellow scientists believed that there was NO force on Earth which could plow something as large as a continent through the ocean beds.
The mechanism was later discovered to be the CONVECTION CURRENTS located in Earth's mantle. This discovery and new information about the way Earth works was incorporated into a more modern version of the continental drift theory. This modern version, nearly universally accepted is called the PLATE TECTONICS THEORY.
This theory is really just an updated version of the Continental Drift theory. the major difference is that this theory says that the Lithospheric Plates, some carrying oceans, some carrying continents, some carrying parts of both, float on the Asthenosphere (the upper part of the mantle). Convection currents in the mantle are said to cause the plates to move.
Mid-Ocean ridges are mountain ranges on the sea floor. They mark the boundary between lithospheric plates. New crust is being formed here, and when it emerges, it forces the two plates apart. The ocean floor ends up working kind of like a giant conveyor belt. As each end of the ocean floor reaches the larger and heavier continental crust, it is pushed under and remelted to become a part of the mantle. This process is called SUBDUCTION. This process helps to force the continental plates away from each other, contributing to continental drift. This subduction also explains why scientists have been able to find nor really old rocks (over 220 million years old), on the ocean floor.