by T Marie Allman

Created on: April 01, 2009   Last Updated: June 09, 2009

The idea that the earth's continents move has been around for centuries. As early as the sixteenth century, cartographers noticed the puzzle-like similarities between the coastlines of Africa and North America. The theory of continental drift, however, was not popularly accepted by the geologic community until the 1960's. One reason geologists found the theory difficult to believe was that no one had yet proposed a suitable explanation for how the continents were moving.

When a German meteorologist, Alfred Wegener, first proposed his theory of continental drift, he was ridiculed. Popular belief in the early twentieth century was that the great landmasses of earth had always been in their present locations. There was no known earthly power strong enough to move continents. Wegener's evidence that the continents once stood connected was intriguing, but geologists of the day resisted his ideas. Evidence such as similar plant and animal life or rock formations on shores separated by an ocean was dismissed. Regrettably, Wegener was known widely as a zealot, and was greatly disliked by the geologic community. His universal lack of respect made geologists less likely to consider his work legitimate. Although Wegener was largely ignored during his liftime, his idea that the continents move stood the test of time.

The modern theory of continental drift is known as the theory of plate tectonics. Plate tectonics is simply the idea that the earth's crust consists of plates that float on top of the plastic mantle and move about, into and past each other. The earth's crust is a very thin layer that is composed of two types of rock, continental and oceanic. Continental crust is less dense, less flexible and much thicker than oceanic crust. The continents consist of the landmass visible above the ocean, plus the continental shelf they rise above. The continental plates are analogous to an iceberg, with a smaller fraction of the plate visible above water, and the rest underwater. What could be so powerful that it could move such behemoths across the surface of the earth? The most accepted reason is simple: mantle convection.

The layer directly beneath the crust is the mantle. The mantle consists of denser rock that is hot enough to melt, but is still solid because it is under great pressure. The mantle is a solid, but often behaves as a fluid. There are convection currents within the mantle that carry warmer mantle toward the surface and pull cooler mantle downward. Temperature

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