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Continental Drift

© 1985 by Andrew J. Morris

The geophysical structure of our planet gives rise to a strange and startling phenomena. This seemingly solid and immobile earth beneath our feet is only a thin and shifting skin floating on a viscous semi-liquid a few miles beneath the surface. In places the surface cracks and the underlying material -- molten rock known as lava -- oozes onto the surface in a lava flow, or bursts dramatically forth in a volcanic eruption.

Since the surface of the Earth consists of numerous independent sections of crust, or "plates" that move in relation to one another, the entire face of the planet is constantly changing. The placces where two plates ome in contact with one another are more geologically active than other areas, because of the interaction between the two plates. Three general types of interaction between plates can be envisioned -- either the plates are moving toward one another, away from one another, or parallel to the area of contact.

Because the continental part of the Earth's crust consists of lighter material, the continents ride along on top of theses shifting plates, and are carried to and fro around the surface of the globe. The plates move in response to the subsurface force of convection currents in the underlying magma, so the direction, speed and force of their movement varies through time, resulting in motion that seems almost random when viewed from the surface.

When two plates of similar density collide they may buckle up and form mountain ranges. This is how the Himalayas were formed, when the Indian subcontinent collided with Asia. If a dense plate of the ocean floor collides with a lighter continental plate, the denser plate sinks down and is absorbed back into the magma below, while the continental crust rides up over it, remaining on the surface.

When two plates move away from one another, new crust is gradually formed from the underlying magma which swells up at the point of divergence. This is currently happening on the floor of the Atlantic Ocean. North America and Europe are moving away from one another at the rate of one or two inches per year. That might not seem like much, Columbus would only have about 50 feet extra to travel were he making his trip today instead of in 1492. But project that movement back 100 million years and we find Europe and North America were connected!

This movement of the continents goes a long way towards explaining the observed distribution of life forms on the planet, and makes clear the reason for some noticeable similarities between geological structures on different continents. Surprisingly, continental drift amy also explain a puzzeling climatic phenomena -- ice ages.

Over most of the history of the Earth the climate has been, on the average, a bit warmer than it is today. Occasionally though, there have been cold spells. It seems that these ice ages may correspond to those times when one or both poles were covered by continental land masses.

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