|MadSci Network: Earth Sciences|
Your question about the mechanism that causes continental drift is a very good one, in fact it puzzled scentists for about 100 years. Until the late 1960's to early 1970's many geologists did not believe in continental drift because no mechanism could be found that would cause giant landmasses to move.
A little history: People in the mid-1800's noticed curious things about the earth, such as how well South America and Africa fit together like 2 pieces of a jigsaw puzzle.
In about 1910 a great thinker named Alfred Wegener studied such curiosities in greater detail and found many more strange facts that only made sense if the continents had moved through time. For example, fossils of tropical plants and animals were found in Antarctica and Greenland - why were things that only lived in warm temperatures found in such cold areas? When the pieces of continents were fitted together, lage scale features such as mountain ranges also fit together, even though today they are separated by oceans. Many other observations like these were noticed by Wegener and he wrote a book that described his idea that continents had moved through time. Very few people, especially in America, believed him however because he could not answer your question - how did it happen? Geologists knew the ocean floors were made up of dense rock, how could the continents bulldoze their way through this rock and move around?
The answer finally came in the late 1960's with the discovery of plate tectonics. It was discovered that the earth's crust is made up of a number of large, rigid plates. Each of these plates floats on the mantle, a part of the earth1s interior that is made up of hot, molten fluid. Just as a pot of boiling water contains convection currents that cause the water to cirulate within the pot, the mantle contains convection currents that cause the molten fluid to move and circulate. It is these currents that cause the floating crust above to move. Individual plates can therefore move around the surface of the earth, although this happens very slowly, at most about 1.5 inches per year. The places where the edges of plates meet is very interesting, since the plates do not move in the same direction or at the same speed. It is at these boundaries where most of the earths volcanoes, earthquakes, and mountains are formed.
Finally I want address your point about all the continents starting out as one landmass. This isnt quite true. When the earth first formed and began to cool about 5 billion years ago, many small pieces of solid land formed. These floated around and eventually crashed into one another forming larger and larger landmasses. Because geologic time is so vast, these landmasses have connected with one another and broken apart many times during their years of drifting. The last time all of the continents were connected was about 240 million years ago. This super large continent is called Pangea. It broke apart and its pieces form the shapes of the continents we know today - and they continue to move.
Thanks for your question and I hope I answered it.
-Dr. Bill Raatz
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