It wasn't until the mid-1960s that the ideas of new floor being built in some places, and old floor being extracted or subducted into the earth were finally put together to explain why the earth wasn't expanding or shrinking. Magnetic studies of the ocean floor added to the puzzle solution by showing striped patterns of polarity in one direction and then polarity in the opposite direction. This occurred as new floor was made along the mid-oceanic ridges over millions of years when the earth had one polarity or another. This provided a zebra-like situation with stripes wider in places where the polarity in one direction lasted longer in time. The mid-oceanic ridges act like a conveyor belt, spitting out the floor in a linear fashion along the ridge to either side of the ridge.
PLATE TECTONIC BASICS There are currently seven accepted major plates and many minor plates. The major plates are the African, North American, Eurasian, South American, Pacific, Australian, and Antarctic plates. They are not moving at the same rate or direction. Some are moving about 10 centimeters a year, while others move at just 2 centimeters per year. The determination of what is a major, minor, or micro-plate is largely arbitrary. A major plate is defined as one that is 20 million square kilometers in area. The major plates and their location now can be seen in figure 17:
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