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Plate Tectonic Basics
by AudioLearn
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.
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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:
Figure 17.
The largest of the major plates is the Pacific plate at more than 100 million square kilometers. In descending order after this are the North American, Eurasian, African, Antarctic, Australian, and South American plates. The Indian plate is a minor plate with major consequences. It is the one that is participating in creating the Himalayan mountains.
Minor plates besides the Indian plate are the Somali plate, the Amurian plate in East Asia, The Sunda plate in Southeast Asia, the Philippine Sea plate, and the Nazca plate in the Eastern Pacific Ocean area.
The areas where the plates bump into one another are extremely prone to earthquakes. Seismographs are able to see where the epicenter of an earthquake is; these are largely around the plate edges. They also are seen in the areas of the mid-ocean ridge plates.
