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Name: Dave
Status: Educator
Age: 9-12
Location: California
Country: United States
Date: January 9, 2005


Question:
We have gone through the idea of plate tectonics, but what our book does not mention is how the ocean floor is disappearing. There is the mid-Atlantic ridge creating new floor, and the ocean floor is about 18 times younger than the continental crust, but where is it going?
You will notice the lack of subduction zones and subsequent volcanoes on the Atlantic coasts of North America and Europe.



Replies:
In simplified terms, the mid-Atlantic ridge is the eastern edge of the North American plate and the west coast of the US is the western edge. Relatively speaking, North America is being moved westward by the spreading at the mid-Atlantic ridge. On the western edge, the North American plate is mostly being pushed against the Pacific plate (think rotation), creating faults where each side moves in the opposite direction, like the San Andreas. At some other plate boundaries, like the west coast of South America, or in parts of southeast Asia, where four plates intersect near Indonesia, the ocean floor is being subducted. The subducted material from the Pacific plate is absorbed deep within the earth (into the asthenosphere). So, the ocean floor is being consumed within these subduction zones and produced at mid-ocean ridges, such as the one in the mid-Atlantic.

At distance from a plate boundary, the tectonic stresses are greatly reduced. Since the east coast of the US is far from a plate boundary, it is much less prone to earthquakes and other seismic disturbances.

Of course, this movement is not smooth or linear, especially near subduction zones, as so tragically demonstrated in southeast Asia at the end of 2004, so this explanation is a simplification of the actual behavior. I would urge you to consult additional reference material for greater detail.

Andy Johnson



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