What Is The Theory Of Sea Floor Spreading?

Scientists believe that the mid-ocean ridge is being formed continuously by the upwelling of magma from the earth's plastic mantle through a crack in the earth's crust, and that the ocean floor is moving away from both sides of the crack as it is formed. The newly formed rock is clean, and sediment accumulates slowly and becomes noticeable only after the new rock has been exposed for some time and has moved away from the ridge. The parallel bands of normal and reverse magnetic polarity result when the magma oozes out and solidifies for a time while the earth's poles are normal, and then for a time while they are reversed.

The findings indicate that at the present time the floor of the Atlantic Ocean is spreading a little more than an inch (2.5 centimeters) a year, and the Pacific Ocean about six inches (15 centimeters) a year. But if the earth is forming new crust on the ocean floor on this prodigious scale, it must be getting rid of its old crust somewhere else. After all, the total surface of the earth is not increasing. Geophysicists speculate that this takes place along certain boundaries where one part of the crust slides under another part and descends into the hot interior, where it melts and is consumed into the fluid mantle again. They believe that this is not a smooth process, but is accompanied by earthquakes and volcanic eruptions. It forms deep ocean trenches and high mountain ranges along the consumption boundary lines.

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