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A glimpse into this mechanism came from studies of the magnetic characteristics of the ocean floor. During the 1950's, at least partly driven by the military need to understand the ocean as the arena of submarine warfare, intensive studies of the ocean floor were carried out. It was found that the center of each major ocean was occupied by a ridge at whose center was a valley. On each side of this, parallel stripes of rocks magnetized in opposite directions were found. The pattern of stripes on one side was the mirror image of the pattern on the other side. The reversal of the earth's magnetic field recorded in the rocks was repeated on each side of the ridge.

This led H. H. Hess (1960) to propose the idea of sea-floor spreading. Molten rock is continuously extruded and cools to form the ridge. As it solidifies, it records the magnetic field at that time. Since it spreads to each side of the ridge, each side has the same record magnetic field record (one is the mirror image of the other). Since new crust is being formed at the ridges, it must be consumed somewhere. Hess proposed that his happens at deep sea trenches, where oceanic crust "dives" under a continent.
Continental Drift and Plate Tectonics