Until the 1950s, most scientists believed that the geology of the ocean floor had remained essentially unchanged for many millions of years. But this idea became insupportable as new discoveries were made. First, scientists noticed that the ocean floor exhibited odd magnetic variations. Though unexpected, this was not entirely surprising, because it of the magnetite grains is “locked in,” recording the earth’s polarity at the time of cooling.
As more of the ocean floor was mapped, the magnetic variations revealed recognizable patterns, particularly in the area around the other great oceanic discovery of the 1950s: the global mid-ocean ridge, an immense submarine mountain range that winds its way around the earth much like the seams of a baseball. Alternating stripes oceanic crust. Over millions of years, this process, called ocean floor spreading, built the mid-ocean ridge.
This theory was supported by several lines of evidence. First, at or near the ridge crest, the rocks are very young, and they become progressively older away from the crest. Further, the youngest rocks all have normal polarity. Finally, because geophysicists had already determined the ages of continental volcanic rocks and, by is a remarkable correlation between the ages of the earth’s magnetic reversals and the striping pattern.
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