Science Puzzle
Why Is the Sea Floor Striped Like a Barcode?
In the 1950s, ships towed instruments back and forth across the oceans to measure the magnetism of the rock on the sea floor. The results were a surprise: long stripes of rock magnetised one way alternated with stripes magnetised the opposite way, all running parallel to the great underwater mountain ranges called mid-ocean ridges. Stranger still, the pattern on one side of a ridge was a mirror image of the pattern on the other.
What made the stripes?
The Answer
Two things working together: new sea floor is made at the ridge and spreads out to both sides, and every so often Earth's magnetic field flips over. The rock records the direction of the field as it forms, so the spreading sea floor works like a giant tape recorder.
At a mid-ocean ridge, the plates pull apart and molten rock wells up to fill the gap. As the lava cools, tiny magnetic crystals inside it line up with Earth's magnetic field and are locked in place, like a frozen compass. The new rock is then carried away from the ridge, half to each side, as more rock forms behind it.
Earth's magnetic field doesn't always point the same way. At irregular intervals, usually hundreds of thousands of years apart, magnetic north and south swap over; the last time was about 780,000 years ago. Rock made during each period records the field's direction at the time. Spreading out symmetrically from the ridge, it leaves matching stripes on both sides.
In 1963 the British geologists Fred Vine and Drummond Matthews explained the stripes this way, and the Canadian Lawrence Morley had the same idea independently. It was powerful evidence that the sea floor really spreads, and the widths of the stripes even reveal how fast: a few centimetres a year.
The principle: A magnetic tape recorder. New sea floor locks in the direction of Earth's magnetic field as it cools at a ridge, so as the field flips over time, the spreading floor records the flips as mirror-image stripes.