Which discovery did scientists make at mid-ocean ridges that caused them to conclude that new rock was being formed by molten material?

Pillow-shaped rocks form when molten lava erupts underwater and cools rapidly, creating rounded, pillow-like shapes. Finding these rocks at mid-ocean ridges showed that molten material was rising there and forming new rock. Magnetic banding and the ages of rocks farther from the ridge provided other evidence for seafloor spreading, but they are not the discovery described here.

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