JStrips of alternating magnetic polarities found in rocks in the ocean basins provide evidence for sea-floor spreading.
These strips, known as magnetic stripes, show that as new crust forms at mid-ocean ridges, it records the Earth's magnetic field at that time. The alternating patterns of magnetic polarity correspond to periods of geomagnetic reversals, supporting the idea of sea-floor spreading, which is a key component of the theory of plate tectonics.