When molten lava is expelled from mid-ocean ridges, it cools, creating new ocean floor. And as the rock cools, specific minerals with magnetic properties line up with the prevailing magnetic field of Earth. This preserves a record of the magnetic field orientation at that particular point in time. Changes in the rocks’ magnetic field records, called magnetic anomalies, happen when Earth’s field reverses—or when the northern and southern magnetic poles change places—usually over hundreds of thousands of years. Scientists still do not know what causes these magnetic reversals, but it may have something to do with the giant convection currents in Earth’s interior.

The theory of seafloor spreading was confirmed by measuring such magnetic anomalies in rocks on the ocean floor. Scientists discovered a symmetrical, striped pattern of magnetic anomalies on the ocean floor, spreading out on either side of the Mid-Atlantic Ridge. This ridge is a long volcanic mountain range that runs down the Atlantic Ocean seafloor between the continents of North America, Europe, Africa, and South America. The pattern and distribution of these stripes showed that the magnetic fields had reversed many times over millions of years— and only could have formed if the seafloor had been spreading apart over those millions of years.