The Earth's magnetic field isn't an unchanging constant. It changes in intensity and even completely reverses so that what was the north magnetic pole becomes the south magnetic pole and the south magnetic pole becomes the north magnetic pole. These reversals happen randomly with an interval between 100,000 years and 1 million years. The average interval is about 450,000 years. And when these reversals happen, they happen quickly from a geological perspective, taking only 1,000 to 10,000 years. What does this have to do with seafloor spreading? Well, when the sea floor spreads, magma comes to the surface and cools forming new rock. And this rock tends to be magnetic and the bands of magnetism orient themselves to Earth's current magnetic field. And once the rock solidifies, that magnetism remains even after Earth's magnetic field changes again. So as a result, there are bands of rock on the sea floor that have alternating magnetic polarities. These bands of rock provide persistent evidence of seafloor spreading.