Sea floor spreading. Artwork showing the process of sea floor spreading. New sea floor (top) is formed at a mid-ocean ridge by upwelling lava (red arrow). Over millions of years the new rock fills the gap left as the continents drift apart (yellow arrows), and also helps drive the process. The evidence for this process is found by looking at the magnetism of the rocks. As volcanic rocks solidify, they retain the magnetism present at the time. The Earth's magnetic field reverses periodically, and this is shown by changes in the magnetism of these rocks (see image E350/087).