Lavas on the seafloor have different magnetic properties. These can be detected using a magnetometer, a cylindrical instrument towed 300m behind the highly magnetic ship. A magnetic survey of an area of the ocean floor typically shows a pattern of stripes arising from variations in the magnetism of the lava flows. The magnetism of the lavas is set in place when they cool and crystallize. Some comes from the iron content of the lavas. But the pattern of stripes is not related to the iron content. The pattern comes from the regular reversals of the Earth’s magnetic field, switching direction from south to north and back every few hundred thousand years. The stripes come from bands of lavas being erupted from a linear fissure during alternations of the Earth’s magnetic field. The survey shows the existence of ancient fissures and is critical in understanding how the ocean floor has been formed.