Seafloor spreading in the Atlantic Ocean.
Sea-floor spreading is an important phenomenon arising from plate tectonics. Along mid-ocean ridges material emerges from the Earth's mantle to form new oceanic crust. This material, primarily basalt, spreads out to either side of the ridges at a rate of the order of 10–50 millimeters per year. Newly laid down basalt is able to "fossilize" the prevailing geomagnetism: the main evidence for sea-floor spreading comes from the symmetric pattern of alternately magnetized strips of basalt on either side of the ridge.