The Earth’s magnetic field resembles a bar magnet located at the Earth’s centre, with its axis emerging at the magnetic poles. The north and south magnetic poles have repeatedly changed places, at irregular intervals, while the axis has stayed in place. When igneous rocks form, they take up the prevailing pattern of the Earth’s magnetic poles. As a result, areas of the ocean bed where sea-floor spreading has taken place are characterized by magnetic stripes—parallel bands of igneous rock with differing magnetic polarity. See Nicolosi et al. (2006) Geol. 34 on the Marsili Basin, Italy.