Main Menu >  Plate Tectonics and Volcanoes  >  Juan de Fuca Ridge >  How the Sea Floor Got Its Stripes Periodically, Earth's north and south magnetic poles reverse themselves. Basaltic lavas erupting from spreading ridges record these reversals. As cooled lavas move away from spreading ridges, the reversals produce magnetic "stripes" on the sea floor. The discovery of these magnetic stripes was a turning point: By showing that the sea floor is spreading, they proved the concept of plate tectonics.