<P> Seafloor spreading is a process that occurs at mid-ocean ridges , where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge . Seafloor spreading helps explain continental drift in the theory of plate tectonics . When oceanic plates diverge , tensional stress causes fractures to occur in the lithosphere. The motivating force for seafloor spreading ridges is tectonic plate pull rather than magma pressure , although there is typically significant magma activity at spreading ridges . At a spreading center basaltic magma rises up the fractures and cools on the ocean floor to form new seabed . Hydrothermal vents are common at spreading centers . Older rocks will be found farther away from the spreading zone while younger rocks will be found nearer to the spreading zone . Additionally spreading rates determine if the ridge is a fast , intermediate , or slow . As a general rule , fast ridges see spreading rate of more than 9 cm / year . Intermediate ridges have a spreading rate of 4 -- 9 cm / year while slow spreading ridges have a rate less than 4 cm / year . </P>
where does new crust come from in sea floor spreading
basaltic magma