<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 . </P> <P> 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 have spreading (opening) rates of more than 9 cm / year . Intermediate ridges have a spreading rate of 5--9 cm / year while slow spreading ridges have a rate less than 5 cm / year . </P> <P> Earlier theories (e.g. by Alfred Wegener and Alexander du Toit) of continental drift postulated that continents "ploughed" through the sea . The idea that the seafloor itself moves (and also carries the continents with it) as it expands from a central axis was proposed by Harry Hess from Princeton University in the 1960s . The theory is well accepted now, and the phenomenon is known to be caused by convection currents in the asthenosphere, which is ductile, or plastic, and the brittle lithosphere (crust and upper mantle). </P> <P> Seafloor spreading occurs at spreading centers, distributed along the crests of mid-ocean ridges . Spreading centers end in transform faults or in overlapping spreading center offsets . A spreading center includes a seismically active plate boundary zone a few kilometers to tens of kilometers wide, a crustal accretion zone within the boundary zone where the ocean crust is youngest, and an instantaneous plate boundary - a line within the crustal accretion zone demarcating the two separating plates . Within the crustal accretion zone is a 1 - 2 km - wide neovolcanic zone where active volcanism occurs . </P>

When was the theory of seafloor spreading proposed