Seafloor spreading occurs at the mid-ocean ridges, long mountain ranges extending through the middle of every ocean floor. The large oceanic plates meet at the ridges but slowly move apart. Magma from beneath the crust of the Earth oozes up into the resulting gap and is cooled by the ocean water.
Because of the way new seafloor is created as ocean plates move apart, the seafloor varies in age depending on how far from the mid-ocean ridges it is. The youngest parts of the crust are the ridges themselves. Plate tectonic theory states that seafloor spreading is responsible for many of the geological features in the world. The Red Sea was created by the slow separation of the African and Arabian continental plates by seafloor spreading. The theory also states that most of the newly-created crust is compensated for by subduction, where old oceanic crust plunges beneath the continents and melts into the mantle.
The discovery of seafloor spreading was a major factor in disproving old theories about the way the continents came to be how they are. An early theory, continental drift, stated that continental plates slowly moved over stable oceanic plates. The exact opposite turns out to be the case, as the major sites of changes to the crust are at the mid-ocean ridges.