Sea floor spreading occurs at mid-ocean ridges, where new oceanic
crust is formed through volcanic activity and then gradually moves away from
the ridge. Sea floor spreading helps explain continental drift in the theory of plate
tectonics.
Earlier theories (e.g., by Alfred Wagner) of continental drift were that continents
"plowed" through the sea. The idea that the sea floor itself moves
(and 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 plastic, very weak upper mantle, or asthenosphere. In the
general case, sea floor spreading starts as a rift in a continental land mass,
similar to the Red Sea-East Africa Rift System today. The process starts with
heating at the base of the continental crust which causes it to become more
plastic and less dense. Because less dense objects rise in relation to more
dense objects, the area being heated becomes a broad dome. As
the crust bows upward, fractures occur that gradually grow into rifts. The
typical rift system consists of three rift arms at approximately 120 degree
angles. These areas are named triple junctions and can be found in several
places across the world today.