Seafloor spreading is a geological process that occurs at mid-ocean ridges, where tectonic plates split apart from each other, and new oceanic crust is formed through volcanic activity. As two tectonic plates slowly separate, molten material rises up from within the mantle to fill the opening, creating a rugged volcanic landscape along the plate boundary. The new oceanic crust then gradually moves away from the ridge, and the spreading rate determines if the ridge is fast, intermediate, or slow. Seafloor spreading occurs at spreading centers, distributed along the crests of mid-ocean ridges, and ends in transform faults or in overlapping spreading center offsets.

The process of seafloor spreading starts as a rift in a continental land mass, similar to the Red Sea-East Africa Rift System today. The process starts by heating at the base of the continental crust, which causes it to become more plastic and less dense. As the crust bows upward, fractures occur that gradually grow into rifts. Seafloor spreading creates a successively younger ocean floor, and the flow of material is thought to bring about the migration, or drifting apart, of the continents. The continents bordering the Atlantic Ocean, for example, are believed to be moving away from the Mid-Atlantic Ridge at a rate of 1–2 cm per year, thus increasing the breadth of the ocean basin by twice that amount.

A veritable legion of evidence supports the seafloor spreading hypothesis. Studies conducted with thermal probes, for example, indicate that the heat flow through bottom sediments is generally comparable to that through the continents except over the mid-ocean ridges, where at some sites the heat flow measures three to four times the normal value. The anomalously high values are considered to reflect the intrusion of molten material near.