Sea floor spreading is a process that occurs at mid-oceanic ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading helps explain continental movements in the theory of plate tectonics.

The concept of sea floor spreading was first propounded by Harry Hess in 1960. The mapping of the oceanic floor and palaeomagnetic studies of rocks from oceanic regions revealed the following facts:

It was realised that all along the mid-oceanic ridges, volcanic eruptions are common and they bring huge amounts of lava to the surface in this area.

• The rocks equidistant on either sides of the crest of mid oceanic ridges show remarkable similarities in terms of period of formation, chemical composition and magnetic properties.
• Rocks closer to the mid oceanic ridges have normal polarity (i.e. that of earth) and are the youngest. The age of the rocks increase as one moves away from the crest.
• The ocean crust rocks are much younger than the continental rocks.
• The sediments on the ocean floor are unexpectedly very thin.
• The deep trenches have deep seated earthquake occurrences while in the mid oceanic ridge areas, the quake foci have shallow depths.

On the basis of the above factors, Hess conceptualised that constant eruption at the crest of oceanic ridges causes the rupture of the oceanic crust and the new lava wedges into it, pushing the oceanic crust on either side. The ocean floor thus spreads.

On the basis of the evidence of palaeomagnetism and sea floor spreading it has been now validated that the continents and ocean basins have never been stationary or permanent at their places rather these have always been mobile throughout the geological history of the earth and are moving in relation to each other.

E.g.: Mediterranean Sea is the residual of once very vast ocean (Tethys Sea) and the Pacific Ocean is continuously contracting because of gradual subduction of American plate along its ridge.
E.g.: Indian Ocean did not exist before cretaceous period. Indian plate began to move towards Asiatic plate through ‘Tethys sea’ and Australian-Antarctic plates after breaking away from African plate began to move southward during cretaceous period.

Through evidences presented by deep crustal drilling and paleo-magnetic studies on ocean deposits, Sea Floor Spreading was further expanded and a new Plate Tectonic Theory was advanced by Tuzo Wilson, Morgan, et al in 1960s. A plate is a massive, irregularly-shaped slab of solid rock, generally composed of both continental and oceanic lithosphere. Plates moves horizontally over the asthenosphere as rigid units and interact with each other. Based on density and margin of interaction, the heavier plates subduct under the lighter plates and give rise to new landforms.