sea-floor spreading

sea-floor spreading Sea-floor spreading is the process whereby new oceanic lithosphere is created as a pair of plates diverges at a mid-ocean ridge spreading centre. It was originally proposed by R. S. Dietz in 1961, and was an important component of the theory of plate tectonics that developed shortly afterwards.

At a mid-ocean ridge, two plates diverge, probably driven mainly by the pull of subducting slabs and the gravitational sliding of the ridge flanks (see plate tectonics). As the plates separate, ductile upper mantle rises from the underlying asthenosphere, and undergoes adiabatic decompression (decompression without loss or gain of heat) and consequent partial melting. The melt rises toward the surface, where it solidifies to form new oceanic crust, while the residual ductile mantle is drawn along with the diverging plates. As the residual mantle cools, it becomes more brittle and is effectively accreted to the lithosphere, gradually thickening it.

The newly formed oceanic crust (especially the extrusive basalts at its top) acquires a thermal remanent magnetization as it cools; and as the Earth's magnetic field episodically reverses direction, strips of crust are formed with alternating polarities of magnetization. These give rise to alternating positive and negative magnetic anomalies that are symmetrical about the spreading centre and can be used to date the oceanic crust, as was first suggested by Vine and Matthews in 1966.

Roger Searle