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As a result of processes in the Earth's core, the magnetic field spontaneously reverses at irregular intervals averaging at 500,000 years. The direction of magnetization is recorded within the oxide minerals that crystallize from the magmas, much as in a compass needle. As they cool, these minerals retain a memory of the field at the time of their formation. As sea‐floor spreading proceeds, alternating strips of ‘normally magnetized’ (i.e. with the magnetic field as it is currently) and those with ‘reversed magnetization’ are left on the ocean floor (Figure 2, see Plate section). It was the discovery of these ocean floor magnetic anomalies by Vine and Matthews (1963) that provided confirmation of Hess's (1962) ocean‐floor spreading hypothesis. Vine and Matthews demonstrated that continuous growth of the ocean floor, in conjunction with the periodic reversals, produces distinct magnetic stripes, developed as mirror‐images on either side of the spreading ridge.

These magnetic signatures record the past positions of the ridge axis and, since specific ages can now be assigned to the reversals, the age of the ocean floor from new‐formed crust to oldest (first‐formed) crust can be determined.