Marine magnetic anomalies were first discovered off the coast of the western United States in the late 1950s and completely baffled scientists. The anomalies were charted from southern California to northern Washington and out several hundred kilometres. Russian-born American geophysicist Victor Vacquier noticed that these linear anomalies ended at the fracture zones mapped in this area. In addition, he noticed that they had unique shapes, occurred in a predictable sequence across their trends, and could be correlated across the fracture zones. Soon thereafter, linear magnetic anomalies were mapped over the Reykjanes Ridge south of Iceland. They were found to occur on both sides of the ridge crest and parallel to it. Simultaneously, Alan Cox and several other American geophysicists documented evidence that Earth’s magnetic field had reversed in the past: the north magnetic pole had been the south magnetic pole about 700,000 years ago, and there were reasons to believe older reversals existed. Also at this time, American geophysicist Robert S. Dietz and American geologist Harry H. Hess were formulating the theory of seafloor spreading—the hypothesis that oceanic crust is created at the crests of the oceanic ridges and consumed in the deep-sea trenches.

It remained for English geologists Frederick J. Vine and Drummond H. Matthews and Canadian geophysicist Lawrence W. Morley to put these observations together in a theory that explained marine magnetic anomalies. The theory rests on three assumptions: (1) that Earth’s magnetic field periodically reverses polarity, (2) that seafloor spreading occurs, and (3) that the oceanic crust is permanently magnetized as it forms and cools at spreading centres. The theory expresses the assumptions—namely, that the oceanic crust records reversals of Earth’s field as it is formed during seafloor spreading. Positive anomalies result when the crust is magnetized in a “normal” polarity parallel to the ambient field of Earth, and negative anomalies result when the crust is “reversely” magnetized in an opposite sense. As the magnetized crust moves down the flanks of a ridge away from the spreading centre, it remains permanently magnetized and “carries” the magnetic anomalies along with it.

A brilliant leap in understanding was now possible. If the age of the field reversals were known, the age of the ocean crust could be predicted by mapping the corresponding anomaly. By the mid-1960s, Cox and his colleagues had put together a schedule of reversals for the last four or five million years by studying the ages and magnetic polarities of lava flows found on land. Vine and the Canadian geologist J. Tuzo Wilson applied the time scale to marine magnetic anomalies mapped over the Juan de Fuca Ridge, a spreading centre off the northwest United States. They thus dated the crust there and also computed the first seafloor spreading rate of about 30 mm (1.2 inches) per year. The rate is computed by dividing the distance of an anomaly from the ridge crest by the age of the anomaly twice. Thus the oceanic crust at the Juan de Fuca Ridge is moving at about 15 mm (0.6 inch) per year away from the ridge crest and at about 60 mm (2.4 inches) per year away from the crustal segment on the opposite side of the crest.

During the 1960s and ’70s marine magnetic anomalies were mapped over wide areas of the ocean basins. By using estimates of the ages of oceanic crust obtained from core samples by deep-sea drilling, a magnetic anomaly time scale was constructed, and at the same time the spreading history for the ocean basins covering the last 200 million years or so was proposed.

It is thought that the most important contributor to marine magnetic anomalies is the layer of lavas in the upper oceanic crust. A secondary contribution originates in the upper layer of gabbros. The dike layer is essentially demagnetized by the action of hydrothermal waters at the spreading centres. The dominant mechanism of permanent magnetization is the thermoremanent magnetization (or TRM) of iron-titanium oxide minerals. These minerals lock in a TRM as they cool below 200–300 °C (392–572 °F) in the presence of Earth’s magnetic field. Although several processes are capable of altering the TRM, including reheating and oxidation at the seafloor, it is remarkably robust, as is evidenced by magnetic anomalies as old as 165 million years in the far western equatorial Pacific.