<P> In geophysics, a magnetic anomaly is a local variation in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks . Mapping of variation over an area is valuable in detecting structures obscured by overlying material . The magnetic variation in successive bands of ocean floor parallel with mid-ocean ridges is important evidence supporting the theory of seafloor spreading, central to plate tectonics . </P> <P> Magnetic anomalies are generally a small fraction of the magnetic field . The total field ranges from 25,000 to 65,000 nanoteslas (nT). To measure anomalies, magnetometers need a sensitivity of 10 nT or less . There are three main types of magnetometer used to measure magnetic anomalies: </P>

Magnetic reversals (i.e. the cause of marine magnetic anomalies) are caused by