<P> Past field reversals can be and have been recorded in the "frozen" ferromagnetic (or, more accurately, ferrimagnetic) minerals of consolidated sedimentary deposits or cooled volcanic flows on land . </P> <P> The past record of geomagnetic reversals was first noticed by observing the magnetic stripe "anomalies" on the ocean floor . Lawrence W. Morley, Frederick John Vine and Drummond Hoyle Matthews made the connection to seafloor spreading in the Morley - Vine - Matthews hypothesis which soon led to the development of the theory of plate tectonics . The relatively constant rate at which the sea floor spreads results in substrate "stripes" from which past magnetic field polarity can be inferred from data gathered from towing a magnetometer along the sea floor . </P> <P> Because no existing unsubducted sea floor (or sea floor thrust onto continental plates) is more than about 180 million years (Ma) old, other methods are necessary for detecting older reversals . Most sedimentary rocks incorporate tiny amounts of iron rich minerals, whose orientation is influenced by the ambient magnetic field at the time at which they formed . These rocks can preserve a record of the field if it is not later erased by chemical, physical or biological change . </P> <P> Because the magnetic field is global, similar patterns of magnetic variations at different sites may be used to correlate age in different locations . In the past four decades much paleomagnetic data about seafloor ages (up to ~ 250 Ma) has been collected and is useful in estimating the age of geologic sections . Not an independent dating method, it depends on "absolute" age dating methods like radioisotopic systems to derive numeric ages . It has become especially useful to metamorphic and igneous geologists where index fossils are seldom available . </P>

Where were magnetic reversals in the earth's crust were first discovered