<P> Beginning in 1966, Lamont--Doherty Geological Observatory scientists found that the magnetic profiles across the Pacific - Antarctic Ridge were symmetrical and matched the pattern in the north Atlantic's Reykjanes ridge . The same magnetic anomalies were found over most of the world's oceans, which permitted estimates for when most of the oceanic crust had developed . </P> <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>

When was the shortest period of reversed polarity