Geomagnetic polarity dating
This scenario is supported by observations of the solar magnetic field, which undergoes spontaneous reversals every 7-15 years (see: solar cycle).
However, with the sun it is observed that the solar magnetic intensity greatly increases during a reversal, whereas all reversals on Earth seem to occur during periods of low field strength.
Scientific opinion is divided on what causes geomagnetic reversals.
Many scientists believe that reversals are an inherent aspect of the dynamo theory of how the geomagnetic field is generated.
Originally, however, the past record of geomagnetic reversals was first noticed by observing the magnetic stripe "anomalies" on the ocean floor. 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.
During the 1950s and 1960s research ships gathered information about variations in the Earth's magnetic field.Under favorable conditions, it is thus possible to extract information of the variations in magnetic field from many kinds of sedimentary rocks.However, subsequent diagenetic processes after burial may erase evidence of the original field.In the early 20th century geologists first noticed that some volcanic rocks were magnetized in a direction opposite to what was expected.The first examination of the timing of magnetic reversals was done by Motonori Matuyama in the 1920s, who observed that there were rocks in Japan whose magnetic fields were reversed and those were all of early Pleistocene age or older.
Because the magnetic field is present globally, finding similar patterns of magnetic variations at different sites is one method used to correlate age across different locations.