<P> Changes that predate magnetic observatories are recorded in archaeological and geological materials . Such changes are referred to as paleomagnetic secular variation or paleosecular variation (PSV). The records typically include long periods of small change with occasional large changes reflecting geomagnetic excursions and reversals . </P> <P> Although generally Earth's field is approximately dipolar, with an axis that is nearly aligned with the rotational axis, occasionally the North and South geomagnetic poles trade places . Evidence for these geomagnetic reversals can be found in basalts, sediment cores taken from the ocean floors, and seafloor magnetic anomalies . Reversals occur nearly randomly in time, with intervals between reversals ranging from less than 0.1 million years to as much as 50 million years . The most recent geomagnetic reversal, called the Brunhes--Matuyama reversal, occurred about 780,000 years ago . A related phenomenon, a geomagnetic excursion, amounts to an incomplete reversal, with no change in polarity . The Laschamp event is an example of an excursion, it having occurred during the last ice age (41,000 years ago). </P> <P> The past magnetic field is recorded mostly by strongly magnetic minerals, particularly iron oxides such as magnetite, that can carry a permanent magnetic moment . This remanent magnetization, or remanence, can be acquired in more than one way . In lava flows, the direction of the field is "frozen" in small minerals as they cool, giving rise to a thermoremanent magnetization . In sediments, the orientation of magnetic particles acquires a slight bias towards the magnetic field as they are deposited on an ocean floor or lake bottom . This is called detrital remanent magnetization . </P> <P> Thermoremanent magnetization is the main source of the magnetic anomalies around mid-ocean ridges . As the seafloor spreads, magma wells up from the mantle, cools to form new basaltic crust on both sides of the ridge, and is carried away from it by seafloor spreading . As it cools, it records the direction of the Earth's field . When the Earth's field reverses, new basalt records the reversed direction . The result is a series of stripes that are symmetric about the ridge . A ship towing a magnetometer on the surface of the ocean can detect these stripes and infer the age of the ocean floor below . This provides information on the rate at which seafloor has spread in the past . </P>

Where does the earth’s geomagnetic field originate from