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Thursday, November 10, 2016

Earth's Magnetic Pole Reversals


Schematic illustration of Earth's magnetic field.
Credits: Peter Reid, The University of Edinburgh


When Earth's magnetic field reverses compasses point toward Antarctica. Hundreds of geomagnetic reversals have occurred throughout Earth's history. The last reversal happened 780,000 years, and there is evidence that Earth may be in the early stages of a pole reversal. As  2014 study indicates that the magnetic field has been weakening rapidly.

Geomagnetic reversals can happen gradually over many thousands of years or within a century, according to a recent study published in Geophysical Journal International about the most recent reversal known as the Brunhes-Matuyama event/boundary.

Magnetic pole reversals happen when patches of iron atoms in Earth's liquid outer core become reverse-aligned, like tiny magnets oriented in the opposite direction from those around them. Earth's magnetic field flips when the reversed patches grow to the point that they dominate the rest of the core. Sediment cores taken from deep ocean floors can tell scientists about shifts in magnetic polarity.

There are many theories about magnetic pole reversal and Earth scientists continue to gather more data and refine the mathematics to develop more accurate models. Ron Merrill, a geophysicist from the University of Washington has said, "No one knows what causes reversals, and there is no agreement on whether we can ever even find convincing evidence to forecast a reversal."

One theory involves the South Atlantic Anomaly, a dent in Earth's shield against cosmic radiation, 124 miles above the ground (200 kilometers). Strong radiation enters Earth's atmosphere here and often causes the electronics of satellites and spacecraft traveling through this area to malfunction.

According to this theory, Earth's iron-rich magnetic core is leaking in this spot. This is one possible cause of magnetic reversals. A magnetic reversal is actually fairly rare. This happens when Earth's magnetic north and south poles switch places, rearranging the magnetic field over the course of between 1,000 and 10,000 years. The last magnetic-field reversal occurred 780,000 years ago.

The seasonal ritual burning of village huts in southern Africa provides clues about the fluctuation of the magnetic field. The fires reached temperatures of over 1,800 degrees Fahrenheit (1,000 degrees Celsius). This melted the magnetic compounds like magnetite in the clay floors. When the magnetite cooled it became remagnetized by the Earth's magnetic field, leaving a record for geophysicists.

John Tarduno, a geophysicist from the University of Rochester in New York, describes the hut floors as "Sort of like minimagnetic observatories back in time."

This Space.com report touches on evidence of the South Atlantic Anomaly's role in Earth's rare magnetic reversals.
Patches of ground where huts were burned down in southern Africa contain a mineral that recorded the magnetic field at the time of each ritual burning. Those mineral records teach researchers more about a weird, weak patch of Earth's magnetic field called the South Atlantic Anomaly and point the way toward a possible mechanism for sudden reversals of the field.
"It has long been thought reversals start at random locations, but our study suggests this may not be the case," John Tarduno, a geophysicist from the University of Rochester in New York and lead author of the paper, said in a statement. [How Earth's Magnetic Field Shielded Us from 2014 Solar Storm]

Within the past 150 years, researchers have seen the Earth's magnetic field rapidly decrease in intensity. However, investigation of the Iron Age remnants of African huts has allowed them to extend this to A.D. 1,000 to A.D. 1,850. Research shows that the South Atlantic Anomaly was strong during this time also.


Related reading:  Magnetic Pole Reversals Can Happen in a Lifetime;  Facts About Magnetic Pole Reversals; Paleomagnetism of selected quaternary sediments on Mt Kenya, East Africa; Global Chronostratigraphical Correlation Table


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