Geomagnetic reversal
change of direction of Earth magnetic field / From Wikipedia, the free encyclopedia
A geomagnetic reversal is a change in a planet's magnetic field such that the positions of magnetic north and magnetic south are interchanged.
This has happened 183 times over the last 83 million years, averaging about two or three times per million years. Before a change of magnetic field, the Earth's magnetic field becomes weaker and moves around, like a spinning top would before it falls. Scientists know this as a result of studies of magnetism on the sea floor, near the mid-Atlantic ridge. The lava slowly moves out of this crevasse (gap in the sea floor) and then it cools with its iron oxide molecules all pointing in the new direction of the Earth's magnetic field. We can look at the history of this magnetic field today to look back at the many flips in the past.[1]
Reversals occur at intervals from less than 0.1 million years to as much as 50 million years. These periods are called chrons.
There is no pattern to these changes, which seem to take place at random. Chrons last from between 0.1 and 1 million years (see diagram) with an average of 450,000 years. Most reversals take between 1,000 and 10,000 years to happen.
The latest one, the Brunhes–Matuyama reversal, occurred 780,000 years ago; and may have happened very quickly, within a human lifetime.[2] A brief complete reversal, known as the Laschamp event, occurred only 41,000 years ago during the last glacial period. That reversal lasted only about 440 years with the actual change of polarity lasting around 250 years. During this change the strength of the magnetic field weakened to 5% of its present strength.[3] Brief disruptions that do not result in reversal are called geomagnetic excursions.