About a hundred years ago, geologists first noticed that some volcanic rocks were magnetized opposite to the direction of the local Earth's magnetic field. Thus, the concept of magnetic field reversal was born. Estimates regarding the timing of magnetic field reversals were made about that same time and scientists initially thought that this phenomenon had occurred by the early Pleistocene age.

Over the next 30 years, with a better understanding of the Earth's magnetic field, theories were advanced that further suggested our magnetic field may have reversed itself at some time in the remote past.

By the 1950s, a major portion of paleomagnetic research activities included further examination of this phenomenon. As it turns out most magnetized volcanic rocks preserve traces of the Earth's magnetic field at the time such rocks had cooled. Based on limited scientific evidence at the time, it was generally thought that magnetic field reversals occur approximately every million years.

The next major advances in understanding reversals came in the 1950s with new techniques for radiometric dating. Scientists at the US Geological Survey produced the first magnetic-polarity time scale in 1959. Others showed that the same pattern of reversals was recorded in sediments from deep-sea cores.

During the 1950s and 1960s research vessels were used to gather information on variations in the Earth's magnetic field. Based on plots of these data onto maps it become apparent that remarkably regular and continuous magnetic stripes appeared on the ocean floors. Finally, in 1963, a time scale of reversals was formulated.

In summary, reversal occurrences are statistically random. An estimated 183 reversals have occurred over the last 83 million years, averaging about 450,000 years per reversal. The latest reversal occurred some 780,000 years ago. It has been estimated that the time it took for each of the last four reversals averages around 7,000 years.

The reversal process of the magnetic field may adversely impact the Earth's biosphere. In the 1960s, scientists began exploring links between magnetic field reversals and extinctions.

For example, if we assume the Earth's magnetic field becomes much weaker during reversals, it is possible that high-energy particles trapped in the Van Allen Belt would be released to bombard the Earth. Some have argued that the Martian atmosphere may have been eroded away by the solar wind because it had no magnetic field protection.

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