Terry Ebeling, of Goldfields, Queenstown, asks :-

How frequently does the earth's magnetic field reverse and when can we expect the next reversal?

David Nobes, a geophysicist at the University of Canterbury, responded.

Predicting the Earth's magnetic field is a bit like predicting the weather for a day next year. We know what the general pattern will be for the season of the year, but we cannot accurately predict the weather for that day. The weather is complex, what we call non-linear. The earth's magnetic field is also complex and non-linear. Only recently have geophysicists been able to accurately reproduce all of the main features of the Earth's magnetic field, including reversals and excursions. Reversals occur when the Earth's magnetic field turns off, then back on again in the opposite direction. Excursions occur when the earth's magnetic field turns off, then back on again, but in the same direction as before.

Reversals and excursions do not occur in a regular fashion and there are several hundred thousand years between each event. We know of reversals about 2.2 million years ago, 1.7 million years ago and 900,000 years ago. The last reversal of the earth's field that is clearly recorded in the geologic record occurred about 700,000 years ago.

There may have been short reversals or excursions since, but these are not yet clearly defined. The field appears to turn off slowly at first, then more quickly, and the actual reversal may take less than 100 years. We don't know if there are any biological effects from reversals, but navigation and some communications would be affected. Imagine what would happen to your compass! It would point to the South after a reversal.