Is it true that the magnetic field occasionally reverses its polarity?
Yes. We know this from an examination of the geological record. When lavas are deposited on the Earth’s surface, and subsequently freeze, and when sediments are deposited on ocean and lake bottoms, and subsequently solidify, they often preserve a signature of the ambient magnetic field at the time of deposition. This type of magnetization is known as ‘paleomagnetism’. Careful measurements of oriented samples of faintly magnetized rocks taken from many geographical sites allow scientists to work out the geological history of the magnetic field. We can tell, for example, that the Earth has had a magnetic field for at least 3.5 billion years, and that the field has always exhibited a certain amount of time-dependence, part of which is normal secular variation, like that which we observe today, and part of which is an occasional reversal of polarity. Incredible as it may seem, the magnetic field occasionally flips over! The geomagnetic poles are currently roughly coincident with the geogra
