Historical Geology/Paleomagnetic dating
In this article we shall discuss how we can use the paleomagnetism in rocks to attach dates to them (paleomagnetic dating). The reader may find it useful to go back and read the main article on paleomagnetism before continuing.
Polar wander and dating
Once we have dated a sufficient number of rocks and measured the orientation of the magnetism they contain, we can build up a picture of how the position or apparent position of the poles over time.
So if we are then faced with a rock the date of which we do not know, then we do know (of course) the latitude and longitude at which we found it, and we can measure the orientation of its magnetism, and so we can look at the global picture we've built up of continental drift, and to figure out when the rock must have formed in order to have its magnetism oriented in just that direction.
Magnetic reversals and dating
As noted in a previous article, magnetic reversals come at irregular intervals. This means that the pattern of normal and reverse polarity in an assemblage of rocks can be distinctive in the same way (though for a completely different reason) that growth rings in a tree can be distinctive. We might, for example, see a long period of reverse polarity, followed by six very quick switches of polarity, followed by a long period of normal polarity; and this might be the only time that such a thing occurs in our timeline.
So if we are presented with an undated rock, and we find a really distinctive pattern of paleomagnetic reversals within it, we may be able to identify the one time at which such a sequence of magnetic reversals took place.
Strengths and weaknesses of the method
The reader will observe that it is necessary to be able to date some rocks, in fact a lot of rocks, before paleomagnetic dating can be brought into play. You may therefore be wondering why, if we have perfectly good dating methods already, we don't just use them.
However, the advantage of paleomagnetic dating is that we can use it on different rocks from those susceptible to our ordinary methods of absolute dating: while most radiometric methods usually require igneous rocks, paleomagnetism can be measured in sedimentary rocks.
One problem which may arise is that the direction of the poles from a given location, or the pattern of magnetic reversals, may repeat over a long enough period of time, so that the paleomagnetic data we get when we measure these factors are not unique to a single time in the history of the Earth.
It is possible to get round this problem if we can find an approximate date of the rocks by other means. For example, if by considering their stratigraphic relationship to a datable igneous rock we can establish that they are (for example) less than 20 million years old, then it may turn out that the paleomagnetic data, though not unique over the whole history of the Earth, are unique over the course of the last 20 million years, and then we can go ahead and use paleomagnetic dating.