Reverse magnetic dating
It is not known when the next magnetic reversal will be, or how long the process will take, though it will certainly have a significant impact on the artificial and biological navigational systems of humans and animals.
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The record of geomagnetic reversals preserved in volcanic and sedimentary rock sequences (magnetostratigraphy) provides a time-scale that is used as a geochronologic tool.
Geophysicists who specialize in paleomagnetism are called paleomagnetists.
Geomagnetic polarity during the last 5 million years (Pliocene and Quaternary, late Cenozoic Era).
Dark areas denote periods where the polarity matches today's normal polarity; light areas denote periods where that polarity is reversed.
Once we have dated a sufficient number of rocks and found out whether they have normal or reverse polarity, we can likewise build up a timeline for the occurrence of the reversals.
As noted in a previous article, magnetic reversals come at irregular intervals.
Because the pattern of positive and negative readings is more or less symmetrical about the axis of the mid-ocean ridge and remains the same throughout the length of the ridge, geophysicists have been able to construct a calendar of the Earth's magnetic record dating back to as far as 150-200 million years ago.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.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.The models show a ridge (a) about 5 million years ago (b) about 2 to 3 million years ago and (c) in the present.Paleomagnetism (or palaeomagnetism in the United Kingdom) is the study of the record of the Earth's magnetic field in rocks, sediment, or archeological materials.