Form the basis of paleomagnetic dating
Rocks from hot liquid magma (see lava), or even minerals made up of crystals that grow at low temperatures, can acquire magnetization.
Also, when magnetized minerals become disaggregated from their parent rocks by erosion and are carried into a basin, they will tend to align themselves parallel to the earth's magnetic field as they settle in still water.
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.
This process, in which the rotation of a planet with an iron core produces a magnetic field, is called a dynamo effect.
The Earth's magnetic core is generally inclined at an 11 degree angle from the Earth's axis of rotation.
The Earth's molten core has electric currents flowing through it.
Though first proposed by American geologist Frank Bursley Taylor in a lecture in 1908, the first detailed theory of.....
Therefore, the magnetic north pole is at approximately an 11 degree angle from the geographic north pole.
On the earth's surface, when you hold a compass and the needle points to north, it is actually pointing to magnetic north, not geographic (true) north.
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.