The Earth has a weak magnetic field, rather like that of a bar magnet. Before GPS and satnav, we used to find our way around using a compass, which points north. If we get far enough north, we can see the Northern Lights, caused in part by the Earth's magnetic field.

Magnetism is associated with iron and to some extent nickel. We believe that the Earth's core is made mainly of iron, with some nickel. However, the core is molten and too hot (above the Curie point) for magnetism to be maintained permanently. So it has to be generated continuously, probably by circulating currents within the outer core, mediated by spin of the Earth which keeps the magnetic poles quite close to the geographic poles.

If the magnetic field is being generated by movements within the core, then the way it is generated can change. The north and south poles can swap round. In other words, the polarity can reverse. What probably happens it that magnetic field gradually fades away and then gets stronger again with the poles swapped. The evidence for this is found in iron-bearing rocks, for example the basalts of the ocean floors. These basalts can be dated and a time scale of magnetic reversals (magnetic stripes) determined.

Iron occurs in the several of the minerals which make up rocks. As they cool from magma (for example, basalt) or settle in a sediment (for example, red sandstone), these mineral crystals or grains become aligned to the Earth's magnetic field at that time. So, using very sensitive magnetometers, it is possible to find the palaeolatitude at the time that the rock was formed and the direction to the then magnetic north pole.

Triassic red sandstones showing dune bedding, Cheshire

Incidentally, it is of interest to work out why it is not possible to find the palaeolongitude of the time.
In Britain, the palaeolatitude during the Carboniferous appears to have been about 0^o - on the Equator. This matches the occurrence of coal seams, formed in hot, humid equatorial swamps.
Does this mean that the Equator has moved? If so, the way in which the Earth spins on its axis must have changed.
Or does it mean that the position of Britain (and Europe) has changed since the Carboniferous?

Careful study of the remanent magnetism held by these rocks tells us the latitude and direction to the geomagnetic north pole at the time. From this, the position of the pole can be calculated. Evidence seems to suggest that the location of the poles has changed dramatically over geological time, a phenomenon called apparent polar wandering.
Is it likely that the magnetic north pole has moved by thousands of kilometres? Or is there another explanation?

A magnetic survey is yet another remote sensing method of finding out which rocks lie beneath the surface. Iron ore, and other mineral deposits containing iron, make the magnetic field stronger than normal, as do some igneous rocks. If the survey shows a strong magnetic anomaly over one particular area, it is worth investigating further, for example by traditional fieldwork.

• explains the origin and measurement of the Earth's magnetic field;
• outlines how magnetic surveys are carried out and the workings of magnetometers;
• describes remanent magnetism, magnetic stripes and other anomalies polar wandering curves;
• introduces the problems posed by polar wandering curves.

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