Our Earth has a magnetic field around it which is self-sustaining due to the semi-molten composition of parts of the inner Earth. The axis of the magnet is offset from the Earth's rotation by around 11 degrees which means that the magnetic north and south poles are not in the same positions as the geographical north and south poles. Not only this but the strength of the magnetic field varies across the Earth too.
So knowing this I bet you're wondering what this has to do with geology? Well as you know, some metals are more magnetic than others, so by doing geomagnetic surveying using specialised equipment we can essentially 'map' the sub-surface and locate bodies of ore which could potentially be mined.
Magnetic surveying is also done for oil exploration and in archaeology to detect buried artefacts, grave sites and building remnants, such as the site map of magnetic data shown above.
We can also use it for dating rocks. In the past, the Earth's magnetic field has flipped, so magnetic north has been at the south and vice versa. Scientists are still not sure why the Earth does this but it can do it quite erratically and either lots or very rarely. This is particularly notable at mid-ocean ridges where plate divergence is occurring and new rocks are being generated which get magnetised to magnetic north as they cool, so the magnetic pattern preserved in the rocks across the ocean floor preserve a record for magnetic field reversals, and because we have dated these rocks (so know how old they are) we can piece together a history for the magnetic field of our Earth.
So there you go. We are all sat inside one giant magnetic field which is useful for geological reasons as well as many others. Of course it does other more essential things, such as protect the Earth from the deadly solar winds which would otherwise make life on Earth impossible, but I thought it was important to mention its geological uses too.
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