Palaeomagnetism
I've previously written a post on magnetism. It’s fun stuff to play with in junior science class, but is it really any use?
Well, yes, of course it is!!! Ever heard of maglev trains? Or the Aurora Borealis? Or the Aurora Australis? Admittedly, the northern lights are more well-known than their southern counterpart... Anyway, here is just one way magnetism is used, that you may not have thought of...
The Earth’s outer core is molten iron (with some nickel and sulfur for good measure). It is also spinning around the inner core. This generates a magnetic field, and the Earth essentially behaves as a very large magnet with invisible lines of force that extend from one magnetic pole to the other. A compass needle aligns itself with these lines of force and points toward the magnetic poles.
When igneous rocks (magma that has come out of a volcano - when it does that we call it lava - and cooled down so that it hardens into solid rock) containing magnetic minerals crystallise, the crystals align themselves with the Earth’s magnetic field. The magnetic field of the rock then points toward the magnetic pole that existed when the rock formed. If the rock is moved, its magnetic field will act as a fossil compass.
Magnetised minerals can also be used to determine the latitude of their origin. The Earth’s magnetic field is curved, and the inclination of the magnetic grains gives an estimate of the palaeolatitude (i.e. the latitude at which the rock was formed).
Equator: horizontal
Mid-latitude: high angle
Pole: straight up
That’s all well and good, but why do we care where rocks were first formed? Well, this is using palaeomagnetism, which is just one piece of evidence that shows how the continents have moved with time, i.e. continental drift.
And it uses magnetism :)