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Evidence for Plate Tectonics

Updated Tuesday 24th August 2004

We use information from sources as varied as fossil distribution and magnetic fields to build a picture of how the continents used to fit together

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Fossils and tectonics- More than just pretty objects

One of the most important concepts in earth sciences, such as geology and palaeontology, is the concept of uniformitarianism. All this complicated word really means is that what happens now has always happened: processes stay uniform. So, the fact that we find fossils in Britain of animals that only live in tropical environments today, such as corals, indicates that Britain was once itself in a tropical environment itself. Because tropical environments today only occur near the equator, Britain must once have occupied a position near the equator.

Fossil distribution

Fossils are also useful for showing that two areas were once joined together. Some species of fossil plant and animal are found on different continents today, would be unable to spread such distances over huge oceans. This indicates that the continents used to fit together.

How do magnets tell us Britain was once at the equator?

One ingenious way of working out the ancient latitude, or palaeolatitude of a country, is by studying magnetic particles in rocks. When lava or mud is first deposited, the rock is fluid and all of the mineral particles are suspended. Some lava’s and sediments may contain particles of naturally occurring magnetic minerals such as magnetite.

The magnetic field of the Earth

The earth itself behaves like a giant bar magnet, with the positive end towards the North Pole, and the negative end towards the South Pole. As a consequence, all of the magnetic particles suspended in the liquid volcanic rock (lava) and soft sediments orientate themselves relative to this magnetic field, in the same way that iron filings organise themselves around a small magnet.

Diagram showing the magnetic field of the earth Copyrighted image Icon Copyright: Used with permission
The magnetic field of the earth

For example, near the equator, the particles will orientate themselves horizontally and at the poles they will be orientate themselves vertically. This will be especially clear in elongate particles, which will appear to be standing on their tips or lying flat. Between the equator and the poles, the inclination of the particles gradually changes.

Preserving magnetic clues

As the rock cools and/or solidifies, the orientation of these magnetic particles is preserved. By measuring the inclination of the particles in the rocks we can deduce what latitude the rock itself originally formed. The magnetic particles in some British volcanic rocks are horizontal, indicating that Britain was at the equator when the rocks formed.

Britain in the Southern hemisphere

In fact, Southern Britain was once positioned 60 degrees SOUTH of the equator according to palaeomagnetic data: compare this to our current latitude of between 50 degrees NORTH on the south coast and 57 degrees North in Scotland (Aberdeen). Britain has done a lot of travelling during its geological history.

Next: The mechanism of plate tectonics

 

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