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Science, Maths & Technology

Explosive beginnings

Updated Thursday 12th October 2006

For a small island, our coast has a lot of rock types, thanks to their explosive beginnings

The Giant's Causeway Copyrighted image Icon Copyright: Mike Dodd

On most modern coastlines you will see deposition of mud, sand and pebbles. Was it a similar environment millions of years ago or can you reveal a far more exciting and dynamic history?

Next time you are down on the beach you might like to take a look at the cliffs and the rocks and pebbles lying around at their base.

Some rocks you find will have been produced by the cooling of molten rock (or magma) as it erupted from a volcano (for example, the black basaltic rocks of The Giant’s Causeway in County Antrim or the flat-topped cliffs and mountains of the Western Isles of Scotland).

Other rocks crystallise slowly at depth beneath a volcano (such as the ancient granites of Land’s End and Dartmoor in Cornwall). Unlike sedimentary rocks, which are made of fragments of pre-existing materials, these so-called igneous rocks are made of interlocking crystals, which gives them great strength and resistance to weathering and erosion.

Rocks in the Giant's Causeway [Image: Richard Messenger - CC-BY-NC licence] Creative commons image Icon RichardMessenger via Flickr under Creative-Commons license
Close-up of the Giant's Causeway [Image: Richard Messenger - CC-BY-NC licence]

The slower the magma cooled, the larger the crystals were able to grow. Igneous rocks often form steep cliffs with little sign of layering, although they may have regular cracks and fractures which were formed as the molten magma cooled.

The most famous cooling cracks are those which give the distinctive columnar features in the ancient lava flows of The Giant’s Causeway and Fingal’s Cave, on Staffa.

These lava flows were erupted when the Atlantic Ocean was formed, splitting the British Isles away from North America, with which it was once joined. Although there are currently no active volcanoes in the British Isles, some 62 million years ago things were clearly very different.

In other places, particularly the wild stretches of deserted coastline in northwest Scotland, you may find other rock types. These can be formed of interlocking crystals, but the crystals are lined up into distinct bands of different coloured minerals (pink, white and black).

Sometimes you may spot very shiny flaky crystals, or even small pinkish red garnets. These rocks are hundreds, or even several thousands of millions of years old, and represent the oldest rock types in the British Isles. They are known as metamorphic rocks and because they are so very old they have a very complex history, most of them having been formed by burial of pre-existing rocks deep beneath the Earth’s surface.

They are only now exposed because of uplift from depth during mountain building processes with subsequent erosion of all the overlying rocks. The intense heat and pressure that these rocks have suffered whilst buried have caused their original character to be completely obliterated.

Take care not to get too close to unstable overhanging rocks, and NEVER climb up the cliffs to take a closer look. A magnifying glass to look at the rocks and a small well-illustrated guide to rocks and minerals might also be useful to help you identify the different rock types.

This article was originally published Summer 2005 as part of the Coast postcards pack

 

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