Scilly Geology

If you had a go at Monday’s Discovering Where quiz, on islands and island groups, you will hopefully have discovered the location of the Scilly Isles.  At just over 25 miles beyond Land’s End in Cornwall, they contain the most southerly point in the British Isles.

The Scilly Isles are largely made of granite rock.  This is an igneous rock, which means that it formed when liquid rock cooled down and solidified.  It wasn’t a volcano.  The liquid rock stayed underneath the ground surface and cooled relatively slowly, allowing crystals to form.

This is one piece of rock, but can you see 3 different colours in it?  There are at least 3 different types of crystals in there.  The white and pink colours are quartz and feldspar, while the tiny black pieces are crystals of mica.

Although it is softer than the granite on mainland Britain, the granite on Scilly is still a relatively hard rock, so the wind and rain wear it away very slowly.

There are lumps of it sticking up all over the place.

The edges have been rounded by the wind and rain gradually wearing it away over a long period of time.

Weaknesses in the rock also wear more easily resulting in deep cracks and crazy shapes.  This one is called the tooth.

If you are familiar with Dartmoor, with its hill tops littered with granite tors, then Scilly is much the same, except the granite tors are right by the sea.

Go and have a look at some granite rock and see if you can see the crystals for yourself.  If there isn’t any nearby in the natural landscape, try the DIY store or garden centre – granite is often used for kitchen worktops or decorative paving.