Basalt is the name for a rock that forms when some kinds of lava cool.

Basalt begins as a very runny kind of lava. It is not found much on land, but it is the most common kind of volcanic rock in the oceans, and as these make up over two thirds of the earth's surface, it makes basalt the world's most common rock.

Because basalt is very runny, it is easy for any gases in it to escape and so basalt only has small bubbles of gas trapped inside. It also cools very quickly, so there is not time for minerals to grow, as they do in, for example granite. When basalt cools it gets smaller and so cracks appear. These cracks break up the basalt into six-sided columns.

The best places to see basalt on land are in Iceland, Scotland (Fingal's Cave), Northern Ireland (Giant's Causeway) and Hawaii.