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Igneous Rocks - What is Basalt?

Basalt is an igneous rock that forms when molten magma
cools quickly. This happens when molten magma from
the mantle reaches the surface of the Earth's crust either
during sea floor spreading or from an erupting volcano.

When magma gets to the Earth's surface it cools
quickly and can form a dark rock containing many
randomly arranged interlocking crystals.
Basalt is one of the main examples of this type of rock.
The crystal size is small.

Basalt is called an extrusive igneous rock because
the molten magma has been pushed out on to the surface
(compare this with intrusive). Extrusion is the process of
"pushing out" or "forcing out", like toothpaste from a tube.

In the right conditions, basalt may crystallize in the shape
of hexagonal columns. There are famous examples of this at
the Giant's Causeway, County Antrim, Northern Ireland
and at Fingal's Cave on Staffa Island,
part of the Inner Hebrides, Western Scotland.

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