sometimes called volcanic rocks, are very, very common.
They cover almost all of the Earth's oceanic crust,
formed where magma comes out from the mid-oceanic ridges.
And they cover much of the continental
crust, where granite
- one of
the commonest rocks on the Earth, is only one example.
and basalt represent two different types
of igneous rocks
and intrusive. The intrusives,
such as granite, are formed under the
Earth's surface. The extrusives,
such as basalt, form above the Earth's surface, as lava freezes when it
The size of mineral
grain tells how quick the cooling has been. When the
cooling is slow,
which it is deep down under the surface, igneous
long time to
grow and they are large and well formed. When cooling is quick, the
minerals are small and not very well formed.