These,
sometimes called volcanic rocks, are very, very common.
They cover almost all of the Earth's oceanic crust,
because basalt
is
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.
Granite
and basalt represent two different types
of igneous rocks
- extrusive
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
cools.
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
minerals have
long time to
grow and they are large and well formed. When cooling is quick, the
minerals are small and not very well formed.
This site uses British
English, which is the English we use in
Australia.
Disclaimer:
Although
best efforts have been made to ensure that
all the information on this
site is correct,
collecting-rocks-and-minerals.com is not to be blamed should there be a
mistake.
Copyright notice:
All contents of this website are strictly protected
by the Law of
Copyright. What does that
mean?
Copyright
2010-2021
collecting-rocks-and-minerals.com. ALL RIGHTS
RESERVED.