There are three main groups of rocks - igneous, sedimentary and
have got their name from Latin "ignis" for "fire", and it
makes sense because these rocks are formed by volcanic activity.
are two types of them: intrusive
igneous rocks, which form from molten
magma under the surface, and extrusive
igneous rocks, which form from
lava above the surface. They form when lava/magma cools. As you can see
on the photo below, igneous rocks are characterised by distinctive
grains and interlocked
mineral crystals. Grain size is one of the keys for their
generally the quicker the cooling of magma/lava was at the time of
their formation, the smaller is the grain size. This is why extrusive
igneous rocks have smaller grain size than intrusives.
The second group is sedimentary
rocks - rocks that have formed from
sediments, under the pressure from new layers that build above the
older layers of sediments. Even here we have different groups such as
chemical, biochemical and organic sedimentaries. You can often
sedimentary rock from its fine grain size (although there are
texture, and often
from its layers.
Sedimentary rocks are the only rocks that sometimes
The third group, metamorphic
rocks, are the rocks that have formed from
igneous, sedimentary, or other metamorphic rocks by changes caused by pressures and
While igneous rocks also form by pressures and
temperatures, the difference with metamorphics is that they reform in
- they are compressed, streched and twisted but as opposed
to igneous rocks they don't melt in the process. Because they are often
many, but not all metamprophic rocks are
you find a rock with strong foliation, you can be sure
that of the three different types of rocks it's a metamorphic rock. The
main groups of metamorphic rocks are contact
which form by heat alone, and regional
metamorphics, which form both by pressures and temperatures.
are often "schisty"-
they are very distinctively layered, in a
different way from sedimentary layers, because the bonds between the
layers are weak so the layers are very easily broken off.