| * |
Igneous rocks
are broken into a number of categories based on grain size and composition
- these also match color and cooling history features. |
| * |
Grain Size -
this depends on the time the rocks had to cool and can be divided into
two groups:
| * |
Fine grained
rapidly cooled surface formed rocks - Aphanitic |
| * |
Course grained
slow cooled deep formed rocks - Phaneritic |
|
| * |
Composition
- determined by the relative abundance of Iron and magnesium on one hand
and Aluminum and Silica on the other:
| * |
Felsic - rich
in Aluminum and Silica and low in in mafics - light colored. |
| * |
Mafic - rich
in Iron and magnesium and relatively low in Aluminum and Silica - dark
colored. |
|
