IGNEOUS ROCKS
Igneous rocks form when molten rock cools and solidifies (crystallizes).
Molten rock trapped underground is called magma. Molten rock erupted at Earth's
surface is referred to as lava. The pictures below illustrate the significant
textures and compositions that enable geologists and geology students to
classify igneous rocks. For convenience of presentation the igneous rocks
are divided into intrusive rocks (or plutonic rocks) which form underground,
and extrusive rocks (or volcanic rocks) which form at Earth's surface.
Texture of Igneous
Rocks
The texture of an igneous rock is determined by its rate of cooling. Intrusive
rocks, formed due to slow cooling of magma underground, are composed of large
crystals because crystals had a long time to grow before the crystallization
process was finished. This forms a coarse-grained or phaneritic texture.
Extrusive rocks, formed due to rapid cooling of lava at Earth's surface,
are composed of tiny crystals (fine-grained) because crystallization was
completed within a few seconds, hours, or perhaps days. Special igneous rock
texture referred to as porphyritic when large white crystals are formed as magma
began to slowly crystallize underground.
Fine-grained rocks with tiny crystals are formed as the still-molten magma is
erupted at Earth's surface where it cools very rapidly.
Composition of Igneous
Rocks
Igneous rocks are divided into three compositional groups:
1) Felsic - composed mostly of the minerals quartz and potassium feldspar
which are generally white to pink in color.
2) Intermediate - composed mostly of the minerals amphibole and sodium-rich
plagioclase feldspar, with some quartz and pyroxene possible. Intermediate
igneous rocks are usually light to dark gray in color.
3) Mafic - composed mostly of the minerals pyroxene, calcium-rich
plagioclase feldspar and olivine. Mafic rocks are usually black to dark green
in color.