Igneous Rocks

Igneous rocks are those that have been formed by the cooling and crystallization of a magma. All magmas originate at great depths in the Earth’s crust, where both temperature and pressure are very high. When opportunity permits, magma will expand and flow towards the surface. The magma may cool and solidify below surface, allowing individual mineral grains to grow together, forming an interlocking pattern.

Rocks that cool at great depths do so slowly, giving crystals time to grow large. This coarse grained texture is typical of these intrusive igneous rocks. Molten rock that comes closer to surface will cool faster, so the crystals will have less time to grow. The resulting rock is finer grained.

Some magma may reach the surface, cooling quickly in form of lave flow. The resulting rock is very fine grained and called volcanic, or extrusive rock. Magma containing trapped gases can also erupt with explosive for from a vent or volcano.

The material ejected from volcanoes varies in size from fine dust to rock fragments weighing hundreds of tones. This includes both lava and solid or semi-solid rock from around the vent or the volcano’s crater. When this material settles, the large angular pieces form rocks known as agglomerate or volcano breccia, while the finer materials form tuff. 

The composition of an igneous rock depends on the composition of the magma that produced it. If the magma is high in iron and magnesium, the resulting rock will have abundant iron and magnesium silicate minerals and will be dark in color; these rocks are called mafic rocks. A magma high in silica will form light-colored rock with abundant quartz and feldspar; such rocks are called felsic rocks. There are also rocks with compositions intermediate between these two extremes.

Intrusive rocks made up wholly of dark minerals – rocks like dunite and peridotite – are termed ultramafic. As the proportion of dark minerals decreases, the rocks are called gabbro, diorite, granodiorite, and granite, with granite being the most felsic. Syenite is similar to granite, but has less silica.

Volcanic rocks are classified in the same way as intrusive rocks, so that each intrusive rock has a volcanic counterpart with a similar composition. Komatiite is the ultramafic volcanic rock, and like peridotite, contains only dark minerals. Basalt corresponds to gabbro, andesite, to diorite, dacite to granodiorite and rhyolite to granite. Trachyte is the volcanic equivalent of syenite.

An igneous rock that contains distinct crystals embedded in a much finer-grained groundmass is called porphyry. The name applies only to this characteristic texture, to the chemical composition of the rock. The groundmass of a porphyry can have either a volcanic or intrusive texture. Porphyries are usually given a compound name signifying their composition, as in dacite porphyry. Similar, volcanic fragmental rocks are usually given compound names like rhyolite tuff.