Igneous Rocks

Igneous rocks are the ones which have been formed by the crystallization and cooling of magma. Each and every magma originates at great depths in the Earth’s crust, where both pressure and temperature are very high. When opportunity allows it, magma is bound to flow and expand towards the surface. The magma may solidify and cool below the surface, permitting individual mineral grains to grow together, which form an interlocking pattern.

The process of cooling of rocks which are at great depths is very slow, giving the crystals time to grow very large. This coarse-grained texture is typical of these intrusive igneous rocks. The melted rock that comes closer to the surface will obviously cool off faster, so the crystals will have less time to grow. The rock which resluts from this will certainly be finer grained.

Some magma may reach the surface, cooling very fast to form a flow of lava. The rock which results from this is very fine-grained and is known as a volcanic or extrusive rock. Magma which contains trapped gases can also erupt with an explosive force from a volcano or vent.

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

The composition of an igneous rock depends entirely on the composition of the magma that produced it. If the magma is high in magnesium and iron, the rock which results from it will have an abundant amount of magnesium and iron silicate minerals and will have a dark color; these rocks are known as mafic rocks. A magma which is high in silica will form a light colored rock with an abundant amount of quartz and feldspar; these rocks are known by the name of felsic rocks. There are also rocks which have intermediate compositions between these two extremes.

Intrusive rocks which are made up entirely of dark minerals – rocks like peridotite and dunite – are known by the term of ultramafic. As the proportion of dark minerals lessens, the rocks are called granodiorite, diorite, gabbro and granite; the most felsic would be the granite. Syenite is very similar to granite, but it has less silica.

Volcanic rocks are classified in the same manner as intrusive rocks, so that each and every intrusive rock has a volcanic counterpart with a very similar composition. Komatiite is the ultramafic volcanic rock, and like the peridotite, it contains only dark minerals. Andesite corresponds to diorite, basalt corresponds to gabbro, rhyolite corresponds to granite and dacite corresponds to granodiorite, trachyte is the volcanic equivalent of syenite.

An igneous rock that contains distinct crystals embedded in a much finer-grained groundmass goes by the name of prophyry. The name applies only to this characteristic texture, it does not apply to the chemical composition of the rock. The groundmass of a prophyry can have either a volcanic or intrusive texture. Prophyries are usually given a compound name explaining their composition, as in dacite prophyry. In a similar form, volcanic fragmental rocks are most of the time given compound names such as rhyolite tuff.