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Igneous Rock

Igneous Rock

Igneous rock is sometimes referred to as fire rock from its Latin translation 'ignis' which means 'pertaining to fire'. This is an appropriate translation as igneous rock is derived from the cooling and crystallization of magma or lava. Magma is a naturally occurring molten rock material which comes from deep within the Earth's surface. Magma's liquid state allows for it to eventually rise to the surface where it is described as volcanic lava.

Two different types of rock can be formed from magma or lava:

Intrusive (plutonic):

Produced when magma has been has cooled, solidified and crystallized in pockets or chambers beneath the Earth's surface. The term 'plutonic' comes from the Greek God of the underworld, Pluto. Igneous rock formed in this way will have visually large crystals due to the slow cooling of the magma which allows the crystals to grow. Granite is the most well known of the intrusive rocks.

Extrusive (volcanic):

Produced when lava that flows over the surface of the Earth rapidly cools down. Because of this rapid cooling the resulting crystals in the rock which is formed are very small. Basalt is the Earths most abundant extrusive igneous rock.

Characteristics and textures of Igneous rock

As well as the cooling rate, other factors which contribute to the appearance and texture of igneous rocks are the gas content of the magma and its composition and temperature. Magma with high gas content can produce a rock with large crystals even when cooled rapidly. Igneous rocks can be described in the following ways:

Phaneritic: Rocks that have visible crystals (usually intrusive) are described as 'Phaneritic'

Aphanitic: A term used to describe rocks (usually extrusive) which don't have crystals visible to the human eye.

Vitric: A vitrified or glassy textured rock (such as Obsidian) is produced when very rapid cooling takes place or when the ions in the minerals have no time to organise into crystals.

Porphyritic: Non-uniform cooling will create a rock that is composed of large crystals surrounded by a groundmass which is much finer grained.

Pyroclastic: This term comes from the Greek 'Pyro' for fire and 'Klastos' for broken. It refers to the process of broken volcanic matter being fused by heat to another rock of a finer grain. They can be split into two types; tuff and breccia.

Pegmatic: A magma solution which is rich in water and rare metals such as Beryl or Lithium can produce a rock with very large crystals and therefore a coarse, grainy texture.

Vesicular: Sometimes gases escaping from below the Earth's surface can preserve the vesicules or cavities of Aphanitic rocks. When this happens to basalt, the resulting formation is called Scoria. Rhyolite undergoing the same process is known as Pumice.

Igneous rocks can also be classified according to their mineral content, the two most common being Felsic and Mafic.

Felsic: Sometimes called Granitic rock. Usually light in colour and containing less of the heavy elements. The word Felsic comes from combining Feldspar and Silica which are present in high abundance. Common elements found in felsic rock are oxygen, aluminium and potassium. Granite and its aphanitic extrusive equivalent Rhyolite is a felsic rock.

Mafic: Sometimes called Basaltic rock. Dark in colour, high in heavy elements and containing the minerals olivine, pyroxene, and plagioclase feldspars. The term is derived from combining Magnesium and Ferrum (Iron). Basalt and its phaneritic intrusive equivalent, Gabbro, are mafic rocks.

Intermediate: Sometimes called Andesitic. There are many intermediate steps in the process between mafic and felsic which create rocks such as Andesite and Diorite. These are typically medium grey in colour.

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