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Igneous rocks constitute one of the main types of rocks found in nature. The other types include sedimentary and metamorphic rocks. These rocks differ in terms of their formation and constituents. As magma moves towards the surface, igneous rocks develop from crystallising molten rock material, either magma or lava. If magma remains lodged within the volcano’s fissures and vents, it can cool and crystallise slowly to develop into intrusive igneous rocks. However, if the magma rises to the surface and becomes lava, it is exposed to the cool atmospheric conditions resulting in its solidification to extrusive igneous rocks. There are numerous examples of igneous rocks in nature.
Definition of igneous rocks.
Igneous rocks are one of the major rock types that develop due to the cooling and crystallisation of magma or lava on or below the earth’s surface. The cooling can be rapid or slow, resulting in two types of igneous rocks.
List of types of igneous rocks
Igneous rocks fall into two main categories: intrusive and extrusive rocks. These types vary depending on the texture and size of granules.
– Extrusive igneous rocks.
These rocks materialise from the solidification of lava on the earth’s surface. The rate at which the cooling of lava occurs is quickly hindering crystal growth. Furthermore, they have tiny granules and a smooth texture. Some rocks even adopt a glossy texture due to the minuscule size of the granules.
– Intrusive igneous rocks.
During a volcanic eruption, magma may fail to reach the earth’s surface. The magma then rests within the vents and fissures. It starts cooling and crystallising slowly, resulting in rocks with large crystals. Moreover, these rocks have a characteristic rough texture.
Examples of igneous rocks based on their types.
Intrusive igneous rocks
Examples include granite, gabbro, diabase, diorite, pegmatite, and peridotite.
- Granite.
The main characteristics observed in granite are its rough texture, coarse grains, and light colour. It is hard, comprising large crystals of numerous minerals. The main minerals found in its structure include quartz, mica, and feldspar.
- Gabbro.
This rock has a dark colour with large, coarse granules. It has the coarse texture common in intrusive igneous rocks. Its mineral contents are olivine, feldspar, and pyroxene.
- Diabase.
This igneous rock has pronounced granules. Its grains are larger than those of basalt rock. Its mineral composition entails inorganic minerals such as feldspar, pyroxene, and plagioclase.
- Pegmatite.
Pegmatite is a key intrusive igneous rock in nature. The main features used in its identification are its light colour, coarse granules, and rare minerals. This rock forms following the crystallisation of magma around the margins of the vents and fissures.
Extrusive igneous rocks.
Examples comprise basalt, obsidian, pumice, tuff, rhyolite, scoria, dacite, and andesite.
- Basalt.
This common igneous rock has a dark colour and very fine grains. Its main mineral constituents include pyroxene and plagioclase. It is a major constituent of the oceanic crust.
- Obsidian.
Obsidian is widely known for its dark-coloured, glass-like appearance. This appearance can be attributed to the immensely rapid solidification of lava, leading to the lack of any crystals.
- Pumice.
This rock is common in many homes. Its light colour and vesicular appearance easily distinguish it. Its vesicular appearance can be directly traced to the rapid crystallisation of molten lava trapping air bubbles within its structure. These air bubbles are later expelled in the late stages of the rock cycle.
- Scoria.
Scoria’s texture closely resembles the vesicular texture seen in pumice. Contrastingly, it is dark-coloured. When magma exits the vent, it may crystallise while airborne.
Uses of igneous rocks.
Igneous rocks serve a major role in the construction industry. Polished granite is commonly used to make countertops. Furthermore, basalt is a staple rock in the construction industry. It is an ingredient in concrete. Pumice is also an ingredient in lightweight construction materials.
Additionally, these rocks can be applicable in other industries. Pumice is used to manufacture toothpaste and cosmetic products. Its abrasive surface makes it a great exfoliant to remove dead skin cells.
These rocks are also sources of minerals. Pegmatite is the main source of commercial minerals, including feldspar, mica, and lithium.
