Types of Igneous Rocks

The three main types of rocks include igneous, sedimentary, and metamorphic rocks. Each type is formed by specific physical changes on the earth’s crust, resulting in their formation. These changes constitute their rock cycle or rock formation. Igneous rocks differ from other rocks as they form from hot, molten material which escapes the earth’s crust and cools and crystallises. Their formation can occur in different ways to form two main types of igneous rocks. Furthermore, there are numerous examples of these rocks which have a role in the construction industry. This article focuses on the meaning of igneous rocks, their types, and their formation. 

What do igneous rocks mean?

Igneous rocks refer to the types of rocks formed when a hot, molten liquid called magma cools and crystallises. Igneous is a term derived from the Latin word for fire. The crystallisation of magma can occur at the surface after expulsion from a volcano or within the earth’s crust. Magma is a molten rock found within the earth’s lower crust or mantle. When magma reaches the surface, it becomes lava. A volcano eruption brings about its extrusion from the earth’s interior. Some magma also escapes through fissures and vents within the volcano’s structure. 

List of types of igneous rocks.

Igneous rock classification depends on the area where the magma crystallised. If it crystallised above the surface, it forms an extrusive igneous rock. However, if the magma solidifies before reaching the surface and becomes lodged within the volcano’s vents and fissures, it forms an intrusive igneous rock.

– Extrusive igneous rock.

These types of rocks are also known as volcanic igneous rocks. They are produced when magma leaves the earth’s interior to form lava that solidifies at or near its surface. During their formation, the lava cools quickly as it is exposed to relatively cool atmospheric conditions on the surface. This quick cooling rate inhibits crystal growth. Therefore, these rocks can be identified by their small crystals.

Furthermore, they have very fine grains. The term ‘aphanitic’ is used to describe their fine-grained nature. Aphanitic is derived from the Latin word for invisible. Geologists can only see their small crystals under a microscope. Some rocks develop a glassy texture as a result of this quick crystallisation. These igneous rocks lack any crystals. A great example of such a rock is the obsidian. 

– Intrusive igneous rock.

These igneous rocks develop as magma fails to reach the surface and cools and crystallises within the earth’s crust. They are also referred to as plutonic igneous rocks. Magma may not reach the surface due to obstructions or insufficient pressure. It remains between the chambers of pre-existing rocks in the vents and fissures of the volcano. 

Within the earth’s crust, there is some heat and pressure. These conditions do not support quick crystallisation. The magma that remains within the earth’s crust takes a very long time to cool. The slow cooling translates to the growth of large crystals. Intrusive igneous rocks have large crystals easily visible to the naked eye.

Moreover, they develop into relatively large rocks and have a coarse-grained texture. This texture is described as phaneritic. A key example of an intrusive igneous rock with a coarse-grained texture is granite.

Igneous rocks examples.

The rocks show common characteristics depending on their formation and types.

– Examples of intrusive igneous rocks.

1. Granite.

This type of rock is of the coarse-grained intrusive type. It has a light colour. Its composition encompasses quartz, feldspar, and mica minerals.

2. Diabase.

This rock is an important intrusive igneous rock with a crucial function in the construction industry. Diabase is used as a dimension stone. Its surface features labradorite crystals that give it a beautiful aesthetic appearance. Furthermore, by analysing its grains, diabase can be differentiated from other intrusive igneous rocks such as basalt and gabbro. Its coarse grains are smaller than gabbro’s but larger than the basalt granules. Its mineral structure is composed of feldspar, pyroxene, and plagioclase.

Other examples include gabbro, diorite, pegmatite, and peridotite.

– Examples of extrusive igneous rocks. 

1. Basalt.

This type of rock has fine grains and a smooth texture. Its dark colour can easily distinguish it. The main elements found within basalt include pyroxene and plagioclase.

2. Obsidian.

It is a prominent extrusive igneous rock characterised by its dark colour and smooth glassy texture. It forms due to the rapid cooling of lava, leading to the underdevelopment of its crystals.  

3. Pumice.

This light-coloured rock forms due to the very rapid crystallisation of lava trapping gas within the lava during cooling. Pumice has a vesicular texture.

Other examples include andesite, rhyolite, tuff, scoria, and dacite.

Conclusion:

Rocks develop due to physical changes of earth constituents such as magma, lava, or organic materials. These physical changes differ depending on the type of rock. Igneous rocks are a major type of rock resulting from the crystallisation of magma near, at, or on the surface. There are two main igneous rocks categories: intrusive igneous rocks and extrusive igneous rocks. These categories vary based on where these rocks formed, their texture, and the size of their granules. These rocks include granite, pumice, gabbro, diabase, and tuff. These rocks have a functional significance and are applicable in the construction and cosmetic industries.