What are Igneous Rocks

Rocks can be classified into three main categories, such as metamorphic rocks, sedimentary rocks and igneous rocks. Metamorphic rocks are rocks formed by the metamorphosis or transformation of different rock types as an aftermath of encountering high heat and pressure.

Sedimentary rocks are formed by sedimentation of mineral or organic particles at the surface of the earth's crust within water bodies. Igneous rocks on the other hand, are rocks formed by the solidification of molten magma.

A thin layer of metamorphic and sedimentary rocks blanketing the earth's surface tend to hide the abundance of igneous rocks and mask the fact that almost 90% of the earth's crust comprises igneous rocks. There are more than 700 types of igneous rocks present in the earth's crust. Basalt and granite are the two most common types of igneous rocks known to man.

Igneous rocks get their name from the Latin word 'ignis', which stands for fire. This is because these rocks are formed from molten rocks present within the earth's crust. So let's see how these rocks are actually formed.

We all know that as we go deeper into the earth's crust, the temperature goes on rising. These high temperatures conduce to partial melting of pre-existing rocks in the earth's crust. Drop in pressure or deviation in rock composition also results in the formation of molten rock or magma.

Magma is nothing but a mixture of elements like potassium, silica, sodium, iron, etc. and also contains gases such as carbon dioxide, sulfur dioxide, etc. Since it is less dense than the surrounding rocks in the earth's crust, it begins to rise to the surface of the earth.

As this hot, molten magma makes its way upwards to the earth's surface, the temperature gets cooler and these magma particles may cool and solidify to form crystalline, glassy igneous rocks that settle in the earth's crust.

If the solidification takes place below the earth's surface, the rock formed is called plutonic rock or intrusive rock. If the rock is formed near the earth's surface, it is called extrusive rock or volcanic rock. Depending on how slow the cooling and crystallization process is taking place, the size of the rock will vary. For example, the plutonic rocks are rocks visible to the naked eye, however, volcanic rocks are small grains of rock particles that can be identified only under the microscope.

If they do not crystallize to form rocks, then they erupt from the surface of the earth in the form of volcanic lava flow with extrusive rocks being splattered all across the region. So basically the molten magma within the earth is what we normally refer to as lava that gushes out of an active volcano.

During the process of crystallization, the amount of mineral of the magma that gets crystallized to form igneous rock varies. Silica oxide is the most abundant component in these igneous rocks, however, its concentration may vary from 45% - 75%. Depending on this chemical composition variation, the igneous rocks formed can be further classified into three types.

These are usually dark-colored, dense rocks that have a silica content of only 50% by weight. This low-level of silica oxide in its composition is the reason why these rocks are considered to be silica deficient rocks.

The remaining of its composition consists of aluminum oxides, magnesium oxides, calcium and iron oxides. Basalt and gabbro are the most common mafic extrusive and intrusive igneous rocks.

These rocks are light-colored rocks that have a lesser percentage in density as compared to mafic rocks. They contain more than 65% of silica oxide and only a small proportion of the other oxides like potassium oxide, sodium and aluminum oxide. The rock's light color is due to low magnesium and iron contents. Rhyolite and granite are one of the most abundant intrusive and extrusive felsic rocks available.

This type of igneous rock has a chemical composition that lies between that of mafic and felsic rocks. They appear slight greenish to medium gray in color and the most common intermediate extrusive rock is andesite.

Now that we know how these igneous rocks are formed, let's see what they are used for. Igneous rocks like granite is used for curb stones, for building long-lasting monuments, flooring, countertops, etc. Moreover, pumice stones are used to scrub off various stains that resist soap and water. It is also used in polishes and abrasive cleaners.

The use is different for each type of igneous rock and listing them will just prove to be quite cumbersome. However, we now know what are igneous rocks and how they are formed. These rocks formed from magma are truly quite beneficial to us!