Igneous Rocks

Igneous rocks, which are also known as fire rocks since the name has been derived from the Latin word ‘ignis’, are formed by cooled magma solidifying, either above the ground, which is known as extrusive rocks, or below the earth’s surface, which is known as intrusive rocks. There are more than 700 kinds of igneous rocks, and most of them are found under the crust of the Earth.

About 95 percent of the upper portion of the crust of the Earth is made up of igneous rocks, however, their huge abundance is not visible because a comparatively thin layer of metamorphic and sedimentary rocks covers them.

Igneous rocks can either be intrusive igneous rocks or extrusive igneous rocks:

Intrusive Igneous Rocks: Intrusive igneous rocks form when the magma, which gets trapped in pockets of pre-existing surrounding rock within the earth, cools and becomes solid. The grains of minerals that form intrusive igneous rocks can be identified without the help of a microscope. Some of the different types of intrusive igneous rocks, which are categorized according to their size and shape, are: dikes, sills, laccoliths, stocks, and batholiths.

The cores of large mountain ranges are made up of intrusive igneous rocks, generally granite. Intrusive igneous rocks that are closer to the surface of the earth are known as hypabyssal, while those that are formed very deep inside the earth are known as abyssal.

Extrusive Igneous Rocks: Extrusive igneous rocks form when volcanoes erupt, resulting in the magma, or molten rocks within the earth’s crust, rising above the surface of the earth. Once the magma erupts on the earth’s surface, it is referred to as lava. When this lava cools, it turns into igneous rocks.

Various Types of Igneous Rocks

Here are some of the most common types of igneous rocks:

Granite: Granite forms deep beneath the Earth’s crust by magma cooling, hence it is an intrusive igneous rock. Since it hardens so deep under the ground, it takes a long time to cool. This is what allows the four minerals it is comprised of – mica, feldspar, quartz, and generally hornblende – to be large enough to be visible to the naked eye. Since granite can withstand enormous amounts of pressure, it is a very good material for buildings and bridges. Also, since it weathers slowly, it is also widely used to construct monuments.

Basalt: This is fine-grained, dark colored extrusive igneous rock is the most common type of solidified lava. It is composed mainly of pyroxene and plagioclase feldspar, although the grains of the minerals are so fine that they are not even visible with a magnifying glass. Some types of basalt are also the intrusive type, having been formed within the Earth’s surface.

Obsidian: This is a natural type of volcanic rock, which is very shiny, and is produced by lava cooling very quickly. In fact, the cooling process is so quick that obsidian does not have any crystals. When obsidian is broken, it breaks with a conchoidal fracture. When glass is made, silica rocks such as quartz and sand are heated to melting point and then cooled very rapidly. Obsidian is created in a similar way naturally. Usually, obsidian is dark green or black in color, although almost clear ones can also be found. Obsidian has been used since ancient times to make spearheads, knives, arrowheads, and various other cutting tools. These days, doctors use it as a scalpel while performing extremely delicate operations in the eyes.

Pumice: This is an extrusive type of igneous rock which forms by lava cooling quickly. If you examine pumice, you will notice where the tiny pockets of air had been while it had solidified. It is such a light rock, that many types of pumice rocks actually float in water. Pumice too is a kind of a glass, and hence does not contain a mixture of minerals. Since it is so light, it is often used as a decorative stone for landscaping. It is also ground to powder and used as an abrasive element in soaps and polishing compounds.

Rhyolite: This is closely akin to granite, the only difference being that the crystals in rhyolite are much finer. In fact, they are so small that they are not visible to the naked eye. It is an extrusive igneous rock and forms by cooling far more rapidly compared to granite, which makes it glassy in appearance. The minerals that rhyolite contains are mica, feldspar, quartz, and hornblende.