Types of Igneous Rocks

Igneous rocks are also known as fire rocks, since the name has been derived from the Latin word 'ignis'. They are formed due to the cooling and solidification process of magma, either above or below the Earth's surface. The former ones are known as extrusive rocks, and the latter ones are known as intrusive rocks. There are more than 700 kinds of igneous rocks, and most of them are formed under the Earth's crust.

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

Intrusive Igneous Rocks: They are formed when magma gets trapped within preexisting surrounding rock spaces within the Earth, cools, and consolidates. Some of their different types, which are categorized according to their size, shape, and mode of formation are: dikes, sills, laccoliths, stocks, batholiths, etc.

The interior of large mountain ranges are made up of intrusive rocks like granite and gabbro. Intrusive igneous rocks that are closer to the surface of the earth are known as hypabyssal rocks.

Extrusive Igneous Rocks: Extrusive rocks are formed when magma reaches the Earth's surface in the form of lava, due to volcanic and fissure eruptions. The cooling and consolidation of this lava forms these extrusive rocks.

Granite: It forms deep beneath the Earth's crust by magma cooling. As the lava takes a long time for consolidation, the four main minerals of this rock: mica, feldspar, quartz, and generally hornblende, are 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, it weathers quite slowly, hence it is also widely used for construction of monuments.

Basalt: It is a fine-grained, melanocratic extrusive igneous rock, and is the most common form solidified lava types. It is mainly composed of pyroxenes and plagioclase feldspar, quartz, glass fragments, etc. The mineral grains are so fine that they are not even visible with a magnifying glass. Some types of basalt are also of the intrusive type, as they formed beneath the Earth's surface.

Obsidian: This volcanic rock is produced by lava that cools very quickly or almost instantaneously. In fact, this process is so quick that it does not have any form of crystallization. It breaks with a conchoidal fracture. During glass formation, silica-rich rocks such as quartz and sand get heated till their melting point, and then cooled very rapidly.

Obsidian forms in a similar way. Usually, it is dark green or black in color; sometimes even 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 in scalpels while performing extremely delicate eye operations.

Pumice: This is an extrusive rock also forms by a rapidly cooling lava. If you examine this rock, you will notice where the tiny pockets of air had been created, while it had solidified. It is very lightweight, and many types of pumice can actually float on water.

Pumice also 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: It is closely akin to granite; the only difference being that the crystals in rhyolite are fine-grained, and are not visible to the naked eye. It is an extrusive igneous rock, and it forms by cooling far more rapidly as compared to granite, which makes it glassy in appearance. The major minerals constituting this rock are mica, feldspar, quartz, and hornblende.