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Volcanic Rock

Abhijit Naik Apr 23, 2019
A compilation of some facts about volcanic rocks intended to shed light on various attributes of this igneous rock type which is formed as a result of volcanic activity on the Earth's crust. Continue reading...
Rocks, which are nothing but naturally occurring solid aggregates of minerals, are categorized into three different types - sedimentary rocks, metamorphic rocks and igneous rocks.
While sedimentary rocks and metamorphic rocks are formed as a result of sedimentation and metamorphism respectively, the formation of igneous rocks is attributed to cooling and solidification of magma or lava.
Igneous rocks are further categorized into two types - intrusive igneous rocks aka plutonic rocks (which form beneath the Earth's surface) and extrusive igneous rocks aka volcanic rocks (which form above the Earth's surface).

Volcanic Rock Facts

Simply put, volcanic rocks are igneous rocks which are formed on or near the Earth's surface as a result of some volcanic activity. The appearance and composition of these rocks depends on several factors - right from which volcano it is, which part of the world it is, how much time it took for the lava to cool and solidify, etc.
In terms of texture, they range from fine-grained or aphanitic texture to glass texture. It's true that these rocks are formed as a result of cooling and hardening of lava on the Earth's surface, but there is lot more to this process than this simple one line explanation.
While the term magma refers to molten rock within the Earth's crust, the term lava is used to refer to the same molten rock when it comes to surface of the Earth as a result of volcanic activity.

Formation Process

A volcano is basically an opening in the Earth's crust which allows hot magma, volcanic ash and other gases, to escape from below the surface of the planet. As we mentioned earlier, lava is the hot magma which comes to the surface of the Earth as a result of volcanic eruption.
As this lava comes to the surface, it starts cooling and solidifying, and eventually results in formation of igneous volcanic rocks. The composition of lava plays a crucial role in this process, while its viscosity determines how far it will travel and how much time it will take for this molten rock matter to cool down and solidify.
Volcanic lava is further categorized into three different types on the basis of its composition - felsic lava (with more than 63 percent of silica content), intermediate lava (52-63 percent of silica), mafic lava (45-52 percent of silica) and ultramafic lava (less than 45 percent of silica).
The fine-grained texture of these rocks can be attributed to the fact that lava cools and crystallizes rapidly on the surface of the Earth's crust - as compared to below the surface, in course of the formation of these rocks.

Different Types of Volcanic Rocks

The chemical composition and texture of these rocks helps in classifying them into different types. For instance basalt, which happens to be one of the most popular igneous rock type, is pretty low in terms of silica content, while the same in rhyolite is pretty high.
If rhyolite lava cools down quickly, it results in formation of obsidian - a natural glass which is formed when molten lava cools very rapidly without crystallization. On the other hand, when air seep inside this lava as it cools and makes it frothy, it results in formation of pumice.
Approximately 95 percent of the Earth's crust is composed of igneous rocks, while sedimentary and metamorphic rocks constitute the remaining 5 percent in form of a thin layer. Both plutonic and volcanic rocks are of great importance when it comes geological studies, as they help the scientists study the interior of the Earth.