In geology, a rock is naturally occurring solid of one or more minerals and since there are many kinds of minerals, there are many different types of natural rocks that exist on the earth. In this article on ‘TypesOf.com’, we will understand what is a rock, how it is formed and classify the different types of natural rocks on Earth.
Rocks or stones are a very common sight on the Earth. Everywhere we go we see some types of natural rocks on the ground. Mountains, in fact, are nothing but really huge pieces of folded rocks. And it is easy to conclude so, because the lithosphere or the crust of the Earth, which is the outermost layer of the Earth, is made up of nothing but different rocks. Even the magma in the mantle, which later comes out to the surface as lava during a volcanic episode, is nothing but melted rocks that melted because of the subduction of one lithospheric plate under another under high pressure and temperature conditions. But what exactly is a rock made of?
We know that rocks are made up of minerals, so these rocks differ because of the difference in the constituent minerals. Minerals are simply different elements that can occur together stably. These elements are stacked together in the form of a crystal lattice. A mineral can be represented by a chemical formula that describes the elements present in the mineral and in what quantity.
The aggregate minerals forming the rocks are held together by means of different kinds of chemical bonds and the manner in which a rock is formed determines which minerals and in what quantity will be present in the rock and this in turn defines the types of natural rocks.
On the basis of how the rock is being formed, there are three major types of natural rocks, namely, Igneous Rocks, Sedimentary Rocks and Metamorphic Rocks.
Igneous Rocks form by the cooling and subsequent consolidation of magma. This magma is formed when the pre-existing rocks on the Earth’s crust are melted. The melting of the rocks is a result of increase in pressure, temperature and a change in composition which take place during the subduction of one part of the crust under the other. Igneous rocks are further divided into two types. These are plutonic or intrusive rocks and volcanic or extrusive rocks. Plutonic rocks are formed when magma is cooled and consolidated slowly within the Earth’s crust. These rocks are generally dense and coarse grained. Common examples are granite and gabbro. Volcanic rocks on the other hand, form when the magma reaches the surface of the Earth as lava and is quickly cooled. These rocks are fine grained. Common examples include basalt, andesite and pumice stone. Basalts form most of the oceanic floor and granites are the main composition of the continental crust.
Sedimentary Rocks are formed when the rocks existing on the Earth’s surface are eroded and weathered and are subsequently reconsolidated by means of compaction and cementation. Rocks on the surface of the Earth undergo weathering and erosion by means of wind, water, chemicals and biological reagents that break the rock into smaller sediments. These sediments are then transported over distances by wind or water and are then deposited as layers. These layers are then buried under many more such layers. This causes the sediments to become compact under pressure and become cemented to form sedimentary rocks. Some of the most commonly found sedimentary rocks are the sandstones which are formed by the cementation and compaction of sand.
Metamorphic rocks are types of natural rocks that form when any rock, be it sedimentary, igneous or another metamorphic rock, is subjected to different pressure and temperature conditions that those in which the rock had originally formed. Many minerals and crystal lattices of the original rock becomes unstable due to these differences which leads to recrystallization of the rock in accordance to the pressure and temperature conditions now present. This process is called metamorphism and the temperature and pressure conditions required to bring this process into play are found deep inside the earth’s crust. Common types of metamorphic rocks include schist, gneisses, quartzites.