Types Of Rocks

In geological terminology, rocks are naturally occurring aggregate of minerals or mineraloids. Rocks make up for the majority of the earth’s crust and have been a key part in the history of human civilization. The usage of rocks has been diverse; from being used as a hunting tool in the primitive ages to being used as a building material to erect monuments. Most rocks are solid combination of many minerals, e.g. granite, while some rocks are composed of just one mineral, e.g pyrite and quartz. Petrology or the study of rocks suggests that there are different types of rocks. However, geologists have classified them based on their process of formation. Rocks have been classified into three kinds - igneous, sedimentary and metamorphic. Given below is a detailed info about each kind of rock.

 

The word igneous has come from the Latin word ‘ignis’, which means ‘fire’. When the melted rock within the earth’s surface (magma) cools and solidifies, it is known as igneous rock. There are two types of igneous rocks - extrusive and intrusive. Extrusive igneous rocks are formed when a volcano erupts and the magma spilled all over the surface cools and solidifies. On the other hand, intrusive igneous rocks are formed when magma does not reach the surface. It forces its way between cracks and spaces between other rock under the surface of the earth and turns solid before reaching the surface. The rate at which the magma cools down determines what kind of rock is formed. A faster cooling which takes place on the surface, produces fine grained rocks, while a slower cooling which takes place underground leads to the formation of large crystals. Some examples of igneous rocks are obsidian, pumice, scoria, and granite. Glass is an example of man made igneous rock.

 

Layered buildup of sediments, minerals, fragments of rock or animal or plant remains at the surface of the earth, either in water or on land leads to the formation of sedimentary rocks. Sedimentary rocks are worn down by wind and rain and are washed downstream, where they settle at the bottom of rivers, lakes and oceans or on land, thus forming sedimentary rock. Lying almost parallel to the earth’s surface, most sedimentary rocks are cemented together with the help of minerals and chemicals or by electrical attraction. However, there are some that remain loose and unconsolidated. Some types of sedimentary rocks are gypsum, conglomerate, shale, sandstone, and limestone. The Grand Canyon is made up of sandstone, limestone and shale.

Metamorphism means ‘to change’ and metamorphic rocks are formed owing to changes in physical state of rocks. Igneous and sedimentary rocks when exposed to extremes of pressure change their appearance, thus leading to the formation of metamorphic rocks. Much against the general belief, the process of metamorphism does not melt the rocks, but instead makes them more impenetrable and compact. New minerals are created either by rearrangement of mineral components or by reactions with fluids that enter the rocks. The deviation in the pressure and heat results in distinction in size and appearance of the rocks formed. Some other examples of metamorphic rocks are schist and gneiss.