ROCKS AND TYPES OF ROCKS
Rock or stone is a natural substance, a solid aggregate of one or more minerals or mineraloids. For example, granite, a common rock, is a combination of the minerals quartz, feldspar and biotite. The Earth’s outer solid layer, the lithosphere, is made of rock.
Petrology is science of rocks. It is a branch of geology. A petrologist studies rocks in all aspects – composition, texture, structure, origin, occurrence, alternation and relationship with other rocks.
Based on their mode of formation, there are three different types of rocks:
Igneous Rocks
1) The word igneous has been taken from the Latin word ignis which means fire. These rocks are formed due to cooling and solidification of hot and liquid materials (lava or magma).
2) The process of formation of igneous rocks takes place whether within the Earth’s crust or at the surface.
3) There are two types of igneous rocks: Intrusive rocks (Granite) and Extrusive rocks(Basalt-Deccan Traps).
4) Plutonic or intrusive rocks result when magma cools and crystallizes slowly within the Earth’s crust. A common example of this type is granite.
5) Volcanic or extrusive rocks result from magma reaching the surface either as lava or fragmental ejecta, forming minerals such as pumice or basalt.
6) Examples of igneous rocks include basalt, granite, pumice, obsidian, tuff, diorite, gabbro and andesite.
Sedimentary rock
1) The word sedimentary has been derived from the Latin word sedimentus which means settling.
2) Sedimentary rocks are formed by sediment that is deposited over time, usually as layers at the bottom of lakes oceans.
3) Before being deposited, sediments are formed by weathering of earlier rocks by erosion in a source area and then transported to the place of deposition by water, wind, ice, mass movement or glaciers (agents of denudation).
4) This sediment can include minerals, small pieces of plants and other organic matter.
5) The sediment is compressed over a long period of time before consolidating into solid layers of rock.
6) Sedimentary rocks forms layers called strata which can often be seen in exposed cliffs.
7) Sedimentary rocks cover the majority of the Earth’s rocky surface but only make up a small percentage of the Earth’s crust compared to metamorphic and igneous types of rocks.
8) Examples of sedimentary rocks include limestone, sandstone, mudstone, greywacke, chalk, coal, claystone and flint.
Metamorphic rocks
1) The word metamorphic means ‘change of form’.
2) Metamorphism occurs when rocks are forced down to lower levels by tectonic processes or when molten magma rising through the crust comes in contact with the crustal rocks.
3) Metamorphism is a process by which already consolidated rocks undergo recrystallization and reorganization of materials within original rocks.
4) On the basis of the agency of metamorphism, metamorphic rocks can be of two types
a)Thermal Metamorphism
The change of form or re-crystallisation of minerals of sedimentary and igneous rocks under the influence of high temperatures is known as thermal metamorphism.
b)Dynamic Metamorphism
This refers to the formation of metamorphic rocks under the stress of pressure.
The combination of directed pressure and heat is very powerful in producing metamorphism because it leads to more or less complete recrystallisation of rocks and the production of new structures. This is known as dynamothermal metamorphism.
5) Examples of metamorphic rocks include anthracite, quartzite, marble, slate, granulite, gneiss and schist.
Rock cycle:
1) Any type of rocks changes into another type under heat, pressure, chemical change and other conditions. This conversion of rocks from one type to another is called rock cycle.
2) Example-Molten magma cools to form igneous-These igneous rocks breaks to form sedimentary rocks –under certain conditions sedimentary rocks converted into metamorphic rocks-These metamorphic rocks under great heat and pressure below the earth melts to form magma-These magma again when comes out on the earth surface forms igneous rocks.
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