Types of Rock And their Formation
•Earth’s crust is composed of rocks. It’s an aggregate of one or more minerals. Rocks do not have definite composition of mineral constituents. However, feldspar and quartz are the most common minerals found in rocks. 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 – solidifies from magma and lava.
•Sedimentary rocks – the result of deposition of fragments of rocks by exogenous processes.
•Metamorphic rocks – formed out of existing rocks undergoing re-crystallization.
Igneous rocks:
•Igneous rock is formed when magma cools and solidifies, it may do this above or below the Earth's surface.
•Magma can be forced into rocks, blown out in volcanic explosions or forced to the surface as lava.
•The atoms and molecules of melted minerals are what make up magma.
•These atoms and molecule rearrange themselves into mineral grains as the magma cools, forming rock as the mineral grains grow together.
•There are over 700 different types of igneous rocks.
•Examples of igneous rocks include basalt, granite, pumice, obsidian, tuff, diorite, gabbro and andesite.
•Basalt forms the metamorphic rock granulite when subjected to extreme heat and pressure over time (metamorphism).
•Granite is a common rock that contains at least 25% quartz and is sometimes used in construction because of its strength.
•Pumice is an unusual, lightweight rock formed when molten rock is rapidly blown out of a volcano, forming bubbles as it quickly loses pressure and cools at the same time.
•Obsidian is a volcanic glass that forms quickly without crystal growth, it can have very sharp edges making it useful as a cutting tool or arrowhead.
•Tuff is a rock formed from volcanic ash.
•The upper section of the Earth's crust is made up of around 95% igneous rock.
Sedimentary rocks:
•Sedimentary rocks are formed by sediment that is deposited over time, usually as layers at the bottom of lakes and oceans.
•This sediment can include minerals, small pieces of plants and other organic matter.
•The sediment is compressed over a long period of time before consolidating into solid layers of rock.
•Sedimentary rocks forms layers called strata which can often be seen in exposed cliffs.
•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.
•Examples of sedimentary rocks include limestone, sandstone, mudstone, greywacke, chalk, coal, claystone and flint.
•Limestone forms the metamorphic rock marble when subjected to extreme heat and pressure over time (metamorphism).
•Sandstone forms the metamorphic rock quartzite.
•Mudstone forms the metamorphic rock slate.
•Chalk is a soft, white form of limestone.
•Flint is a hard, sedimentary form of the mineral quartz.
Metamorphic rocks:
•The word metamorphic means ‘change of form’ these rocks form under action of Pressure, Volume, temperature changes.
•Metamorphic rocks have been changed over time by extreme pressure and heat.
•Metamorphic rocks can be formed by pressure deep under the Earth's surface, from the extreme heat caused by magma or by the intense collisions and friction of tectonic plates.
•Uplift and erosion help bring metamorphic rock to the Earth's surface.
•Dynamic metamorphism: mechanical disruption due to breaking without any appreciable chemical changes.
•Thermal metamorphism: chemical alteration and recrystallization of rocks. It is of two types: A) Contact – rocks come in contact with hot intruding magma and rocks recrystallize under high temperature.
B) Regional – recrystallization of rocks due to deformation caused by tectonic shearing together with high temperature or pressure or both.
•Foliation or lineation: During recrystallization, minerals might get arranged in layers or lines, this is termed as foliation.
•Banding: When minerals and materials of different groups are arranged into alternating thin to thick layers appearing in light and dark shades, they are called structures with banding, and rocks displaying banding are obviously called banded rocks
•Examples of metamorphic rocks include anthracite, quartzite, marble, slate, granulite, gneiss and schist.
•Anthracite is a type of coal with a high carbon count, few impurities and with a high lustre (meaning it looks shiny).
•Marble is a metamorphic rock that is formed from the sedimentary rock limestone.
•Quartzite is a metamorphic rock that is formed from the sedimentary rock sandstone.
•Slate is a metamorphic rock that is formed from the sedimentary rock mudstone.
•Granulite is a metamorphic rock that is formed from the igneous rock basalt.
Rock cycle
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.
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.
Above process goes on and on several times, thus these process is called rock cycle.
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