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Types of Rocks

Rocks are an aggregate of one or more minerals held together by chemical bonds.Feldspar and quartz are the most common minerals found....

Rocks are an aggregate of one or more minerals held together by chemical bonds.

  • Feldspar and quartz are the most common minerals found in rocks.
  • The scientific study of rocks is called petrology.
  • Based on the mode of formation three major groups of rocks are defined: igneous, sedimentary, and metamorphic.
Igneous Rocks
  • Igneous rocks start as molten material that cools and solidifies.
  • Granite, for instance was once a super heated liquid. When this liquid is beneath the surface of the earth, it’s called magma. Once it reaches the surface, perhaps as an eruption, it is called lava.
  • Lava cools very quickly, since the temperature of the surface (let’s say 32 degrees Fahrenheit, or zero degrees Celsius) is significantly cooler than the lava, which can be more than 1000 degrees F. This rapid cooling is called quenching, and is how glass-like rocks, like obsidian, form.
  • Bringing material as hot as lava to the surface is somewhat like sticking a red-hot iron into a pot of water. Igneous rocks can (and often do) cool beneath the surface of the earth, the molten material moving up from the mantle but never making it to the surface. Other times they extrude at the surface, either at mid-oceanic ridges or hotspots.
Economic Significance of Igneous Rocks
  • Since magma is the chief source of metal ores, many of them are associated with igneous rocks.
  • The minerals of great economic value found in igneous rocks are magnetic iron, nickel, copper, lead, zinc, chromite, manganese, gold, diamond and platinum.
  • Amygdales are almond-shaped bubbles formed in basalt due to escape of gases and are filled with minerals.
  • The old rocks of the great Indian peninsula are rich in these crystallised minerals or metals.
  • Many igneous rocks like granite are used as building materials as they come in beautiful shades.

Sedimentary Rocks
  • Sedimentary rocks are made up of grains that break off of other rocks through a process called weathering.
  • When rocks are exposed to rain, wind, temperature changes, roots, and some chemicals, they can be broken down into their basic components.
  • Physical weathering breaks off grains of rock, while chemical weathering breaks rocks down into more basic elemental components.
  • These grains can come from other sedimentary rocks, from igneous rocks, or from metamorphic rocks.
  • Grains are transported downstream, eventually settling in a basin, or low energy environment (for example a lake or an ocean).
  • Over time, as more sediment is deposited, layers of sediment are buried deep enough to be lithified, or turned to stone.
  • Because features of the environment are preserved in the sediments, one can tell something about where the rock was deposited.
  • For example, a rock with leaf fossils preserved must have been formed in a slow, stagnant environment – one in which the leaves would not be disturbed or moved away.
  • Sedimentary rocks are all about what remains.
  • Black shales, organic rich sediments from which we extract oil and gas, form in deep marine settings, where there is very little energy and there is very little oxygen to decompose the organic material that gets deposited.
Economic Significance of Sedimentary Rocks
  • Sedimentary rocks are not as rich in minerals of economic value as the igneous rocks.
  • But important minerals such as hematite iron ore, phosphates, building stones, coals, petroleum and material used in the cement industry are found.
  • The decay of tiny marine organisms yields petroleum. Petroleum occurs in suitable structures only.
  • Important minerals like bauxite, manganese, tin, are derived from other rocks but are found in gravels and sands carried by water.
  • It also yield some of the richest soils.

Metamorphic Rocks
  • Metamorphic rocks are rocks that have been deformed.
  • They have been heated or squashed or buried (which often means they are both heated and squashed), which can cause minerals in the rocks to recrystallize.
  • It form in places where change is happening: they can form in mountain belts (collision zones) where rock is compressed or buried, along fault planes, along subduction zones, next to magma pockets as the neighboring rock is cooked, to name a few.
  • Marble is metamorphosed limestone (a chemical sedimentary rock). Slate was once used as the backing of chalkboards.
  • Any type of rock can be metamorphosed, including metamorphic rocks.
Metamorphic Rocks in India
  • The gneisses and schists are commonly found in the Himalayas, Assam, West Bengal, Bihar, Orissa, Madhya Pradesh and Rajasthan.
  • Quartzite is a hard rock found over Rajasthan, Bihar, Madhya Pradesh, Tamil Nadu and areas surrounding Delhi.
  • Marble occurs near Alwar, Ajmer, Jaipur, Jodhpur in Rajasthan and parts of Narmada Valley in Madhya Pradesh.
  • Slate, which is used as a roofing material and for writing in schools, is found over Rewari (Haryana), Kangra (Himachal Pradesh) and parts of Bihar.
  • Graphite is found in Orissa and Andhra Pradesh.

Types of Rocks,Types of Rocks

Read More : The Earth’s Interior : Sources

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