Rocks And Their Types | What are Rocks

Definition of Rocks, What is Rocks, Types of Rocks, Igneous Rocks, Sedimentary Rocks, Metamorphic Rocks

Rocks and their Types (What are Rocks)

Rocks are generally a mixture of two or more minerals and do not possess a definite chemical composition.

Major Types of Rocks

On the basis of their mode of formation, rocks can be classified into the following three types:

1. Igneous rocks
2. Sedimentary rocks
3. Metamorphic rocks

1. Igneous Rocks

The word igneous is derive from the Latin word ‘ignis’ meaning fire. These rocks are of thermal origin and are associate with volcanic eruptions. These rocks have been form due to solidification of hot and molten material call as magma.

It is believe that at the time of its birth the Earth was in a molten state. The igneous rocks were the first to be form as a result of the solidification of the outer layer of the Earth. Igneous rocks are also known as the primary rocks. They can be divide into two types intrusive igneous rocks and extrusive igneous rocks.

Igneous that cool below the surface of the Earth are call as intrusive igneous. The rate of cooling is slow inside the Earth. Thus the crystals form on cooling are large. Two common examples of intrusive rocks are dolerite and granite. Igneous rocks that cool on the surface of the Earth are call as extrusive igneous rocks. These rocks are also known as volcanic rocks. Due to rapid cooling, the crystals are fine grained such as in basalt.

On the basis of their composition the igneous rocks are also classified as acidic Igneous Rocks and Basic igneous rocks. In Acidic Igneous rocks silica content in rocks is more than 65 per cent. These rocks are light coloured and have less density. These are also known as ‘Silicic rocks’. Granite and rhyolite are examples of these rocks. In Basic Igneous rocks the silica content is less than 65 per cent. They are composed predominantly of ferromagnesian minerals (rich in iron and magnesium). They are dark coloured and dense. Gabbro and basalt are basic rocks.

Characteristics of Igneous rocks

They are compact and massive and do not possess rounded particles.
  do not occur in distinct beds or strata.
They are generally granular and crystalline.
It has hard and impermeable
They are less affected by chemical weathering.
They do not contain any fossils or traces of animals or plants.
Most of the igneous rocks consist of silicate minerals.
The valuable minerals such as iron, gold, silver, aluminium, etc., are found in them.

Economic Importance of Igneous Rocks

They are a reservoir of minerals.
Majority of metallic minerals are found in igneous rocks.
Economically important minerals are found in these rocks-Magnetic iron, nickel, copper, lead, zinc, chromite, manganese, tin.
Rare minerals like gold, diamonds, platinum are also found in these rocks.
Basalt and granite are used for construction of buildings and roads.
The formation of black soils is probably the result of erosion of these rocks. These soils are very much suited to cultivation of cotton and some other crops.

Rocks and their Types

2. Sedimentary Rocks

The word sedimentary has been derive from the Latin word ‘sedimentum’, meaning settling down. Rain, wind, ice, running water, plants and animals constantly break the rocks (Rocks and their types) into fragments of all sizes. These broken rock materials carry away by wind, ice and running water, and deposit in the depressions. The deposit materials are called sediments, and they give rise to sedimentary rocks.

The sediments are generally deposit in horizontal layers or stratas. Thus these rocks are also refer to as stratified rocks. The loose materials are convert into hard and compact rocks such as shale and sandstone. This is due to the pressure exert from the top or because of cementation.

The sedimentary rocks can be formed mechanically (sandstone), chemically (gypsum or salt) or organically (coal, limestone). The sedimentary rocks are most widespread and cover about 75 per cent of the total land area on the earth.

Characteristics of sedimentary rocks

They are comparatively softer than the igneous rocks.
They are made up of minute particles of various shapes and sizes.
– Modern industries depend on the products from the sedimentary rocks.

They have layers horizontally arranged one above the other.
They have been mostly formed under water.
– Of all the sedimentary deposits, coal and petroleum are the most important ones.

They have mud cracks and marks of ripples and waves.
They have fossils between the layers. – Most of them are permeable and porous.

Economic Importance of Sedimentary Rocks

It is true that they contain lesser minerals than in the igneous. But iron ore, phosphates, building stones, coal, raw materials for cement and bauxite are obtain from these.
Mineral oil and Natural Gas is also obtained from sedimentary. In India there is a possibility of finding oil fields in the sedimentary rock strata of the sub-Himalayan zone, in the delta regions of Ganga, Kaveri, Godavari and Krishna rivers, Rann of Kutch and the Gulf of Khambhat. The mineral oil commonly known as petroleum is form by the decay of tiny marine organisms (in contrast Coal is formed from dead plant) between to impermeable rocks.
Sandstone, limestone are used in construction of buildings. The forts of Agra, Delhi and  Fatehpur Sikri are built of red sandstone
Fertile Soils: The Indus and Ganga basins are also made of sedimentary rocks. Their alluvial soils are highly fertile.

Metamorphic Rocks

The word metamorphic means ‘changed form’. The rocks, originating at or near the surface of the Earth (Rocks and their types) are sometimes subject to tremendous heat and pressure. This can change the original properties of rocks such as their colour, hardness, texture and mineral composition. Such change rocks are call as metamorphic rocks. Both igneous and sedimentary can change into metamorphic rocks. The special feature about the origin and formation of metamorphic rocks is that they remain in their original position and change under the impact of internal and external forces. Metamorphism can be of thermal and dynamic origin.

1. In the case of thermal metamorphism, the original rocks are change under the influence of high temperature inside the Earth’s crust. For example, limestone is convert into marble, sandstone into quartzite, shale into slate and coal into graphite.

2. In the case of dynamic metamorphism, the original rocks are change under the influence of pressure at great depths inside the Earth’s crust. For example, granite is convert into gneiss and shale into schist.

Characteristics of metamorphic rocks

They are usually hard.
It has a high specific gravity.
They may be banded.
They do not have void spaces in them.

Economic Importance of Metamorphic Rocks

1. Building Construction Materials: Gneiss, quartzite, slate, marble are use as building materials. In India marble is available in Alwar, Ajmer, Jodhpur and Jaipur districts of Rajasthan. The thick sheet of slate is use for laying the surface of billiards table. Slate appears in parts of Riwari (Haryana), Karigra (H.P) and Bihar.
2. Industrial Uses: Graphite is use for making pencils. Its melting point is 3500°C. Therefore pots for melting of metals are made of graphite. Graphite is indispensible for atomic energy power house Quartzite, one of the hard rocks, use in glass making.
3. Beauty Aids: Steatite is use for making talcum powder and other such beauty aids.
4. Asbestos is fire resistant.
5. Garnet: It is a precious stone. It is use for making abrasives

Rock Cycle

Rock cycle is the intimate relationship and mutual interdependence between the three types of rocks—igneous, sedimentary and metamorphic. The change of one type of rock into another type under different conditions is call as the rock cycle.

In the cycle of rock change, the materials of the lithosphere are constantly being form and transform in both their physical and mineral composition. The rock cycle has neither a beginning nor an end.

There are two environments for the working of a rock cycle, such as:

(a) a surface environment of low temperature and pressure
(b) a deep environment of high temperature and pressure.

The leftover igneous and the newly created sedimentary are likely to change into metamorphic due to heat and pressure in course of time. The formation of sedimentary  on the Earth’s surface and its conversion into metamorphic rock takes place within the crust of the Earth. The sedimentary rocks may be bury again and may melt to form the igneous rocks. In the rock cycle, the matter of the Earth’s crust is not lost. The cycle of rock change has been active since our planet became a solid.
The loops in the cycle of rock change are power by two main sources of energy such as:
the heat inside the Earth, which can melt the existing rocks; and
the solar energy responsible for weathering and erosion, and finally converting them into sedimentary rocks.

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