Types of Rocks, Economic Significance of Rocks Part 2

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TYPES of ROCKS

On the basis of their mode of formation, rocks may be grouped into three types:

Igneous Rocks: The word igneous is derived from the Latin word ‘ignis’ meaning fire. Igneous rocks are formed by the cooling of highly heated molten fluid material, known as magma. It requires a greater quantity of heat to melt the rocks under overlying pressure than at the surface. Molten rocks produce an increase in volume resulting in fractures or cracks in the crust. The overlying pressure gets weakened along these openings, thus forcing out the magma through them.

When magma is ejected to the surface, it is called lava. Igneous rocks comprise the earth’s first crust and all other rocks are derived from them. Thus, these are called the parent of all rocks or the ‘primary rocks’. About 95% of the volume of outermost 16 km of the earth is composed of them.

On the basis of their mode of occurrence, they can be classified as:

Extrusive Igneous Rocks: They are formed by cooling of lava on the earth’s surface. The mineral crystals forming these rocks are very fine. These rocks are also called volcanic rocks. For example, gabbro and basalt. Deccan plateau’s regur soil in India is derived from lava.

Intrusive Igneous Rocks: They are formed when magma solidifies below the earth’s surface. The rate of cooling below the earth’s surface is very slow which gives rise to formation of large crystals in the rocks. Deep seated intrusive rocks are termed as plutonic rocks and shallow depth intrusive rocks are termed as hypabyssal. For example, granite and dolerite.

The igneous rocks in accordance with their mode of formation can be classified as (a) Plutonic, (b) Hypabyssal and (c) Volcanic rock masses.

Common forms of intrusive igneous rocks are batholiths, sills and dykes etc. Batholiths are huge masses of solidified magma. They generally form the core of the major mountains. Their irregular dome shaped roofs sometimes appear on the surface after erosion of millions of years. Sill is the horizontal intrusion of solidified magma between the layers of pre-existing rocks. Dyke is similarly a more or less vertical formation from few metres to several kilometres in length and from few centimetres to hundreds of metres in thickness.

On the basis of chemical properties, igneous rocks are classified into acidic and basic rocks. Acidic igneous rocks are composed of 65% or more of silica. These rocks are light coloured, hard and very strong. For example, granite. Basic igneous rocks contain less than 55% of silica and have more of iron and magnesium. These rocks are dark coloured and weak enough for weathering. For example, gabbro, basalt and dolerite.

Forms of intrusive igneous rocks

Forms of Intrusive Igneous Rocks

Forms of intrusive igneous rocks

Sedimentary Rocks: These rocks are formed by successive deposition of sediments. Sedimentary rocks have layered or stratified structure. The thickness of the strata varies from few millimetres to several metres. Generally, these rocks have some type of fossils between their strata. Fossil is the solid part or an impression of a prehistoric animal or plant embedded in strata of sedimentary rocks.

The individual rock particles are first broken from rocks and then transported by running water, ocean currents and glaciers or even by wind from one place to another. The process by which rock forming material is laid down is called sedimentation or deposition. These are identified as riverine, lacustrine (formed by lake), glacial or Aeolian (formed by wind) sedimentary rocks with reference to their deposition near rivers, lakes, glacier or deserts respectively. The sediments are often loose, unconsolidated, soft rock material, in the beginning like sand and clay, but in the course of time they get hardened to a compact material by excessive pressure and cementation to form sedimentary rocks. For example, sandstone, shale, limestone and dolomite.

Sediments of different sizes may get bound by cementing material under suitable conditions. Conglomerate is formed of consolidated material and is termed as mechanically formed sedimentary rock. The consolidation of organic matter derived from plants and animals forms sedimentary rocks of organic origin. For example, coal and limestone. Direct precipitation of minerals from their solution in water may give rise to sedimentary rocks of chemical origin. For example, gypsum, rock salt and nitre.

The Fold Mountains of the world like Himalayas and Andes are made up of sedimentary rocks. The river basins, particularly their plains and deltas, such as Indo-Gangetic plain and Ganga-Brahmaputra delta are good examples of sedimentary accumulations.

Metamorphic Rocks: They are formed under the influence of heat or pressure on sedimentary or igneous rocks. Tremendous pressure and high temperature change the colour, hardness, structure and composition of all types of the pre-existing rocks. The process which brings about the change is known as metamorphism and the ultimate products formed are defined as the metamorphic rocks.

Temperature, pressure stress and access of chemically reactive substances are the main agents, of metamorphism. Heat causes the minerals to recrystallise in the rock. The process of change by heat is called thermal or contact metamorphism. The formation of metamorphic rocks due to tremendous pressure is known as dynamic or regional metamorphism. Slate, gneiss, schist, marble and diamond are some examples of metamorphic rocks. Metamorphic rocks are hard and tough in comparison to the parent rocks from which they are formed. Different types of metamorphic rocks are found all over the world. In India, marble is found in Rajasthan, Bihar and Madhya Pradesh, whereas slates are available in Orissa, Andhra Pradesh and Haryana.

ECONOMIC SIGNIFICANCE of ROCKS

Man has been interacting with the surface of the earth since long. With the advancement in technology man is making different uses of rocks and minerals. The importance of rocks can be seen as under the following heads:

Soils: Soils are derived from rocks. Soils provide suitability for agricultural produce that provide food for man and provide raw material for many industries.

Building Material: Rocks are the source of building materials. Granite, gneiss, sandstone, marble and slates are extensively used for the construction of buildings. The Tajmahal is made of white marble and the Red Fort of Delhi are made of red sandstone.

Mineral Source: Metallic minerals that are obtained from different rocks provide all metals ranging from very precious gold, platinum, silver, copper to aluminium and iron.

Raw Material: Certain rocks and minerals are used as raw material for many industries. In cement industry and limestone kilns different type of rocks and minerals are used. Graphite is used in crucible and pencil manufacturing as raw materials.

Fuel: Fuel in the form of coal, petroleum, natural gas, and nuclear minerals are derived from different rocks.

Fertilizer: Fertilizers are also derived from rocks. Phosphatic fertilizers are obtained from phosphorite mineral found in abundance in some parts of the world.