UPSC Geography Minerals and Energy Resources / खनिज और ऊर्जा संसाधन NCERT Extracts - Minerals and Rocks

 The Major Elements of the Earth's Crust

• The earth is composed of various kinds of elements. These elements are in solid form in the outer layer of the earth and in hot and molten form in the interior.
• About 98 per cent of the total crust of the earth is composed of eight elements like oxygen, silicon, aluminium, iron, calcium, sodium, potassium and magnesium and the rest is constituted by titanium, hydrogen, phosphorous, manganese, sulphur, carbon, nickel and other elements.
• The elements in the earth's crust are rarely found exclusively but are usually combined with other elements to make various substances. These substances are recognized as minerals.
• Though the number of elements making up the lithosphere are limited they are combined in many different ways to make up many varieties of minerals.
• There are at least 2,000 minerals that have been named and identified in the earth crust; but almost all the commonly occurring ones are related to six major mineral groups that are known as major rock forming minerals.
• The basic source of all minerals is the hot magma in the interior of the earth.
• When magma cools, crystals of minerals appear and a systematic series of minerals are formed in sequence to solidify so as to form rocks.

Some Major Minerals and their Characteristics

Feldspar

• Silicon and oxygen are common elements in all types of feldspar and sodium, potassium, calcium, aluminium etc. are found in specific feldspar variety.
• Half of the earth's crust is composed of feldspar. It has light cream to salmon pink colour. It is used in ceramics and glass making.

Quartz

• It is one of the most important components of sand and granite. It consists of silica.
• It is a hard mineral virtually insoluble in water.
• It is white or colourless and used in radio and radar. It is one of the most importan components of granite.

Pyroxene

• Pyroxene consists of calcium, aluminum, magnesium, iron and silica.
• Pyroxene forms 10 per cent of the earth's crust.
• It is commonly found in meteorites.
• It is in green or black colour.

Amphibole

• Aluminium, calcium, silica, iron, magnesium are the major elements of amphiboles
• They form 7 per cent of the earth's crust.
• It is in green or black colour and is used in asbestos industry,
• Hornblende is another form of amphiboles.   

Mica

• It comprises of potassium, aluminium, magnesium, iron, silica etc.
• It forms 4 per cent of the earth's crust. It is commonly found in igneous aim metamorphic rocks.
• is used in electrical instruments.

Olivine

• Magnesium, iron and silica are major elements of olivine.
• It is used in jewellery.                                
• It is usually a greenish crystal, often found in basaltic rocks.
• Besides these main minerals, other minerals like chlorite, calcite, magnetite, haematic bauxite and barite are also present in some quantities in the rocks. 

 Rocks

• The earth's crust is composed of rocks. A rock is an aggregate of one or more minerals
• Rock may be hard or soft and in varied colours. For example, granite is hard, soapstone is soft. Gabbro is black and quartzite can be milky white.
• Rocks do not have definite composition of mineral constituents.
• Feldspar and quartz are the most common minerals found in rocks.

 Igneous Rocks

• As igneous rocks form out of magma and lava from the interior of the earth, they are known as primary rocks.
• The igneous rocks (Ignis - in Latin means 'Fire') are formed when magma cools solidifies.
• When magma in its upward movement cools and turns into solid form it is called igneous rock.
• The process of cooling and solidification can happen in the earth's crust or on the surface of the earth.
• Igneous rocks are classified based on texture. Texture depends upon size and arrangement of grains or other physical conditions of the materials.
• If molten material is cooled slowly at great depths, mineral grains may be very large.
• Sudden cooling (at the surface) results in small and smooth grains. Intermediate conditions of cooling would result in intermediate sizes of grains making up igneous rocks. Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some of the examples of igneous rocks.

Sedimentary Rocks

• T'he word 'sedimentary' is derived from the Latin word sedimentum, which means settling.
• Rocks (igneous, sedimentary and metamorphic) of the earth's surface are exposed to denudational agents, and are broken up into various sizes of fragments.
• Such fragments are transported by different exogenous agencies and deposited.
• These deposits through compaction turn into rocks. This process is called lithification.
• In many sedimentary rocks, the layers of deposits retain their characteristics even after lithification.
• Hence, we see a number of layers of varying thickness in sedimentary rocks like sandstone, shale etc.
• Depending upon the mode of formation, sedimentary rocks are classified into three major groups:
• mechanically formed - sandstone, conglomerate, limestone, shale, loess etc. are examples;
• organically formed - geyserite, chalk, limestone, coal etc. are some examples;
• chemically formed - chert, limestone, halite, potash etc. are some examples.

Metamorphic Rocks

• The word metamorphic means 'change of form'.
• These rocks form under the action of pressure, volume and temperature (PVT) changes.
• 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 or the underlying rocks are subjected to great amounts of pressure by overlying rocks.
• Metamorphism is a process by which already consolidated rocks undergo recrystallisation and reorganisation of materials within original rocks.
• al disruption and reorganisation of the original minerals within rocks due to breaking and crushing without any appreciable chemical changes is called dynamic metamorphism.
• The materials of rocks chemically alter and recrystallise due to thermal metamorphism.
• There are two types of thermal metamorphism - contact metamorphism and regional metamorphism.
• In contact metamorphism the rocks come in contact with hot intruding magma and lava and the rock materials recrystallise under high temperatures.
• Quite often new materials form out of magma or lava are added to the rocks.
• In regional metamorphism, rocks undergo recrystallisation due to deformation caused by ectonic shearing together with high temperature or pressure or both.
• In the process of metamorphism in some rocks grains or minerals get arranged layers or lines.
• Such an arrangement of minerals or grains in metamorphic rocks is called foliation or lineation.
• Sometimes minerals or materials of different groups are arranged into alternating thin to thick layers appearing in light and dark shades.
• Such a structure in metamorphic rocks is called banding and rocks displaying banding are called banded rocks.                                
• Types of metamorphic rocks depend upon original rocks that were subjected to metamorphism.
• Metamorphic rocks are classified into two major groups - foliated rocks and non foliated rocks.
• Gneissoid, granite, syenite, slate, schist, marble, quartzite etc. are some examples of metamorphic rocks.

Rock Cycle

• Rocks do not remain in their original form for long but may undergo transformation
• Rock cycle is a continuous process through which old rocks are transformed into new ones.
• Igneous rocks are primary rocks and other rocks (sedimentary and metamorphic form from these primary rocks.
• Igneous rocks can be changed into metamorphic rocks.
• The fragments derived out of igneous and metamorphic rocks form into sedimentary rocks.
• Sedimentary rocks themselves can turn into fragments and the fragments can be a source for formation of sedimentary rocks.
• The crustal rocks (igneous, metamorphic and sedimentary) once formed may be carried down into themantle (interior of the earth) through seduction process (parts or whole of crustal plates going down under another plate in zones of plate convergence) and the same melt down due to increase in temperature in the interior and turn into molten magma, the original source for igneous rocks.

Some Important Facts

• The earth, is a dynamic planet. It is constantly undergoing changes inside and outside
• Just like an onion, the earth is made up of several concentric layers with one inside another.
• The uppermost layer over the earth's surface is called the crust. It is the thinnest of all me layers. It is about 35 km on the continental masses and only 5 km on the ocean floor
• The main- mineral constituents of the continental mass are silica and alumina. It is thus called sial (si-silica and al-alumina).
• The oceanic crust mainly consists of silica and magnesium; it is therefore called sima (si - silica and ma - magnesium).
• Just beneath the crust is the mantle which extends up to a depth of 2,900 km below the crust. The innermost layer is the core with a radius of about 3,500 km.
• It is mainly made up of nickel and iron and is called nife (ni - nickel and fe – ferrous i.e. iron). The central core has very high temperature and pressure.
• The earth's crust is made up of various types of rocks. Any natural mass of mineral matter that makes up the earth's crust is called a rock. Rocks can be of different colour, size and texture.
• There are three major types of rocks : igneous rocks, sedimentary rocks and metamorphic rocks.
• When the molten magma cools, it becomes solid. Rocks thus formed are called igneous rocks. They are also called primary rocks. There are two types of igneous rocks: intrusive rocks and extrusive rocks.
• Lava is actually fiery red molten magma coming out from the interior of the earth on its surface. When this molten lava comes on the earth's surface, it rapidly cools down and becomes solid. Rocks formed in such a way on the crust are called extrusive igneous rocks.
• They have a very fine grainedstructure. For example, basalt. The Deccan plateau is made up of basalt rocks.
• Sometimes the molten magma cools down deep inside the earth's crust. Solid rocks so formed are called intrusive igneous rocks.
• Since they cool down slowly they form large grains. Granite is an example of such a rock. Grinding stones used to prepare paste/powder of spices and grains are made of granite.
• Rocks roll down, crack, and hit each other and are broken down into small fragments. These smaller particles are called sediments.
• These sediments are transported and deposited by wind, water, etc. These loose sediments are compressed and hardened to form layers of rocks. These types of rocks are called sedimentary rocks. For example, sandstone is made from grains of sand.
• These rocks may also contain fossils of plants, animals and other micro-organisms that once lived on them.
• Igneous and sedimentary rocks can change into metamorphic rocks under great heat and For example, clay changes into slate and limestone into marble.
• The deepest mine in the world, is in South Africa. It is about 4 km deep. In search for oil engineers have dug a hole about 6 km deep.
• To reach to the centre of the earth (which is not possible!) you will haye to dig a hole 6,000 kmdeep on the ocean floor.
• The crust forms only 1 per cent of the volume of the earth, 84 per cent consists of the mantle and 15 per cent makes the core. The radius of the earth is 6,371 km.
• Igneous: Latin word Ignis meaning fire.
• Sedimentary: Latin word sedimentum meaning settle down,
• Metamorphic: Greek word metamorphose meaning change of form.
• Fossils; The remains of the dead plants and animals trapped in the layers of rocks are called fossils.
• Switzerland has no known mineral deposit in it.
• A green diamond is the rarest diamond.
• The oldest rocks in the world are in Western Australia. They date from 4,300 million years ago, only 300 million years after the earth was formed.
• Brazil is the largest producer of high grade iron-ore in the world.
• The coal which we are using today was formed millions of years ago when giant ferns and swamps got buried under the layers of earth.
• Petroleum and its derivatives are called Black Gold as they are very valuable.
• Norway was the fast country in the world to devlop hydroelectricity.
• The site of the world's fast solar and wind powered bus shelter is in Scotland.
• The fast tidal energy station was built in France.
• USA has the world's largest geothermal power plants followed by New Zealand, Iceland Philippines and Central America.
• In India, geothermal plants are located in Manikaran in Himachal Pradesh and Puga Valley in Ladakh.