Rocks

• 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 — solidified from magma and lava.
• Sedimentary Rocks — the result of deposition of fragments of rocks.
• Metamorphic Rocks — formed out of existing rocks undergoing recrystallisation.

Igneous Rocks or Primary Rocks

• The solidification of magma formed the first rocks on earth.
• Rocks formed out of solidification of magma (molten rock below the surface) and lava (molten rock above the surface) and are known as igneous or primary rocks.
• Having their origin under conditions of high temperatures the igneous rocks are unfossiliferous.
• Granite, gabbro, basalt, are some of the examples of igneous rocks.
• There are three types of igneous rocks based on place and time taken in cooling of the molten matter, plutonic rocks, volcanic rocks and intermediate rocks.
• There are two types of rocks based on the presence of acid-forming radical, silicon, acidic rocks and basic rocks.

Intrusive igneous rocks (Plutonic rocks)

• If magma cools slowly at great depths, mineral grains formed in the rocks may be very large.
• Such rocks are called intrusive rocks or plutonic rocks (e.g. Granite).
• These rocks appear on the surface only after being uplifted and denuded.

Extrusive igneous rocks (Lava or Volcanic rocks)

• Sudden cooling of magma just below the surface or lava above the surface results in small and smooth grains in rocks as rapid cooling prevents crystallisation, as a result, such rocks are fine-grained.
• Such rocks are called extrusive rocks or volcanic rocks (e.g. Basalt).
• The Deccan traps in the Indian peninsular region is of basaltic origin.
• Basic rocks contain a greater proportion of basic oxides, e.g. of iron, aluminium or magnesium, and are thus denser and darker in colour.

Hypabyssal or Dyke Rocks or Intermediate rocks

• These rocks occupy an intermediate position between the deep-seated plutonic bodies and the surface lava flows.
• Dyke rocks are semi-crystalline in structure.

Acidic Rocks

• Acidic rocks are characterised by high content of silica (quartz and feldspar) — up to 80 per cent.
• The rest is divided among aluminium, alkalis, magnesium, iron oxide, lime etc.
• These rocks have a lesser content of heavier minerals like iron and magnesium. Hence, they are less dense and are lighter in colour than basic rocks.
• These rocks constitute the sial portion of the crust.
• Due to the excess of silicon, acidic magma cools fast, and it does not flow and spread far away.
• High mountains are formed of this type of rock.
• Add rocks are hard, compact, massive and resistant to weathering.
• Granite, quartz and feldspar are typical examples.

Basic Rocks

• These rocks are poor in silica (about 40 per cent); magnesia content is up to 40 per cent, and the remaining is spread over iron oxide, lime, aluminium, alkalis, potassium etc.
• Due to low silica content, the parent material of such rocks cools slowly and thus, flows and spreads far away. This flow and cooling give rise to plateaus.
• Presence of heavy elements imparts to these rocks a dark colour. Not being very hard, these rocks are weathered relatively easily.
• Basalt, gabbro and dolerite are typical examples.

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 or Detrital Rocks

• Sedimentary rocks are formed by lithification ? consolidation and compaction of sediments.
• Hence, they are layered or stratified of varying thickness. Example: sandstone, shale etc.
• Sediments are a result of denudation (weathering and erosion) of all types of rocks.
• These types of rocks cover 75 per cent of the earth’s crust but volumetrically occupy only 5 per cent (because they are available only in the upper part of the crust).
• Ice deposited sedimentary rocks are called till or tillite. Wind-deposited sediments are called loess.

Depending upon the mode of formation, sedimentary rocks are classified into:

1. mechanically formed — sandstone, conglomerate, limestone, shale, loess.
2. organically formed — geyserite, chalk, limestone, coal.
3. chemically formed — limestone, halite, potash.

Mechanically Formed Sedimentary Rocks

• They are formed by mechanical agents like running water, wind, ocean currents, ice, etc.
• Arenaceous sedimentary rocks have more sand and bigger sized particles and are hard and porous. They form the best reservoirs for liquids like groundwater and petroleum. E.g. sandstone.
• Argillaceous rocks have more clay and are fine-grained, softer, mostly impermeable (mostly non-porous or have very tiny pores). E.g. claystone and shales are predominantly argillaceous.

Chemically Formed Sedimentary Rocks

• Water containing minerals evaporate at the mouth of springs or salt lakes and give rise to Stalactites and stalagmites (deposits of lime left over by the lime-mixed water as it evaporates in the underground caves).

Organically Formed Sedimentary Rocks

• The remains of plants and animals are buried under sediments, and due to heat and pressure from overlying layers, their composition changes. Coal and limestone are well-known examples.
• Depending on the predominance of calcium content or the carbon content, sedimentary rocks may be calcareous (limestone, chalk, dolomite) or carbonaceous (coal).

Chief Characteristics of Sedimentary Rocks

• They are stratified ? consist of many layers or strata.
• They hold the most informative geological records due to the marks left behind by various geophysical (weather patterns, wind and water flow) and biological activities (fossils).
• They are fossiliferous ? have fossils of plants and animals.
• These rocks are generally porous and allow water to percolate through them.

The spread of Sedimentary Rocks in India

• Alluvial deposits in the Indo-Gangetic plain and coastal plains is of sedimentary accumulation. These deposits contain loam and clay.
• Different varieties of sandstone are spread over Madhya Pradesh, eastern Rajasthan, parts of Himalayas, Andhra Pradesh, Bihar and Orissa.
• The great Vindhyan highland in central India consists of sandstones, shales, limestones.
• Coal deposits occur in river basins of the Damodar, Mahanadi, the Godavari in the Gondwana sedimentary deposits.

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.
• Sedimentary rocks also yield some of the richest soils.

Metamorphic Rocks

• The word metamorphic means ‘change of form’.
• Metamorphism is a process by which recrystallisation and reorganisation of minerals occur within a rock. This occurs due to pressure, volume and temperature changes.
• 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, metamorphosis occurs.
• In the process of metamorphism in some rocks grains or minerals get arranged in layers or lines. Such an arrangement is called foliation or lineation.
• Sometimes minerals or materials of different groups are arranged into alternating thin to thick layers. Such a structure in is called banding.
• Gneissoid, slate, schist, marble, quartzite etc. are some examples of metamorphic rocks.

Causes of Metamorphism

• Orogenic (Mountain Building) Movements: Such movements often take place with an interplay of folding, warping and high temperatures. These processes give existing rocks a new appearance.
• Lava Inflow: The molten magmatic material inside the earth’s crust brings the surrounding rocks under the influence of intense temperature pressure and causes changes in them.
• Geodynamic Forces: The omnipresent geodynamic forces such as plate tectonics also play an important role in metamorphism.

On the basis of the agency of metamorphism, metamorphic rocks can be of two types

Thermal Metamorphism

• The change of form or re-crystallisation of minerals of sedimentary and igneous rocks under the influence of high temperatures is known as thermal metamorphism.
• A magmatic intrusion causing thermal metamorphism is responsible for the peak of Mount Everest consisting of metamorphosed limestone.
• As a result of thermal metamorphism, sandstone changes into quartzite and limestone into marble.

Dynamic Metamorphism

• This refers to the formation of metamorphic rocks under high pressure.
• Sometimes high pressure is accompanied by high temperatures and the action of chemically charged water.
• The combination of directed pressure and heat is very powerful in producing metamorphism because it leads to more or less complete recrystallisation of rocks and the production of new structures. This is known as dynamo thermal metamorphism.
• Under high pressure, granite is converted into gneiss; clay and shale are transformed into schist.

Some examples of Metamorphosis

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.

Rock cycle

• Rock cycle is a continuous process through which old rocks are transformed into new ones.
• Igneous rocks are primary rocks, and other rocks form from these rocks.
• Igneous rocks can be changed into sedimentary or metamorphic rocks.
• The fragments derived out of igneous and metamorphic rocks form into sedimentary rocks.
• Sedimentary and igneous rocks themselves can turn into metamorphic rocks.
• The crustal rocks (igneous, metamorphic and sedimentary) may be carried down into the mantle (interior of the earth) through subduction process and the same meltdown and turn into molten magma, the source for igneous rocks

Some Rock-Forming Minerals

• Feldspar: Half the crust is composed of feldspar. It has a light colour, and its main constituents are silicon, oxygen, sodium, potassium, calcium, aluminium. It is used for ceramics and gloss making.
• Quartz: It has two elements, silicon and oxygen. It has a hexagonal crystalline structure. It is uncleaved, white or colourless. It cracks like glass and is present in sand and granite. It is used in the manufacture of radio and radar.
• Bauxite: A hydrous oxide of aluminium, it is the ore of aluminium. It is non-crystalline and occurs in small pellets.
• Cinnabar (mercury sulphide): Mercury is derived from it. It has a brownish colour.
• Dolomite: A double carbonate of calcium and magnesium. It is used in cement and iron and steel industries. It is white.
• Gypsum: It is hydrous calcium sulphate and is used in cement, fertiliser and chemical industries.
• Haematite: It is a red ore of iron.
• Magnetite: It is the black ore (or iron oxide) of iron.
• Amphibole: It forms about 7 per cent of the earth’s crust and consists mainly of aluminium, calcium, silica, iron, magnesium, etc. It is used in the asbestos industry.
• Mica: It consists of potassium, aluminium, magnesium, iron, silica, etc., and forms 4 % of the earth’s crust. It is generally found in igneous and metamorphic rocks and is mainly used in electrical instruments.
• Olivine: The main elements of olivine are magnesium, iron and silica. It is normally a greenish crystal.
• Pyroxene: It consists of calcium, aluminium, magnesium, iron and silica. It is of green or black colour.
• Other minerals like chlorite, calcite, magnetite, hematite, bauxite, barite, etc., are also present in rocks.