ROCKS
ROCKS AND MINERALS
Elements in the Earth’s Crust-
-The earth is composed of various kinds of elements.
-About 98% of the total crust is made up of eight elements as oxygen, silicon, aluminium, iron, calcium, sodium, potassium, and magnesium.
-The rest is constituted by elements like titanium, hydrogen, phosphorous, manganese, sulphur, carbon, nickel and others.
-The elements in the earth’s crust are rarely found exclusive but are usually combined with other elements to make various substances.
-These substances are recognised as minerals.
-The table below shows the percentage wise share of each element in both the earth’s crust and the whole earth.
Earth Crust | Entire Earth |
Oxygen-47% | Iron-35% |
Silicon-28% | Oxygen-30% |
Aluminium-8% | Silicon-15% |
Iron-5% | Magnesium-13% |
Calcium-2% | Nickle-2% |
Potassium-2% | Calcium-1% |
Sodium-2% | Aluminium-1% |
others-2% | others-1% |
Minerals in the Earth’s Crust-
-A mineral is a naturally occurring organic or inorganic substance, having an orderly atomic structure and a definite chemical composition and physical properties.
-A mineral is composed of two or more elements. But, sometimes single element minerals like sulphur, copper, silver, gold, graphite, etc are also found.
-The basic source of all minerals is the hot magma in the interior of the earth.
-When magma cools, crystals of the minerals appear and a systematic series of minerals are formed in sequence to solidify so as to form rocks.
-The minerals which contain metals are called as metallic minerals (eg: Haematite) and the metallic minerals which are profitably mined are called as the ores.
-The crust of the earth is made up of more than 2000 minerals, but out of these, only six are the most abundant and contribute the maximum.
-These six most abundant minerals are feldspar, quartz, pyroxenes, amphiboles, mica and olivine
Characteristics of some of the major minerals-
1) Feldspar:
-Silicon and oxygen are major elements of all types of feldspar.
-Sodium, potassium, calcium, aluminium, etc are found in specific feldspar varieties.
-Half of the earth’s crust is composed of feldspar (plagioclase (39%) and alkali feldspar (12%)).
-It has light cream to salmon pink colour.
-It is commonly used in ceramics and glass making.
2)Quartz:
-It is one of the most important components of sand and granite.
-It consists of silica and it is a hard mineral virtually insoluble in water.
-It is usually white or colourless.
-They are used in the manufacturing of radio, radar, etc.
3)Pyroxene:
-The common elements in pyroxene are Calcium, aluminium, magnesium, iron and silicon.
-About 10% of the earth’s crust is made up of pyroxene.
-It is commonly found in meteorites.
-Its colour is usually green or black.
4)Amphibole:
-Aluminium, calcium, silicon, iron and magnesium are the major elements of amphiboles.
-They form 7% of the earth’s crust.
-It is green or black in colour and is used in asbestos industries commonly.
-Hornblende is another form of amphiboles.
5)Mica:
-It is made up of elements like potassium, aluminium, magnesium, iron, silicon, etc.
-It forms 4% of the earth’s crust.
-It is commonly found in igneous and metamorphic rocks.
-Mica is widely used in electronic instruments.
6)Olivine:
-Magnesium, iron and silica are the major elements of olivine.
-It is commonly found in basaltic rocks with a greenish colour.
-Olivine is used commonly in jewellery.
ROCKS in the earth crust-
-A rock is nothing but a composition of minerals.
-They are aggregates or a physical mixture of one or more minerals.
-Rocks may be hard or soft and in varied colours.
-Feldspar and quartz are the most common minerals found in rocks.
-Petrology is science of rocks.
Rocks → Material that form crust of earth
Classification of Rocks-
-As we said above, rocks differ in their properties, the size of particles and mode of formation.
-On the basis of mode of formation, rocks may be classified into three:
1) Igneous Rocks — solidified from magma and lava.
2) Sedimentary Rocks — the result of deposition of fragments of rocks.
3) Metamorphic Rocks — formed out of existing rocks undergoing recrystallization.
Igneous Rocks:
-Igneous rocks are formed by the cooling of highly heated molten fluid material called as Magma.-Asthenosphere, which is just below the upper mantle, a region beneath Lithosphere is the main source of magma.
-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.
-They might be formed directly by cooling of magma from the interior of the earth itself or by cooling of lava from the surface of the earth.
-As they comprise the earth’s first crust and all other rocks are derived from them, they are also called as the parents of all rocks or the Primary Rocks.
-They are the most abundant rocks in the earth’s crust.
-Granite, gabbro, pegmatite, basalt, etc.are some of the examples of igneous rocks.
-On the basis of their mode of occurrence, igneous rocks can be classified as Intrusive Igneous(Granite) and Extrusive Igneous(Basalt-Deccan Traps) Rocks.
-Having their origin under conditions of high temperatures, the igneous rocks are Acid igneous rocks, such as granite, are less dense and are lighter in colour than basic rocks.
Based on place and time taken in cooling of the molten matter, igneous rocks can be divided into Plutonic and Volcanic rocks.
Plutonic Rocks or intrusive 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 the formation of large crystals in the rocks.
-That is, the mineral grains of intrusive igneous rocks are very large.
-Deep-seated intrusive igneous rocks are called as Plutonic rocks and shallow depth intrusive igneous rocks are called as Hypabyssal Rocks.
-Eg: Granite, dolerite, etc.
Lava or Volcanic Rocks or Extrusive rocks-
-They are formed by the cooling of the lava on the earth’s surface.-As lava cools very rapidly on the surface, the mineral crystals forming extrusive igneous rocks are very fine.-These rocks are also called as Volcanic Rocks.-Eg: Gabbro, Basalt, etc.
Plutonic rocks: | Volcanic rocks: |
Intrusive rocks | Extrusive rocks |
Granite | Basalt |
Slow cooling allows big-sized crystals(large grains) | Rapid cooling prevents crystallization, as a result such rocks are fine-grained |
Less dense and are lighter in colour than basic rocks | 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.
-Based on the presence of acid forming radical, silicon, igneous rocks are divided into Acid Rocks and Basic Rocks.
Acid Rocks-
-These are characterized by high content of silica—up to 80 per cent, while the rest is divided among aluminium, alkalis, magnesium, iron oxide, lime etc.
-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.
-These rocks have a lesser content of heavier minerals like iron and magnesium and normally contain quartz and feldspar.
-Add rocks are hard, compact, massive and resistant to weathering.
Basic Rocks-
-These rocks are poor in silica (about 40 per cent); magnesia content is up to 40 per cent and the remaining 40 per cent 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 gives rise to plateaus.
-Presence of heavy elements imparts to these rocks a dark colour.
-Basalt is a typical example, others being gabbro and dolerite.
-Not being very hard, these rocks are weathered relatively easily.
Acidic rocks | Basic rocks |
High content of silica—upto 80 per cent | Poor in silica; magnesia content (40 per cent) |
Due to the excess of silicon, acidic magma cools fast | Due to low silica content, the parent material of such rocks cools slowly |
High Volcanic mountains are formed of this type of rock. Mt Fuji, Japan | Forms plateaus. Deccan Traps |
Lesser content of heavier minerals like iron and magnesium lighter in colour | Presence of heavy elements imparts to these rocks a dark colour. |
Granite, quartz, feldspar etc. Add rocks are hard, compact, massive and resistant to weathering. easily. | -Basalt, gabbro, dolerite etc.. and resistant -Not being very hard, these rocks weathered relatively easily. |
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 material as they come in beautiful shades.
Sedimentary Rocks:
-These rocks are formed by successive deposition of sediments.
-These sediments may be the debris eroded from any previous existing rock which may be igneous, metamorphic or old sedimentary rocks.
-The process of successive deposition and formation of sedimentary rocks is called as Lithification.
-Due to successive depositions, they have a layered or stratified structure and hence are also called as Stratified Rocks. Example: sandstone, shale etc.
-Till or Tillite == Ice deposited sedimentary rocks.
-Loess == Wind deposited sediments.
Depending upon the mode of formation, they are classified into:
Mechanically formed — sandstone, conglomerate, limestone, shale, loess etc.
Organically formed — geyserite, chalk, limestone, coal etc.
Chemically formed — chert, limestone, halite, potash etc.
1. Mechanically formed/ Clastic Sedimentary Rocks
-They are formed by the consolidation of sediments under excessive pressure and cementation.
-Formed by mechanical agents like running water, wind, ocean currents, ice, etc.
-Arenaceous rocks == More sand and big sized particles, and are hard. E.g. sandstone.
-Argillaceous rocks == More clay and are fine-grained, softer, impermeable and non-porous. They are easily weathered and eroded. E.g. shale.
-Eg: Conglomerate, Breccia, Sandstone, Shale, etc.
2. Organically/ Biologically formed Sedimentary Rocks
-The consolidation of organic matters derived from plants and animals form this type of rocks.
-The remains of plants and animals are buried under sediments and due to heat and pressure from overlying layers, their composition undergoes a change.
-Coal and limestone are well-known examples.
-Plant remains give rise to coals of different grades depending upon the proportion of carbon and the degree of overlying pressure.
-The peat and lignite (brown coal) is the first stage of coal having below 45 per cent of carbon; the bituminous variety is the next stage with 60 per cent carbon.
-Limestone is composed of shells and skeletons of dead marine animals that once lived in shallow, warm and clear waters of a sea or lake.
-Depending on the predominance of calcium content or the carbon content, sedimentary rocks may be calcareous (limestone, chalk, dolomite) or carbonaceous (coal).
-Eg: Coal, limestone, chalk, chert, etc.
3. Chemically formed Sedimentary Rocks
-They are formed by various chemical reactions.
-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.
-Eg: Gypsum, rock salt, limestone, etc.
Chief Characteristics of Sedimentary Rocks-
-These rocks consist of a number of layers or strata
-These rocks are characterized by marks left behind by water currents and waves etc..
-These rocks have fossils of plants and animals.
-These rocks are generally porous and allow water to percolate through them.
-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, 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 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’.
-Hence, these rocks form under the action of temperature, pressure and volume changes on original rocks.
-Metamorphic rocks are formed under the influence of heat or pressure on original rocks which cause to change their colour, hardness, structure and composition.
-The process of recrystallization and reorganisation of materials within the original rock is called as metamorphism.
-When the metamorphism happens without any appreciable chemical change, it is called as Dynamic Metamorphism.
-If metamorphism happened due to the influence of heat, it is called as Thermal Metamorphism. It has two types: Contact Metamorphism and Regional Metamorphism.
-When the reorganisation occurs due to direct contact with the hot magma, it is called as Contact Metamorphism.
-If the rocks undergo reorganisation due to tremendous heat/ pressure formed as a result of tectonic shearing, it is called as Regional Metamorphism.
-Metamorphic Rocks can be classified into Foliated (Slate, Schist, Gneiss) and Non-Foliated (Quartzite, Marble) Metamorphic Rocks on the basis of the presence or absence of bands of mineral grains.
-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 interplay of folding, warping, crumpling 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.
-There may be various sources of the’high temperatures—hot magma, hot gases, vapours and liquids, geothermal heat etc.
-A magmatic intrusion causing thermal metamorphism is responsible for the peak of Mt. 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 the stress of 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 dynamothermal metamorphism.
-Under high pressure, granite is converted into gneiss; clay and shale are transformed into schist.
Some examples of Metamorphosis:
Igneous or Sedimentary rock | Influence | Metamorphosed rock. |
Granite | Pressure | Gneiss |
Clay, Shale | Pressure | Schist |
Sandstone | Heat | Quartzite |
Clay, Shale | Heat | Slate ==> Phyllite |
Coal | Heat | Anthracite ==> Graphite |
Limestone | Heat | Marble |
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 melt down and turn into molten magma, the original 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.
-Quartz: It has two elements, silicon and oxygen. It has a hexagonal crystalline structure. It is uncleavaged, white or colorless. It cracks like glass and is present in sand and granite. It is used in 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: It is mercury sulphide and 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 in colour.
-Gypsum: It is hydrous calcium sulphate and is used in cement, fertilizer and chemical industries.
-Haematite: It is a red ore of iron.
-Magnetite: It is the black ore (or iron oxide) of iron.