CHAPTER NO.2 ROCKS

CHAPTER NO.2 ROCKS

Since stone age, rocks have been used as tools. Actually since origin and then

development of ancient civilisation is related with rocks. Rock is anaturally occuring aggregate of one or more minerals. The Earth’s outer solid layer, the lithosphere, is made of rocks (Lithosphere means Rocksphere). A rock may be made of a single element/n eta/or mineral and are known by the name of that element/neta/or mineral,these are made of. The size of rocks varies and so vary their characteristics as these

may be hard or soft. A minute particle of sand and a big stone, both are known as Rocks. These might be hard as Granite and soft as Grafite. They might be porous like Limestone and non-porous like Slate. Earth’s crust has been formed with 2000 minerals but mamly 20 mmerals are immensely important.

Actually 12 elements have been considered as major elements in composition ofrocks.These are also known as ‘Rock-formers’. It includes Iron 35%, Oxygen 30%, Silicon 15%, Magnesium 13%, Nickel 2.4%, Sulphur 1.9%, Calcium 1.1% and Aluminium 1.1%. In this way they form 99% of the Earth mass. Out of this also Iron, Oxygen,Silicon and Magnesium contribute 93% part in constitution. Oxygen, Silicon,Alluminium, Iron, Magnesium, Calcium, Postassium and Sodium are the major elements of the crust’s composition.

Definition of Rocks : ‘A Rock can be defined as an aggregate of minerals.

Sometimes chiefly or entirely of single mineral species, as in case of rock salt and limestone but more commonly of two or more different mimerals.”Types of Rocks : Rocks are aggregate of one or more minerals. They might be soft

or hard and have various colours. On the bases of composition and origin, rocks

can be divided into three groups :

(1) Igneous Rocks

(2) Sedimentary Rocks

(3) Metamorphic Rocks

1. Igneous Rocks : Word Igneous is derived from the Latin word ‘ignis’ which means ‘fire’. These rocks originate through the cooling and solidification of magma or lava.

These may be crystalised or non-crystalised. These may form on the surface of the Earth or beneath the surface of Earth’s crust. There are near about 700 types of Igneous Rocks and most of them are found beneath the surface of the earth.These are also known as Primitive Rocks because these are the oldest rocks.Earlier Earth was hot and in melted form. Igneous Rocks were formed by the cooling and solidification of magma or lava. After the blast in the soft and weaker parts of the Earth, the lava comes out with gases. After the cooling and solidification of lava,igneous rocks are formed.

Classification of Igneous Rocks :Where, Why and in What type of conditions, does cooling and solidification process of lavatake place, plays an unportant role in the formation of igneous rocks.For example, slow cooling process results in coarse grained texture and on the other

hand fast cooling results in fine grained texture. All this leads to the formation of various types of igneous rocks. Basically these are divided into two parts :

Extrusive Igneous Rocks

Intrusive Igneous Rocks

Extrusive Igneous Rocks : These rocks are formed by the cooling of molten magma on Earth’s surface. When molten magma comes out of Earth’s surface, the gases present in it evaporate at fast rate because of which molten magma spreads on Earth’s surface like shield and cooling takes place. Souther Plateau region (Deccan Plateau) of India is the biggest example of such formations. Basalt is extrustve Igneous rock

which has fine particles. Black soil of Deccan plateau of India is formed by weathermg of these rocks. Basalt rocks are used for the construction of roads.

Density of lava depends upon its silica content. If the silica content is high, then the lava will be more dense and won’t be able to spread up to large area. The layer of this type of lava is also very thick. Thus, the lava forms soft and even too hard and uneven rocks. The outer layer of Earth i.e. Sial is made of these acidic rocks. Felsic;

Felspan (Fe) and Silica (Si) are one of the best examples of these types of rocks. These have light colours.

On the other hand, the lava in which silica content is low and magnesium (Ma),Ferrous (Fe) Iron, are present, the density of that lava is low and it is not thick. Because of this it spreads upto large area. The rocks formed from this type of lava are soft and have deep colour. ‘Mafic’ rocks are one of the best examples of these types of rocks.

Intrusive Igneous Rocks : These rocks are formed of magma that cools and

solidifies within the crust of planet. These are intesive igneous rocks. We distinguish these rodes in two categories further i.e.

(a) Plutonic rocks

(b) Hypabyssal rocks.

(a) Plutonic Rocks : This word is derived from the Greek word ‘Pluto’ which means “God of Under World’. These rocks are formed deep within the Earth. The cooling and solidificaton of magma takes long time and it forms in large size crystals. It has various colours; Grey, Red, Pink, White etc. In India, it is found in Deccan Platean,Chhatisgath, Chhota Nagpur (Jharkhand), Rajasthan and in some parts of Himalayas.It has been generally used in construction of various buildings, temples and most

unportantly, castles.

(b) Hypabyssal Rocks : These are formed due to cooling and soldification of rising magma in ‘cracks’ and ‘joints’ just beneath the Earth surface, means not as deep as plutonic rocks. Their crystals are small in size. ‘Dolesite’ and ‘Pegmatic’ rocks are the major examples of these types.

These are further classified on the bases of place and structure:

(i) Laccoliths : It is a sheet intrusion of lava that cools horizontally between two layers of sedimetary rock. The pressure of magma is high enough that the over lying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally

plane base. Such formations are found in westem part of North America.

(ii) Batholiths : Thses are big intrusive Igneous rocks which may cover an area, larger than 100 kilometers long. Because of their thickness it is hard to see their base. These are formed when magma solidifies beneath the layers of Earth in an irregular way. They

might have nregular shape or may have shape of dome. That is also areason that such type of rocks have been named as batholiths. These get exposed to surface through the process of erosion of overhead portion. At times their breadth is 50 to 80 Kilometers and generally such rocks are base of mountains. Granite rocks are such large batholith rocks. Low lying mountains around Chennai are fine examples of Batholith rocks which have come visible because of erosion and are generally dome shaped.

(iii) Lap olith : When magma solidifies beneath the Earth’s surface in shape of a saucer with a depressed central region, are called Lapolith. This type is an other form of Batholith. We find its examples in North America, in shape of Duluth Gabbro shield in Canada which extends to approximately 2 lakh sq. km in area.

(iv) Phacolith : Magma solidified beneath the surface of earth in the tidal shape is called Phacolith.

(v) Stock : Small size dome shaped or round batholith is called stock. The area covered by stocks is less than 100 sq. km.

(vi) Sills : When magma solidifies horizontally between the folds of rocks, it is called Sill. These rocks may be upto 100 meters in length while if their breadth is not much,sills are called sheets.

(vii) Dyke/Dike : Magma solidified in the vertical position is called dike or dyke. Their length varies from few meters to kilometers and breadth from a few centimeter to meters. In India, its examples are found in Tamil Nadu and Andhra Pradesh.

(viii) Volcanic Neck : It is a vertical landform created when magma hardens within avent or neck (an opening in the Earth’s crust from which lava, ash and hot gases flow or are ejected during an eruption) of an active volcano. A neck can cause an extreme build-up of pressure if rising volatile-charged magma is trapped beneath it and this can

sometimes lead to an explosive eruption.

Characteristics of Igneous Rocks :

1. These rocks are formed by cooling and solidification of lava that is why most of the minerals are found in these types of rocks. Iron, Nickel, Copper, Caromite,Magneese, Diamond, Zinc, Platinum and Mica are found in huge quantities in these rocks.

2. These are hard rocks and they do not have layers.

3. Mostly these rocks are crystallite but if lava cools down quickly, these rocks attain non crystallite characteristic.

4. These are found in both vertical and horizontal shapes.

5. Almost 85 percent of Earth’s surface is made up of igneous rocks.

6. Texture of these rocks depends upon the cooling process of lava.

7. These are not porous and water does not seve in such rocks therefore these are used for construction purposes. Their non-erodic character makes them useful.

Sedimentary Rocks : Sedimentary rocks cover 75% to 80% of the lithosphere while only about 5% of Earth’s crust comprises of Sedimentary rocks.. The word ‘Sedimentary’ has been derived from Latin word ‘Sedimentum’ which means ‘settle down’. Its definition runs as ‘Sedimentary rocks are formed through consolidation of

accumulated sediment deposits in oceans, seas, rivers or lakes’. Almost 95% part of Earth is covered by igneous and Sedimentary rocks. Sedimetary rocks are formed by the deposition of mineral and organic particles created or formed by weathering and erosion in a source area and then transported to the place of deposition by water, wind,ice, mass movement or glaciers. These are also known as Strat Rocks. Basically these

rocks originate from the igneous rocks as these are formed by the deposition of the particles produced by the weathering and erosion on igneous rocks. These are

important sources of natural resources like coal, fossil-fuels etc. The lower layers of these rocks have large pieces while upper layer are made of small pieces in collective fonn.

The layers of these rocks get formed when small stones, sand, fossils and

biowaste etc. deposits exert pressure on each other while cementing agents like silica,calcium, carbonate, iron oxide slowly mix with deposits to solidify them. Both pressure and cementation plays vital role in the formation of layers. Rocks thus formed are also known as fossil rocks or secondary rocks.

Relative quantum of various elements in sedimentary types of rocks is as

under :Shale - 60% (Found in Spiti)

Sand Stone: 20% (Formed by the combination of small sand particles)

Carbonate: 15%

All other: 5%

Sedimentary rocks are important source of natural resources like coal, gas, oil

(Fossil Fuels), Fresh water and various other minerals like Aluminium and Iron.

Different layers of these rocks help in the scientific study of history of Earth, Stone age and Scientific research. On the basis of formation these are divided into following parts :

1. Clastic Rocks : The word ‘clastic’ has been derived from the Greek word

‘Klastos’ which means ‘broken part’. These rocks are broken part of parent rocks and are formed by weathering processes which break rocks into pebbles, sand or clay particles by exposure to wind, ice and water. These may be further classified as :

(a) Alluvial Rocks : Small broken parts of parent rock which get deposited at some place in layers, having been transported by by rivers, glaciers and wind are called Alluvial Rocks. Such rocks are inorganic rocks. On the bases of parent rocks these may be further divided in various types :

Marine or Aqueous Rocks : Such rocks are formed when material (debris)

transported by rivers settles down on the sea bed. Big rocks settle close to the coast line while sand and other organic materials settle on continental shelf: Deposition on the shelf year by year turns the material into sedimentary rocks.

(ii) Arenaceous Sedimentary Rocks : Those aqueous rocks which have sand as

principal constituent of sediments are known as Arenaceous sedimentary rocks. these rocks fall in abiotic rocks category also. Example of such rocks is sand stone.

(iii) Argillaceous Sedimentary Rocks : The rocks which have soil as principal

constituent of their sediment are known as Argillaceous sedimentary rocks. ‘Shale’ is its example while these rocks fall in category of biotic rocks.

(iv) Riverine Deposit Rocks : River deposits debris (sediments) in plains before falling into sea. From these sediments Riverine Deposit rocks are formed. Such rocks are commonly found in flood plains.

(v) Lacustrine Rocks : When sediments brought down by rivers fall into lakes and with slow and continuous deposition layers are formed, this situation gives birth to lacustrine rocks. Gypsum is example of each formations.

(vi) Aeolian Rocks : These rocks are formed by winds. High speed winds remove

soil particles from one place and deposit them at another place. This depostion results in formation of Aeolian Rocks. Kachh and Kathiawar in India and northwestern regions of China are good examples of such rock formations.

(vii) Glacial Rocks : Glacial activity is responsible form formation of this type of rocks. Glaciers transport good amount of debris while moving and when these glaciers cross snowline, they get melted and debris gets deposited on those spots. Layers of such materials form Glacial Rocks.

(2) Mechanically Formed Sedimentary Rocks :Soft rocks, Sand particles, Mud or clay soil solidifies with passage of time in layers, these soft rocks get compacted and with the help of cementing agents like calcite or silica, take form of these rocks e.g. sand stone, shale, corglomerate and breccia. Corglomerate is found in Mt. Kailash, Talchir and Shivalik hills near Chandigarh. In other words these are the rocks formed by accumulation of materials from other rocks which are cemented together.

(3) Organic Rock : These rocks are formed by the remains of plants and animals.When these remains keep buried for a long time, they change their form. This is a continous process which takes place with the help of heat and pressure which finally,results into formation of organic rocks. ‘Peat coal’ is a good example of organic rocks,it is formed with the little change in the remains of plants and animals, that’s why its use

is limited. Good quality organic rocks are formed with the passage of long time. Lignite and Bituminous (coal) are fine quality organic rocks which get formed when the remains of plants and animals keep buried for long time and their form changes a lot.We get petroleum also from organic rocks.

Organic rocks are farther divided into two types :

(a) Calcareous Sedimentary Rocks : Calcium content is very high in this type of rocks and these are formed with the depositiion of bones and shells of animals. In simple words, these are formed with the combination of remains of animals and water.At first step Calctum Hydroxide is formed, after this with reaction to Carbondioxide,Calcium Carbonate is formed. Examples of this type of rocks are Limestone, Chalks,

Dolomite, Talc etc. Chalk and Dolomite are found at Mt. Everest, Jaisalmer, Shahbad,Viashnoo Devi and Pithoragarh.

(b) Carbonaceous Sedimentary Rocks : These rocks have high carbon content.

When dense forests and high planted areas get buried because of various activities of Earth, they change their form due to the internal heat and various layers continously exert pressure on them which change their actual form and results in the formation of carbonaceous Sedimentary Rocks. Coal is biggest example of such rock formations.

(4) Chemically formed Sedimentary Rocks : These rocks get formed by various

chemical reactions. There are various chemicals in the sea, when water evaporates due to heat, these chemicals result in the formation of rocks. Like in a lime stone region,water calcium hydroxide and whole process results in the formation of chemically formed sedimentary rocks. like Stalegmite, Stalactic, Rock Salt, Gypsum and

Saltpetre etc. In simple words water evaporates leaving behind the sediments and with the passage of time rocks are formed of such sediments.

Characteristics of Sedimentary Rocks :

1. These rocks have layers.

2. Fossils are found in the rocks.

3. These rocks do not have crystals like igneous rocks.

4. These rocks are porous and water can easily pass through them.

5. These are not hard like igneous rocks, that’s why they get easily eroded.

6. Himalayan mountains in Asia, Alps mountains in Europe, Western Cordillera of North America and Andies of South America are examples of these rocks. In India,Ganga-Brahmaputra plains are the example of these rocks.

7. Being easily eroding type, Sedimentary rocks have marks known as ‘Ripples’ on them. Deltas of Mahanadi, Godavari, Krishna and Kaveri are examples of such rocks.

8. These rocks are the sources of oil, coal and natural gas.

9. Such rocks like small stones are combined by some cementing agent are of two types (i) Corglomerate : These are formed by round stones (ii) Breccia : These are formed by Angular stones.

Metamorphic Rocks

The word Metamorphic has been derived from the Greek word ‘Metamorphosis’ which means ‘Change’. The formation process of these rocks takes place 12 to 16 kilometers deep beneath the Earth surface. Chemical and physical form

of sedimentary and igneous rocks changes because of internal heat of Earth, pressure of rock layers over them and exposure to various materials. After this process, the rocks which are formed with different chemical and physical properties are known as metamorphic rocks. These are not porous. Metamorphic rocks have highly compressed layers. Gold, Silver, Diamonds and other precious stones are found in these rocks. The process of change which results in the formation of metamorphic

rocks, known as metamorphism is of three types :

1. Dynamic Metamorphism : The ‘changing’ process which takes place because of

extreme pressure is called dynamic metamorphism. The process takes place deep beneath the Earth in which ‘Granite’ converts into ‘Nice’ and ‘Shale’ converts into ‘Schist’.

2. Thermal/Contact Metamorphism : This process takes place because of extreme

heat present in the internal parts of Earth. The rocks formed by this process are known as Thermal/Contact metamorphic rocks. When magma comes into contact with igneous and sedimentary rocks, they melt because of high temperature. This results in the reformation of crystals or change in crystals. Limestone and Chalk changes into Marble, Clay soil into Slate and Coal changes into Graphite and Graphite further changes into Diamond. ‘Slate’ rocks are found in Kangra and Chamba districts of Himachal Pradesh and Rewari mn Haryana. The tip of Mt. Everest is also formed of

converted lime stone.

3. Regional Metam orphism : Deep beneath the Earth when changing process takes

places in large area because of compression and friction combined and results in conversion of rocks that is called Regional Metamorphism and rocks formed by this process are known as Regional Metamorphic Rocks. Quartzite is the example of regional metamorphic rocks. These are found in Rajasthan, Bihar, Madhya Pradesh.

Characteristics of Metamorphic Rocks :

1. Some metamorphic rocks get more strong and harder than their parent rocks e.g. Marble and Quartize formed from Lime stone and Sand Stone making erosion process is very tough on them.

2. Changing process takes place because of Heat, Pressure and Friction.

3. These are found m various colours.

4. These may originate from any type of rock.

5. Precious stones like Diamonds, Ruby, Sapphires are found in these rocks.

6. Gnesis rocks are used for construction and quartzite is used for ‘glass making’.

7. Many fam ous buildings have been constructed of these rocks like ‘Taj Mahal’ in Agra is made of Marble, Fort of Tughlabad, Fort of Agra and Red Fort of Delhi are the other examples. Graphite is used for pencils and crucibles. It is found in Odisha and

Andhra Pradesh.

Rock Cycle

Distribution of rocks is very unequal on the crust or outer most layer of Earth.

Deep in the interior of our Earth, rocks are found in liquid form called ‘magma’.

Cooling and solidification of magma results in the formation of ‘igneous rocks’. Igneous rocks break down into small rocks and sediments. With the deposition of these sediments in the form of layers ‘sedimetary’ rocks are formed. Because of heat compression and pressure, igneous and sedimentary rocks change their physical and chemical properties resulting in the formation of metamorphic rocks. After sometime

these rocks start changing their form because of erosion process. The process of changing of rocks from one form to another is known as ‘Rock Cycle’.

In simple words, rock cycle mixes up the outer and ternal layers of rocks.

Chemical reactions leads to conjugation and separation of various particle. Physical form of rocks also changes Rock cycle exists on our Earth since stone age. Because of this the matter exists on Earth and changes from one form to another. Rock cycle is continously on going process.

Activity :

(i) Prepare a graphic of Rock Cycle.

(ii) Prepare a list of metamorphied rocks to show it as exhibit board

im your class.

EXERCISE

1. Answer the following questions in few words :

(a) In which era tools were made of stone ?

(b) Distinguish rocks on the basis of their character of Seving water.

(d) Patarology is known as Science of What else ?

(e) Cristals may be a property of rocks of what type ?

(f) Mafic is combination of which elements ?

(g) In which type of rocks, fossils are found the most ?