NCERT Class 11 Geography Chapter 5 Minerals and Rocks

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NCERT Class 11 Geography Chapter 5 Minerals and Rocks

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Minerals and Rocks

Chapter: 5

GEOGRAPY [ PART – I ]

SHORT ANSWER TYPE QUESTIONS

Q.1. Define a rock.

Ans. Any material mass of mineral matter that makes up the earth’s crust. Example-clay.

Q.2. Name the types of rocks.

Ans. The types of rocks are:

1. Igneous.

2. Sedimentary. and

3. Metamorphic rocks.

Q.3. What is lithification?

Ans. The process of compaction of sediments through which they turn into rocks is called lithification.

Q.4. What is the name of the scale on which the hardness of minerals is measured?

Ans. Mohs’ Scale.

Q.5. What is a mineral?

Ans. It is an inorganic element or compound. It has definite chemical composition, atomic and chemical structure and definite set of physical properties.

Q.6. What is meant by the texture of rock?

Ans. The texture of a rock relates to the size and the pattern of mineral crystals present in it.

Q.7. Name the main igneous rocks.

Ans. The main igneous rocks are granite, basalt, regmatile, synetile, diorits, dolerite and pigmetile.

Q.8. What is a sedimentary rock?

Ans. The rock which is formed by the accumulation of sediments is called a sedimentary rock.

Q.9. Name some main sedimentary rocks.

Ans. Main sedimentary rocks are slate, limestone, chalk, dolomite, gypsum, coal and rock salt.

Q.10. Name the important minerals found in metamorphic rocks.

Ans. Minerals found in metamorphic rocks are granite and sillimanite.

Q.11. Which are two main types of igneous rocks?

Ans. (i) Matic. and

(ii) Felesic.

Q.12. Give one word for the following:

(i) Few large crystals imbedded into material of smaller crystals.

Ans. Igneous rocks.

(ii) The process of physical and chemical changes affecting sediments during their conversion into solid rocks.

Ans. Lithification.

(iii) Structure in metamorphic rocks in which minerals are drawn out into long, thin pencil like objects all in parallel alignment.

Ans. Banding.

(iv) The smooth planer surfaces of weakness along which minerals have pronounced tendency of split.

Ans. Cleavage.

Q.13. Classify the following rocks into igneous, sedimentary and metamorphic rocks.

(i) Granite.

(ii) Slate.

(iii) Marble

(iv) Limestone.

(v) Clay.

(vi) Basalt.

(vii) Sand stone.

(viii) Coal.

(ix) Chalk.

(x) Gypsum.

(xi) Gneiss and

(xii) Schist.

Ans. Igneous Rocks: granite, basalt, gneiss and schist. 

Sedimentary Rocks: clay, limestone, gypsum, chalk, coal, sandstone.

Metamorphic Rocks: slate, marble.

Q.14. What do you mean by Deccan Trap?

Ans. Deccan trap is an extensive area in north-west part of India covering about 5 lakh sq. km. It has been formed by lava flows. Lava has solidified to form Basalt. It is useful for cotton cultivation.

Q.15. How is coal formed?

Ans. Vegetation is burried in swamps and forests. Due to pressure of overlying sediments the remains of plants are changed into carbon. Coal is formed in this way. Coal is different types such as part lignite and tritunimus and anthracite.

SHORT ANSWER TYPE QUESTIONS

Q.1. What do you mean by rocks? Name the three major classes of rocks.

Ans. A rock is an aggregate of one or more minerals:

The three major classes of rocks are:

1. Igneous rocks.

2. Sedimentary rocks.

3. Metamorphic rocks.

Q.2. What is an igneous rock? Describe the method of formation and characteristics of igneous rock.

Ans. Igneous rocks are those rocks which have been formed by the cooling and solidification of molten natural matter of the earth. The igneous rocks are formed when magma cools and turns into solid forms. The process of the cooling and solidification can happen in the earth’s crust as well as on the surface.

Characteristics of igneous rocks:

1. These rocks do not have particles and layers.

2. These rocks are compact and massive.

3. These rocks are crystalline.

4. The igneous rocks are extremely hard and resistant.

5. These rocks do not possess fossils of plants or animals.

6. These rocks are thermal in origin.

Q.3. Why are igneous rocks called primary rocks?

Ans. Igneous rocks have been forming since the earth took its birth. They are still being formed in regions of volcanic activities. Out of three kinds of rocks Igneous, Sedimentary and Metamorphic, they were first to be formed and hence they are called primary rocks. They are also called parent rocks because they are the parents of all other type of rocks. They are turned into sediments by the agents of erosion to give birth to the sedimentary rocks. Due to heat and pressure these sedimentary rocks are transformed into metamorphic rocks.

Q.4. What relationship is explained by rock cycle between the major types of rock?

Ans. The igneous rocks are primary rocks and other rocks are formed from this rock. The change of one rock into another type of rock is known as rock cycle. The igneous rock can be changed into a metamorphic or a sedimentary rock. Sedimentary rocks are formed from the material obtained by the erosion of igneous rock. Metamorphic rocks are again changed into igneous rocks due to melting.

Q.5. Discuss physical properties of minerals.

Ans. Minerals can be identified by their hardness, colour, the way they reflect light, the way they break and their density. The physical properties of minerals are as under:

1. External crystal form.

2. Cleavage.

3. Fracture.

4. Specific gravity.

5. Hardness.

6. Lustre.

7. Colour.

8. Streak.

Q.6. Why fossils are preserved in sedimentary and not in igneous rocks?

Ans. Sedimentary rocks are formed by the deposition of sediments carried away by exogenous agents such as water, wind or ice. They are deposited in distinct-strata or layers. Fossils in the form of leaves of plants, insects or some bovine animals and pieces of bones, shells or some hard parts of old living beings are deposited into the layers of the sedimentary rocks. In this way, fossils are preserved in sedimentary rocks.

On the other hand, igneous rocks are formed by the solidification of magma. They have no layers. They are hard and compact. Fossils cannot be preserved in such a hard and compact rock because fossils die out in the process of their formation. Hence, igneous rocks are free of fossils.

Q.7. Why should mineral in both metamorphic and igneous rocks be almost the same?

Ans. Since magma is the chief source of metal ores, many of them, are associated with igneous rocks e.g., magnetic iron, nickel, copper, lead, zinc, chromite, manganese, gold, diamonds, platinum, etc. All igneous rocks are also of magnetic origin. Granite is an igneous rock formed from magma which cools down and solidifies at moderate depths beneath the earth’s surface. Metamorphic rocks are changed rocks due to heat and pressure or as result of contact with a hot igneous material. The change takes place at depth of about 12-16 km below the earth’s surface. These rocks are formed under conditions similar to those producing igneous rocks. Ores of copper, zinc, lead, quartz, feldspar, mica are the results of contact metamorphism. Gold, diamond and platinum are also found in metamorphic rocks. Hence the minerals in both metamorphic and igneous rocks are almost the same.

Q.8. How can we distinguish an intrusive igneous rock from extrusive igneous rock?

Ans. An extrusive igneous rock is the name given to the magma erupting and solidifying after escape of gases as lava on the surface of the earth. As lava cools down rapidly, the mineral crystals of extensive igneous rocks change their structure and become very small in size making them look fine-grained and giving a glassy look. Basalt is a typical example of extrusive type.

The intrusive igneous rock is formed by solidification of magma at some depths. The cooling is slow and the crystals formed are large. The granite and the dolerite are the most common examples of such rocks. For this reason granite is coarse-grained unlike the fine-grained basalt. So a study of the size of crystals and grains present in an igneous rock will tell whether it is an intrusive igneous rock or an extrusive igneous rock.

Q.9. What are the types of minerals?

Ans. These are:

1. Metallic Minerals: The contain metal contents and can be subdivided into three types:

(a) Precious Metals: Gold, Silver and Platinum.

(b) Ferrons Metals: iron and other metals often mixed with iron to form various kinds of steel.

(c) Non-ferrous Metals: These contain little iron and include metals like copper lead zinc, tin, aluminium.

2. Non-Metallic minerals: They do not contain metal content sulpher, phosphate and nitrates are example of non-metallic minerals. Cement is the non-metallic mineral.

Q.10. Distinguish between

(i) Batholiths.

Ans. Batholiths: Batholiths are the largest intrasive igneous rocks. These are formed in the deep interior. These are not visible on the surface. The batholiths are great granite masses. These form the core of mountains.

(ii) Braccia.

Ans. Braccia: Braccia is a sedimentary rock. The pebbles are cremated together by silica or calcite. It has angular fragments.

(iii) Conglomerate.

Ans. Conglomerate: Conglomerate is a sedimentary rock loose unconsolidated pebbles are rounded in shape due to erosion. The presence of quartize cements these together to form conglomerate.

Q.11. What is meant by sedimentary rocks? Describe the mode of formation of sedimentary rocks.

Ans. Sedimentary rocks are formed from those materials which are deposited at favourable sites by agents of denudation such as river, wind, glacier and sea waves. The process is called the lithification. The sedimentary rocks are formed in different ways.

1. The layers of deposits: Loose materials or sediments are deposited in the layers.

2. Stratification: The sediments are deposited in the layers. Materials such as clay, silt, sand, etc. settle down in due course.

3. Solidification: The loose material gets converted into hard and compact rocks.

Q.12. What are the characteristics of igneous rocks?

Ans. 1. These rocks are found in bulks.

2. These rocks are thermal in origin.

3. These rocks have well developed joints.

4. These rocks are extremely hard and resistant.

5. These rocks are crystalline.

6. These rocks are found in volcanic regions.

7. These rocks do not have particles and layers.

Q.13. Write short notes on:

(i) Sedimentary rocks.

Ans. Sedimentary rocks are made from sediments formed by the erosion and weathering of other rocks. When deposited in the sea, they are compressed and hardened to form layers of rocks. Sediments are fragmented minerals or organic matter derived directly or indirectly from pre-existing rocks and from life-processes, transported and deposited by air, water or snow. Sandstone is made from grains of sand which have been naturally cemented together. Chalk is made up of millions of tiny calcium carbonate skeletons of microorganisms.

(ii) Economic importance of the minerals.

Ans. Mineral resources have much importance. They are essential resources, energy resources and metal and industrial resources. Energy resources are fossil fuels such as coal and petroleum and gas and nuclear fuels such as uranium and thorium. Industries and power generation depend upon them.

Q.14. Distinguish between:

(i) Rocks and Minerals.

Ans.

Minerals	Rocks
1. Minerals are naturally occurring compounds, such as silicates, oxides, carbonates or hydroxides.	1. It is an aggregate of several or one group of minerals.
2. Minerals have fixed physical and chemical properties.	2. The physical and chemical properties of rocks are variable.
3. Silver, gold, coal, oil are the examples of minerals.	3. Granite, basalt, marble are the examples of rocks.

(ii) Chemically precipitates and organically derived sediments.

Ans. Chemically precipitate of sediments: They are formed by the deposition of the sediments from minerals of the fresh or salty water.

Organically derived sediments: These rocks are formed by the deposition of the sediments of the living organisms.

(iii) Clastic rocks and recrystallised rocks.

Ans. Clastic rocks: The calastic rocks are formed by mechanical disruption (breaking and crushing) of the original without appreciable chemical change.

Recrystallised rocks: These are formed by the recrystallisation of the original minerals. Minerals with different chemical composition crystallise and are produced due to recrystallisation.

(iv) Foliation and Schistocity.

Ans. Foliation: As metamorphism continues, a large percentage of the minerals assume plate-like shape and are assembled in parallel orientation in the rock, a structure known as foliation.

Schistocity: It is the growth of very largest crystals and segregation of some minerals into lighter and darkar bands.

(v) Formation of limestone and coal.

Ans. The organic matter derived from plant or animal remains is the raw material for the formation of sedimentary rocks in due course of time. Limestone and coal are good examples of sedimentary rocks of organic origin.

Limestones consist mainly of the shells or skeletons of animals or plants, and are formed of carbonates of lime. Deposits of carbonates of lime are sometimes produced when water containing a large proportion of that mineral evaporates leaving films of carbonate of lime where it stood or used to flow. The lime shells of such organisms as were living in moderately deep warm and clear waters of a sea or lake, are cemented into limestone. The reefs built from the skeletons of dead corals in tropical seas is an example of limestone.

Many a time, plants, their roots, leaves, twines and even tree trunks are buried in swamps. In due course of time, these materials turn into coals of different grades depending upon the proportion of carbon and the degree of overlying pressure. The peat and lignite is the first state of coal formation (carbon content 45%) and bituminous variety is the next stage (carbon content 50 to 70%).

Q.15. Distinguish between Igneous and Sedimentary rocks.

Ans. 

Igneous Rocks	Sedimentary Rocks
1. These rocks are massive and found in bulks.	1. These rocks are occur in layers or strata.
2. These rocks have different types of crystate.	2. These rocks contain in particles.
3. These rocks are formed due to the coaling of magma.	3. These rocks are formed by deposition of sediments.
4. These are hard and resistant rocks.	4. These are soft rocks.
5. These rocks contain fossile.	5. These rocks contain fossile of animals and plants.
6. These are also known primary rocks.	6. These rocks are known as Secondary rocks.
7. These rocks are not easily eradid due to weathering.	7. These rocks are easily eroded by weathering.
8. These have a well developed joints system.	8. These rocks have no joints.
9. These are impavious rocks.	9. These are previous rocks.
10. These rocks are formed in volcanic regions.	10. These rocks are formed in depressions seas and lakes.

Q.16. Distinguish between coal and limestone.

Ans.

Limestone	Coal
1.Limestone is a calcareous rock.	1.Coal is a carboniferous rock.
2. It has been formed by remains of microscopic animals on the sea floor.	2. It is formed by the vegetation buried in swamps and forests.
3. Due to presence of cementing materials it takes a solid shape.	3. Due to pressure of overlying sediments the remains of plants changed into carbon.

Q.17. Describe the different types of minerals.

Ans. The types of minerals are as under:

1. Metallic minerals: They certain metal content and can be subdivided into three types:

(i) Precious metals: Gold, Silver, Platinum etc.

(ii) Ferrous Metals: Iron and other often mixed with iron to form various kinds of steel.

(iii) Non-Ferrous Metals: Contain little iron and include metals like copper had zinc, tin, aluminium etc.

2. Non-Metallic minerals: They do not contain metal contents. Sulphur phosphates and nitrates are example.

LONG ANSWER TYPE QUESTIONS

Q.1. What are metamorphic rocks? Describe the types of metamorphic rock and how are they formed.

Ans. When the original characteristics of sedimentary or igneous rocks-their colour, hardness, texture and mineral composition is partly or wholly changed, it gives rise to metamorphic rocks under favourable conditions of heat and pressure.

The types of metamorphic rocks depend upon the original rocks that were subjected to metamorphism. Metamorphic rocks are classified. into two major types, foliated rocks and non- foliated rocks.

Formation of metamorphic rocks: These rocks are formed due to thermal metamorphism. It is of two types:

1. Contact metamorphism. and

2. Regional metamorphism.

In contact metamorphism the rocks come in to the contact with hot magma and lava and the rock material is recrystallized under high pressure and temperature.

In regional metamorphism, rocks under go recrystallization due to deformation caused by tectonic shearing together with high temperature or pressure or both.

Q.2. Describe the formation of igneous rocks. Give suitable examples of its various type.

Ans. The igneous rocks are formed by solidification of magma and by granitization. They have come out of the interior of the earth in the molten state to solidify either

(i) on the surface of the earth. or

(ii) in cracks and fissures of the crust. or

(iii) far down in the crust. Different names are given to these three types.

They are:

(i) Plutonic rocks or intrusive igneous rocks.

(ii) Sills and dykes or intermediate igneous rocks.

(iii) Volcanic rocks or extrusive igneous rocks.

1. Plutonic Rocks or Intrusive Rocks: The molten material that solidifies in the interior of the earth is fully crystallized because it cools very gradually. This is called plutonic rock. It is exposed on the surface of the earth only when the overlying rocks are removed by the weathering or by earth movements. Granite and dolerite belongs to this class.

2. Sills and Dykes or Intermediate Igneous Rocks: Lava may sometimes solidify gradually in the openings and fissures through which it is passing and become crystallized. Horizontal beds in this case are called Sills while vertical ones are known as Dykes.

3. Volcanic Rocks or Extrusive Igneous Rocks: Lava flows out of the craters of the volcanoes or through fissures caused in the (crest) by the earth movements. Lava coming out of the earth solidifies very soon and crystals do not form in it and becomes of a glassy appearance. Basalt belongs to this class.

Q.3. Discuss the physical properties of minerals.

Ans. Main physical properties of minerals are as under:

(i) External crystal form: It is determined by the internal arrangement of the molucubs-cubes hexagonal prisms.

(ii) Cleavage: Tendency to break in given direction producing relatively plane surfaces result of internal arrangement of the molecules, may cleave in one or more directions and at any angle to each other.

(iii) Fracture: Internal molecular arrangement is so complex that there are no plans of molecules the crystal will break in an irregular manner not along planes of cleavage.

(iv) Lustre: Appearance of a material without regard to colour each mineral has a distructive lustre like metallic silky glossy etc.

(v) Colour: Some minerals have colour determined by their molecules structure etc. and some minerals are coloured by impurities. For example Quartz may be white, green, red etc.

(vi) Streak: Colour of the grand powder of any mineral. It may be of the same colour as the mineral or may differ-malachite is green and gives green streak flourite as purple or green but gives a white streak.

Q.4. Define the term ‘mineral’ and name the major classes of minerals with their physical characteristics.

Ans. A mineral is a naturally occurring inorganic substance, having on 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 found. The major classes of minerals with their characteristics are as follows:

1. Feldspar: Half of the earth’s crust is composed of feldspar. It has a light cream and salmon pink colour. It is used in ceramics and glass making.

2. Quartz: It consists of silica. It is the most important components of sand and granite. It is a hard mineral and is insoluble in water. It is used in radio and radar.

3. Pyroxene: It consists of calcium, aluminium, magnesium, iron and silica. It forms 10% of the earth’s crust. It is commonly found in meteorites.

4. Amphibole: Its major elements are silica, aluminium, calcium and iron. They form the 7% of the earth’s crust. It is used in the asbestos industry. It is black or green in colour.

5. Mica: It comprises potassium, aluminium, magnesium, iron, silica etc. It forms 4% of the earth’s crust. It is uses in the electrical instruments.

6. Olivine: Magnesium, iron and silica are the major elements of olivine. It is used in jewellery. It is usually a greenish crystal often found in basaltic rocks.

Q.5. Describe the significance of sedimentary and igneous rocks from the viewpoint of their economic use.

Ans. The geologists are interested in rocks to know the geological history of the earth because it is in fact written in rocks. But the geographers are interested in them due to their mineral content and the soil they produce. The sedimentary rocks are not very rich in minerals. But some of the minerals found in sedimentary rocks which are of economic value are such as haematite iron ore; phosphates, building stones, coal, materials for making cement, petroleum. Secondary ores of bauxite, manganese and deposits of tin, derived from rocks, are also found in sedimentary rocks.

Economic significance of sedimentary rocks:

(i) Coal and petroleum are major sources of power.

(ii) Bricks from clay, cement from limestone and glass from sand is obtained.

(iii) Limestone is used for buildings purpose.

The igneous rocks are crystalline rocks and hence are very rich in minerals of great economic value. These minerals are: magnetite (iron), nickel, copper, lead, zink, chromite, manganese, gold, diamond, platinum etc. These are of great value in metallurgical industry. Ores of lead, tin, copper are also found mixed with quartz and calcite. Igneous rocks abound in silicate minerals containing iron and magnesium. Some mica are also associated with igneous rocks.

Economic significance of igneous rocks:

(i) Granite is used for house-building and idol-making.

(ii) Pumic stone is used as a sharpener.

(iii) Basalt and dolerite are used for road-making.

(iv) Many minerals found in these rocks e.g iron ore, manganese are basic raw materials for many industries.

Q.6. What are the types of sedimentary rocks?

Ans. 1. Mechanically formed rocks: These rocks have been formed by the deposition of sediments derived from other rocks wind river and glacier remove these sediments from existing rocks and redeposit it in the form of layers.

(a) Arenaceous Rocks: These are somely rocks and have more of silica, quartz predominates in then rocks sandstone is the typical example.

(b) Argillaceous Rocks: These rocks have more clay particles. These are soft rocks and can be easily eroded. Shah is the most abundant specimen of this type of rocks. It is compacted silt and clay. It is widely used for pattery brick making or tile making.

2. Organically formed Rocks: These rocks are formed by the deposition of remains of plants and animals. These are of two types:

(a) Carbonaceous rocks: These rocks are formed from vegitative material buried in swamps and forests. Due to the pressure of the overlying sediments the remains of plants are changed into carbon. Therefore these are termed as carbonceous rocks coal is formed in this way.

(b) Calcareous Rocks: These rocks have been formed by the deposition of remains of microscopic sea animals on the sea floor. These organisms extract calcium carbonate from the water and use it to build their skeletons. When they die their skeletones are deposited on the sea bottom to form a rock chalk limestone and dolomits are the examples.

3. Chemically formed rocks: These rocks are deposited chemically from different solution when water is evaporated, it leaves behind layers of chemicals such deposits are formed near springs salt lakes or geysers. Rock salt and Gypsam are formed by the evaporation of salt water from lakes such as dead sea calcium sulphate and sodium chloride predominate in these rocks.

Q.7. Describe the natures and mode of origin of the chief types of rocks at the earth’s crust. How will you distinguish them?

Ans. The chief types of rocks are as under.

1. The igneous rocks.

2. The sedimentary rocks.

3. The metamorhpic rocks.

1. The Igneous Rocks: The igneous rocks form out of magma and lava from the interior of the earth. These are formed when magma cools and solidifies. These are hard in nature such as granite.

2. Sedimentary Rocks: These rocks are formed by the sedimentation in the bottom of sea or lake transported by various exogenous agencies.

The sedimentary rocks are soft in nature. Sandstone is the typical example.

3. Metamorphic Rocks: These rocks are formed under the action of pressure-volume-temperature changes. Metamorphism is a process in which already consolidated rock undergoes recrystallisation of materials within the original rock.

These rocks are crystalline in nature. Marble is one examples of the metamorphic rocks.

Q.8. Explain some important minerals and their characteristics.

Ans. 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.

• Question uartz: 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.

• Pyroxene: Pyroxene consists of cak ium, aluminum, magnesium, iron and silica. Pyroxene forms 10 percent of the earth’s crust.

• Amphibole: Aluminium, calcium, silica, iron, magnesium are the major elements of amphiboles. They form 7 per cent of the earth’s crust.

• Mica: It comprises of potassium, aluminium, magnesium, iron, silica etc. It forms 4 per cent of the earth’s crust.

• 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.