NCERT Solutions for Class 11th: Ch 5 Minerals and Rocks

NCERT Solutions for Class 11th: Ch 5 Minerals and Rocks

Exercises

Page No: 44

1. Multiple choice questions.

(i) Which one of the following are the two main constituents of granite?
(a) Iron and nickel
(b) Iron and silver 
(c) Silica and aluminium
(d) Iron Oxide and potassium
► (c) Silica and aluminium

(ii) Which one of the following is the salient feature of metamorphic rocks?
(a) Changeable
(b) Quite 
(c) Crystalline
(d) Foliation
► (a) Changeable

(iii) Which one of the following is not a single element mineral?
(a) Gold
(b) Silver 
(c) Mica
(d) Graphite
► (c) Mica

(iv) Which one of the following is the hardest mineral?
(a) Topaz
(b) Diamond
(c) Quartz
(d) Feldspar
► (b) Diamond

(v) Which one of the following is not a sedimentary rock?
(a) Tillite
(b) Borax 
(c) Breccia
(d) Marble
► (d) Marble

2. Answer the following questions in about 30 words.

(i) What do you mean by rocks? Name the three major classes of rocks.

Answer

A rock is an aggregate of one or more minerals. The earth’s crust is composed of rocks.
The three major classes of rocks are:
• Igneous rocks
• Sedimentary rocks
• Metamorphic rocks

(ii) What is an igneous rock? Describe the method of formation and characteristics of igneous rock.

Answer

Igneous rocks are those rocks which are formed through the cooling and solidification of magma or lava. These rocks are formed when magma in its upward movement cools and turns into solid form. The process of cooling and solidification can happen in the earth’s crust or on the surface of the earth.
Characteristics of Igneous rocks:
• These rocks are consist of crystals.
• These rocks are extremely hard in nature.
• Fossils are not present in these rocks.

(iii) What is meant by sedimentary rock? Describe the mode of formation of sedimentary rock.

Answer

Sedimentary rocks are those rocks which are formed by the deposition at favorable sites by agents of denudation such as wind, river and sea waves. These deposits gradually turn into rocks.
The rocks 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.

(iv) What relationship explained by rock cycle between the major type of rock?

Answer

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 sedimentary rocks or metamorphic rocks.
• The fragments derived out of igneous and metamorphic rocks form into sedimentary rocks.
• Sedimentary rock can change into metamorphic rock or into igneous rock.
• Metamorphic rock can change into igneous or sedimentary rock.

3. Answer the following questions in about 150 words.

(i) Define the term ‘mineral’ and name the major classes of minerals with their physical characteristics.

Answer

A mineral is a naturally occurring 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 found.

Major minerals with their physical properties:
• Feldspar: It has light cream to salmon pink colour. It is used in ceramics and glass making. Half of the earth’s crust is composed of feldspar. 
• Quartz: It consists of silica. It is a hard mineral virtually insoluble in water. It is white or colourless and used in radio and radar.
• Pyroxene : It consists of calcium, aluminium, magnesium, iron and silica. It forms 10% of the earth's crust. It is commonly found in meteorites.
• 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.
• Mica : It comprises potassium, aluminium, magnesium, iron, silica etc. It forms 4% of the earth's crust. It is uses in the electrical instruments.
• 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.

(ii) Describe the nature and mode of origin of the chief types of rock at the earth’s crust. How will you distinguish them?

Answer

There are three chief types of rock at the earth’s crust:

• Igneous Rocks:  These are formed when magma cools and solidifies. The process of cooling and solidification can happen in the earth’s crust or on the surface of the earth.
→ These are extremely hard in nature.
→ 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 results in small and smooth grains.

• Sedimentary Rocks: 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. The deposited material is known as sediment and the rocks formed are called sedimentary rocks.
→ The sedimentary rocks are soft in nature.
→ These rocks have many layers of varying thickness.

• Metamorphic Rocks: These rocks form under the action of pressure, volume and temperature (PVT) changes. Metamorphism is a process by which already consolidated rocks undergo recrystallisation and reorganisation of materials within original
rocks.
→ These rocks are crystalline in nature.
→ The materials of rocks chemically alter and recrystallise due to thermal metamorphism.

(iii) What are metamorphic rocks? Describe the types of metamorphic rock and how are they formed?

Answer

Metamophic rocks are formed by the physical or chemical alteration by heat and pressure of an existing igneous or sedimentary rocks. 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.

Types of Metamorphic rocks:

• Foliated rocks: These rocks are formed in the inerior of the earth under extremely high pressures that are unequal occurring when the pressure is greater in one direction than in the others. These rocks develop a platy or sheet-like structure. Slate, schist are examples.

• Non-foliated rocks: These rocks are formed around igneous intrusions where the temperatures are high but the pressures are relatively low and equal in all directions. These are not flat or elongate. Marble, quartzite are examples.

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