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Chapter 6

Rocks

Class 9 - Total Geography Morning Star


Short Answer Questions

Question 1

State two points of distinction between rocks and minerals.

Answer

RocksMinerals
Rocks are aggregates of mineral elements.Minerals are solid inorganic substances occurring naturally.
A rock has no definite chemical composition.Minerals have a definite chemical composition.

Question 2

Name any three elements of the earth's crust.

Answer

Three elements of the earth's crust are iron, silicon and aluminium.

Question 3

Name three types of rocks.

Answer

Three types of rocks are-

  1. Igneous rocks
  2. Sedimentary rocks
  3. Metamorphic rocks

Question 4

Give two differences between Extrusive Igneous and Intrusive Igneous rocks.

Answer

Extrusive Igneous rocksIntrusive Igneous rocks
They are formed by the cooling of molten magma on the earth's surface.They are formed when magma solidifies within the the earth's crust.
These rocks are smooth, crystalline and fine grained.These rocks are coarse textured, hard rocks with large crystals.
For example- BasaltFor example- Granite

Question 5

Name any two characteristics of Igneous Rocks.

Answer

Two characteristics of Igneous Rocks are-

  1. They are hard and compact.
  2. They are formed by solidification of molten magma.

Question 6

State two characteristics of Basic Igneous Rocks.

Answer

Two characteristics of Basic Igneous Rocks are-

  1. They have higher percentage of oxides of denser elements and silica content varies between 40 to 60 percent.
  2. It is heavy and dark in colour.

Question 7

Name two important landforms made by Igneous Rocks.

Answer

Two important landforms made by Igneous Rocks are lava sheets and lava plateaus.

Question 8

What are Sills? Give an example.

Answer

Sometimes magma flows between layers of rocks horizontally. It then hardens there. This layer of intrusive rock is called sill. For example- Archuleta Mesa in southern Colorado.

Question 9

Which rocks are associated with ores of metals and which rocks are associated with fossil fuels?

Answer

Igneous rocks are associated with ores of metals and sedimentary rocks are associated with fossil fuels.

Question 10

Mention any two characteristics of Sedimentary Rocks.

Answer

Two characteristics of Sedimentary Rocks are-

  1. Sedimentary rocks are formed from materials derived from other rocks. They may also have plant and animal remains. These rocks my thus contain fossils.
  2. Sedimentary rocks are generally soft and layered as they are formed by deposition of sediments.

Question 11

Name the rocks which are most widespread on the earth. Give two examples of Sedimentary Rocks.

Answer

Sedimentary rocks are most widespread on the earth. Examples of sedimentary rocks are Gypsum and sand stone.

Question 12

Name the three stages of lithification of Sedimentary rocks.

Answer

The three stages of lithification of Sedimentary rocks are-

  1. Evaporation
  2. Compaction
  3. Cementation

Question 13

Name the types of Sedimentary rocks based on agents of formation.

Answer

The types of Sedimentary rocks based on agents of formation are-

  1. Riverine rocks
  2. Lacustrine rocks
  3. Glacial rocks
  4. Aeolian rocks
  5. Marine rocks

Question 14

Which agents are responsible for deposition of sediments?

Answer

The main agents responsible for deposition of sediments are water, wind and ice.

Question 15

What are known as metamorphic rocks? Give two examples.

Answer

Metamorphic rocks are formed by complete change of texture and composition of pre-existing rocks through volcanism and diastrophism. For example, slate is formed from clay and graphite from coal.

Question 16

Briefly define Mechanical Metamorphism.

Answer

Mechanical metamorphism is the mechanical transformation of pre-existing rocks, in which texture of rocks is changed by crushing and rubbing processes associated with heat and compression created by mechanical action.

Explain these terms associated with rocks

Question 1

Extrusive Igneous Rocks

Answer

These rocks are formed by the cooling of molten magma on the earth's surface. The magma which is brought to the surface through fissures or volcanic eruptions, solidifies to form rocks. Hence, these rocks are smooth, crystalline and fine grained.
For example- basalt

Question 2

Laccoliths and Batholiths

Answer

Laccoliths are formed when the magma becomes solid just below teh crust. The upper surface is dome shaped but the bottom is flat.

Batholiths are deep seated intrusion of igneous rocks. They are usually dome shaped with no definite floor and they form cores of mountain ranges.

Question 3

Fossil fuels

Answer

With increased heat and pressure, fossils are turned into fossil fuels. Fossil fuels are found in sedimentary rock, which is often located beneath a body or former body of water such as an ocean or swamp. Fossil fuels are nonrenewable resources that include oil, natural gas, and coal.

Question 4

Lithification of Rocks

Answer

The process that turns loose sediments into hard rocks is called lithification.

Question 5

Metamorphism.

Answer

Metamorphism refers to the alteration of the composition or structure of a rock by heat, pressure or other natural agency.

Distinguish between each of the following

Question 1

Plutonic and Volcanic rocks.

Answer

Plutonic rockVolcanic rock
They are also called intrusive rocks.They are also called extrusive rocks.
They are formed when magma solidifies within the the earth's crust.They are formed by the cooling of molten magma on the earth's surface.
These rocks are coarse textured, hard rocks with large crystals.These rocks are smooth, crystalline and fine grained.
For example- graniteFor example- basalt

Question 2

Thermal and Dynamic Metamorphism.

Answer

Thermal MetamorphismDynamic Metamorphism
It occurs when the transformation of the original rock takes place due to the influence of high temperature.It occurs when the transformation occurs mainly because of pressure at a great depth within the earth's crust.
For example- slate is formed from clayFor example- marbles used in Taj Mahal at Agra are metamorphised from Dolomites.

Question 3

Sills and Dykes.

Answer

SillsDykes
Sometimes magma flows between layers of rocks horizontally and hardens there. This layer of intrusive rock is called sills.The magma when forced upwards fills vertical cracks or fissures in existing rocks and then hardens there to form dykes.

Question 4

Calcarious and Carbonacious rocks.

Answer

Calcarious rocksCarbonacious rocks
These rocks are formed by the deposition of shells and skeletons of sea organisms like corals, oysters etc.These rocks are formed by the sea plants which remain buried for a very long time .
For example- chalk, limestone.For example- coal, peat.

Question 5

Acid Igneous Rocks and Basic Igneous Rocks.

Answer

Acid Igneous RocksBasic Igneous Rocks
These rocks have silica content between 65% to 85 %. They lack in iron and magnesium.These rocks have higher percentage of oxides of denser elements and silica content varies between 40% to 60%.
For example- granite.For example- basalt

State the type of rocks for the formation of which the following processes are involved

Question 1

Solidification of magma on the surface of the earth.

Answer

Extrusive igneous rocks

Question 2

Formation of large crystals, coarse texture and slow cooling and compaction.

Answer

Intrusive igneous rocks

Question 3

Accumulation takes place over long periods of time in seas, lakes and streams.

Answer

Sedimentary rocks

Question 4

Decomposition of organic matter at different stages and over different periods of time.

Answer

Carbonaceous Sedimentary rocks

Structured Questions

Question 1(a)

Distinguish between rocks and minerals.

Answer

RocksMinerals
Rocks are aggregates of mineral elements.Minerals are solid inorganic substances occurring naturally.
A rock has no definite chemical composition.Minerals have a definite chemical composition.
Minerals are organised to form rocks.Elements are organised to form compounds which are known as minerals.
For example- basalt, granite etc.For example- iron, silicon etc.

Question 1(b)

Describe how igneous rocks are formed. State their main characteristics.

Answer

Igneous rocks are formed due to cooling, solidification and crystallisation of magma found at great depths in the interior of the earth.

The main characteristics of igneous rocks are-

  1. They are hard and compact.
  2. They are formed by solidification of molten magma.
  3. They are granular and crystalline.
  4. They are either fine grained, smooth and compact or may have large crystals with coarse texture.
  5. The size of their crystals depends upon the rate of cooling of the molten material.
  6. These rocks are less affected by chemical weathering because water does not percolate in these rocks.
  7. Most of them consist of silicates.
  8. They do not have layers. They are generally weathered by mechanical weathering.
  9. They are associated with volcanic activity and are found mostly in volcanic regions.

Question 1(c)

Give a reason for each of the following:

(i) Igneous rocks also called the primary rocks.

(ii) Igneous rocks are used by builders and sculptors.

(iii) Sedimentary rocks are called secondary rocks.

Answer

(i) Igneous rocks also called the primary rocks as they form the basis of formation of other types of rocks.

(ii) Igneous rocks are used by builders and sculptors because these rocks are easily available, economical, and feasible in nature. These rocks are very strong, which is considered the best option for the construction of strong structured statues and buildings.

(iii) Sedimentary rocks are called secondary rocks because they are formed by the solidification of sediments of original igneous, sedimentary or metamorphic parent rocks.

Question 1(d)

Define different types of igneous rocks on the basis of their chemical composition.

Answer

Igneous rocks are classified into two types on the basis of their chemical composition-

  1. Acid Igneous rocks — These rocks have silica content between 65% to 85 %. They lack in iron and magnesium. For example- granite.

  2. Basic Igneous Rocks — These rocks have higher percentage of oxides of denser elements and silica content varies between 40% to 60%. These are heavy and dark in colour. For example, Basalt.

Question 2(a)

How are sedimentary rocks formed?

Answer

When igneous rocks are exposed on the surface of the earth, they are broken down by weathering and carried away by forces of gradation. They are then deposited as sediments over long periods in seas, lakes, streams etc. Subsequent layers are added and due to their own weight from above, the sediments get cemented together to form sedimentary rocks.

Question 2(b)

Explain the formation of sedimentary rocks on the basis of agents of formation.

Answer

There are five main types of sedimentary rocks on the basis of agents of formation:

  1. Riverine Rocks — These are formed by the sediments deposited by running water or river.
  2. Lacustrine Rocks — These are formed by the deposition of silt etc on the bed of lakes.
  3. Glacial Rocks — These are formed by the debris carried on and deposited by glaciers.
  4. Aeolean Rocks — These are formed by the deposition of sand particles accumulated by wind.
  5. Marine Rocks — These are formed by the deposition of sediments at the ocean floor. These are of two types- Calcarious sedimentary rocks and Carbonaceous sedimentary rocks.

Question 2(c)

Give a reason for each of the following:

(i) Mechanically formed sedimentary rocks are also termed as stratified rocks.

(ii) Rocks like peat, lignite and anthracite are called carbonaceous rocks.

(iii) Fossils are present in sedimentary rocks.

Answer

(i) Mechanically formed sedimentary rocks are also termed as stratified rocks because such rocks are formed in layers.

(ii) Rocks like peat, lignite and anthracite are mainly made out of carbon composition (almost 60-90%). Due to this majority of carbon composition in these rocks, these rocks are known as carbonaceous rocks.

(iii) Fossils are found in sedimentary rocks, where gentler pressure and lower temperature allows preservation of past life-forms. Fossils become a part of sedimentary rocks when sediments such as mud, sand, shells and pebbles cover plant and animal organisms and preserve their characteristics through time.

Question 2(d)

What is metamorphism? What are its causes? Give an example of rocks formed by heat and pressure.

Answer

Metamorphism refers to the alteration of the composition or structure of a rock by heat, pressure or other natural agency. Metamorphism is caused by heat and pressure.
For example- graphite.

Question 3(a)

State the chief characteristics of metamorphic rocks.

Answer

The chief characteristics of metamorphic rocks are-

  1. They are harder and more compact than their original form.
  2. Most of them are impermeable.
  3. They do not have fossils.
  4. They are formed due to change in texture and composition of the pre-existing rocks.
  5. New minerals are formed during the process of metamorphosis.

Question 3(b)

State the economic significance of rocks.

Answer

The economic significance of rocks are-

  1. Rocks are of great resource value, some directly and some as constituents of minerals.
  2. Soils are derived from weathering of rocks.
  3. Almost all types of building materials used for paving roads, floors or building walls of houses or various other structures including bridges come from rocks.
  4. Rocks are a source of precious metals like gold, silver, platinum etc.
  5. Apart from minerals and fossil fuels, even rock wastes have now been used in manufacturing various articles.

Question 3(c)

What is Rock Cycle? How does it keep the earth young?

Answer

Rocks undergo cyclic transformation. The continuous process of transformation of old rocks into new ones is known as rock cycle. The cycle may also pass through the process of formation of sedimentary, metamorphic and igneous rocks.

To keep the earth young, rocks melt again resulting in the formation of igneous rock. This disintegrated material again forms sedimentary rock, which takes hundreds and thousands of years.

Question 3(d)

Draw a well labelled diagram of Rock Cycle.

Answer

Draw a well labelled diagram of Rock Cycle. Rocks, Total Geography ICSE Class 9.
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