class 9 ICSE Solutions

ICSE Solutions for Class 9 Geography – Volcanoes

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Exercises

I. Short Answer Questions.

Question 1.
What are known as volcanoes ?
Answer:
Volcanoes are the vents in the earth’s crust erupting hot magma from the interior core motivated by endogenic forces.

Question 2.
What is the difference between magma and lava ?
Answer:
Molten material moving under the earth’s crust is called magma. When magma comes to the surface of the earth it is known as lava.

Question 3.
Give one example each of an active volcano and a dormant volcano.
Answer:

• Active Volcano – Mt. Stromboli and Mt. Etna in Italy
• Dormant Volcano – Mt. Kilimanjaro of Africa.

Question 4.
What is the difference between dormant volcano and an extinct volcano ?
Answer:
Dormant volcano can erupt at any interval but extinct volcano can never erupt again.

Question 5.
What is the magma chamber of a volcano ?
Answer:
The magma chamber is created by the molten magma itself by melting the surrounding rocks in the form of a huge chamber.

Question 6.
Name two types of landforms made by volcanoes.
Answer:
Two types of landforms are :

1. Extrusive landforms :
   Extrusive landforms include crater composite cones caldere lava platforms.
2. Intrusive landforms :
   Intrusive landforms include dykes batholiths laccoliths volcanic hill etc.

Question 7.
What is called the Pacific Ring of Fire ? Why is it called so ?
Answer:
There are 80% active volcanoes around the Pacific ocean which is called the Ring of Fire.

Question 8.
Name the three types of volcanoes on the basis of the frequency of their eruption.
Answer:
There are three types of volcanoes e.g. active volcano which is still active in erupting magma dormant volcano which erupts in uncertain intervals and extinct volcano which has stopped eruption permanently it is also called dead volcano.

Question 9.
What are known as Shield volcanoes ?
Answer:
A volcanoes erupting with plentiful lava spreading over the surface into several kilometres with huge circumference and taking a shape of shield are called as shield volcanoes.

Question 10.
Mention any two extrusive landforms caused by volcanic eruptions.
Answer:

1. Composite cone : The volcano which erupts both lava and pyroclastic rocks form alternating layers of these two materials and build up to form composite cones. Examples : Fujiyama in Japan Vesuvius and Stromboli in Italy.
   
2. Caldera : During repeated eruptions the summit of a valcano may be blown up. In its place a large depression called caldera is formed. These are generally formed when the magma chamber is no longer able to emit sufficient magma and results in the collapse of a cone either partly or wholly.
   

Question 11.
Name any two intrusive landforms made by volcanic eruptions.
Answer:

1. Batholiths : Large sized intrusions in igenous rocks. They occur at considerable depth and come to the surface in course of mountain building activity.
2. Laccolith : These are formed when magma spreads laterally in a dome shape. The dome also forces the overlying strata to bulge upward. The projecting landform is subjected to erosion and denudation. Thus laccolith comes to the surface.
   

Question 12.
How are hot springs formed ?
Answer:
The movement of magma heats up the underground water which converts into steam and gushes out through any crack or holes on the crust.

Question 13.
What is called the Pacific Ring of Fire ? Why is it called so ?
Answer:
There are 80% active volcanoes around the Pacific ocean which is called the Ring of Fire.

Question 14.
Give an example each of conical volcano and fissure volcano.
Answer:
Mount Fuji in Japan is a conical volcano and Columbia plateau in South America and Deccan plateau in India are the examples of fissure volcanoes.

Question 15.
What is the difference between dormant volcano and an extinct volcano ?
Answer:
Dormant volcano can erupt at any interval but extinct volcano can never erupt again.

II. Match the following

Answer:

III. Fill in the blanks below

1. The forces arising from the interior of the earth are called endogenic forces.
2. The molten rock that reaches the surface of the earth is called magma.
3. A lava shield is made up of basic lava flows solidified away from the vent.
4. Vents are intrusions of igneous rock that are vertical in shape.
5. The Circum-Paciflc Belt is also called Pacific Ring of Fire.

IV. Long Answer Questions

Question 1.
Explain the various parts of a volcano.
Answer:
A volcano represents some typical parts e.g. the molten rock or magma mostly lava makes the conical body of a volcano. Magma chamber is the source of erupting magma. Vent is the main channel through which the magma erupts outwards. Crater is the mouth or uppermost part of a volcano which is a big hole from which magma erupts and spreads all over.

Question 2.
Describe the causes of volcanic eruptions.
Answer:
The main causes of volcanic eruptions are the following :

1. Heat and Pressure inside the Earth : Temperature and pressure both increase from the surface towards the centre of the earth. Rocks are bad conductors of heat. So the earth’s heat does not escape on its own. Instead it melts the rocks and builds up great pressure. The pressure forces the heat to find an escape route through fissures and cracks in the rocks.
2. Plate Tectonics : Most volcanic eruptions take place near plate margins. The same forces that cause earthquakes also cause volcanic eruptions.
3. Magma Chamber : The molten material while still under the earth’s crust melts weak rocks and creates a huge chamber for itself. Fresh magma continues to pour inside the chamber. Since magma contains silicate materials gases and water vapours the pressure always acts vertically upwards. Once a route is found it rises to the surface.

Question 3.
Explain briefly the landforms created by volcanoes on the surface of the earth.
Answer:
The landforms created on earth’s surface are called Extrusive Landforms. These include volcanic plateaus volcanic mountains and volcanic plains.

(a) Volcanic Plateaus : These are derived from lava which flows from volcanic eruptions. The Plateau of Peninsular India especially the north western Deccan is an example of lava plateau. Other examples are South African plateau Columbian Plateau and Ethiopian plateau.
(b) Volcanic Mountains : These mountains are built from material ejected from the fissures in the earth’s crust. Volcanic mountains are the most diverse because there are great differences in volcanic eruptions as well as the materials they throw up.
(c) Volcanic Plains : These plains are vast and smooth formed by extensive volcanic flooding from volcanic centres. The Western Victorian Plains in Victoria Australia are the finest examples of Volcanic Plains.

Question 4.
Write any three destructive effects of volcanoes.
Answer:
Destructive Effects :

1. When the volcanoes erupt they destroy life and property. The hot lava together with ash and dust destroy not only human beings but also animals as well as plant life.
2. Agricultural fields are covered with layers of volcanic ash and dust making them unsuitable for cultivation.
3. Huge clouds are formed after the eruption of volcanoes causing heavy rains which result in floods and landslides.
4. Volcanoes also emit poisonous gases which pollute the environment and cause health problems.
5. Explosive volcanoes in ocean islands are followed ‘by high Tsunami waves. They flood the area and destroy property people animals and crops.

Question 5.
Describe the distribution of volcanoes in the world.
Answer:
Volcanoes are found along the weak zones of the earth’s crust where due to continuous tussle by tectonic forces maximum active volcanoes have created. These are

1. Cirum-Pacific Belt : It is also known as ‘ring of fire due to consisting 75 % active volcanoes of the world.
2. Mid-World Mountain Belt or Mid Continental Belt : It extends along the fold mountain zone of Alpine-Himalayan Region. Although this belt is noted for drastic earthquake but these are some of the very destructive volcanoes e.g. Stromboli Vesuvius Mt. Etna Mt. Pelee (West Indies). Out of these belts there are also various volcanoes scattered here and there.

Question 6.
Describe important volcanic landforms on earth.
Answer:
Prominent landforms associated with volcanoes are composite cones built of multiple material erupted out from a volcano. Conical hills are common in volcanic areas Crater lake is formed along the mouth of a conical volcano which is filled with water after cooling down of volcano. Due to repeated eruptions mouth of a volcano is converted into a large depression called caldera. Lavashields are made by large amount of basic lava flows making volcanic shields with a gentle slope.
Lava plateau : is formed by large scale fissure eruptions covering extensive area e.g Columbia plateau in South America and Deccan plateau of India.

Practice Questions (Solved)

Question 1.
Name three causes of volcanic eruptions.
Answer:

• Hot interior of the Earth
• Steam and gases
• Faults and fissures.

Question 2.
Name the largest active volcano in the world.
Answer:
Mauna Loa (Hawaii islands).

Question 3.
Which volcano is known as the ‘light house of the Mediterranean ?
Answer:
Stromboli.

Question 4.
Name the three belts where volcanoes are found.
Answer:

• Circum Pacific belt
• Mid-world mountain belt
•  African rift valley.

Question 5.
In which belt most of the volcanoes of the world are found ?
Answer:
Circum Pacific Belt.

Question 6.
Name three causes of Earthquakes.
Answer:

• Volcanic eruptions
• Tectonic causes
• Elasticity of Rocks

Question 7.
What is epicentre ?
Answer:
The point on Earth’s surface vertically above the focus is called epicentre.

Question 8.
Indicate the world distribution of active volcanoes.
Answer:
There are about 500 active volcanoes. Most of them are confined to Circum-Pacific Belt and Mid World Mountain Belt.

Question 9.
Give two reasons why tremors occur inside the earth?
Answer:

1. Movements of the earth’s crust along lines of weakness produces great tremors.
2. During volcanic activity movement of lava beneath the crust also causes tremors inside the earth.

Question 10.
Distinguish between :

1. Seismology and Volcanology.
2. Volcanic Dust and Volcanic Ash.

Answer:

1. Seismology and Volcanology : Seismology is the science of study of earthquake and Volcanology is the science of study of volcanic phenomena.
2. Volcanic Dust and Volcanic Ash : The finely pulverised fragments of rock are called volcanic ash and very fine particles which blow into air are called volcanic dust.

Question 11.
Describe the materials thrown out during volcanic eruption.
Answer:
The materials thrown out of a volcano due to eruptions are of three types :

1. Solid materials :- The solid materials include large fragments of rocks known as volcanic Bombs. The finest particles include cinder volcanic ash dust.
2. Liquid materials :- The liquid materials include basic lava and Acid lava. Acid lava has more silica while Basic lava has low content of Silica.
3. Gaseous materials :- The gaseous material is mainly composed of steam. The other gases include Oxygen Hydrogen Sulphuric acid Carbon dioxide etc.

Question 12.
Describe the effect of volcanic eruption of Karakatoa in 1883.
Answer:
Karakatoa island is situated between the islands of Java and Sumatra in the Sunda strait. It suddenly erupted in 1883. The top of the mountain was blown away by the explosion 16 metres high tidal waves were caused killing 36,000 persons in west Java. Volcanic dust rose to a height of about 27 kilometres. This dust encircled the globe for 3 years. Its effect caused strange sunrise and sunset conditions.

Question 13.
Why is volcanic activity often associated with mountain building ?
Answer:
Most of the active volcanoes are formed along the fold mountains such as the Himalayas, The Alps, The Andes etc. Fold mountains have been formed by mountain building movements. These involve intense folding and faulting which mark lines of weakness on the surface. Most of the eruptions take place along these lines of weak.

Question 14.
Describe the materials thrown out during volcanic eruptions.
Answer:
The materials thrown out of a volcano due to eruptions are of three types :

1. Solid Materials : The solid materials include large fragments of rocks known as volcanic bombs. The finest particles include cinder volcanic ash and dust.
2. Liquid Materials : The liquid materials include Basic lava and Acid lava. Acid lava has more of silica while Basic lava has low content of silica.
3. Gaseous materials : The gaseous material is mainly composed of steam. The other gases include Oxygen Hydrogen Sulphuric acid Carbon dioxide etc.

Describe any three advantages of volcanoes.

1. Many minerals from the interior of the Earth come on to the surface.
2. Fertile soils like Black soil are made by breaking up of lava.
3. Electricity is generated from gases emitted out during volcanic eruptions.

Question 15.
Why are Earthquakes related to volcanoes ?
Answer:
There is a close relationship between an Earthquake and a volcano Earthquakes and volcanoes occur in same belts i.e. mid-world belt and Circum Pacific belt. Their distribution shows a similar pattern. Volcanic eruptions lead to Earthquakes. Volcanic eruptions are the local cause of Earthquakes.

Question 16.
What is a geyser ? Give two of its main characteristics. Name a well known geyser.
Answer:
Geyser : Geysers are fountains of super heated steam and hot water that is usually emitted with an explosion trigged off by gases escaping from below
Main Characteristics of Geysers

1. Water in a geyser gets heated up beyond its boiling point.
2. They may spout to a height of over 150 feet.

A well known geyser is ‘Old Faithful’ in Yellow Stone National Park of Wyoming. It erupts regularly every hour and attracts a large number of tourists.

Question 17.
State two ways in which lava may come out of the earth’s crust giving an example of each type of these volcanic activities.
Answer:
Two main types of volcanic eruptions are :

• Explosive and
• Quiet or Hawaiian.

1. The Explosive eruption are the most violent and destructive Explosion of pent up gases mainly steam cause enormous quantities of magma to be thrown into the air to form great clouds. Example Krakatoa volcano in Sunda Strait in Indonesia erupt in 1883.
2. The Quiet eruption cause and other gases to escape. There is no violent explosion. Lava flows the creaters and flows down the sides of the cone.
   Example : Hawaiian volcanoes and Maupa Loa.

Question 18.

(a) What do you understand by ‘Vulcanism’ ?
(b) What are ‘Volcanoes’ ?
(c) How are volcanoes formed ?
(d) Differentiate between active dormant and extinct volcanoes.
(e) What is‘magma’?
(f) What do you understand by ‘Crater of the Volcano’ ?

Answer:

(a) The interior part of the earth is extremely hot temperature upto 2000°C. in which every matter converts into molten form. This molten material ‘magma’ always tries to burst out whereever it finds any crack or hole in the crust. The process of erupting out of magma is known as ‘vulcanism’.
(b) ‘Volcanoes’ are the outlets of magma through a vent or cracks in the form of a conical volcanic hill or through various holes known as fissure eruption.
(c) Volcanoes are formed by the eruption of magma from the interior and deposited on the land surface and after cooling down take the shape of volcanoes.
(d) Active volcanoes go on erupting magma continuously e.g. stromboli (North of Sicily). Dormant volcanoes erupt in accidental intervals e.g. Mt. Vesuvius in Italy. Extinct volcanoes stop eruption for ever. e.g. Mt. Fujiyama in Japan.
(e) ‘Magma’ is the one word of several matters erupting out of a volcano e.g. lava, steam, cinderellas, stones, cinder, smoke etc.
(f) Crater of a volcano is the mouth of volcano in its vertex in the form of a circular hole.

Question 19.

(a) Describe the distribution of volcanoes in the world.
(b) What are the influences of volcanic eruptions on man ?

Answer:
(a) Volcanoes are found along the weak zones of the earth’s crust where due to continuous tussle by tectonic forces maximum active volcanoes have created. These are :

1. Cirum-Pacific Belt— It is also known as ‘ring of fire due to consisting 75% active volcanoes of the world.
2. Mid-World Mountain Belt or Mid Continental Belt— It extends along the fold mountain zone of Alpine-Himalayan Region. Although this belt is noted for drastic earthquake but these are some of the very destructive volcanoes e.g. Stromboli Vesuvius Mt. Etna Mt. Pelee (West Indies). Out of these belts there are also various volcanoes scattered here and there.

(b) The effects of volcanoes on human life are both positive and negative :
Destructive influences : The deposition of lava makes the area very porous which creates water problem. The major flow of hot lava of Etna destroyed the whole area and property in Sicily. Mt. Vesuvius destroyed the city of Pompeii seven times since 79 AD. and the city of Herculaneum.
Constructive Influences : Volcanoes have provided some fertile lands e.g. Java and Deccan plateau and areas of South Brazil. Various precious minerals come out on the upper part of the earth’s crust through eruption e.g. Diamonds of Kimberley and gold of Johannesburg in South Africa nickel deposits of Sudbury in Canada.

Question 20.

(a) What is an ‘earthquake’ ?
(b) Give two major causes of earthquakes.
(c) Describe the world’s distribution of earthquakes.
(d) Mention some of the main effects of earthquakes.
(e) Name the major earthquakes of India from 1991 to 1997.

Answer:

(a) An earthquake is tremor or convulsion of the earth’s crust due to sudden movement of the crust.
(b) Two major causes of earthquakes are faulting associated with tectonic forces and the movements due to volcanic eruptions.
(c) The earthquakes are distributed along two major belts namely Circum Pacific earthquake belt (Ring of fire) and the Mid-World mountain earthquake belt along the great fold mountain zone.
(d) The destructive effects of earthquakes are very dangerous e.g. disruption of the rocks bed land slides changing the river courses floods tides collapsing of buildings destruction of transport lines and fires in electric wiring etc.
The constructive effects are the creation of additional coastal plains inlets bays for good harbours creation of fissure- openings to form sulphur or hot springs etc.
(e) Major earthquakes of India from 1919 to 1997.

• Latur 1991
• Uttarkashi 1993

Question 21.
What are the following

(a) Fissure type of volcanoes
(b) Spine or plug
(c) Caldera
(d) Mud volcanoes
(e) Epicentre
(f) ‘Ring of Fire’
(g) Cinder Cone

Answer:

(a) Some times the volcanic eruption takes place through several small holes which is known as fissure type volcanoes.
(b) The volcanic cone made by the quick solidification of viscous lava and having steep slopes is known as spine or plug.
(c) Caldera is the spacious crater of a volcano created by tremendous eruption. Crater lake in Oregon U.S.A occupies a caldera about 9 kilometre in diameter.
(d) Mud Volcanoes — A volcanic cone made of the mud due to the eruption of muddy water is called a mud volcano.
(e) Epicentre — It is the place of surface position immediately above the origin or focus of an earthquake.
(f) ‘Ring of Fire’ — It is the belt around the pacific ocean where due to the weak crust 75% of active volcanoes exist forming a huge ‘ring of fire’ phenomenon.
(g) Cinder Cone — The volcanic cone built of small pieces and fragments of solidified lava and ash is called ‘Cinder Cone’.

Question 22.

(a) Distinguish between the following pairs of terms associated with vulcanicity

1. Lava and Magma
2. Acidic Lava and Basic Lava
3. Cinder Cone and Composite Cone
4. Fissure-type Volcanoes and Central-types Volcanoes
5. Crater and Caldera
6. Laccolith and Lapolith
7. Geysers and Hot Springs

(b) Distinguish between the following pairs of terms associated with crustal movement of the earth

1. Graben and Horst
2. Tilted Block mountains and Lifted Block mountains

Answer:
(a)

Question 23.
Give a brief account of ‘Plate Tectonics’.
Answer:
In the beginning all the continents were combined together known as ‘Pangasa’ which later on splitted away and separated. But still all continental and oceanic plates are sliding upon each other and result in various earthquakes and volcanoes. The continental drifting theory was discovered by German scientist Alfred Wagner in 1912 which supported this plate tectonics to prove the movement of the continental and oceanic plates working for changing the landforms of the crust.

Question 24.
Give reasons for the following

1. The Belts of volcanic activity and earthquakes are roughly the same.
2. Basic lava cones are broader than Acid lava cones.
3. The Circum-Pacific Belt of volcanoes is called ‘The Ring of Fire’.

Answer:

1. The volcanoes and earthquakes are associated with each other as every volcanic activity takes place by shaking and breaking the weak crust which naturally creates tremors and earthquakes within the earth’s crust.
2. Basic lava cones are formed by liquid lava which expands and covers a large area while the Acid cones are formed by solid material e.g. ash cinders etc which heap up but do not cover large area and form a high conical hill as compared to broad and low volcanic deposits on the surface.
3. The circum-pacific belt is truly known as “The Belt of Fire” or “The Ring of Fire” as 75% active volcanoes erupting fire are located in this belt.

Question 25.
Match the items given in Column A with the correct ones in Column B.

Answer:

Question 26.
Give one word for each of the following :

1. A narrow block elevated between two normal faults.
2. The funnel shaped hollow at the top of a volcanic cone.
3. The lava which is poor in silica and rich in iron and magnesium.
4. A volcano which has the possibility of erupting in future.
5. A large sill of acid lava which has solidified gradually giving a dome – like shape.
6. A volcano where magma reaches the surface through a vent or a pipe.
7. A volcano whose eruption buried and destroyed two Roman towns.
8. An instrument used for recording all the earth tremors and earthquakes.
9. The surface position immediately above the origin of an earthquake.
10. The region where there are highest number of geysers and hot springs.

Answer:

1. Horst
2. Crater
3. Basic-Lava
4. Dormant Volcano
5. Acid-Lava Dome
6. Central type volcano
7. Vesuvius
8. Seismograph
9. Epicentre
10. Yellow stone park (U.S.A.)

Question 27.

(a) Which type of lavas weather into more fertile soil. Name also one useful feature of volcanicity other than soil fertility.
(b) Which four of the following words are connected with volcanic activity :  Karst, crater, drumlin, stalactite, gully, potholesl, ash, basalt, swallow, holes, dyke, domes, bluffs.

Answer:

(a) Basic – type lava sheet weather into fertile soil e.g.Deccan trap soil. Other useful feature of volcanicity is that the precious minerals come out with the magma near the land surface e.g. diamond and gold etc.
(b) Crater, ash, dyke, domes.

Question 28.
What are tectonic movements ? How are these classified?
Answer:
Tectonic movements are changes through earth’s natural activities which are known as ‘diastrophism’. These movements are of two types e.g. vertical movement and horizontal movement.

Question 29.
Give reasons for the following :

1. Earth movements have modified the Earth’s surface.
2. Internal processes are different from external processes.
3. Folding and faulting frequently go together.
4. Earth as a whole does not expand.

Answer:

1. Earth movements like Continental Drift theory changed the whole face of the earth into distinct continents and oceans of today with highest mountains plateaus plains drainage system and so on.
2. Internal processes are associated with tectonic forces resulting in drastic changes e.g. earthquakes volcanoes etc. External processes are carried on by natural agents of change e.g. water wind and ice which produce gradual changes.
3. Folding and Faulting frequently go together because the stress on folding exceeds more than enough then folds break through fault line to two pieces slipping one upon another which is called faulting.
4. Inspite of various changes occurring within the earth it does not expand as it is affected by the centripital force of gravity working towards the centre of the earth.

Question 30.
How the theory of plate tectonics has explained the formation of mountains like Himalaya or Alps and of the volcanic islands.
Answer:
The formation of the highest mountains of Himalayas and Alps have been created by the bucking up of the geo synclines of tethys sea between Angaraland and Gondwanaland which pushed towards each other and forced the geosynclines to be lifted up forming the mountains. The Mid-Atlantic Ridge is the proof of the edge raised at the joint of continental plates which gives birth to several volcanic islands.

Question 31.
(a) Describe the distribution of volcanoes in the world.
OR
Name the important belts of volcanoes.
(b) What are the influences of volcanoes eruption on man?
OR
What is the importance of volcanoes ?
OR
Mention adverse and beneficial effects of volcanoes.
Answer:
(a) Obviously volcanoes will be found in those regions where the crust of the earth is weak because lava can easily be discharged from such places. These are found in areas of Fold mountains. There are three main belts of volcanoes :

1. Circum-Pacific Belt : This belt runs round the Pacific Ocean in Asia and the America. It starts from Cape Horn goes along the Andes and the Rockies to Alaska. From their it turns westwards and passing through Aleutian Islands Japan and Formosa goes to the Philippine Island. Here one branch goes to East Indies that is Java Sumatra and Bameo and other branch goes to New-Zealand. Some of the well known Volcanoes are Karakatoa (on a hilly island between Sumatra and Java) Mayon (N. Philippines) Fujiyama (Japan) Chimborezo and Cotopaxi (Equador S. America).
2. Mid World Mountain Belts : This belt starts from the West Indies and passing through the Canavy Islands. The Mediterranean sea Caucasus mountains and Turkey reaches the Himalayas. Mt. Vesuvius (Italy) Mt Stromboli (Sicily) Etna (the Mediterranean sea) and Mt. Pelee (West Indies) are the important Volcanoes of this belt.
3. African Belts : This belt follows the Great African Rift Valley. This belt running through Red Sea extends upto Africa. Important Volcanoes are Kilimanjaro Kenya Canary Islands St. Helena (Atlantic ocean).

(b) Adverse effects of volcanoes

1. Most of the recently formed volcanic areas are barren and forbidding to man.
2. The sudden flow of basic lava from an eruptive volcano may cause the total destruction of human life property and crops in the neighbouring areas.
3. The Karakatoa volcanic eruption (in 1883) caused such high sea waves that Karakatoa and several other neighbouring islands were completely destroyed.
4. Volcanoes eruption of Vesuvius in 79 A.D. buried and destroyed completely the two Roman towns of Pompeii and Herculaneum.

Beneficial effects of Volcanoes :

1. The soil made up of lava is very fertile.
2. Lakes are formed when the craters are filled up with waters.
3. Several minerals which are found deep down come up near the surface of the earth.
4. Lava flows have preserved many fossils which throw much light on the past life.

Question 32.
What are the following :

(a) Fissure type of volcanoes
(b) Spine or plug
(c) Caldera
(d) Mud volcanoes
(e) Epicentre
(f) ‘Ring of Fire’
(g) Cinder Cone

Answer:

(a) Fissure type of volcanoes : A volcano is a vent in the earth’s crust out of which hot molten rocks (lava) flow. The hot rocks may also eject violently in the form of solid pieces. If the vent is in the form of a long crack then it is knows as a Fissure Type Volcano. In this type volcanic activity occurs quietly the lava upwells silently and spreads over a large area giving rise to volcanic plateaux and extensive lava sheets.
(b) Spine or plug : Acid lava dome is formed when viscous lava solidifies quickly and gives rise to steep sloping cones. This is known as a spine or plug. Sometimes these spines and plugs are exposed by denudation.

(c) Caldera : In some volcanoes the summit of the volcano blows up during a violent explosion resulting in the formation of a large depression called a Caldera. Some calderas may have been formed by the collapse of the summit portion. Calderas are occupied by large lakes. The lake in the caldera is called the crater lake. In the state of Oregon in the United States there is a caldera which is about 9 km in diameter.

(d) Mud volcanoes : If water which erupts in a volcano is muddy then a conical mound of mud is formed within a creater at the top. This is known as a mud volcano. Such mud volcanoes are found in New Zealand Sicily and other areas of volcanic activity.
(e) Epicentre : The point on the earth’s surface directly above the ‘focus’ of earthquake is called epicentre.
(f) Ring of Fire ; The Circum-Pacific Belt of volcanoes encircles the Pacific Ocean in Asia and the Americas along the weak coastal crust. It is called the ‘Ring of Fire’ because there are a large number of active volcanoes in it. The belt begins from the volcanic islands of South America and includes the Andes mountains of the Central America and Mexico the western part of the Rockies in the U.S.A. Canada and Alaska.
(g) Cinder cone : When the lava is ejected from a central vent its pieces and fragments solidify round the vent to form a cone. This is known as a Cinder Cone.

Question 33.
Distinguish between the following pairs of terms associated with vulcaniaty

1. Magma and Lava
2. Acidic Lava and Basic lava
3. Cinder Cone and Composite Cone
4. Fissure-type Volcanoes and Central Type Volcanoes
5. Crater and Caldera
6. Laccolith and Lapolith
7. Geysers and Hot springs
8. Active Volcano and Dormant Volcano
9. Folding and Faulting
10. Volcanic Cone and Volcanic Plateau
11. Seismic Focus and Epicentre
12. Dykes and Sills

(b) Distinguish between the following pair of terms associated with crustal movement of the Earth

1. Graben and Horst
2. Tilted Block Mountains and Listed Block Mountains

Answer:
(i) Magma and Lava :
Magma :

1. Magma is hot sticky molten material.
2. It contains solutions of water and gases.
3. It comes out during volcanic eruptions.

Lava :

1. Lava is solidified magma.
2. Gases and water disappear after evaporataion.
3. It cools down as it comes into contact with atmosphere.

(ii) Acidic Lava and Basic lava
Acidic Lava :

1. It is highly viscous lava.
2. It is light coloured like granite.
3. It has low density.
4. It has a high percentage of silica.
5. It flows slowly and results in steepsided cones or lava domes.

Basic Lava :

1. It is highly fine and thin.
2. It is dark coloured like Basalt.
3. It has high density.
4. It is poor in silica.
5. It flows rapidly as thin sheets resulting in shield cones.

(iii) Cinder Cone : Volcanic cones are called Cinder cones when the material erupted consists of cinder and other solid particles. These cones have steep slopes because they consist of particles of large size.
Composite Cone : The volcanoes which start as cinder cone and grown into large volcanic hills with alternating layers of lava and ash are called Composite cones. These cones are formed due to an explosive eruption followed by eruption of lava. Explosive eruption leads to the formation of a layer of ash while lava solidifies as a sheet on the layer of ash. This is followed by a quiet period and then the process gets repeated.
(iv) Fissure type of volcanoes : A volcano is a vent in the earth’s crust out of which hot molten rocks (lava) flow. The hot rocks may also eject violently in the form of solid pieces. If the vent is in the form of a long crack then it is known as a Fissure Type Volcano. In this type volcanic activity occurs quietly the lava upwells silently and spreads over a large area giving rise to volcanic plateaux and extensive lava sheets.
Central-type Volcanoes : If the vent in the earth’s crust is of such type that the rock materials come out and mounds hills or cones are formed than the volcanoes formed are known as Volcanoes of the Central type. Vesuvius and Fuji Yama are the best examples of this type.
(v) Crater and Caldera : Crater forms the summit and Caldera the enlarged mouth or the sunken crater at the centre of a volcano. When water accumulates in a crater it forms a crater lake and in a caldera a lake like Taba lake of Sumatra. A crater is formed as a result of overflow of lava and calera as a result of subsidence.
(vi) Laccoliths : Laccoliths are large lens-shaped intrusions which assume a dome shape. They vary in thickness and extent. When laccoliths are exposed on the surface they form low hills.
Lapoliths : Lapoliths are saucer-shaped intrusive layer of solidified magma and sinks as shallow basins in rock-beds.

(vii) Hot Springs :

1. It is a stream of hot water issuing from the ground. The hot water flows unobstructed quietly and continuously.
2. Hot springs are common where joints fissures and porous beds allow the free exit of water to the surface. The water becomes hot when it comes into contact with the heated rocks or upper heated steam lying deep into the earth’s crust.

Geysers :

1. It is a hot spring which at regular or irregular intervals throws a jet of hot water and steam into the air.
2. In the case of geyser the fissure or vent connecting the source of hot water to the surface is very narrow which greatly increases the pressure and temperature of the hot water and so when water comes out it rise high into the air.

Question 34.
Give reasons for the following :

1. The Belts of volcanic activity and earthquakes are roughly the same.
2. Basic lava cones are broader than the Acid lava cones.
3. The Circum-Pacific Belt of volcanoes is called ‘The Ring of Fire’.

Answer:

1. The belts of volcanic activity and earthquakes are roughly the same because the movement of magma beneath the Earth is the main cause of earthquakes and volcanoes.
2. Basic lava cones are broader than the Acid lava cones because basic lava is very fluid and flows easily for a great distance before it solidifies where as acid lava is highly viscous and flows only for a short distance.
3. The Circum-Pacific belt of volcanoes is called “The Ring of Fire” because there are large number of fire volcanoes in it.

Question 35.

(a) Name one useful feature of vulcanicity other than soil fertility.
(b) Out of the following words write down the four that are connected with volcanic activity.
Karst, crater, drumlin, stalacities, gully, pot holes, ash, basalt, swallow holes, dyke, domes, bluffs.

Answer:

(a) The molten rocks of vulcanicity is of considerable environmental significance other than soil fertility since it is the direct or indirect cause of several classes of landforms. Basic lava weather into more fertile soils.
(b) Crater, ash, basalt, dyke are connected with volcanic activity.

Question 36.
Give reasons for the following :

1. Earth’s movements have modified the Earth’s surface.
2. Earth as a whole does not expand.

Answer:

1. The surface of the earth is undergoing constant change. Some of these changes take place all of a sudden as in the case of volcanic eruptions and earthquakes but most of them are gradual and slow. Due to these changes the sediments which were originally deposited in horizontal layers are found tilted bent broken and twisted. In certain regions the structures which were once at sea-level in the Baltic Sea are now well above the water. Recently it has been found that the larger part of the coast of Scandinavia is rising relative to sea-level but that of the Southern extremity is sinking. Along some coasts submerged forests and various human structures show that the land has not only risen but at places it has also submerged. All this has happened or is happening due to the Earth’s movements. Thus we find that the Earth’s movements have modified the Earth’s surface.
2. About six important Convection Current Cells with over-riding six large plates have been identified below the Lithosphere. The Mid-Oceanic ridges from edges of the plates. For example the Mid-Atlantic ridge is such a ridge. The molten matter from below adds new crust along such’ ridges. Taking the earth as a whole it has resulted in the spreading of the ocean floor at the rate of 1 cm to about 10 cm every year. It is called the constructive plate movement. At the same time the crust at the other edges of the plates in oceanic trenches gets destroyed. It is called the destructive plate movement Consequently a balance exists and the Earth as a whole does not expand.

Question 37.
Answer the following :

1. Some volcanoes erupt explosively
2.  Some volcanoes develop parasitic cones.
3. Hot springs are common in volcanic regions.
4. Earthquakes are common in the belt of young fold mountains.
5. Plate margins are zones of great volcanic activity.
6. Volcanic eruption is one of the main causes of earthquakes.
7. The vent of a volcano when blocked results in explosive eruption.

Answer:

1. Some Volcanoes erupt explosively because the vent of a volcano.may be blocked by some sticky material or rock which causes the explosion.
2. Some volcanoes develop parasitic cones because the main vent grows too high and develops a parasite or a branch l cone.
3. Hot springs are common in volcanic regions because undergound water gets heated by contact with hot magma.
4. Earthquakes are common in the belt of young fold mountains because these mountains are in a state of constant flux.
5. Plate margins are zones of great volcanic activity because collisions of plate margins produce the magma and allow it to escape through a vent, a fissure or a crack.
6. Volcanic eruption is one of the main causes of earthquakes bacause volcanic earthquakes are caused by gas explosions. Such earthquakes occur either simultaneously with eruption or more commonly in the period preceding an eruption.
7. It causes the pressure to build up which results in violent explosion.

ICSE Solutions for Class 9 Geography – Weathering

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Exercises

I. Short Answer Questions.

Question 1.
What is weathering ?
Answer:
Weathering involves disintegration or decay of solid rock due to change in temperature and weather and their impact on the composition of rock.

Question 2.
Give one point of difference between physical and chemical weathering.
Answer:
Physical weathering does disintegration of rock without chemical reaction and the chemical weathering does change in chemical compounds within rock.

Question 3.
What is known as exfoliation ? Name the processes involved in it ?
Answer:
Sudden change in temperature causes fissures in the rocks through which water penetrates to motivate chemical weathering along with sudden contraction and expansion due to change in temperature peels out the upper layer of rock known as exfoliation.

Question 4.
Name the four processes involved in chemical weathering.
Answer:
These are solution, carbonation, oxidation and hydration.

Question 5.
What is known as oxidation ?
Answer:
It is the process of reaction of minerals present in rocks to atmospheric oxygen.

Question 6.
Briefly describe biological weathering.
Answer:
Biological weathering is done by animals, insects plants and man, because these break up the rocks through making holes, root channels and construction of building, canals etc.

Question 7.
What are exogenic forces ?
Answer:
These are the external forces that sculpture the surface features of the land.

Question 8.
What is called denudation ? Name the processes involved in it.
Answer:
Denudation is the wearing away of landmass by various agents as water, wind and ice through various processes like weathering, mass movement, erosion and transportation.

Question 9.
Name the two processes of gradation.
Answer:
Two processes are denudation and aggradation.

Question 10.
What is the chief characteristic of weathering in tropical climates ?
Answer:
Tropical climates are noted for distinct dry and wet season and high rate of evaporation. During rainy season the oxides of iron and aluminium are dissolved in water and mix up with the soil to form laterite soils.

Question 11.
What is known as mass wasting ?
Answer:
The movement of loose material derived from the bed rock through weathering is called mass wasting.

Question 12.
Name any two slow movements of mass wasting.
Answer:
Slow movements of mass wasting an be divided into two
main types — Creep and Solifluction.
Creep : The slow downhill movement of debris is called soil creep.
Solifluction : The water mixed with soil forms clay which moves as a thick viscous fluid.

Question 13.
Give one example of rapid mass movement.
Answer:
Rapid mass movement includes large amount of debris, soil, boulders and rock pieces etc, e.g. landslides and sheet erosion or displacement of the upper rock strata in large amount to uncover the rocks lying below.

Question 14.
What is known as Sheet Wash ?
Answer:
It is the rapid movement of rock debris etc in heavy amount taking the form of a thick sheet of thick fluid of soil and clay removing settlement and vegetation cover, too.

II. Give a technical term for each of the following :

Question 1.
Disintegration or decomposition of rock.
Answer:
Weathering.

Question 2.
Peeling off of the outer layer of rock through contraction and expansion.
Answer:
Exfoliation

Question 3.
Expansion of minerals in rocks on coming into contact with rainwater.
Answer:
Hydration.

Question 4.
The leveling of land surface by erosion and deposition.
Answer:
Gradation.

Question 5.
The process in which a landform of lower level is upgraded to a higher level.
Answer:
Aggradation.

III. Say whether the following are ‘True’ or ‘False’.

1. Temperature is not a factor in physical weathering.
Answer. False

2. In dry climates mechanical weathering is dominant.
Answer. True

3. In Polar regions there is no chemical weathering.
Answer. True

4. External forces are engaged only in erosion.
Answer. False

5. Shear plane is the surface on which movement of a landslide takes place as a result of its breaking off.
Answer.True

IV. Long Answer Questions.

PQ. Describe the process of denudation and gradation.
Answer:
Denudation is the process of wearing away of rock-strata by water, wind and ice along with change in weather and temperature.
Gradation is the process of leveling down the whole landform gradually to ground level by various agents of change mainly water, (rivers), wind and ice (glaciers), etc.

Question 1.
Define weathering and describe the chief characteristics of weathering.
Answer:
Weathering is the process of breaking down of rocks but not its removal. It is described as disintegration or decomposition of a rock in size by natural agents at or near the surface of the earth.
Chief characteristics of weathering are disintegration of rocks, chemical change in rocks, change in the surface of land, formation of soil, several processes involved in weathering like temperature and weather change and reactions, transportation of rock material from one place to another resulting in formation of large plains like Northern plain of India.

Question 2.
Distinguish between physical and chemical weathering.
Answer:
Physical Weathering :

1. Rock disintegration without any change in chemical constituents of rocks takes place.
2. Factors such as temperature, moisture, pressure cause physical break-up of rocks.
3. It takes place in hot dry and cold areas due to rapid temperature changes.
4. Rocks are affected to great depths.
5. Its agents are temp erature and moisture.

Chemical Weathering :

1. Mineral in rocks are dissolved or altered.
2. Temperature, moisture, etc., cause minerals in rocks to dissolve in water or convert them into other minerals.
3. It takes place in hot and humid areas due to chemical action of minerals in rocks.
4. It mostly takes place near the surface of the earth.
5. Its agents are solution, oxidation, carbonation and hydration.

Question 3.
Describe chemical weathering mentioning the processes involved in it.
Answer:
Chemical weathering takes place with the help of agents as water, wind and various organic acids, reacting in the chemical composition of rocks which are then disintegrated. Four main processes are solution by dissolving salts in the rocks with water, carbonation works with the reaction of carbon dioxide with rocks, oxidation takes place through atmospheric oxygen reacting with minerals present in rocks, hydration is evident by mixing of rainwater with minerals in rocks, which expand by this and become heavy.

Question 4.
What is biological weathering ? State the main agents of biological weathering.
Answer:
Biological weathering is weathering caused by plants and animals. Plants and animals release acid forming chemicals that cause weathering and also contribute to the breaking down of rocks and landforms. It is also known as Organic weathering. Its main agents are animals, insects, plants and man. In all the cases, however, both physical disintegration and / or chemical decomposition are involved. Their role can be explained as follows :

1. Animals and Insects : Animals like rabbits, worms, moles and insects bring large quantities of fine material to the surface. Burrowing animals help to loosen the surface materials around the rocks facilitating their physical disintegration. Upon death, the decaying physical also provide many chemicals and acids for rock disintegration.
2. Vegetation : Roots of large trees reach deep into rocks and cause physical disintegration due to pressure. Most of : vegetation, however, prevents disintegration of rocks because it binds the surface layer and does not allow exposure of rocks beneath to the elements of weathering.

Question 5.
Describe the chief characteristics of weathering in different climates.
Answer:
Equatorial region is noted for more active chemical weathering due to excess of temperature and water ; tropical region is remarkable for formation of laterite soil by mixing of oxides of iron and aluminium in the soil due to change in heating and much evaporation.

Question 6.
State and explain the classification of mass movements.
Answer:
Mass movements are divided into slow and rapid movements. Slow movement includes Creep, movement of soil and debris and Solifluction or clay formation Rapid movement includes landslide, Earth Flow or heavy thick paste of soil down the slope, Mud Flow in the areas without vegetation cover and Sheet Wash in the form of large sheet flow snatching a thick rock strata uncovering the rocks below. It may also take place by human activities of settlement.
Mass movement has built many remarkable landforms as Himalayan lakes, Terraces, escarpment etc.

Practice Questions (Solved)

Question 1.
Name two processes involved in denudation.
Answer:
Weathering and Erosion.

Question 2.
What does the term denude mean ?
Answer:
To lay the rocks bare.

Question 3.
What is weathering ?
Answer:
Breaking up of rocks by agents related to atmosphere.

Question 4.
Name the gases involved in the process of chemical weathering.
Answer:
Weak acids, oxygen, carbon dioxide, hydrogen.

Question 5.
In which region underground water is an effective agent of denudation.
Answer:
Underground water is an effective agent of denudation in limestone regions.

Question 6.
Name few well known physical features caused by chemical weathering.
Answer:

1. Limestone and chalk landforms of Karst region of Yugoslavia.
2. Weathering of granite rocks in Malaysia.

Question 7.
Why is mechanical weathering also known as physical weathering ?
Answer:
Because it involves physical disintegration of rocks. Physical disintegration is quickest when rock is already weakened by chemical weathering. So it is primarily known as mechanical weathering.

Question 8.
What is weathering ? Illustrate the process graphically.
Answer:
The break-down but not removal of rocks is called weathering. It is a process of wearing away of land through forces of nature like weather, running water, ocean currents etc. Weathering is a single process and takes place in air.

Question 9.
Distinguish between :

1. Weathering and Denudation
2. Loess and Alluvium.

Answer:

1. Weathering and Denudation : Weathering is a process 4 of gradual disintegration of rocks by atmospheric or weather forces. Denudation is a process of wearing away of land and it includes weathering also.
2. Loess and Alluvium : Alluvial soils are formed by depositional activity of running water and loess by wind. Alluvium contains high mineral and humus content, and loess is highly permeable.

Question 10.
On what factors does the weathering depend ?
Answer:
Weathering depends upon :

• structure of rocks
• climate
• vegetation
• topography and slope of land.

Question 11.
What do you understand by Denudation ?
Answer:
Denudation is a general term which covers all the agents causing destruction or the wearing away of the rock surface of the earth together with their transportation and final deposition.

Question 12.
Name any two agents of denudation.
Answer:

1. The weather in which the chief agents are heat and cold which did water and frost
2. The running water

Question 13.
Name the gases which help in chemical weathering.
Answer:
The following atmospheric gases help in the process of chemical weathering:

1. Oxygen
2. Hydrogen
3. Carbon dioxide

Question 14.
What is humus ? How is it formed ? What is its significance in soil formation ?
Answer:
Humus : Humus is a dark substance formed in soils. It is a dead organic matter formed by decay of animals and plants. Trees, shrubs, grass and bacteria help in the formation of humus. In warmer climates, humus is destroyed by countless bacteria. In colder areas soils are rich in humus and it is collected in the soil. Tropical humid soils are poor in humus because it is consumed by bacteria.
Humus is vital to the fertility of soils. It provides nitrogen, phosphorus and calcium to the soils. It sustains other forms of life. It helps the weathering of minerals to add to fertility of soils. It increases water holding capacity of soils.

Question 15.
What is a badland topography ?
Answer:
A highly dissected land surface is known as badland. In sloping areas, gully erosion results in soil erosion. These gullies develop a ravine land a badland topography. This topography consists of ridges, earth pillars, ravines, escarpments, etc. This topography is found in Chambal Valley of Madhya Pradesh in India.

Question 16.
Explain the processes of physical weathering giving examples.
Answer:
The disintegration of the bedrock due to changes in temperature and freezing action of water by mechanical process is called physical weathering e.g. frost or snow lying between the cracks forces to expand the rocks cracks which helps in disintegration. Besides this, the change in temperature does the expansion and contraction of rocks resulting in breaking the rocks into smaller pieces.

Question 17.
Distinguish between the following

(a) ‘Weathering’ and denudation.
(b) ‘Mechanical Weathering’ and ‘Chemical Weathering’.
(c) Sheet erosion and gully erosion.

Answer:
(a) Weathering :

• Weathering is the physical and chemical weathering of rocks due to mechanical and chemical action of water, wind and ice.

Denudation :

• Denudation includes both weathering and erosional proceses by which the natural agents of change (water, wind and ice), continously try to change the face of the earth.

(b) Mechanical Weathering :

• It is done by the change in temperature and action of ice which breaks the rocks into small pieces.

Chemical Weathering :

• It is done by oxidation hydration, carbonation and solution etc chemical processes which causes the breaking of rocks.

(c) Sheet Erosion :

• Sheet Erosion causes washing out the upper layer of soil and vegetation cover due to heavy rainfall or floods.

Gully Erosion :

• Through gully erosion deep cuts, cracks and trenches are carved in the soil-bed, These tend to be deeper channels known as ‘gullies’.

Question 18.
What do you understand by the following terms :

(a) Exfoliation
(b) Regolith
(c) Oxidation
(d) Carbonation
(e) Desilication
(f) Humus

Answer:

(a) Exfoliation — It is the process of scaling off the curved outer shells of the rock. Exfoliated dome-shaped rocks are largely found in great deserts.
(b) Regolith — It is the loose material created by the weathering of rocks i.e. small pieces of rocks.
(c) Oxidation — Formation of oxygen compounds with the contact of air with rocks e.g. iron-oxide etc. is known as oxidation.
(d) Carbonation — Formation of carbon compounds with the contact of air with rocks e.g. Calcium Carbonate, which is easily carried on by ground water.
(e) Desilication — The solution of silica of the rocks with water is called desilication.
(f) Humus — It is the decayed vegetation material found in the soil.

Question 19.

(a) What are different types of soil according to their texture ?
(b) How are the different kinds of soils in Temperate zones formed ?

Answer:
(a) According to the texture (sizes of the soil particles), the soil is divided into three main types i.e. Clay, loam (silt) and sandy loam (sand gravel), the diameter of soil particles ranges from .0002 mm to more than 2 mm. Soil structure refers to the physical arrangement of the soil.
(b) The kinds of temperate zone soils may be classified as follows.
Name :

1. Podzol
2. Cheronozems region
3. Brown and Grey soils
4. Chestnut soils

Region :

1. Cool and cold coniferous forest
2. Grassy steppes and semi arid lands
3. Deciduous forest region of high and middle latitudes
4. Arid margins of steppes.

Question 20.
Rewrite the following sentences, choosing the right word from those given in brackets :
Answer:

• (Humus / Regolith) is produced by the decayed organic material, including dead leaves, stems roots, living bacteria, fungi, worms and other organisms.
• (Weathering / Denudation) refers to the gradual disintegration and decomposition of rocks which lie exposed to the elements of weather.
• (Young / Mature) soils are those which are undisturbed by erosion or deposition.
• (Hydration / Carbonation) occurs when the water itself combines chemically with some element or mineral.
• (Oxidation / Solution) is the chemical process in which some of the minerals are dissolved by the water.

Question 21.
Define the following terms briefly :

(a) Soil texture
(b) Soil structure
(c) Soil profile
(d) Soils
(e) Chernozems

Answer:

(a) Soil texture — It is the make up of the soil referring to the sizes of the soil particles.
(b) Soil structure — The structure of the soil may be blocky, granular, prismatic or columnar and platy which decides the movement of the soil water.
(c) Soil profile — The vertical structure of the soil from the bedrock below upto the surface soil is called the soil profile.
(d) Soils — The soils are the aggregate of loose, unconsolidated material along with micro-organism mineral compounds, humus etc.
(e) Chernozems — are the richest soils found in the grassy steppes and are best for wheat cultivation.

Question 22.
What are the various factors governing the formation of soil ? Which one is the most important and why ?
Answer:
The factors influencing the formation of soils are the parent rock, topography, vegetation, climate and time span. Out of these climate is the most important because it controls the type of weathering the first step of the soil formation process.

Question 23.
Match the following pairs correctly


Answer:

Question 24.
Explain the processes of physical weathering giving examples.
Answer:
Physical weathering : It is also called Mechanical weathering. It includes the breaking up of rocks without changing their composition. It means disintegration of rocks by mechanical means. This type of weathering results from the action of temperature changes, frost, wind and rainfall. It is important in dry areas.
The change in temperature causes the bare rocks to expand when warmed by the sun, and contract when night falls and their heat is lost by radiation. The mineral grains of a bed rock expand or contract at different rates as a result of heating or cooling. This causes the breaking of rock into fragment of grains.

Question 25.
Distinguish between the following :

(a) Weathering and Denudation
(b) Physical Weathering and Chemical Weathering.
(c) Sheet erosion and Gully erosion.
(d) Granular Disintegration and Block Disintegration.
(e) Solution and Hydration
(f) Erosion and Weathering.

Answer:

(a) Weathering : ‘Weathering’ means the disintegration and decay of the rocks cover of the earth’s surface. It is an action which affects rocks in the place where they are. The rate of weathering will depend on the climate of the region, on the kind of rock, on its chemical composition, on its hardness and structure which may allow water to penetrate easily into it.
Denudation : Denudation is a general term which covers all the agents causing destruction or the wearing away of the rock surface of the earth together with their transportation and final deposition.
(b) Physical weathering : It is also called Mechanical weathering. It includes the breaking up of rocks without changing their composition. It means disintegration of rocks by mechanical means. This type of weathering results from the action of temperature changes, frost, wind and rainfall. It is important in dry areas.
Chemical weathering : The decomposition of rocks by chemical processes is called chemical weathering. This type of weathering produces a chemical change in minerals of rocks. It results from the action of weak acids and atmospheric gases like oxygen, carbon dioxide, hydrogen. It involves the process of oxidation, carbonation, solution and hydration. It is important in hot humid areas.
(c) Sheet Erosion : When the vegetation cover of an area is removed, the rain, instead of sinking into ground, washes the soil down slope. Each succeeding rain-storm washes away a thin layer of absorbed top soil. This is known as Sheet Erosion.
Gully Erosion : When rain falls more heavily, the flow off storm water finds its way down hill in a series of channels.
Every fresh down pour widen and deepens the channels which develop into gullies. Gullies cut up agricultural land into small fragments and make them finally unfit for cultivation. This type of erosion is known as Gully Erosion.
(d) Granular Disintegration ; The temperature changes during summer and winter lead to expansion, due to heating and contraction, due to cooling. Different minerals present in the rocks may have different rates of expansion and contraction. For example dark coloured minerals will absorb more heat and expand to a greater extent than light coloured minerals. Such alternating expansion and contraction between day and night leads to a break up of the rock into different minerals. This type of weathering is called granular disintegration.
Block disintegration : A rock may split along joints or fissures by the repeated freezing of water in cracks or other openings, because water expands nearly one-tenth of its volume when it freezes. It is followed by melting during daytime repeatedly. This type of weathering is called block disintegration because the rock is split into rectangular block along the joints.
(e) Solution : It is the chemical process in which the soluble minerals are dissolved by water entering into rocks through crevices. The dissolved minerals are removed with the flow of water resulting in the break up of the rock. Rock-salt and gypsum weather away chemically because of this process. Since silica is highly soluble in water, and silica is found in so many rocks, this process is also known as Desilication.
Hydration : It is the process by which some minerals in crystalline form absorb water and become a powdery mass. Feldspar, a common rock forming crystalline mineral absorbs water and becomes a mass of clay by the process of hydration. The other minerals present along with feldspar get separated into loose particles and the rock breaks up.

(f) Erosion and Weathering :
Erosion :

1. Erosion includes the work of weathering away of rocks of the earth.
2. It involves motion.
3. River, glacier, wind are its main agents.

Weathering :

1. Weathering is the breaking of rocks by elements of atmosphere.
2. It involves no motion.
3. Temperature, frost, wind, plants and animals are its main agents.

Question 26.
What do you understand by the following terms :

(a) Exfoliation
(b) Regolith
(c) Oxidation
(d) Carbonation
(e) Weathering
(f) Humus
(g) Soil profile
(h) Landslide
(i) Soil texture
(j) Gradation
(k) Frost action

Answer:

(a) Exfoliation : When the rock consists of homogeneous minerals the thin surface layer of rock absorbs heat during the day and expands, while during the night, this surface layer contracts. In a few days the rock breaks up or peels off into layers. This type of weathering is called exfoliation.
(b) Regolith : The loose material which has resulted from the breaking down of bed rocks is called Regolith.
(c) Oxidation : The Atmospheric oxygen combines with minerals of rocks especially iron compounds to fonn oxides. The rocks begin to decay and crumble to a powdered mass of brown dust. This process is known as oxidation.
(d) Carbonation : The process of absorption of carbon dioxide from the atmosphere by minerals present in the rocks is called carbonation.
(e) Weathering : The disintegration or decay of rocks on the surface of the Earth due to atmospheric conditions is called weathering.
(f) Humus : The organic matter present in the soil is called humus.
(g) Soil profile : The vertical cross section of soil is called soil profile.
(h) Landslide : Sudden movement of weathered particles down the slope is known as landslide.
(i) Soil texture : Soil textures refers to size of the particles comprising the soil.
(j) Gradation : The process of erosion, transportation and deposition of rock material is called deposition.
(k) Frost action : The freezing of water in the cracks and openings of rocks during the night leading to the splitting up of the rocks is called frost action.

Question 27.
Account for the two types of weathering.
Answer:
Weathering may be :

1. Mechanical : weathering is the disintegration of rocks by mechanical forces, without chemical changes.
2. Chemical : weathering is the gradual decomposition of the rock owing to the reaction and combination of the constituent minerals of the rock with oxygen or carbon dioxide present in the atmosphere. In Chemical weathering the presence of water is essential.

Question 28.
Describe how changes of temperature lead to weathering.
OR
How do changing temperature lead to weathering ?
Answer:
The alternate action of heat and cold helps in weathering of rocks. For example, during the day, the rocks expand by heat and they contract due to cold during night. It tends to loosen the particles of rock and it begins to disintegrate because every rock is composed of different minerals, which are affected differently due to changes in temperature and therefore the component parts of the rock begin to lose cohesion.

Question 29.
How does frost action cause weathering ?
Answer:
In the middle latitudes, temperature may be above freezing point of water during day time and below freezing point during the night. Water present in the joints and fissures in the rocks, gets frozen into ice during the night. Freezing of water results in expansion in volume and the fissures widened.
It is followed by melting during day time repeatedly. It is called block disintegration because the rock is split into rectangular blocks along the joints. Frost action is most common in the tropical regions and in high mountains all the year round.

Question 30.
Name the different processes of chemical weathering.
Answer:
Different processes of chemical weathering are as follows :

1. Solution
2. Hydration
3. Oxidation
4. Carbonation

Question 31.
Describe the work of plants as agents of weathering.
Answer:
Plants protrude their roots through joints in the rocks and as the plant grows, the joints in the rocks get widened. The widening of joints allows water to seep through. Seepage of water may cause chemical weathering or frost action. Decay of plant remains like flowers, leaves, etc., result in the formation of organic acids which may react with minerals present in the rocks. The leads to chemical weathering.

Question 32.
Which human activities lead to weathering of rocks ?
Answer:
Following human activities lead to weathering of rocks :

1. Mining
2. Quarrying
3. Deforestation

When trees are cut the binding action of the roots of the trees is no longer present. Rocks get exposed to sun and rain and weathering takes place rapidly. Mining and quarrying exposes bare rocks on the surface and these get weathered quickly.

Question 33.
State the effects of weathering.
Answer:
Effects of weathering :

1. It causes landslides.
2. It causes mud flow, that is, the flow of weathered material, soggy with water, deacon a hillside or a slope.
3. Weathering breaks down the particles of rocks gradually and makes it easy for rivers and other agents of denudation to transport them down the slope. It exposes the rock to further weathering. Thus weather assists the agents of denudation.
4. Weathering assists the formation of soils.

Question 34.
Give reasons for the following :

1. Change of temperature leads to physical weathering.
2. Presence of water aids chemical weathering.
3. Human activities encourage weathering.
4. Climate is the most important factor of soil formation.
5. Farmers are encouraged to adopt soil conservation methods.
6. A soil dominated by clay makes tilling difficult.
7. Grassland soils are less acidic than forest soils.

Answer:

1. Change of temperature leads to physical weathering because it leads to expansion due to heating and contraction due to cooling.
2. Presence of water aids chemical weathering because it dissolves soluble minerals present in the rocks.
3. Human activities encourage weathering because these activities expose rocks to sun and rain due to which weathering takes place rapidly.
4. Climate is the most important factor of soil formation because it controls the type of weathering of the parent rock arid availability of soil moisture for plants and micro-organisms.
5. Farmers are encouraged to adopt soil conservation methods because it protects the top layer of the soil.
6. A soil dominated by clay makes tilling difficult because it is sticky and tough.
7. Grassland soils are less acidic than forest soils because grasses are heavy feeders on the bases.

Question 35.
Explain the following terms.

1. Colloids
2. Bases

Answer:

1. Colloids : Colloids are extremely tiny particles which are so small that they can not be seen by optical microscope ‘ and remain suspended indefinitely in water. They have the property of being electrically charged and can attract and hold ions of calcium, magnesium and potassium, known as bases. These bases are required by plants for their growth. – Soil colloids are also useful in holding water in the soil. When present in large quantities, they make the soil sticky and tough so that it is difficult to cultivate.
2. Bases : Ions of calcium, magnesium and potassium are known as bases. They are required by the plants for their growth.

ICSE Solutions for Class 9 Geography – Denudation

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Exercises

I. Short Answer Questions

Question 1.
What do you mean by denudation ?
Answer:
Denudation is the wearing away of landmass by various processes like weathering, erosion mass movement and transportation. It results in lowering the level of land, rounding exposed rock surfaces and levelling the peaks.

Question 2.
Name important factors which affect work of a river.
Answer:
Refer Long Answers Question 1

Question 3.
What is the main function of a river in its upper course in land formation ?
Answer:
In the upper course the dominant activity of a river is erosion. Many land forms are shaped by this activity of the river. Along its course, it erodes bed and banks, and carries large boulders of rock with it. Under the impact of rush of water.

Question 4.
Name two important land forms of river erosion.
Answer:

The two landforms resulting from erosion in the upper course of a river are the following :

1. V-shaped Valleys : It is formed from a stream eroding downward, through a process, called down cutting. These valleys are formed in the initial stages of rivers and have steep slopes.
2. I-shaped Valley : In regions of hard rocks it develops almost vertically in an I-shape. River valleys are normally formed in areas of sufficient rainfall and where rocks are not very hard or resistant.

Question 5.
How is a waterfall formed ?
Answer:
Waterfall : A waterfall is generally formed due to large differences in the rate of erosion. The smaller differences lead to formation of step-like features known as rapids. The water that falls down the edge of a hard resistant rock may have at the bottom a soft rock. It is this soft rock that gets eroded fast and creates a hollow basin called plunge pool. Some of the world’s highest and best known falls are Angel Falls (986 m) in Venezuela and Yosemite fall (778 m), California, USA.

Question 6.
State the main function of a river in its middle course.
Answer:
At this stage, the gradient is reduced, and the river flows more slowly. The middle course of the river begins when it leaves the mountains area and enters the plains. In the middle course the energy required to transport the materials is just enough to drag large particles. Fine particles remain suspended in water, large particles are just dragged along the bed of the river and smaller ones are rolled down in the centre. The river also gets windened as a result of great volume of water and the impact of the load.

Question 7.
How are meanders formed ?
Answer:
River meanders are bends of longitudinal courses. S-shaped meanders which are common to most rivers result when a ^ channel-forms a curved path. These are the result of both erosion and deposition work of rivers.
Meanders generally form under conditions of a gentle slope and sufficient water in rivers. The river flow is diverted by an obstruction allowing the river to do lateral erosion work.

Question 8.
Name two landforms in the lower course of a river.
Answer:

1. Delta : The river divides itself into many distributaries.
   These distributaries are also subdivided into mini-distributaries. This is due to depsoition of sediments over a large’area near the mouth of rivers. Such sedimentation occurs on the sides of the stream, at the mouth, in the front part as well as in the bed of the river. The deposition over a large area results in a triangular-shaped formation called delta. It resembles the Greek letter A The Ganga-Brahmaputra delta is growing towards the sea. It is also one of the largest in the world.
2. Oxbow lakes are also formed in the lower course of a river.

Question 9.
Where is wind erosion most predominant ?
Answer:
The wind erosion is most predominant in arid regions. Since there is little vegetation or moisture to bind the loose surface materials, the effects of wind erosion are more pronounced in the desert region.

Question 10.
What are known as Deflation Hollows ?
Answer:
Deflation Hollows : Deflation involves the lifting as well as the blowing away of loose materials from the ground. The sands and pebbles are carried in the air or driven along the ground. The finer dust and sands may be deposited even outside the desert margins. Deflation results in the lowering of the land surface to create depressions called deflation hollows.

Question 11.
Name one chief landform of wind deposition ?
Answer:
Hamada is the hard bedrock exposed after sweeping away of the dust and sand by the wind.

Question 12.
Name two chief types of sand dunes.
Answer:
The following two types of sand dunes are seen

1. Barchan : These are moon-shaped dunes. They are live dunes which advance steadily before winds that blow from a particular direction. They are found in the deserts of the Sahara. These are initially formed by an accumulation of sand at an obstacle, such as a heap of rocks. These occurs transversely to the wind, so that their horns thin out and become lower in the direction of the wind.
2. Longitudinal Dunes or Seifs : These are long and narrow sand-ridges which grow parallel to the direction of the prevailing wind. They are found in the interior parts of deserts. In the Libyan desert, they are known as Seifs (means ‘sword’ in Arabic). In the Thar Desert they are confined to the western margin, where the force of south-west monsoon influences their formation. An important feature of such a dune is that in its crest line there are rise and fall patterns. Seifs are found in the Sahara, Iran, Thar Desert (India) and West Australia. These sand dunes are stationary and they remain in a fixed position.

Question 13.
State three chief characteristics of barchans.
Answer:

1. Barchans are moon-shaped dunes. They are live dunes which advance steadily before winds that blow from a particular direction.
2. They occurs transversely to the wind, so that their horns thin out and become lower in the direction of the wind.
3. The windward side is convex and gently-sloping while the leeward side (being sheltered) is concave and steep.

II. Long Answer Questions

Question 1.
Briefly describe the factors which affect the work of a river.
Answer:
Various factors affect erosion, transportation and deposition work of a river. Important factors include the following :

1. Velocity of water : Erosion and transportation are both maximum when velocity is high. Deposition, on the other hand takes place when velocity of water is low.
2. Volume of water : The larger the volume of water, the greater is the power of erosion and transportation. Deposition, on the other hand, takes place better when there is reduced volume of water as well as when the load is greater.
3. Load : Load is the material transported by a river. The load of a river, does all the work of erosion and deposition. Mass wasting, deepening of river valleys and formation of landforms depend on the load a river carries.

Question 2.
Describe chief landforms of river deposition.
Answer:

The cheif landforms resulting from the deposition of a river are the following :

1. V-shaped Valleys : It is formed from a stream eroding downward, through a process, called down cutting. These valleys are formed in the initial stages of rivers and have steep slopes.
2. I-shaped Valley : In regions of hard rocks it develops almost vertically in an I-shape. River valleys are normally formed in areas of sufficient rainfall and where rocks are not very hard or resistant.

Question 3.
Write a short note on the three stages of a river.
Answer:
From the source to its mouth, a river is divided into three main sections – Upper Course, Middle Course and Lower Course. These three sections are also called the Profile of a River.

• Upper Course – In the upper course the dominant activity of a river is erosion. Many land forms are shaped by this activity of the river. Along its course, it erodes bed and banks, and carries large boulders of rock with it. Under the impact of rush of water.
• Middle Course – At this stage, the gradient is reduced, and the river flows more slowly. The middle course of the river begins when it leaves the mountains area and enters the plains. In the middle course the energy required to transport the materials is just enough to drag large particles. Fine particles remain suspended in water, large particles are just dragged along the bed of the river and smaller ones are rolled down in the centre. The river also gets windened as a result of great volume of water and the impact of the load.
• Lower Course – The river divides itself into many distributaries. These distributaries are also subdivided into mini-distributaries. This is due to depsoition of sediments over a large area near the mouth of rivers. Such sedimentation occurs on the sides of the stream, at the mouth, in the front part as well as in the bed of the river. The deposition over a large area results in a triangular-shaped formation called delta. It resembles the Greek letter A The Ganga -Brahmaputra delta is growing towards the sea. It is also one of the largest in the world. Oxbow lakes are also formed in the lower course of a river.

Question 4.
Describe the work of wind erosion and state two important landforms of wind erosion.
Answer:
Winds move along the desert floors with great speed and the obstructions in their path create turbulence. Storm winds are formed which are very destructive. Winds cause deflation, abrasion and impact. Deflation includes lifting and removal of dust and smaller particles from the surface of the rocks. The impact is the simply the shear force of momentum which occurs when sand is blown into or against a rock surface. These are the ways in which wind action creates a number of interesting erosional and depositional features in the deserts.
Winds are not the only agent responsible for the erosional features of the deserts. Along with winds the running water is also important for this.