Volcanoes

Answer the following questions

Question 1

What are volcanoes?

Answer

A volcano is a vent or opening in the crust of the Earth, connected by a conduit to an underlying magma chamber from which molten lava, volcanic gases and steam are ejected. It is usually in the form of cone or dome according to the material ejected.

Question 2

How are volcanoes caused?

Answer

The main causes of volcanism can be summarised as follows:

1. The temperature inside the earth's crust increases with increase in depth at the rate of 1°C per 32 metres. The high temperature and pressure inside the earth crust, which increases with increase in depth is responsible for instability. Molten magma under great pressure has the capacity to liquefy great volume of gases.
2. The surface water that percolates through the cracks and crevices, turns into steam due to hot rocks. The steam being very powerful, forces itself through the weak points of the earth crust along with molten material.
3. The plate tectonics or the movement and splitting of the major and minor plates of the earth cause cracks, crevices and fissures in the earth's crust through which the magma is poured out.

Question 3

How can volcanoes be classified on the basis of their activity?

Answer

On the basis of their activity, volcanoes can be classified into three types:

1. Active volcanoes — They constantly eject lava, gases and ash. For example- Sabancaya in Peru.
2. Dormant volcanoes — They remain dormant but can suddenly erupt and again go dormant. For example- The Vesuvius volcano in Italy.
3. Extinct volcanoes — They are volcanoes where there has been no volcanic activity since long. Their crater usually developes into a lake. For example- For example- Arthur's seat at Edinburgh in Scotland.

Question 4

Name the four major types of volcanic eruptions.

Answer

The four major types of volcanic eruptions are:

1. Ash and cinder cones
2. Lava Dome
3. Lava Shield
4. Composite or Strato cone

Question 5

What is volcanicity?

Answer

Volcanicity is a process which involves the intrusion of magma in the Earth's crust or the extrusion of such molten material on the Earth's surface. It gives rise to volcanic eruptions.

Question 6

What are the products of volcanic eruption?

Answer

The volcanic eruption ejects molten lava, volcanic gases(mostly sulphur dioxide), ashes, cinder, pumice, steam etc.

Question 7

What is a crater?

Answer

An abrupt circular depression formed by extrusion of volcanic material, by collapse or by impact of a meteorite is called crater. For example, sunset crater, Arizona.

Question 8

Name any four major types of landforms associated with volcanoes.

Answer

The four major types of landforms associated with volcanoes are:

1. Volcanic Plateaus — The erupted lava spreads to form plateau. For example- Columbian plateau.
2. Volcanic Mountains — These mountains are formed from material ejected from the fissures of the Earth's crust. For Example- Mauna Loa in Hawaii.
3. Volcanic Plains — Sometimes lava forms a very fertile plain. For example- Plain in Java, Malwa region in Deccan plateau.
4. Caldera Lake — The crater forms large lakes that may serve as great source of perennial rivers. For example- Lake Nayos in South Africa.

Question 9

State three beneficial effects of volcanoes.

Answer

Three beneficial effects of volcanoes are:

1. They lead to formation of geysers and springs. These are important from health and medical point of view.
2. The crater lakes serve as great source of perennial rivers. For example, Lake Nyos in South Africa.
3. Precious stones are found in volcanoes. For example, diamonds in South Africa.

Question 10

Mention three adverse effects of volcanoes.

Answer

Three adverse effects of volcanoes are:

1. They cause extreme destruction of forests, cities and lives in large area.
2. Volcanoes eject poisonous gases that cause pollution and health problems.
3. The volcanic area becomes inhabitable by man. The rain water is immediately percolated due to high porosity.

Question 11

Describe the distribution of volcanoes.

Answer

Volcanoes generally occur in the weaker zone of the earth crust and are characterized by young fold mountains. The following are the three belts of volcanic eruptions:

1. The Circum-Pacific Belt (Ring of Fire) — It extends through the Andes of South America, Central America, Mexico, the mountains of western U.S.A, the Aleutian Islands, Kamchatka, the Kurile Islands, Sakhalin, Japan, Philippines, Celebes, New guinea, the Solomon Islands, New Caladonia and New Zealand.
2. Mid-Atlantic— This belt includes the volcanoes of Mid-Atlantic ridge. It also includes the volcanoes of Lesser Antilles, Southern Antilles, Azores, St. Helena, etc.
3. Mid-Continental— This belt includes the Volcanoes of Alps Mountains, Mediterranean Sea (Stromboli, Vesuvius, Etna, etc.) and volcanoes of Aegean Sea, Mt. Ararat (Turkey), Elbrus, Hindukush and the Himalayas.

Question 12

Explain in brief, how a volcanic eruption is caused.

Answer

The temperature inside the Earth increases at the rate of 1° per 32 metres of descent. It results in very high temperature as well as extreme pressure deep inside the Earth. The molten lava inside the Earth, turns surface water (which percolates through the cracks and crevices to the interior) into steam. This steam and other compressed gases put pressure on the Earth's crust. The movement of Tectonic plates cause cracks, crevices and fissures in the Earth's crust through which the magma is poured out.

Question 13

Classify volcanoes on the basis of their intensity of eruption.

Answer

On the basis of intensity of eruption, volcanoes are of three types:

1. Active volcanoes — They constantly eject lava, gases and ash. For example- Sabancaya in Peru.
2. Dormant volcanoes — They remain dormant but can suddenly erupt and again go dormant. For example- The Vesuvius volcano in Italy.
3. Extinct volcanoes — They are volcanoes where there has been no volcanic activity since long. Their crater usually developes into a lake. For example- For example- Arthur's seat at Edinburgh in Scotland.

Question 14

Where do volcanic eruptions generally occur? Give one example.

Answer

Volcanic eruptions generally occur in the weaker zone of the earth crust and are characterized by young fold mountains. For example - the Andes mountains are young mountains which are part of 'Ring of Fire'.

Question 15

Give one example of an active volcano.

Answer

Sabancaya in Peru in South America is an example of an active volcano.

Explain the following terms

Question 1

Caldera

Answer

It is a large, more or less circular depression or basin associated with a volcanic vent. Its diameter is many times greater than that of the included vents. Calderas are believed to result from subsidence or collapse and may or may not be related to explosive eruptions.

Question 2

Ring of Fire

Answer

The Circum-Pacific belt of volcanoes is called as Ring of Fire. It is called so because the belt has numerous active volcanoes. It is the chain of volcanoes making up mountain belts and island arcs surrounding the pacific ocean basin.

Question 3

Magma

Answer

A mobile silicate melt coming out at the time of volcanic eruption, which can contain suspended crystals and dissolved gases as well as liquid is known as magma. It is formed due to great pressure and temperature inside the Earth's crust. It contains a mixture of minerals, water vapour, sulphur and dissolved gases.

Question 4

Lava

Answer

The magma that reaches the Earth's surface due to volcanic eruption is called lava. The molten lava solidifies as it reaches the surface of earth forming different structures.

Distinguish between the following pairs

Question 1

Lopoliths and Phacoliths

Answer

Question 2

Crater and Caldera

Answer

Question 3

Active and Extinct Volcano

Answer

Diagram

Question 1

Draw a labelled self-explanatory diagram of a volcano.

Answer

Labelled self-explanatory diagram of a volcano is given below:

Give reasons for the following

Question 1

Earthquakes are closely associated with volcanic belts.

Answer

Earthquakes are closely associated with volcanic belts because volcanic belts are usually situated around tectonic plates. Movement of tectonic plates results in earthquakes and also causes cracks, crevices and fissures in the Earth's crust through which the magma is poured out. Thus, an earthquake may trigger a volcano and vice versa.

Question 2

Lava activity is profitable to man.

Answer

Lava activity is profitable to man as:

1. Lava forms plains, which are very fertile. For example- plains in Java, Malwa region in Deccan plateau and parts of Brazil.
2. Lava activity also provides us precious stones, for example, diamonds in South Africa and nickel deposits of Sudbury in Canada.
3. Geysers and springs are formed due volcanic activity and are very useful from health point of view as they contain sulphur and other useful minerals.
4. The crater lakes are formed as result of volcanic eruption. They may serve as source of perennial rivers.
5. Volcanic spots become tourist spots.
6. Sometimes remnants of volcanoes after erosion help in preserving traces of old civilization.

Question 3

The Circum-Pacific Belt is also called the 'Ring of Fire'.

Answer

The Circum-Pacific Belt is also called the 'Ring of Fire' because this belt consist of numerous active volcanoes which keep erupting.

Question 4

Volcanos have constructive effects in favour of man.

Answer

Volcanos have constructive effects in favour of man because:

1. Lava from plains are very fertile. For example- plains in Java, Malwa region in Deccan plateau and parts of Brazil.
2. Lava activity also provides us precious stones.
3. Geyser and springs are formed that provide water with health benefits.
4. Crater lakes are formed which serve as source of water to perennial rivers. For example-Lake Nyos.
5. Sometimes remnants of volcanoes after erosion helps in preserving traces of old civilization.

Question 5

Mt. Vesuvius in Italy is a dormant volcano.

Answer

Mt. Vesuvius in Italy is a dormant volcano because it first erupted in 79 AD. After that it remained dormant for over 1500 years and then suddenly erupted with great force in 1631 AD. Since then, it has erupted about 7 times with the last eruption seen in 1944.

Choose the correct option

Question 1

Identify the parts of volcano which are marked as 5 & 7

1. Lava & Ash
2. Fissure & Dyke
3. Lava & Magma
4. Vent & caldera

Answer

Vent & caldera

Question 2

Hawaiian volcanoes are the excellent examples of

1. Strato volcano
2. Lava domes
3. Lava Shields
4. Cinder cones

Answer

Lava Shields

Question 3

Which of the following is an extinct volcano?

1. Vesuvius
2. Etna
3. Arthur’s Seat
4. Stromboli

Answer

Arthur’s Seat

Question 4

Which of the following belts of volcano is also known as the 'Ring of Fire'?

1. Mid-Continental belt
2. Circum-Pacific belt
3. Mid-Atlantic belt
4. None of the above

Answer

Circum-Pacific belt

Question 5

It is an intrusive volcanic landform which is formed when the lava solidifies between the horizontal layers of parent rocks.

1. Dykes
2. Lopoliths
3. Phacoliths
4. Sills

Answer

Sills