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Solutions for Geography, Class 9, ICSE
Assertion (A): Most volcanic eruptions take place near plate margins.
Reason (R): When these plates slide past each other or collide against each other due to intense heat inside the earth's core, their edges produce faults along the lines of weakness.
Both A and R are true and R is the correct explanation of A.
Explanation — The earth's crust is made up of a series of movable plates. When these plates slide past each other or collide against each other due to intense heat inside the earth's core, their edges produce faults along the lines of weakness. It is along these plate boundaries that the crust is weakened and through which the hot magma gushes out leading to volcanic eruption.
Assertion (A): Steam is the most abundant gas that comes out of a volcano.
Reason (R): Violent eruptions, known as bombs, eject materials varying in size.
Both A and R are true but R does not explain A.
Explanation — A volcano erupts a number of products like steam, gases, molten rock, dust, ashes and other liquid and solid matter. Steam escapes from a volcano regularly. However, during violent eruptions, known as bombs, solid matters are ejected varying in size.
Assertion (A): The lava and ash deposited during an eruption breaks down to provide valuable nutrients for the soil and thus create fertile soil.
Reason (R): Agricultural fields are covered with layers of volcanic ash and dust making them unsuitable for cultivation.
Both A and R are true and R is the correct explanation of A.
Explanation — Agricultural fields are covered with layers of volcanic ash and dust making them unsuitable for cultivation, but only temporarily. In fact, once weathered and broken down the lava and ash deposited during an eruption provides valuable nutrients to the soil making it fertile.
The Pacific Ring of Fire is near the earthquake belt around the young fold mountains. It is called so because over 80% of the total number of active volcanoes are concentrated in this region. Among these are included 40 active volcanoes in Andes, over 100 in the Philippines, 70 in Indonesia and 40 in Japan.
Dormant volcano | Extinct volcano |
---|---|
These volcanoes have not erupted in recent historical period. | These volcanoes have not erupted in the present geological period. |
They may become active at any time. | They are not likely to become active again. |
For example- Mt. Kilimanjaro in Africa | For example- Mt. Kenya in Africa |
The molten material while still under the earth's surface, melts weak rocks and creates a huge chamber for itself. Fresh magma continues to pour inside the chamber. This is known as magma chamber.
A volcano erupts a number of products like steam, gases, molten rock, dust, ashes and other liquid and solid matter.
The two causes of volcanic eruptions are-
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. When these plates slide past each other or collide against each other due to intense heat inside the earth's core, their edges produce faults along the lines of weakness. It is along these plate boundaries that the crust is weakened and through which the hot magma gushes out leading to volcanic eruptions.
The two types of volcanoes are-
1. Active volcanoes — These are the volcanoes which are presently in active state and which have erupted in the recent past. For example, Mt. Etna in Italy.
2. Dormant volcanoes — These volcanoes have not erupted in recent historical period. They may become active at any time. For example, Mt. Kilimanjaro in Africa.
(i) Many volcanoes are found along plate boundaries because when tectonic plates slide past each other or collide against each other due to intense heat inside the earth's core, their edges produce faults along the lines of weakness. It is along these plate boundaries that the crust is weakened and through which the hot magma gushes out leading to volcanic eruptions.
(ii) Volcanic mountains are the most diverse mountains in the world because there are great differences in volcanic eruptions as well as materials they throw up.
(iii) Hot springs have medicinal properties to cure skin diseases since they are a rich source of sulphur.
The landforms created by volcanoes on the surface of the earth are-
1. Volcanic Plateaus — These are derived from lava which flows from volcanic eruptions. For example, Columbian Plateau.
2. Volcanic Mountains — These mountains are built from material ejected from the fissures of the earth's crust. For example, Mauna Loa in Hawaii.
3. Volcanic Plains — These plains are formed by extensive volcanic flooding from volcanic centres. For example, Victorian Plains in Victoria.
4. Caldera Lake — A large depression at the summit of a volcano is called a caldera. When this depression gets filled with water, it forms a lake known as caldera lake. For example, Crater Lake in USA.
5. Hot springs — The movement of magma in the interior of the earth heats up underground water particularly around magma chambers. When water turns into steam, it gushes up to the surface. Hot springs are a rich source of sulphur. For example, hot springs at Manikaran in Kullu Valley, Himachal Predesh.
6. Geysers — It is a fountain of hot water and steam that escapes when underground water comes into contact with hot volcanic rocks. For example, the Old Faithful geyser in Wyoming, USA.
Three destructive effects of volcanoes are-
(i) Volcanic Cone — The erupted fluid material being hot collects around the vent and begins to cool gradually and solidifies. The accumulation of material around the vent gives to the volcano its typical cone-like shape known as a volcanic cone.
(ii) Caldera — A large depression at the summit of a volcano is called a caldera.
(iii) Geyser — It is a fountain of hot water and steam that escapes when underground water comes into contact with hot volcanic rocks. For example, the Old Faithful geyser in Wyoming, USA.
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. The important volcanic zones of the world are-
1. Circum-Pacific Belt — The volcanoes occur near the earthquake belts around the young fold mountains. It is also known as 'Pacific Ring of Fire' because over 80% of the total number of active volcanoes are concentrated in this region. Among these are included 40 active volcanoes in Andes, over 100 in the Philippines, 70 in Indonesia and 40 in Japan.
2. Mid-World Mountain Belt — Mt. Stromboli, Mt. Vesuvius and Mt. Etna are located in the Midworld Mountain Belt. The only active volcano in India, in Andaman and Nicobar Islands is also located in this belt. The Hawaiian Islands, Canary Islands, Mt. Kenya are all in the Midworld Mountain belt.
If an extinct volcano erupted during a holiday, it could have significant consequences. Here are some of the potential outcomes:
One day, when I was sitting in my balcony, I suddenly noticed a volcano erupting at a distance. It was a horrifying sight.