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What are earthquakes Grade 7?

meritnation.com - NCERT Solutions for Class 7 Social science Chapter 3 - Meritnation

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Question A.1: Which are the three types of plate movements?

Answer: The three types of lithospheric plate movements are as follows: Divergent plate movement: It occurs when two plates move away from each other. Convergent plate movement: It occurs when, due to the forces of compression, two plates move towards each other and they collide.

Transform plate movement: It occurs when two plates slide past each other.

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Question A.2: How are rift valleys formed?

Answer: A rift valley is formed on a divergent plate boundary when a block of land between two almost parallel faults sinks down. For example, the Narmada Valley in India, lying between the Satpura and Vindhya ranges, is a rift valley.

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Question A.3: Where are most of the active volcanoes of the world located?

Answer: Most of the active volcanoes are located in the Pacific Ring of Fire. This is a circular belt around the Pacific Ocean. The ring of fire coincides with the edges of the Pacific Plate.

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Question A.4: Which instrument records the tremors of an earthquake?

Answer: Seismograph is the instrument that records the tremors of an earthquake. This instrument also helps seismologists to map the interior of the Earth.

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Question A.5: When did the Bhuj Earthquake occur? What was its magnitude?

Answer: The Bhuj Earthquake occurred on the 56th Republic Day of India, that is, on 26th January 2001. The earthquake occurred at 08:46 in the morning with a magnitude of 7.9 on the Richter scale.

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Question B.1: Vent and crater

Answer:

Vent Crater

1. A passage through which the magma travels to the Earth's surface is known as a vent. A circular depression of the volcano is called a crater. 2. A vent enables a volcanic eruption. A crater is formed as a result of a volcanic eruption. 3. A vent has only one opening or a crater. A crater may have several vents.

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Question B.2: Dormant and extinct volcano

Answer:

Dormant Volcano Extinct Volcano 1. Dormant volcanoes are the ones that have not erupted in a long time but may erupt in the future. Extinct volcanoes are the one that have not erupted in a long time and will never erupt in the future. 2. A dormant volcano has erupted before in the last 10,000 years. An extinct volcano has not erupted in the last 10,000 years. 3. Examples: Mt. Fujiyama and Mt. Vesuvius Examples: Mt. Kenya and Mt. Aconcagua

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Question B.3: Folding and faulting

Answer: Folding Faulting 1. Folding occurs when the Earth's rock layers become folded. Faulting occurs when the Earth's crust gets cracked forming a fault. 2. It happens when two lithospheric plates collide with each other. It happens when two lithospheric plates move away from each other. 3. Folding occurs when a force of compression is created. Faulting occurs when a force of tension is created. 4. Fold mountains are formed as a result of folding.

Example: The Himalayas

Block mountains and rift valleys are formed as a result of faulting.

Examples: Satpura ranges and the Narmada Valley

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Question B.4: Focus and epicentre

Answer: Focus Epicentre 1. Focus is the location from where the earthquake originates. Epicentre is the point above the focus on the surface of the Earth. 2. Focus lies inside the Earth's surface. Epicentre lies on the Earth's surface. 3.It is the place from where the earthquake starts. It is the place where most of the damage happens due to earthquake.

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Question C.1: Formation of block mountains.

Answer: Block mountains are formed when the land between the two almost parallel faults is uplifted due to a force of tension exerted during a divergent plate movement. As a result of this tension, the side rocks surrounding the central block go down, leaving the central block of the rock uplifted.

Examples: The Vindhayas and the Satpura ranges in India

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Question C.2: A quiet volcanic eruption forms lava sheets.

Answer: A quiet eruption occurs when the lava oozes out of the Earth's crust instead of coming out in an explosive manner. Quiet eruption causes the lava to flow and cover a large area, thereby creating layers upon layers of lava known as lava sheets.

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Question C.3: Earthquakes are an environmental hazard.

Answer: Earthquakes are the vibrations inside the Earth's crust that happen due to seismic disturbances or volcanic eruptions. Earthquakes are an environmental hazard as they cause widespread distress in the affected area and huge loss of life and property. Earthquakes disrupt telephone lines and transportation modes such as railways and roadways. They often damage national monuments. Buildings collapse due to vibrations and people get buried under the rubble. Electrical wires also get ruptured causing fire to breakout, creating havoc.

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Question C.4: The Bhuj Earthquake is considered to be an exceptional event.

Answer: The Bhuj Earthquake was of 7.9 magnitude on a Richter scale and is considered to be an exceptional event because it occurred at a considerable distance from any plate boundary. Such earthquakes are called intra-plate earthquakes. The Bhuj Earthquake was caused when the force of tension created due to two continental plates rubbing against each other was released.

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Question D.1: Describe the three types of plate movements. Illustrate with suitable diagrams.

Answer:

The three types of plate movements are as follows:

1. Divergent plate movement: This occurs when two lithospheric plates move away from each other. This causes gaps and cracks at the plate boundaries. Magma from the mantle of the Earth rises through these cracks, cools and solidifies to form a new crust. 2. Convergent plate movement: This occurs when two plates move towards each other and collide due to forces of compression. The collision of two plates causes the formation of mountains. For example, the convergent movement of the Eurasian and Indian Plates had caused the formation of the Himalayas. 3. Transform plate movement: This occurs when two plates slide past each other. The tremendous friction caused leads to devastating earthquakes.

The diagram of the three movements is given below:

The extreme heat of the core causes the melting of the rocks of the magma. The expansion of these molten rocks pushes the lithospheric plates towards or away from each other. This is known as plate tectonics.The three types of plate movements are as follows:1. Divergent plate movement: This occurs when two lithospheric plates move away from each other. This causes gaps and cracks at the plate boundaries. Magma from the mantle of the Earth rises through these cracks, cools and solidifies to form a new crust.2. Convergent plate movement: This occurs when two plates move towards each other and collide due to forces of compression. The collision of two plates causes the formation of mountains.For example, the convergent movement of the Eurasian and Indian Plates had caused the formation of the Himalayas.3. Transform plate movement: This occurs when two plates slide past each other. The tremendous friction caused leads to devastating earthquakes.The diagram of the three movements is given below:

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Question D.2: What is folding? Explain the landform features produced by folding.

Answer: A force of compression is created when two plates collide during a convergent plate movement. As a result, the rock layers of the Earth's crust get folded. This process is called folding. The surfaces that were originally flat become curved as a result of folding. The alternate upfolds that are formed on the rocks are called anticline and the downfolds are called syncline. The landform features formed as a result of folding are known as fold mountains. These are formed when the rock formations and the crust lift and fold due to the collision of two tectonic plates. The fold mountains that are less rough-looking are formed adjacent to the sharp block mountains formed by faulting. For example, the Himalayas are the fold mountains formed as a result of collision between the Eurasian and the Indian plates.

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Question D.3: Write a short note on a volcano, explaining its various parts with a help of a diagram. Answer: The magma from the interior of the Earth comes out to the surface through an opening known as volcano.On reaching the Earth's surface, the magma is known as lava, which comes out through a passage known as vent. The top of the mountain through which the volcano erupts has a funnel shaped depression known as a crater.

There can be two types of volcanic eruptions:

(i) An explosive eruption occurs through a narrow vent at one point. Cone-shaped hills are formed when the erupted material accumulates around the vent. Mt Fujiyama in Japan is an example of a cone- shaped hill. (ii) A quiet eruption occurs through cracks in the Earth's surface. The lava spreads across a large area and forms lava sheets. This often leads to the formation of plains and plateaus. The Deccan plateau is an example of such a formation.

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The diagram that illustrates various parts of a volcano is shown below: There can be two types of volcanic eruptions:(i) An explosive eruption occurs through a narrow vent at one point. Cone-shaped hills are formed when the erupted material accumulates around the vent. Mt Fujiyama in Japan is an example of a cone- shaped hill.(ii) A quiet eruption occurs through cracks in the Earth's surface. The lava spreads across a large area and forms lava sheets. This often leads to the formation of plains and plateaus. The Deccan plateau is an example of such a formation.The diagram that illustrates various parts of a volcano is shown below:

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Question D.4: Describe the extent of destruction caused by the Bhuj Earthquake.

Answer: The Bhuj Earthquake occurred on 26th January 2001. It was a very destructive earthquake of 7.9 magnitude on the Richter scale, with its epicentre in an area called Lodi. This earthquake was devastating as it took lives of many people and caused damage to life and property in Gujarat as well as some parts of Eastern Pakistan. Ahmedabad, the commercial capital of the state, faced the consequences of the earthquake. More than 30,000 people lost their lives and almost 1,50,000 people were injured. About 40 per cent of the homes, schools, hospitals and the roads in Bhuj were destroyed. Structures and historical monuments were ruined. Infrastructural failure included 60 per cent destruction of food and water supplies. Telephone lines and electricity supply lines were ruined and the total property damage was estimated at 5.5 billion US dollars.

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Question E.1: This is not one of the seven major lithospheric plates

a. Mediterranean Plate

b. Pacific Plate

c. African Plate

d. North American Plate

Answer: The correct answer is option (a).

Explanation: Pieces of the Earth's crust and the uppermost mantle that together form the lithosphere are known as tectonic plates or lithospheric plates. Mediterranean Plate is not a tectonic plate. The seven major lithospheric plates are the African Plate, the Eurasian Plate, the Pacific Plate, the North American Plate, the South American Plate, the Antarctic Plate and the Indo-Australian Plate.

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Question E.2: A good example of a volcano is

a. Mt. Fujiyama

b. Mt. Everest

c. Mt. K2

d. Mt. Nanda Devi

Answer: The correct answer is option (a).

Explanation: In an explosive volcanic eruption, the magma flows out of a narrow vent at one point. The lava and other materials like ashes, gases and cinder accumulate around the vent forming a conical hill. Mt. Fujiyama in Japan is an example of such a formation caused by explosive eruption.

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Question E.3: Folding is caused by a force of

a. gravity

b. compression

c. tension

d. upliftment

Answer: The correct answer is option (b).

Explanation: Folding is a result of the force of compression when two lithospheric plates collide during a convergent plate movement. This causes the lifting and folding of the rock layers on the Earth's crust leading to the formation of fold mountains.

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Question E.4: The magnitude of a very destructive earthquake is

a. 4-5

b. 7 or above

c. 6-7

d. below 3

Answer: The correct answer is option (b). Explanation: The magnitude of an earthquake is measured by a Richter scale. The numbers on the scale range between 1 to 9. Earthquakes of magnitude 7 or above are destructive and can cause extreme damage to life and property.

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Question E.5: The displacement of rock strata on either sides of a crack results in

a. transform plate movement

b. folding

c. divergent plate movement

d. faulting

Answer: The correct answer is option (d).

Explanation: When two lithospheric plates move away from each other during a divergent plate movement, a tension is caused that creates a crack on the Earth's crust. Faults are created due to the displacement of rock strata on either sides of the crack. This is known as faulting. Landform features that are formed as a result of faulting are block mountains and rift valleys.

meritnation.com - NCERT Solutions for Class 7 Social science Chapter 3 - Meritnation

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