# NCERT notes and solutions for class 7 Geography chapter 3 OUR CHANGING EARTH

## OUR CHANGING EARTH

Lithosphere Plate The lithosphere is made up of a number of plates called the lithosphere plate.

• It moves a few millimeters each year because of magma present inside the earth. The movement of magma is in a circular manner.
• Change in the earth is due to the movement of these lithospheric plates.

Division of movement of the earth on the basis of forces are –

1. Endogenic forced
2. Exogenic forces

Endogenic forced – This is a force that acts in the interior of the earth.

Exogenic forces – Force works on the surface of the earth.

Volcano – It is an opening in the earth’s crust through which molten materials erupt suddenly.

• Molten magma presents in the mantle part of the earth’s crust which comes out through the vent and we call it “lava” when it reaches the surface of the Earth.
• Gases and ashes come out from the crater.

Earthquake   The vibration occurs due to the movement of the lithospheric plates.

Focus – Place inside the crust where seismic waves originate.

Epicenter – Place on the surface just above focus from where seismic waves released.

There are three types of earthquake

(i) P waves or longitudinal waves

(ii) S waves or transverse waves few

(iii) L waves or surface waves

Seismograph- Machine which is used to measure the intensity of an earthquake.

Richter scale – It is used to measure the magnitude of an earthquake.

Richter scale                             Result

2.0 or less                 ⇒             felt very little
upto 5.0                    ⇒             damage from things falling
6 0 and higher              ⇒             very strong
7.0                         ⇒             magor damage

Major landforms-

Weathering and erosion and deposition create different landforms on the surface of the earth.

Work of the river-

River when flows, erodes sand, rocks, etc with it. when river water falls at a steep angle over hard rocks, it forms a waterfall.

Points to remember: -

Highest waterfalls -  Angel falls Venezuela (South America)

Nagara falls - Border of Canada and USA un North America

Victoria falls- Borders of Zambia and Zimbabwe in Africa

when the river reaches the plain area, it forms large bends by twists and turns, which we called meanders. With the passage of time, the meander loop cuts off from the river due to continuous erosion and deposition along the sides of the meander, this is called an ox-bow lake.

As the river reaches the sea, its speed decreases, and the river begins to break up and form distributaries. Collection of sediments from mouths form a delta.

Work of sea level-

Sea waves continuously strike at the rocks and develop cracks that become larger and start forming sea waves. As these cavities become bigger and bigger only the roof of the cave remains, we call it sea arches and time came when roofs erode and only walls left, called stacks.

Work of ice –

Glaciers are frozen rivers that Create deep hollows. As the ice melts, these hollows fill with and form beautiful lakes.

Glaciers carry materials such as rocks, sand, silt, etc, and get deposited in glacial moraines.

Work of mind-

Mushroom rocks are common rocks found on the surface of desert wind erodes lower section as rocks more than the upper part. Therefore, such rock has a narrow base and a wider top.

# Exercise

Q 1. Answer the following questions.

(i) Why do the plates move?

Answer: Plates move a few millimeters each year because of magma present inside the earth.

(ii) What are exogenic and endogenic forces?

Endogenic forced – This is a force that acts in the interior of the earth.

Exogenic forces – Force works on the surface of the earth.

(iii) What is erosion?

Answer: The process that removes soil, rock, or dissolved material from one place to another place is called erosion.

(iv) How are flood plains formed?

Answer: When water gets too high in a river, the river overflows its banks, resulting in flooding of the area around the river. At the time of the flood, it deposits soil and other material in a layer known as sediment. This layer of soil and other material is quite fertile which is called flood plain.

(v) What are sand dunes?

Answer: When the wind blows, it carries and transfers the sand from one place to another. When the wind stops moving, the sand along with it falls down and gets deposited in small hill-like structures. These are called sand dunes. Sand dunes are mostly found in desert areas.

(vi) How are beaches formed?

Answer: When sea waves accumulate sediment at the edge of the sea, it creates beaches.

(vii) What are the ox-bow lakes?

Answer: when the river reaches the plain area, it forms large bends by twists and turns, which we called meanders. With the passage of time, the meander loop cuts off from the river due to continuous erosion and deposition along the sides of the meander, this is called an ox-bow lake.

Q 2. Tick the correct answer.

(i) Which is not an erosional feature of sea waves?

(a) Cliff

(b) Beach

(c) Sea cave

(ii) The depositional feature of a glacier is:

(a) Flood plain

(b) Beach

(c) Moraine

(iii) Which is caused by the sudden movements of the earth?

(a) Volcano

(b) Folding

(c) Flood plain

(iv) Mushroom rocks are found in:

(a) Deserts

(b) River valleys

(c) Glaciers

(v) Ox bow lakes are found in:

(a) Glaciers

(b) River valleys

(c) Deserts

Q 3. Match the following.

 (i) Glacier (a) Sea shore (ii) Meanders (b) Mushroom rock (iii) Beach (c) River of ice (iv) Sand dunes (d) Rivers (v) Waterfall (e) Vibrations of earth (vi) Earthquake (f) Sea cliff (g) Hard bedrock (h) Deserts

 (i) Glacier (c) River of ice (ii) Meanders (d) Rivers (iii) Beach (a) Sea shore (iv) Sand dunes (h) Deserts (v) Waterfall (g) Hard bedrock (vi) Earthquake (e) Vibrations of earth

4. Give reasons.

(i) Some rocks have the shape of a mushroom.

Answer: Mushroom rocks are common rocks found on the surface of desert wind erodes lower section as rocks more than the upper part. Therefore, such rock has a narrow base and a wider top.

(ii) Flood plains are very fertile.

Answer: When water gets too high in a river, the river overflows its banks, resulting in flooding of the area around the river. At the time of the flood, it deposits soil and other material in a layer known as sediment. This layer of soil and other material is quite fertile which is called flood plain. Therefore, flood plains are very fertile.

(iii) Sea caves are turned into stacks.

Answer: When the holes in the sea caves are large, only the roof of the cave remains, which makes the sea arch. Apart from this, the roof of the cave disappears due to water erosion, and only the walls remain, which turns the sea caves into stacks.

(iv) Buildings collapse due to earthquakes.

Answer: Earthquake occurs due to motion of lithospheric plate .when earthquake ranges to 6 in richer scale it causes damage.

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