CBSE Class 7 Social Science: Our Changing Earth NCERT Solutions

Solution: The lithospheric plates, which are the large, rigid slabs of rock that make up the Earth's outer shell, move because of the slow, continuous movement of the molten magma found in the semi-fluid asthenosphere beneath them. These convection currents in the magma exert forces that cause the plates to drift apart, collide, or slide past each other.

Solution:

Endogenic forces are those geological forces that originate from within the Earth. These forces are responsible for processes like earthquakes, volcanic activity, and the formation of mountains and plateaus through upliftment and folding.

Exogenic forces are those geological forces that act on the Earth's surface. These forces are primarily driven by external energy sources like solar energy and gravity, and they include processes such as weathering, erosion, and deposition by agents like water, wind, ice, and sea waves.

Solution: Erosion is the process by which the Earth's surface is worn away and transported by natural agents. These agents include flowing water (rivers), wind, glaciers, and sea waves, which gradually break down and carry away soil, rock, and other materials from one location to another.

Solution: Flood plains are formed when a river overflows its banks during periods of flood. As the floodwaters recede, they leave behind layers of fine soil and other sediments that were carried by the river. These deposited sediments accumulate along the river banks, creating a broad, flat, and fertile area known as a flood plain.

Solution: Sand dunes are natural accumulations of sand, typically found in desert regions or along coastlines. They are formed when wind picks up loose sand particles and deposits them in specific areas, creating hill-like structures. The shape and size of sand dunes depend on factors like wind direction, wind speed, and the availability of sand.

Solution: Beaches are formed along the coastlines where sea waves deposit sediments, such as sand, pebbles, and shells, onto the shore. The constant action of waves, along with currents, transports and sorts these materials, gradually building up the beach over time. The size and composition of beach material are influenced by the local geology and wave energy.

Solution: Ox-bow lakes are U-shaped or crescent-shaped lakes that are formed when a meander loop of a river is cut off from the main river channel. This typically happens when the river erodes its banks at the neck of a meander, and the loop becomes isolated, forming a lake that resembles the shape of an ox's bow.

Solution:

(i) (b) Beach. Beaches are formed by the deposition of sediments by sea waves, not by erosion.

(ii) (c) Moraine. Moraines are depositional features created by glaciers.

(iii) (a) Volcano. Volcanoes are formed by the eruption of molten rock from the Earth's interior, which is a result of sudden internal movements.

(iv) (a) Deserts. Mushroom rocks are formed by wind erosion in desert areas.

(v) (b) River valleys. Ox-bow lakes are formed by the cut-off meanders of rivers in their valleys.

Solution:

1. Glacier - (c) Moraine (Glaciers deposit moraines.)

2. Meanders - (f) River valleys (Meanders are characteristic of rivers in valleys.)

3. Beach - (b) Sea caves (While beaches are depositional, sea caves are erosional features often found near beaches.)

4. Sand dunes - (d) Deserts (Sand dunes are formed by wind in deserts.)

5. Waterfall - (h) Hard bed rock (Waterfalls form where a river flows over a hard bed rock that resists erosion more than the surrounding rock.)

6. Earthquake - (e) Vibrations of earth (Earthquakes are characterized by vibrations of the Earth's surface.)

Solution:

(i) In desert areas, the wind often erodes the base of rock formations more than the top. This is because wind-blown sand, which causes erosion, is more concentrated near the ground. Over time, this differential erosion causes the rock to become narrower at the base and wider at the top, giving it a mushroom-like shape.

(ii) Flood plains are formed by the deposition of fine silt and other nutrient-rich sediments carried by river water during floods. When the floodwaters recede, these fertile materials are left behind on the land, enriching the soil and making the flood plains highly productive for agriculture.

(iii) Sea caves are formed when sea waves repeatedly strike against rocks, creating cracks that widen into hollows. As erosion continues, these caves can enlarge. If the roof of a sea cave collapses, or if erosion wears away the rock in front of the cave, a pillar-like structure called a stack is left standing. Thus, sea caves can evolve into stacks through continued wave erosion.

(iv) Buildings often collapse during earthquakes because they are not designed to withstand the intense vibrations and ground shaking. Many structures lack adequate reinforcement (like steel) or have shallow foundations that cannot resist the sudden lateral and vertical forces generated by an earthquake. This makes them vulnerable to structural failure and collapse.

Solution: This question requires observation of photographs which are not provided in the text. However, based on typical river features:

Possible River Features and their Types:

• Meander: Both (Erosional on the outer bank, Depositional on the inner bank)
• Ox-bow Lake: Depositional (formed from a cut-off meander)
• Flood Plain: Depositional (formed by sediment deposition)
• Waterfall: Erosional (formed by erosion of rock layers)
• Delta: Depositional (formed by sediment deposition at the river mouth)

Students should identify the specific features shown in their photographs and classify them accordingly.