NCERT 7th Class (CBSE) Social Science: The Changing Face of the Earth

Question: Define volcano. How are volcanoes classified on the bases of their period of eruption.

Answer: A volcano is an opening (vent) in the earth’s crust through which molten rocks and gases erupt (come out suddenly) from deep inside the Earth. Volcanoes are divided into three types based on their frequency of eruption:

1. Active Volcanoes: Active volcanoes are those that have erupted recently. For example- Mt. Etna and Mt. Stromboli of the Mediterranean Sea are the most significant examples.
2. Dormant Volcanoes: are those that have been quiet for a long time but show signs that they could erupt again. For example: Mt. Vesuvius.
3. Extinct Volcanoes: These volcanoes no indication of future eruption. The craters of some extinct volcanoes fill with water to form lakes called calderas. For example: Mt. kilimanjaro in Tanzania.

Question: Discuss in detail the erosional and depositional features of rivers.

Answer: River break up the bedlock over which they flow and erode them in two ways – through mechanical erosion and through chemical erosion.

Features caused by river erosion:

Valleys: Valleys of different shapes are formed by the erosive action of a river in its upper and middle course.

1. I-shaped valleys or canyons- are formed when a river cuts out a narrow valley with steep walls.
2. V-shaped valley- Valleys are formed when river is still young and its flow is rapid. The erosion is predominantly vertical.

Waterfalls: When there is a steep drop in the course of a river, the water rushes down the drop with great force, and waterfall is formed.

Features caused by Deposition:

Meanders and oxbow lakes: Meanders are formed in the lower course when the river, loaded with debris, becomes very slow. The river swings from side to side as it flows over a level tract, forming winding sections or loops.

Oxbow lake – Continuous erosion on the concave side and deposition on the convex side can cut a loop from the main river and lead to the formation of an oxbow lake.

Flood Plains: A flood plain is an almost flat plain that is formed as a result of a river flooding its banks. As it floods, it deposits layers of fine soil and other material called sediments along its banks. As a result, fertile floodplain is formed. The raised banks of river flowing through the flood plain are called Levees.

Deltas: As the river approaches the sea, the speed of the flowing water decreases and the river begins to break up into several streams known as distributaries. Then a time comes when river becomes very slow and it begins to deposit its load. Each distributary forms its own mouth. The collection of sediments from all the mouths forms a delta, which is a triangular landmass.

Question: What are sand dunes?

Answer: During sandstorms, the wind carries large amount of sand grains. As the wind slows down and loses its energy, it starts dropping its load of sand. Such hills of sands deposited by the wind are called sand dunes.