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

Question: How are sea caves, arches and stacks formed?

Answer: Sea waves continuously strike at the rocks. Cracks develop. Over time they become larger and wider. Thus hollow- like caves are formed. These are called sea caves. As these cavities become bigger, and bigger, only the roof of the caves remains, forming arches. Further erosion breaks the roof, and only the walls remain. These walls are called stacks. The steep rocky coast rising almost vertically above sea level is called sea cliff. Sea waves deposit sediments along the shores, forming beaches.

Question: How are different landforms created in earth?

Answer: Weathering is the breaking up of the rocks on the earth’s surface. Erosion is the wearing away of the landscape by different agents like water, wind and ice. The eroded material is carried away or transported by water, wind, etc. and eventually deposited. This process of erosion and deposition create different land forms on the surface of the earth.

Question: How are flood plains formed?

Answer: At times the river overflows it banks. This leads to the flooding of the neighbouring areas. As it floods, it deposits layers of fine soil and other material called sediments along its banks. This in turn leads to the formation of a flat fertile floodplain.

Question: Describe how meanders and ox-bow lakes are formed.

Answer: As the river enters the plains, it twists and turns forming large bends called meanders. Due to the continuous erosion and deposition along the sides of the meander, the ends of the meander loop come closer and closer. In due course of time the meander loop cuts off from the river and forms a cut-off lake also called an ox-bow lake.

Question: What is an earthquake? How is it caused? Mention some common earthquake prediction methods.

Answer: When the lithospheric plates move, the surface of the earth vibrates. The vibrations travel all around the earth. These vibrations are called earthquakes. The place in the crust where the movement starts is called the focus. The place on the surface above the focus is called the epicentre. The greatest damage is usually the closest to the epicentre and the strength of the earthquake decreases as we move away from the epicentre. The common methods adopted by the people include studying animal behaviour; fish in ponds get agitated, snakes come up to the surface.

Question: Explain wind as an agent of erosion and deposition.

Answer: Work of wind Wind is an active agent of erosion and deposition in the deserts. Important landforms formed by wind are—mushroom rocks, sand dunes and loess.

1. Mushroom rocks- winds erode the lower section more than the upper part. After a period of time, such rocks have narrower base, and wider top representing a mushroom.
2. Sand dune- when the wind blows it transports sand from one place to another. When it stops blowing, or strikes a barrier, the sand falls and gets deposited into low, hill-like structures called sand dunes.
3. Loess- the wind can carry fine and light sands over very long distances when such sand is deposited in a large area, it is called a loess.