NCERT Solutions for Class 7 Social Science Geography Chapter 3 Our Changing Earth

The lithosphere is broken into a number of plates known as the Lithospheric plates.

Lithospheric plates move around very slowly – just a few millimetres each year. This is because of the movement of the molten magma inside the earth. The molten magma inside the earth moves in a circular manner.

The forces which act in the interior of the earth are called as Endogenic forces and the forces that work on the surface of the earth are called as Exogenic forces. Endogenic forces sometimes produce sudden movements and at the other times produce slow movements. Sudden movements like earthquakes and volcanoes cause mass destruction over the surface of the earth.

When the Lithospheric plates move, the surface of the earth vibrates. The vibrations can travel all-round 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.

An earthquake is measured with a machine called a seismograph. The magnitude of the earthquake is measured on the Richter scale. An earthquake of 2.0 or less can be felt only a little. An earthquake over 5.0 can cause damage from things falling. A 6.0 or higher magnitude is considered very strong and 7.0 is classified as a major earthquake.

Safe Spot – Under a kitchen counter, table or desk, against an inside corner or wall.
Stay Away from – Fire places, areas around chimneys, windows that shatter including mirrors and picture frames.
Be Prepared – Spread awareness amongst your friends and family members and face any disaster confidently.

The landscape is being continuously worn away by two processes – weathering and erosion. 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 landforms on the surface of the earth.

The erosion and deposition of the sea waves gives rise to coastal landforms. Sea waves continuously strike at the rocks. Cracks develop. Over time they become larger and wider. Thus, hollow like caves are formed on the rocks. They are called sea caves.

The plates move because of the movement of the molten magma inside the earth.

The forces that act in the interior of the earth are called endogenic forces. The forces that act on the earth’s surface are called exogenic forces.

Erosion is the wearing away of the landscape by different agents like water, wind and ice.

When rivers outflow their banks, they cause flood in the neighboring areas. The flood water takes away fine soil another material and deposits them along the banks. This deposition leads to the formation of flood plains.

Winds lift and carry sand from one place to another. When they stop blowing, sand falls and gets deposited in low hill-like structures called sand dunes.

The sea waves deposit sediments along the shores which leads to the formation of beaches.

When a river enters the plain, it forms a large bend which is cut-off stream in due course of time. The cut of stream takes the form of a lake called ox-bow Lake.

In desert, wind is an active agent for erosion which erode the lower section of the rock more than the upper part. Therefore, some rocks have a shape of a mushroom having narrower base and wider top.

Flood plains are very fertile because floods deposit layers of fine soil and other material called sediments which is ideal for cultivation along its banks.

When cavities in sea caves become bigger and bigger only the roof of the caves remain forming sea arches. Further, erosion breaks the roof and only walls are left which forms the stacks.

During earthquakes, vibrations travel outwards from the epicentre as waves and propagate through the surface of the earth which produce sudden movements that leads to the collapse of buildings.

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