Chapter 3 - Our Changing Earth

The forces like wind, water and ice that work on the surface of the earth and impacts of celestial objects (comets, meteoroids etc.) on earth are called exogenic forces.

Major Landforms

• The landscape is continuously worn away by weathering and erosion processes.
• Weathering is the natural wearing down of mountains and rocks on the earth’s surface by wind, water, and other elements in the environment.
• Erosion is the process of transporting weathered materials (soil, sand, etc.) with the help of various agents like winds, glaciers, water, etc., and their deposition.
• The process of erosion and deposition create different landforms on the surface of the earth.

Work of a River

• The running water in the river erodes the landscape.
• When the river tumbles at steep angle over very hard rocks or down a steep valley it forms a waterfall. The highest waterfall is Angel Falls of Venezuela in South America.
• As a river flows, it erodes the land it flows on, carrying away a lot of material.
• As the river enters the plain it forms meanders, ox-bow lakes, flat fertile floodplain and levees.
• The speed of the flowing water decreases as the river approaches the sea and the river begins to break up into a number of streams called distributaries.
• The river becomes so slow that it begins to deposit its load. Each distributary forms its own mouth. The collection of sediments from all the mouths forms a delta.

Work of Sea Waves

• The erosion and deposition of the sea waves gives rise to coastal landforms, sea caves, sea arches, stacks and sea cliff.
• Beaches are formed by the sediments deposited along the shores by the sea waves.

Work of Ice

• Glaciers (rivers of ice) can erode rocks and land.
• They move slowly across the land taking along with them rocks, sand, debris and soil. When deposited they form glacial moraines.
• When the ice melts the deep hollows carved out by glaciers fill up with water and become beautiful lakes.

Work of Winds

• Winds carry dust, sand, volcanic ash etc. from one place to another. In the deserts, wind is an active agent of erosion and deposition.

• Desert winds erode the lower section of the rock more than the upper part. These rocks having narrow base and wide top are commonly called ‘mushroom rocks’ since their shape looks like a mushroom.
• Sand is transported from one place to another by wind.
• Sand dunes are small hill like structures which are formed when the wind stops blowing and the sand falls and get deposited.
• The wind can carry fine grains of sand over very long distances. Loess is fine-grained material that has been transported and deposited, in large areas, by the wind.
• Large deposits of loess are found in China.