These DAV Class 7 SST Notes and DAV Class 7 SST Chapter 2 Notes – The Earth and the Changes on It hold significant importance as study material for students.
The Earth and the Changes on It Class 7 DAV Notes
→ The surface of the earth constantly undergoes changes. These changes can be sudden or gradual.
→ Gradual changes on the earth’s surface are brought by external processes which include weathering, erosion, transportation and deposition.
→ The rate at which weathering and erosion takes place depends upon the temperature of a place, vegetation cover, rainfall, slope of the land, type of soil and changes in land use.
→ Weathering and erosion take place together. They occur everywhere and all the time. This combined process is what we call denudation.
→ Agents of denudation are- running water, glaciers, wind and sea waves. A river originates in a mountain or a hill becomes larger where it is met by a number of tributaries and flows over plains and finally ends when it reaches the sea.
→ Like running water, moving ice erodes loose particles or parts of rocks, takes them along and deposits them behind, when it melts.
→ In the regions where there is little rainfall and meagre vegetation, winds are the most powerful agents of denudation.
→ Along the seashore the action of sea waves is most common. The continuous striking of sea waves breaks the rocks. Such continuous erosion forms steep sided cliffs facing the sea.
→ Human beings also play a crucial role in changing the face of the land. Over-growing population and industrialisation has disturbed the environment to a great extent.
→ Soil is very important for plant life. It is made of organic and inorganic particles. The organic particles are derived from plants, dead animals and bacteria. The inorganic particles are derived from the rocks.
→ The formation of soil is very slow process. It takes thousands of years to form a thin layer of soil.
→ The soil forming process continues over a long span of time. It starts developing layers called horizons. The topmost horizon is rich in humus.
→ In India, the soil is mainly of four types- alluvial, black, red and laterite. Alluvial soil is very fertile and forms major agricultural land of the country. Black soil is found in the Deccan plateau. Cotton is mainly grown in this soil. Red and Yellow soils are highly porous, fine grained and deep and is found in peninsular India. Laterite soil is found in the regions of heavy rainfall. Two other types of soil are Arid or Desert soil and Forest and Mountainous soil.
→ Soil erosion is a serious problem affecting land all over the world. This is happening due to cutting of trees, overgrazing of animals, etc. Appropriate measures such as plantation of trees, selective agricultural practices, etc. need to be taken to overcome the problem of soil erosion.
→ Weathering : The process that breaks rocks into smaller particles.
→ Geomorphology : The science of study of landforms.
→ Erosion : Gradual wearing and carrying away of all those loose particles of disintegrated rocks which lie exposed on the earth’s surface by the agents of gradation.
→ Gradation : Levelling of land.
→ Deposition : Laying down of sediments which have been carried from distant parts of the earth’s surface by various agents of erosion.
→ Agradation : Adding of sediments and raising the level of land.
→ Degradation : Breaking and wearing away of sediments and lowering the level of land.
→ Denudation : The wearing away of the earth’s surface by weathering process and agents of gradation.
→ Sand dunes : A small hill of sand formed by the wind near a sea or in a desert.
→ Soil: The loose particles of rocks which cover the earth’s surface.
→ Humus : A substance made from dead leaves and plants, added to soil to help plants grow.
→ Gravel: Small stones often used to make the surface of paths and roads.
→ Horizons : The development of soil in layers, one over the other.
→ Distributary : River channels separating from the main river.
→ T shaped valley : Erosional feature with narrow and vertical walls formed by a river. It is also called gorge.
→ ‘V’ shaped valley : Feature formed by a downward cutting of rocks by a river. It resembles the letter ‘V’.
→ Leaching : A natural process in which rainy water dissolves minerals from the soil and carry them away.
→ Meander : Sweeping curves of a river.
→ Delta : An area of land formed at the mouth of a river. It is usually triangular in shape.
→ Glacier : The mass of moving ice.
→ Continental glacier: Huge mass of ice which covers large area of a continent.
→ Mountain glacier: Mass of ice that occupies small areas over the mountains.
→ Tributary : Small streams joining the main river.