CBSE Class 7 Geography Chapter 3 – Our Changing Earth helps students understand about earth movements, how it is related to force, how an earthquake happens, explains terms like volcano, focus, epicentre, etc. It further teaches concepts related to sea waves, river, ice and wind. All these concepts, as mentioned earlier, are essential to score well in the Geography exam. So, to help students in preparing for their Geography exam, here we have provided CBSE Class 7 Geography notes of Chapter 3 – Our Changing Earth.
Lithospheric Plates – The lithosphere is broken into a number of plates known as the Lithospheric plates. These plates move around very slowly because of the movement of the molten magma inside the earth. The molten magma inside the earth moves in a circular manner. The movement of these plates causes changes on the surface of the earth.
The earth movements are divided on the basis of the forces which cause them. These forces are called Endogenic forces and Exogenic forces.
Endogenic forces: The forces which act in the interior of the earth are called Endogenic forces. Endogenic forces sometimes produce sudden movements and at other times produce slow movements. Sudden movements like earthquakes and volcanoes cause mass destruction over the surface of the earth.
Exogenic forces: The forces that work on the surface of the earth are called Exogenic forces.
Volcano: A volcano is a vent in the earth’s crust through which molten material erupts suddenly.
Earthquake: When the Lithospheric plates move, the surface of the earth vibrates and it travels all around the earth. These vibrations are called earthquakes.
Focus: The place in the crust where the movement starts is called the focus.
Epicentre: The place on the surface above the focus is called the epicentre. Vibrations travel outwards from the epicentre as waves. The greatest damage is usually closest to the epicentre and the strength of the earthquake decreases away from the centre.
Some common earthquake prediction methods adopted locally by people include studying animal behaviour; fish in the ponds get agitated, snakes come to the surface.
The landscape is being continuously worn away by two processes – weathering and erosion.
Weathering: Weathering is the breaking up of the rocks on the earth’s surface.
Erosion: 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 creates different landforms on the surface of the earth.
Work of a River
Waterfall: When the river tumbles at steep angle over very hard rocks or down a steep valley side, it forms a waterfall.
Meanders: As the river enters the plain it twists and turns forming large bends known as meanders.
Cut-off Lake: Due to 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.
Floodplain: As it floods, the river deposits layers of fine soil and other material called sediments along its banks. This leads to the formation of a flat fertile floodplain. The raised banks are called levees.
Distributaries: When the river approaches the sea, the speed of the flowing water decreases and the river begins to break up into a number of streams called distributaries.
Delta: 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
Sea caves: The erosion and deposition of the sea waves give rise to coastal landforms and they continuously strike at the rocks developing cracks. The cracks become larger and wider over time. Due to which, hollow like caves are formed on the rocks. They are called sea caves.
Sea Arches: When these cavities become bigger and bigger only the roof of the caves remain, it forms sea arches.
Stacks: Erosion breaks the roof and only walls are left and these wall-like features are called stacks.
Sea cliff – The steep rocky coast rising almost vertically above seawater is called sea cliff.
Work of Ice
Glaciers: Glaciers are “rivers of ice” which erode the landscape by bulldozing soil and stones to expose the solid rock below. Glaciers carve out deep hollows there. As the ice melts they get filled up with water and become beautiful lakes in the mountains.
Glacial Moraines: The material carried by the glacier such as rocks big and small, sand and silt gets deposited and forms glacial moraines.
Work of wind
Wind: In the desert, an active agent of erosion and deposition is wind.
Mushroom Rocks: In deserts, rocks can be shaped like a mushroom, commonly called mushroom rocks.
Sand Dunes: When the wind blows, it lifts and transports sand from one place to another. When it stops blowing the sand falls and gets deposited in low hill like structures. These are called sand dunes.
Loess: When the grains of sand are very fine and light, the wind can carry it over very long distances. When such sand is deposited in large areas, it is called loess.