NCERT CLASSS-7 GEOGRAPHY
CHAPTER-3 [OUR CHANGING EARTH]
- Lithospheric plates:
The earth’s crust consists of several large and some small, rigid, irregularly- shaped plates (slabs) which carry continents and the ocean floor.
- The lithosphere is broken into a number of plates known as the Lithospheric plates.
- 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 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.
- 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.
- VOLCANO-
● A volcano is a vent (opening) in the earth’s crust through which molten material
erupts suddenlY.
- EARTHQUAKE-
● Similarly, when the Lithospheric plates move, the surface of the earth vibrates.
● The vibrations can 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.
● Vibrations travel outwards from the epicentre as waves.
● 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.
- An earthquake is measured with a machine called a seismograph. The magnitude of the earthquake is measured on the Richter scale.
- MAJOR LANDFORMS-
● 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 highest waterfall is Angel Falls of Venezuela in South America. The other waterfalls are Niagara falls located on the border between Canada and USA in North America and Victoria Falls on the borders of Zambia and Zimbabwe in Africa.
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 side it forms a waterfall.
● As the river enters the plain it twists and turns forming large bends known as meanders.
● 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.
● At times the river overflows its 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 leads to the formation of a flat fertile floodplain.
● The raised banks are called levees.
● As 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.
● 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.
SEA CAVES & SEA ARCHES
Work of Sea Waves-
● The erosion and deposition of the sea waves gives rise to coastal landforms.
● Seawaves 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.
● As these cavities become bigger and bigger only the roof of the caves
remain, thus forming sea arches .
STACKS , SEA CLIFF & BEACHES-
● Further, erosion breaks the roof and only walls are left.
● These wall like features are called stacks.
● The steep rocky coast rising almost vertically above sea water is called sea cliff. The sea waves deposit sediments along the shores forming beaches.
GLACIAL LAND FORMS
Work of Ice-
● Glaciers are “rivers” of ice which too erode the landscape by bulldozing soil and stones to expose the solid rock below.
● Glaciers carve out deep hollows.
● As the ice melts they get filled up with water and become beautiful lakes in the mountains.
● The material carried by the glacier such as rocks big and small, sand and silt gets deposited. These deposits form glaciers.
AEOLIAN LANDFORMS
Work of wind-
● An active agent of erosion and deposition in the deserts is wind.
● In deserts you can see rocks in the shape of a mushroom, commonly called mushroom rocks.
● Winds erode the lower section of the rock more than the upper part. These deposits form glacial moraines.
SAND DUNES & LOESS
● 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.
● 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.
● Large deposits of loess is found in China.
● Therefore, such rocks have narrower base and wider top.
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