Geography (Chap-3) class-7 'Our Changing Earth'

Geography (Chap-3)                Our Changing Earth

Ø  Our Earth constantly undergoes changes inside and outside of it.

Ø  The movement of earth is divided into two types on the basis of forces which cause them.

Ø  The forces which act in the interior of the earth are called Endogenic forces.

Ø  The forces that work on the surface of the earth are called Exogenic forces.

Ø  The Lithosphere is broken into a number of plates known as Lithospheric Plates.

Ø  The Lithospheric plates move around just a few millimeter each year because of the movement of the molten magma inside the earth.

Ø  The molten magma inside the earth moves in a circular manner.

Ø  Lithospheric plates: The earth’s Crust consists of several large and some small, rigid, irregularly shaped plates(slabs) which carry the continents and the ocean floors.

Ø  Endogenic forces sometimes produce sudden movements as well as slow movements. Sudden movements like Earthquakes and Volcanoes cause mass destruction.

Ø  Volcano:A vent(opening)in the earth’s crust through which the  molten material erupts suddenly

Ø  When the Lithosphere plates move, the surface of the earth vibrates and is causes Earthquake.

Ø  The place in the crust where the movement starts is called the ‘Focus’.

Ø  The place on the surface above the focus is called the ‘Epicentre’.

Ø  Vibrations travel outwards from the epicenter as waves. Greatest damage occur to the closest of the epicenter.

Ø  There are three types of earthquake waves: P waves(Longitudinal), S waves(Transversive), L waves (Surface waves)

Ø  Studying the animal behaviours of agitated fish in the ponds and coming out of snakes into the surface, can be predicted about earthquakes.

Ø  An earthquake is measured with the machine called ‘Seismograph’.

Ø  The magnitude of the earthquake is measured on the Richter Scale.

An earthquake of 2.0 or less can be felt only a little

                        over 5.0  can cause damage from things falling

                        of 6.0 or higher magnitude is considered very strong (Bhuj,Gujarat-6.9)

                        of 7.0 is classified as a major earthquake

·         Major Landforms:

Ø  The landscape is being continuously worn away by two processes: Weathering & Erosion.

Ø  Weathering is the breaking up of the rocks on the earth’s surface

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

Ø  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 earth.

·         Work of the River:

Ø  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.

Ø  The Highest WaterFall in the world is Angel Falls of Venezuela in South America(Height-979m, plunge-807m).

Ø  The Niagra Falls(3,950ft wide) on the border between Canada and USA in North America is the largest Water Fall in the world.

Ø  Victoria Falls(width 1708m) is a Water Fall in Zambezi River on the borders of Zambia and Zimbabwe in Southern Africa.

Ø  As the river enters the plain it twists and turns forming large bends known as ‘Meanders’.

Ø  When the meander loop cuts off from the river and forms a cut-off lake, is called an ‘Ox-bow Lake’.

Ø  In south Texas, Oxbows left by the Rio Grande are called Resacas. In India, Oxbow lake also found on the river Dihing which fallen into Brahmaputra river in Assam.

Ø  When the river over flows, its banks  leads to the flooding of the neighbouring areas. As it floods, it deposits layers of fine soils and sediments along its banks which leads to the formation of a flat fertile flood plain.

Ø  The raised banks are called levees.

Ø  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. The river becomes so slow that it begins to deposit its load.

Ø  Each distributaries forms its own mouth. The collection of sediments from all the mouths forms a Delta. Ex- Sundarvan Delta of West Bengal

·         Work of Sea Waves:

Ø  The erosion and deposition of the sea waves gives rise to coastal landforms.

Ø  Sea waves continuously strike at the rocks and crack develops. Over time they became larger and wider. Thus, hollow like caves are formed on the rocks and are called Sea Caves.

Ø  When the cavities of the sea caves becomes bigger and wider only the roof of the caves remain and forms Sea Arches.

Ø  When the erosion breaks the roof of the sea arches, only the walls are left. These wall like features are called Stacks.

Ø  The steep rocky coast rising almost vertically above sea water is called ‘Sea Cliff’.

·         Work of Ice:

Ø  Glaciers are ‘rivers of ice’ which erode the landscape by bulldozing soils and stones to expose the solid rock below.

Ø  When the Ice melts they get filled up with water and become beautiful lakes in the mountains.

Ø  The materials carried by the glacier such as small and big rocks, sand and silt gets deposited. These deposits forms Glacier Moraines.

·         Work of Wind:

Ø  An  active agent of erosion and deposition in the deserts is wind.

Ø  Mushroom rocks: rocks in the shape of a mushroom in deserts. These rocks have narrower base and wider top as the wind erodes the lower section of the rock than the upper top.

Ø  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 known as ‘Sand Dunes’.

Ø  When the fine and light grains of sand are carried over a long distances by wind and deposited in large areas, called 'Loess’. Large deposits of loess are found in China.