Points to Remember : NCERT class VII : Our Environment – Our Changing Earth – Chapter 3
The lithosphere is broken into a number of plates known as the Lithospheric plates.The movement of these plates causes changes on the surface of the earth.
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.
The molten magma inside the earth moves in a circular manner.when, molten material erupts suddenly volcano is caused.
A volcano is a vent (opening) in the earth’s crust through which molten material erupts.
Similarly, when the Lithospheric plates move, the surface of the earth vibrates. The vibrations can travel all round 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.
There are three types of earthquake waves.-
- P waves or longitudinal waves
- S waves or transverse waves
- L waves or surface waves
Machine to measure Earthquake – Seismograph, Scale : Richter scale
A 6.0 or higher magnitude on Richter scale is considered very strong and 7.0 is classified as a major earthquake.
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.
When the river tumbles at steep angle over very hard rocks or down a steep valley side it forms a waterfall .
The highest waterfall is Angel Falls of Venezuela in South America.
As the river enters the plain it twists and turns forming large bends known as meanders.Becuase of erosion, meander loop may cuts off from the river and forms a cut-off lake, also called an ox-bow lake.
While flowing yo sea, river begins to break up into a number of streams called distributaries.Each distributary forms its own mouth. The collection of sediments from all the mouths forms a delta.
The erosion and deposition of the sea waves gives rise to coastal landforms. Seawaves continuously strike at the rocks. Cracks develop. Hollow like caves called sea caves are formed on the rocks. As these cavities become bigger and bigger only the roof of the caves remain, thus forming sea arches. 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.
Low hill – like structures formed due to deposition of sand are called sand dunes.
Rocks in shape of mushroom called mushroom rocks are found in desserts.
Very fine and light sand,which can be carried by wind over very long distances is called loess.