Rajasthan Board RBSE Class 10 Science Notes Chapter 15 Structure of Earth
The Earth is at a distance of about 15 crore km from the Sun. The distance between Sun and Earth is called one Astronomical Unit (1 AU).
Origin and Evolution of Earth:
Various proofs collected from different branches of science show that the earth came into existence about 4.5 billion years ago. Initially, the earth was very hot and its rotation was very fast. External portion of the earth kept on cooling down and coming off from the surface of the earth due to fast rotation. It took millions of years for the earth to cool down.
Structure of Earth:
The earth is composed of three layers, viz. crust, mantle and core.
The outermost layer of the earth is a solid layer and is called crust. The thickness of the crust is not uniform at every place. Due to this, we get to see mountains and oceans at different places.
The second layer of earth is called mantle. This is the thickest layer and most of it is composed of hot and molten rocks. These are silicate rocks and have iron and magnesium in greater quantity than in the crust.
The innermost portion of the earth is called core. This is hottest due to being at the centre. The temperature of the core is about 7000 degree. The heat trapped during formation of earth is the cause of high temperature of core.
Energy System of Earth:
Many forces have been continuously working to change the landforms on the surface. These forces are called moulding forces. There are two types of moulding forces.
Internal Moulding Forces:
These forces continue their work inside the earth and are not apparent from outside. These forces come into origin because of expansion and contraction of rocks due to high temperature inside the earth. These forces also come into origin because of movement of hot molten magma.
Volcano is the most unique phenomenon among internal moulding forces. It causes turbulence inside the earth which shakes the earth and smoke, ash, vapour and gases come out by piercing the surface of the earth. Sometimes, very hot rocks melt and come out in the form of lava.
Shaking of the surface of earth is called earthquake. The place where the earthquake originates is called the epicenter.
Intensity of earthquake is measured on Richter scale. This is done by a device called seismograph. The Richter scale varies from 1 to 10.
Cause of Earthquake:
- Earthquake is usually explained on the basis of tectonic movement theory.
- Boundaries of tectonic plates are of three types, viz. transform boundary, divergent boundary
and convergent boundary. Transform boundary is conservative in nature, divergent boundary is constructive and convergent boundary is destructive.
An earthquake of intensity more than 7 on Richter scale at the bottom of ocean is the prime reason of tsunami. Tsunami travels in two directions from the epicenter, i.e. towards the deep coast and towards deeper ocean. The tsunami which moves towards the coast causes destruction. The debris which flows along with tsunami can destroy life and material even in the hinterland.
External Moulding Forces :
There are two types of external moulding forces viz. weathering and erosion forces.
- Weathering Forces : Weathering forces keep on breaking the rocks to convert them into soil. The heat from the sun, rain, frost and wind physically break the rocks.
- Many chemical processes keep on happening in nature, e.g. oxidation, carbonation, coalescing of water molecules, dissolution, etc. These processes also facilitate weathering of rocks by making them weaker.
- Biological forces like plants, animals, microbes and humans also play important role in weathering of rocks.
Air, water and ice are those substances on earth which move from one place to another in huge quantities. Flow of these substances has immense force in it. Even big structures break and flow away when they came in the path of air, water or ice.
Power of Wind:
The motion of air provides the weathering force in it. Force of weathering increases with speed of wind and if speed goes beyond a certain limit then wind becomes highly destructive which may even alter the map of an area.
The uneven heating of earth’s surface results in uneven heating of air. The air may become less warm or warmer at different places. Warm air expands and goes up because it becomes lighter. This creates an area of low pressure near the ground. The low pressure area is filled in by air moving from high pressure area around it. Thus, a convectional current is set which causes movement of air. Moving air is called wind.
Some winds change direction in different seasons and hence are known as monsoon wind. During summer, monsoon wind moves from ocean to land for six months and cause summer monsoon. Rains in most parts of India are caused by south-west monsoon.
When rainfall is more than drainage of water causes flood. During flood, water flows at high speed and carries away the things which come in its way.
A low pressure area on centre creates cyclone around it. The centre of cyclone is called ‘eye’. The diameter of a cyclone can be from 400 km to 3000 km. Diameter and speed of cyclone is more in temperate zone than in torrid zone.
During its course from a mountain to a lake or ocean, a river makes a variety of structures like plains and deltas. Rivers have plaved important role in development of human civilization. The Vedic Civilization developed along the Saraswati River.
Glacier is defined as a persistent body of dense ice which keeps on moving because of its own weight. Slow speed of glacier means that its effect cannot be apparent as quickly as that of a river but one can easily notice the long term effects of glacier.
Effect of global warming on glaciers:
Rate of melting of glaciers is more than the rate of their formation due to global warming. Hence, glaciers are shrinking in size. Melting of glaciers has resulted in rise in sea levels.
Ocean current are also referred to as rivers flowing in oceans. The water flows in a certain direction in these rivers. Some ocean currents have warm water, while some others have cold water.
When the moon and sun align in a line, it causes tides. The gravitational pull on ocean water increases immensely in this situation. This results in tides.