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(Sample Material) IAS Online Coaching : India & World Geography - "Lithosphere"

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Earth, the home to mankind, is a unique member of the solar family.
The fifth biggest planet in the solar system, its uniqueness lies in its habitability vertically overhead at the equator on two days each year, i.e. on March 21st and September 23rd. These days are called equinoxes meaning ‘equal nights’ because on these two days all places on Earth have equal days and nights.
After the March equinox, the sun appears to move northwards and is vertically overhead at the Tropic of Cancer on June 21st. Thus is known as the summer solstice, when the northern hemisphere will have its longest day and shortest night.
By December 22nd, the sun is overhead at the Tropic of Capricorn. This is the winter solstice when the southern hemisphere will have its longest day and shortest night.

The earth moves in space in two distinct ways. It rotates on its own axis from west to east once in every 24 hours causing day and night.
It also revolves round the sun in an orbit once in every 365 ¼ days causing the seasons and the year.
The earth revolves round the sun in an elliptical orbit at a speed of 18.5 miles per second.
A normal year is taken to be 365 days, and an extra day is added every four years as a Leap Year because 1/4th day of every year becomes 1 day.

Latitude is the angular distance of a point on the Earth’s surface, measured in degrees from the centre of the Earth.
Longitude is the angular distance, measured in degrees along the equator, east or west of the Prime Meridian (the meridian that passes through Greenwich near London).
This is the meridian from which all other meridians radiate eastwards and westwards up to 180º.
Longitudes are not used for calculating distances, since there is so much difference in the length of degrees of longitude outside the tropics.
But they have one very important function-they help in determining local time in relation to Greenwich Mean Time (GMT).

Most countries adopt their standard time from the central meridian of their countries.
The whole world has been divided into 24 Standard Time Zones.
Each zone, therefore, is separated by 15º longitudes or by one hour.

Larger countries like USA, Canada and Russia having greater east-west stretch have to adopt several time zones.
Both USA and Canada mainland have five time zones each- the Atlantic, Eastern, Central, Mountain and Pacific.
Russia has 11 time zones.
Interestingly, despite a considerable east-west stretch, India has only one time Zone.

Apogee: The period of the farthest distance between the moon and the earth (407,000km) is called apogee.
Perigee: The period of the nearest distance between the moon and the earth (356,000 km) is called perigee.

Perihelion: The period of the nearest distance between the earth and the sun (147 million kilometer) is perihelion. It happens on January 3.
Aphelion: The period of the farthest distance between the earth and the sun (152 million kilometers) is called aphelion. It happens on July 4.

A traveler going eastwards gains time from Greenwich until he reaches 180º E meridians, where he will be 12 hours ahead of GMT.
Similarly going westwards, he loses 12 hours till he reaches 180ºW.
There is, thus, a total difference of 24 hours or a whole day between the two sides of the 180º meridian. This is the International Date Line where the date changes by exactly one day when it is crossed.
The International Date Line in the mid-Pacific curves from the normal 180º meridian at the Bering Strait, Fiji, Tonga and other island to prevent confusion of day and date in some of the island groups that are cut through by the meridian.

Temperature inside the Earth increases with depth.
Experiments have confirmed that the temperature increases at the rate of 1ºC for every 32 metres.
With this rate of increase all the matter at the Core should have been in a molten state, but since the pressure at such great depths is also very high, the melting point is higher than the prevalent temperature and the inner Core is in a solid state.
In the outer Core, however, the pressure is lesser and the matter is in a liquid state.
The Core and the Mantle are separated by the Guttenberg discontinuity, while the Mohorovicic discontinuity separates the Mantle from the Crust.
Besides, the upper granitic and the lower basaltic layers of the crust are separated by a seismically determinable boundary called the Conrad discontinuity.
The uppermost portion of the mantle on which the crust rests, is rigid.
The crust and this solid uppermost mantle are together called lithosphere, the sphere of rocks.
Below the lithosphere, the upper mantle becomes so hot that it resembles hot plastic.
This soft plastic layer in the upper mantle is called the asthenosphere.
The hot plastic character of the asthenosphere and the convective belt cells facilitate the movement of lithospheric plates overlying this layer.

The Earth’s magnetic poles do not coincide exactly with the geographic poles.
Earth’s magnetic field reverses polarity; that is, the north magnetic pole becomes the south magnetic pole and vice versa.
However, it is not known exactly how the Earth’s magnetic field is produced.

The theory of Plate Tectonics states that the lithosphere consists of several individual segments called plates.
About twenty such plates have been identified.
Of these, the largest is the Pacific plate while the Juan de Fuca plate, of the western coast of North America, is the smallest.
Most of the Earth’s seismic activity, volcanism and mountain building occur along these dynamic margins.
Depending on the type of movement, plate margins are of three types-Divergent, Convergent and Transform fault boundaries.
When plates move apart with the upwelling of material from the mantle, a divergent plate boundary results.

The conspicuous mid-Atlantic Ridge system where from the Atlantic Ocean is widening: the Red Sea that is driving a wedge between Africa and the Arabian Peninsula and the Great Rift Valley of Africa are examples of such divergent plate boundaries.
A convergent plate boundary is one where two plates collide, one plate bending downward and subducting below the other.