Features Of Earth:
i) The earth is the fifth largest planet of the solar system, where life exists.
ii) Its equatorial diameter is 7,927 miles.
iii) Its polar diameter is 7900 miles.
iv) The earth has average density of 5 ½ g/cc.
v) the rotation period of the earth is 23 hours, 56 minutes and 4 seconds.
vi) the revolution period of the earth is 365 days.
vii) The total mass of the earth is 6*1021 tons.
viii) The earth is composed of shells or layers, which are, the crust, mantle and core.
ix) It is surrounded by a blanket of gases, which is known as the atmosphere, mainly composed of Nitrogen and Oxygen.
x) The surface of the earth is rich in oxygen, silicon, iron, magnesium, sodium etc.
Structure Of The Earth:
The earth comprises of the following layers:
a) The crust
b) Moho discontinuity
c) The mantle
d) The Core
a) The Crust:
i) It is the outermost layer of the earth.
ii) It extends to about 25 miles (40kms) and comprises of rocks.
iii) The crust is divided into the oceanic and the continental crust.
iv) Out of these the oceanic crust is 808 meter thick and consists of sedimentary mud.
v) The continental crust is divided into upper continental and lower continental crust
b) Moho discontinuity:
i) The sharp boundary between the crust and mantle is called Moho Discontinuity.
c) The Mantle:
i) The layer of the earth lying below the crust and above the core is known as the mantle.
ii) It is almost 2900 kms (1800 miles) thick and comprises about 80% of the volume of the earth.
iii) The chemical composition of the entire mantle is fairly homogenous.
iv) However, temperature and pressure increases with depth.
v) The behaviour of the earthquake waves as they travel through the mantle further tells us that it consists of several layers and they are:
d) The Core:
i) The innermost part of the earth is known as the core.
ii) It extends from the base of the mantle to the centre of the earth.
vi) This portion consists of melted iron and nickel that is why it is known as Nife.
vii) The density of this molten mass of the core is 345 pounds per cubic feet
Types Of Movements Of Earth:
There are two types of motions of the earth.
One is around its own axis which is called Rotation. One rotation completes in 23 hours, 56 minutes and 4 seconds.
The other motion of earth is round the sun, and is called revolution of the earth.
One revolution completes in 365 days.
Northern end of the earth’s axis is called the North Pole and the Southern end is called South Pole.
An imaginary line drawn round the earth midway between the poles is called equator.
The path which the earth takes round the sun is called earth’s orbit
Effects Of These Movements:
Rotation of earth round its axis cause days and nights.
That portion of earth, which is within the circle of illumination caused by the rays of sun, has its day.
The other side earth, which is away from sun, remains dark and therefore has its night.
Longer And Shorter Days:
The circular areas near the North and South Pole of the Earth are called Arctic and Antarctic circles, respectively.
These are situated at 66 ½0 North and 66 ½0 South of the equator, respectively and form limits of polar region.
On June, the earth is in position A. north pole is inclined towards the sun and south pole is away from it.
Obviously any place in Northern Hemisphere will have longer days because it remains in light for more than half the time of earth’s rotation.
Places on equator remains in light for half the time making days and nights equal in this region.
The southern hemisphere remains in light for less time than half the rotation of earth so the days are shorter here.
Positions of days and nights in the northern and southern hemisphere are reversed on 22nd December, when the earth completes its half revolution around the sun, so days are longer in southern hemisphere than those in the north
Change Of Season:
The layer of the gases which surrounds the earth is known as the atmosphere.
The atmosphere is a thin layer of the gases held to the earth by the gravitational attraction.
Atmosphere is the huge blanket of gas that circles the entire earth.
Composition Of The Atmosphere:
The atmosphere consists of:
i) Nitrogen: 78.03%
ii) Oxygen: 20.99%
iii) Argon: 0.94%
iv) CO2: 0.03%
v) Hydrogen: 0.01%
vi) Neon: 0.0018%
vii) Helium: 0.0005%
viii) Krypton: 0.0001%
ix) Ozone: 0.000001%
Layers Of The Atmosphere:
The atmosphere of the earth is divided into following layers.
Ionosphere is sub-divided into:
i) Troposphere is the lowest layer of the atmosphere.
ii) It extends roughly to a height of 8 kms near the poles and 16-18 kms at the equator.
iii) It is troposphere where the people, plants, animals and insects live.
iv) It is the layer where all weather occurs that’s why it is also referred as “The weather Zone”.
v) In the Troposphere temperature gradually falls with increasing altitude.
vi) There is a thin buffer zone between the troposphere and stratosphere is called tropopause.
2) Stratosphere And Ozone Layer:
i) The second layer of the atmosphere is known as the stratosphere.
ii) The stratosphere extends from the tropopause to about 30-31 mile above ground level.
iii) The important ozone layer is found in this region where heat is generated by absorption of UV.
iv) Here the temperature either remains constant or increases with altitude.
It is the upper boundary of the stratosphere which occurs beyond 52 kms. Here the temperature remains constant with increase in height.
i) The third major layer of the atmosphere is the ionosphere.
ii) It lies above the stratosphere.
iii) It lies between about 30 and 90 miles above the surface of the earth.
iv) It is divided into mesosphere and thermosphere.
v) The ionosphere is bombarded by cosmic radiation and solar x-rays, which causes the gases in the ionosphere to ionize.
vi) Brilliant displays of colored lights in the sky called Aurora Borealis in the northern hemisphere and the Aurora Auralis in the southern hemisphere occur when streams of electrically charged particles from the sun (solar wind) ionize the atmosphere gases.
i) The uppermost layer of the atmosphere extending beyond Ionosphere is called exosphere.
ii) Beyond 300 miles is the very rare field exosphere which consists only of scattered atmosphere of O, H and He.
It is the distance on the surface of the earth measured in degrees north and south of the equator.
The equator is at zero degree where the poles are at 90 degree. The latitude of the north pole is 900 North and that of south pole is 900 South.
Characteristics OF Lines Of Latitudes:
i) All lines of latitudes are parallel to the equator as well as parallel to one another.
ii) Parallels in the north of the equator are north latitudes while those in the south of equator are known as south latitudes.
iii) They are drawn on the globe as circles running in east to west direction.
iv) The length of the equator is the maxim um and it goes on reducing till the pole is only a point.
The distance on the earth’s surface measures in degrees east and west of a line joining the geographical north and south poles and passing through Greenwich in England. Greenwich is at zero degrees longitude.
The sun rays have highest altitude simultaneously on all the places at a particular line of longitude as a result of which these are also called as Meridians (Meridian is a Latin word which means Mid-way). Among the latitude, equator is the longest and is taken as reference line. But all the lines of longitude are of the same length and selecting a longitude as lines of reference is a serious problem.
Earthquakes are those movements of the earth crust which make the ground vibrated and shake backwards and forwards or in simple words an earthquake is trembling in the earth.
The shocks waves are generated at a point within the crust called the focus, and the point on the earth’s surface vertically above the focus is called the epicentre of the earthquake. The shock waves travel in all directions from the focus. On the earth’s surface, the shaking is the strongest near the epicentre. These waves are detected by seismograph.
Occurrence Of Earthquake:
i) Earthquakes occur when rocks subjected to great stress suddenly break, releasing the accumulated energy, which shakes the ground. Vibrations spread out from the epicentre like ripples in water.
ii) It may also be caused by movements of the plates, resulting from convection currents in the hot mantle of the earth.
iii) Earthquakes are also associated with volcanic activity-eruption of magma. Collapse of mines can also produce small earthquakes.
An opening in the earth’s crust through which molten lava, ash, and gases are ejected.
Occurrence Of Volcanoes:
i) Rocks below the Earth have a very high temperature.
ii) The great pressure upon these keeps them in a semi-solid state.
iii) If the pressure weakens, then some of rocks become liquid.
iv) This liquid is called magma.
v) The magma forces its way into cracks of the crust and may either reach the surface of the earth where it forms lava and flow out.
Types Of Volcanoes On The Basis Of Activity:
There are three types of volcanoes on the basis of volcanic activity, which are as under.
i) Active Volcanoes:
Volcanoes are said to be active when they frequently erupt or at least when they have erupted within recent time.
ii) Dormant Volcanoes:
The volcanoes that have been known to erupt and show signs of possible eruption in the future are describes as dormant volcanoes.
iii) Extinct Volcanoes:
The volcanoes that have not erupted at all in historic times but retain the features of volcanoes are termed as extinct volcanoes.
Causes of The volcanoes Eruptions:
i) Seafloor spreading
ii) Convergence of lithospheric plates
iii) Percolation of cold water
iv) Orogenic Movements
v) High temperature in the interior of the Earth Rocks:
The word igneous means the fires and the rocks formed by solidification of molten rock material known as magma are known as igneous rocks.
The first minerals to crystalize out of the melt are high-temperature minerals-the olivines and pyroxenes, which are silicates of magnesium and iron.
They tend to be denser than magma and so they sink, leaving the remaining fluid deficient in magnesium and iron.
The next group of minerals to solidify are the feldspars (silicates minerals of potassium, sodium, calcium and aluminium); the magma thus loses its metallic constituents first.
Finally, any remaining silica crystalizes out as quartz.
The entire solidification process therefore results in dense iron-and magnesium-rich rocks and less dense silica rich rocks from the same original fluid.
In this way, different types of rocks can be seen in the same rock mass.
The most important igneous rocks are: a) Granite rocks b) Gabboro rocks c) Basalt rocks
Sedimentary rocks are formed from the deposition and compaction or lithification of rocks and mineral grains derived from other rocks.
These grains broke away from existing rocks by the action of water, wind and ice.
Many sedimentary rocks begin their existence as loose deposits of sand or gravel at the bottom of a sea or lake, on beach, or in a desert.
Later the sediment is lithified i.e. compressed into a rock.
Following are the major classes of sedimentary rocks:
i) Calcareous sedimentary rocks
ii) Carbonaceous sedimentary rocks
iii) Siliceous sedimentary rocks
iv) Ferruginous sedimentary rocks
v) Arenaceous sedimentary rocks
vi) Argillaceous sedimentary rocks
vii) Rudaceous sedimentary rocks
The word metamorphic has been derived from two Greek words Meta means change and Morpha means shape.
Thus metamorphic rocks include those rocks that have been changed either in form or composition without disintegration.
Metamorphic rocks are formed from igneous as well as sedimentary rocks but are different from them.
i) Sandstone, made of quartz grains and silica cement, becomes quartzite, a very hard metaphoric rock that resist weathering.
ii) Limestone is converted into much denser and harder marble.
iii) Mica, an igneous rock, is converted into schist after metamorphosis.
iv) Sedimentary rock slate is converted into a slightly higher grade metamorphic rock phyllite.
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