IAS Prelims (GS) preparation 2018
Day # 15 (April 7, 2017)
Topics of the day: Geography 7th NCERT chapter 3 TO 5 key points
ROCKS
The earth’s crust is made up of various types of rocks.
Any natural mass of mineral matter that makes up the earth’s crust is called a rock.
Rocks can be of different colour, size and texture.
There are three major types of rocks: igneous rocks, sedimentary rocks and metamorphic rocks.
IGNEOUS ROCKS:
Igneous is derived from Latin word Ignis meaning fire.
When the molten magma cools, it becomes solid. Rocks thus formed are called igneous rocks.
They are also called primary rocks.
There are two types of igneous rocks: intrusive rocks and extrusive rocks.
Lava is actually fiery red molten magma coming out from the interior of the earth on its surface.
When this molten lava comes on the earth’s surface, it rapidly cools down and becomes solid.
Rocks formed in such a way on the crustare called extrusive igneous rocks.
They have a very fine grained structure. For example, basalt. The Deccan plateau is made up of basalt rocks.
Sometimes the molten magma cools down deep inside the earth’s crust.
Solid rocks so formed are called intrusive igneous rocks.
Since they cool down slowly they form large grains.
Granite is an example of such a rock. Grinding stones used to prepare paste/powder of spices and grains are made of granite.
SEDIMENTARY ROCKS:
Sedimentary is derived from Latin word sedimentum meaning settle down.
Rocks roll down, crack, and hit each other and are broken down into small fragments. These smaller particles are called sediments.
These sediments are transported and deposited by wind, water, etc. These loose sediments are compressed and hardened to form layers of rocks.
These types of rocks are called sedimentary rocks.
For example, sandstone is made from grains of sand.
These rocks may also contain fossils of plants, animals and other microorganisms that once lived on them.
METAMORPHIC ROCKS:
Metamorphic is derived from Greek word metamorphose meaning change of form.
Igneous and sedimentary rocks can change into metamorphic rocks under great heat and pressure.
For example, clay changes into slate and limestone into marble.
The process of transformation of the rock from one to another is known as the rock cycle.
Rocks are made up of different minerals. Minerals are naturally occurring substances which have certain physical properties and definite chemical composition. Minerals are very important to humankind. Some are used as fuels. For example, coal, natural
gas and petroleum. They are also used in industries – iron, aluminium, gold, uranium, etc, in medicine, in fertilisers, etc.
The Lithosphere is broken into number of plates known as Lithospheric plates. The movement of these plates causes movements on the earth.
MOVEMENTS OF THE EARTH:
The earth movements are divided on the basis of the forces which cause them.
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.
Endogenic forces sometimes produce sudden movements and at the other times produce slow movements.
Sudden movements like earthquakes and volcanoes cause mass destruction over the surface of the earth.
VOLACANO:
A volcano is a vent (opening) in the earth’s crust through which molten material erupts suddenly.
EARTHQUAKES:
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.
An earthquake is measured with a machine called a seismograph.
The magnitude of the earthquake is measured on the Richter scale.
An earthquake of 2.0 or less can be felt only a little. An earthquake over 5.0 can cause damage from things falling. A 6.0 or higher magnitude is considered very strong and 7.0 is classified as a major earthquake.
WEATHERING AND DEPOSITION:
The landscape is being continuously worn away by two processes – weathering and erosion.
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.
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 the earth.
MEANDERS AND OXBOW LAKES:
When the river tumbles at steep angle over very hard rocks or down a steep valley side it forms a waterfall.
As the river enters the plain it twists and turns forming large bends known as meanders.
Due to continuous erosion and deposition along the sides of the meander, the ends of the meander loop come closer and closer.
In due course of time the meander loop cuts off from the river and forms a cutoff lake, also called an ox-bow lake.
COASTAL LANDFORMS:
The erosion and deposition of the sea waves gives rise to coastal landforms. Seawaves continuously strike at the rocks. cracks develop.
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.
The sea waves deposit sediments along the shores forming beaches.
GLACIAL MORAINES:
Glaciers are “rivers” of ice which too erode the landscape by bulldozing soil and stones to expose the solid rock below.
The material carried by the glacier such as rocks big and small, sand and silt gets deposited. These deposits form glacial moraines.
SAND DUNES AND LOESS:
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. These are called sand dunes.
When the grains of sand are very fine and light, the wind can carry it over very long distances. When such sand is deposited in large areas, it is called loess. Large deposits of loess is found in China.
Our earth is surrounded by a huge blanket of air called atmosphere.
Nitrogen and oxygen are two gases which make up the bulk of the atmosphere. Carbon dioxide, helium, ozone, argon and hydrogen are found in lesser quantities.
Nitrogen is the most plentiful gas in the air .
When we inhale, we take some amount of nitrogen into our lungs and exhale it.
But plants need nitrogen for their survival.
Oxygen is the second most plentiful gas in the air .
STRUCTURE OF THE ATMOSPHERE:
Our atmosphere is divided into five layers starting from the earth’s sur face.
These are Troposphere , Stratosphere, Mesosphere, Thermosphere and Exosphere.
Troposphere:
This layer is the most important layer of the atmosphere.
Its average height is 13 km.
The air we breathe exists here.
Almost all the weather phenomena like rainfall, fog and hailstorm occur in this layer.
Stratosphere:
Above the troposphere lies the stratosphere.
It extends up to a height of 50 km.
This layer is almost free from clouds and associated weather phenomenon, making conditions most ideal for flying aeroplanes.
One important feature of stratosphere is that it contains a layer of ozone gas, it protects us from the harmful effect of the sun rays.
Mesosphere:
This is the third layer of the atmosphere.
It lies above the stratosphere.
It extends up to the height of 80 km.
Meteorites burn up in this layer on entering from the space.
Thermosphere:
In thermosphere temperature rises very rapidly with increasing height.
Ionosphere is a part of this layer.
It extends between 80-400 km.
This layer helps in radio transmission.
In fact, radio waves transmitted from the earth are reflected back to the earth by this layer.
Exosphere:
The upper most layer of the atmosphere is known as exosphere.
This layer has very thin air .
Light gases like helium and hydrogen float into the space from here.
Weather is the hour -to-hour , day to day condition of the atmosphere.
The average weather condition of a place for a longer period of time represents the climate of a place.
The degree of hotness and coldness of the air is known as temperature.
An important factor that influences the distribution of temperature is insolation
. Insolation is the incoming solar energy intercepted by the earth.
The amount of insolation decreases from the equator towards the poles. Therefore, the temperature decreases in the same manner.
Air pressure is defined as the pressure exerted by the weight of air on the earth’s surface.
As we go up the layers of atmosphere, the pressure falls rapidly.
The air pressure is highest at sea level and decreases with height.
Horizontally the distribution of air pressure is influenced by temperature of air at a given place.
In areas where temperature is high the air gets heated and rises. This creates a low-pressure area.
Low pressure is associated with cloudy skies and wet weather.
The air always moves from high pressure areas to low pressure areas.
The movement of air from high pressure area to low pressure areas is called wind. Winds can be broadly divided into three types.
1. Permanent winds– The trade winds, westerlies and easterlies are the permanent winds. These blow constantly throughout the year in a particular direction.
2. Seasonal winds– These winds change their direction in different seasons. For example monsoons in India.
3. Local winds – These blow only during a particular period of the day or year in a small area. For example, land and sea breeze.
When water evaporates from land and different water bodies, it becomes water vapour.
Moisture in the air at any time, is known as humidity. When the water vapour rises, it starts cooling. The water vapour condenses causing formation of droplets of water .
Clouds are just masses of such water droplets. When these droplets of water become too heavy to float in air , then they come down as precipitation. Jet planes flying in the sky leave a white trail behind them.
The moisture from their engines condenses.
Precipitation that comes down to the earth in liquid form is called rain.
Most of the ground water comes from rainwater .
On the basis of mechanism, there are three types of rainfall: the convectional rainfall, the orographic rainfall and the cyclonic rainfall.
Carbon dioxide released in the atmosphere creates a green house effect by trapping the heat radiated from the earth. It is therefore called a greenhouse gas and without it the earth would have been too cold to live in.
The standard unit of measuring temperature is degree Celsius. It was invented by Anders Celsius. On the Celsius scale the water freezes at 0°C and boils at 100°C.
On the moon there is no air and hence no air pressure.
A wind is named after the direction from which it blows, e.g. the wind blowing from the west is called westerly.
Other forms of precipitation are snow, sleet, hail
Terrarium:It is an artificial enclosure for keeping small house plants.
The process by which water continually changes its form and circulates between oceans, atmosphere and land is known as the water cycle.
Three-fourth of the earth surface is covered by water.
Salinity is the amount of salt in grams present in 1000 grams of water. The average salinity of the oceans is 35 parts per thousand.
Dead sea in Israel has salinity of 340 grams per litre of water . Swimmers can float in it because the increased salt content makes it dense.
March 22 is celebrated as World Water Day.
When the water on the surface of the ocean rises and falls alternately, they are called waves.
Waves are formed when winds scrape across the ocean surface. The stronger the wind blows, the bigger the wave becomes.
The rhythmic rise and fall of ocean water twice in a day is called a tide.
The strong gravitational pull exerted by the sun and the moon on the earth’s surface causes the tides.
During the full moon and new moon days, the sun, the moon and the earth are in the same line and the tides are highest. These tides are called spring tides.
The moon is in its first and last quarter, the ocean waters get drawn in diagonally opposite directions by the gravitational pull of sun and earth resulting in low tides. These tides are called neap tides.
High tides help in navigation.
They raise the water level close to the shores.
This helps the ships to arrive at the harbour more easily.
The high tides also help in fishing. Many more fish come closer to the shore during the high tide. This enables fishermen to get a plentiful catch.
The rise and fall of water due to tides is being used to generate electricity in some places.
Ocean currents are streams of water flowing constantly on the ocean surface in definite directions.
The ocean currents may be warm or cold.
The warm ocean currents originate near the equator and move towards the poles.
The cold currents carry water from polar or higher latitudes to tropical or lower latitudes.
The Labrador Ocean current is cold current while the Gulf Stream is a warm current.
The areas where the warm and cold currents meet provide the best fishing grounds of the world. Seas around Japan and the eastern coast of North America are such examples