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Chapter 12. The Earth (Geography Notes)


International Date Line (IDL)
➤ The International Date Line (IDL) is an imaginary line on the surface of the Earth, that runs from the north to the south pole and demarcates one calendar day from the next.
➤ It passes through the middle of the Pacific Ocean, roughly following the 180° longitude but it deviates at Aleutian Islands, Fiji, Samoa and Gilbert Islands.
➤ The International Date line is on the opposite side of the Earth Prime Meridian.
➤ The Prime Meridian helps to define Universal Time and is the meridian from which all other time zones are calculated.
➤ A traveler crossing the International Date Line eastbound (i.e., from Japan to USA) subtracts one day, or 24 hours, so that the calendar date to the west of the line is repeated after the following midnight.
➤ Crossing the IDL westbound results in 24 hours being added, advancing the calendar date by one day.
Prime Meridian :
➤ It is the mean solar time at the Royal ob in Greenwich London.
➤ GMT is formally used as the international civil time standard now superseded in that function by coordinated universal time.
➤ The modern form of mean solar time at 0° longitude.
➤ Prime meridian passes through these countries :
1. U.K 2. Spain 3. France, 4. Algeria 5. Burkino Faso 6. Ghana, 7. Togo 8. Mali 9. Antarctica (South Pole)
Tropic of Cancer
The line beyond which sunrays never incident directly at 900 on earth in northern hemisphere. It is a line at 23 ½ in northern hemisphere. It is thus the southernmost latitude where the Sun can be directly overhead (900). ➤ Tropic of cancer passes through these countries :
1. USA 2. Mexico 3. Bahamas, 4. Mali 5. Mauritania 6. Niger, 7. Algeria 8. Chad 9. Libya, 10. Saudi Arabia 11. Egypt 12. UAE, 13. India 14. Bangladesh 15. Myanmar, 16. China 17. Oman
Tropic of Capricorn
➤ The line beyond which sunrays never incident directly at 900 on earth in southern hemisphere. It is a line at 23 ½ in southern hemisphere. It is thus the southernmost latitude where the Sun can be directly overhead (900). It passes through these countries — 1. Chile 2. Argentina 3. Paraguay, 4. Brazil 5. Namibia 6. Botswana, 7. South Africa 8. Medagascar 9. Mozambique, 10. Australia
➤ It passes through these countries —, 1. Ecuador 2. Colombia 3. Brazil, 4. Gabon 5. Congo 6. Zaire, 7. Somalia 8. Uganda 9. Kenya, 10. Indonesia 11. Kiribati 12. Maldives, 13. Sao Tome and Principe
(i) Rotation : Earth rotates on its own axis that is tilted at an angle of 23½° to the vertical.
➤ It spins from west to east once in every 23 hours, 56 minutes and 4.9 seconds.
➤ The linear velocity of rotation varies, achieving a maximum of 1690 km/hr at the equator, reducing gradually to zero at the poles.
Effects of Earth’s Rotation
➤ We have day and night.
➤ A difference of 1 hour between two meridians which are 15° apart.
➤ Deflection of ocean currents and winds.
➤ Rise and fall of tides every day.
(ii) Revolution : Earth revolves round the Sun once in every 365¼ days.
➤ Earth revolves on an elliptical orbit at a speed of 106,560 km/hr. As it is not possible to show a quarter of a day in the calendar, a normal year is taken to be 365 days, and an extra day is added every four years in a leap year.
Effects of Revolution
➤ Change of seasons.
➤ Variation in the lengths of day and night at different times of the year.
➤ Shifting of wind belts.
➤ Determination of latitudes.
➤ The axis of Earth is inclined to the plane of ecliptic at an angle of 66½°.
➤ It gives rise to different seasons and varying lengths of day and night.
➤ The Sun is 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’.
➤ After the March equinox, the Sun appears to move northwards and is vertically at the Tropic of Cancer on June 21st. This is known as the Summer Solstice.
➤ During summer solstice, the Northern hemisphere will have its longest day and the shortest night.
➤ By December 22nd, the Sun is overhead at the Tropic of Capricorn.
➤ This is the Winter Solstice.
➤ In the Winter Solstice the Southern hemisphere will have its longest day and shortest the night.
Latitude : Latitude is the angular distance of a point on the earth’s surface, measured in degrees from the centre of the places of same latitude are called parallels of latitude since they are parallel to a line, the Equator, which lies midway between the poles. These parallels of latitude are actually circles on the globe, becoming smaller poleward.
➤ Some important parallels are :
(i) 0° latitude : Equator
(ii) 23½°N : Tropic of Cancer
(iii) 23½°S : Tropic of Capricorn
(iv) 66½°N : Arctic Circle
(v) 66½°S : Antarctic Circle
➤ The parallel of 60° is half of the Equator in length and 75° is ¼th of the Equator.
Longitude : Longitude is the angular distance measured in degrees along the equator east or west of the Prime Meridian.
➤ On the globe the lines of longitude are drawn as a series of semi-circles that extend from the North Pole to the South Pole through the Equator.
➤ They are also called meridians.
The Prime Meridian (0°) is that meridian which passes through the Greenwich, near London and from which all other meridians radiate eastwards and westwards up to 180°.
➤ 1° of latitude or longitude represents 111 km approx.
➤ This distance is true for the longitudes along the equator only since the distance between the longitudes gradually decreases towards the poles to 0 km.

Nebular Hypothesis— Laplace
Tidal Hypothesis— James Jeans
Supernova Hypothesis— F. Hoyle
Gaseous Hypothesis— Immanuel Kant
Planetesimal Hypothesis— Chamberlin
Binary Star Hypothesis— H.N. Russell

1. Iron 35%
2. Oxygen 30%
3. Silicon 15%
4. Magnesium 13%
5. Nickel 2.4%
6. Sulfur 1.9 %

Direct sourcesIndirect Sources
The surface rocks or rocks we get from miningMeteors as they are made up of same materials as, or similar to our planet.
Molten material (magma) from
volcanic eruptions
Observations from scientific projects such as : “Deep ocean Drilling Project” and “Integrated
Other important sources include:
1. Gravitation
2. Magnetic field
3. Seismic activity

➤ The structure of Earth can be defined in two ways : by mechanical properties such as rheology, or chemically. Mechanically, it can be divided into lithosphere, asthenosphere, mesospheric mantle, outer core, and the inner core.
➤ The interior of Earth is divided into 5 important layers. Chemically, Earth can be divided into the crust, upper mantle, lower mantle, outer core, and inner core.
➤ The geologic component layers of Earth are at the following depths below the surface :
➤ The inner most layer of the Earth is called the core.
➤ The core is divided into two parts, a solid inner core with a radius of~1,220 km and a liquid outer core extending beyond it to a radius of ~ 3,400 km.
➤ The solid inner core was discovered in 1936 by Inge Lehmann and is generally believed to be composed primarily of iron and some nickel.
➤ The core is thus believed to largely be composed of iron
(80%), along with nickel and one or more light elements, whereas other dense elements, such as lead and uranium, either are too rare to be significant or tend to bind to lighter elements and thus remain in the crust.
➤ The liquid outer core surrounds the inner core and is believed to be composed of iron mixed with nickel and trace amounts of lighter elements.
➤ Earth’s mantle extends to a depth of 2,890 km, making it the thickest layer of Earth.
➤ The mantle is composed of silicate rocks that are rich in iron and magnesium relative to the overlying crust.
➤ The high temperatures within the mantle cause the silicate material to be sufficiently ductile that it can flow on very long timescales.
➤ Convection of the mantle is expressed at the surface through the motions of tectonic plates.
➤ The melting point and viscosity of a substance depends on the pressure it is under.
➤ The crust ranges from 5-70 km in depth and is the outermost
➤ The thin parts are the oceanic crust, which underlie the ocean basins (5-10 km) and are composed of dense (mafic) iron magnesium silicate igneous rocks, like basalt.
➤ The thicker crust is continental crust, which is less dense and composed of (felsic) sodium potassium aluminium silicate rocks, like granite.
➤ The rocks of the crust fall into two major categories -Sial (Silicate + Aluminium) and Sima (Silicate + Magnesium).
➤ It is estimated that sima starts about 11 km below the Conrad discontinuity (a second order discontinuity).
➤ The uppermost mantle together with the crust constitutes the lithosphere.
➤ The crust-mantle boundary occurs as two physically different events.
➤ First, there is a discontinuity in the seismic velocity, which is known as the Mohorovicic discontinuity or Moho. The cause of the Moho is thought to be a change in rock composition from rocks containing plagioclase feldspar (above) to rocks that contain no feldspars (below).
➤ Second, in oceanic crust, there is a chemical discontinuity between ultramafic cumulates and tectonized harzburgites, which has been observed from deep parts of the oceanic crust that have been obducted onto the continental crust and preserved as ophi-olite sequences.
➤ Many rocks now making up Earth’s crust formed less than 100 million (1×108) years ago; however, the oldest known mineral grains are 4.4 billion (4.4 × 109) years old, indicating that Earth had a solid crust for at
1. Oxygen 46.60%
2. Silicon 27.7%
3. Aluminium 8.1%
4. Iron 5.0%
5. Calcium 3.6%
6. Sodium 2.8%
7. Potassium 2.5%
8. Magnesium 2.09%
9. Others 1.41%
Moho Discontinuity : Between Crust and Mantle
Conrad Discontinuity : Between outer and Inner Crust
Weichert-Gutenberg Discontinuity : Between Mantle and Core
Repetti Discontinuity : Outer and Inner Mantle
Transition Discontinuity : Outer and Inner Core.
➤ When the Lithosphere plates move, the surface of the Earth vibrates.
➤ The vibrations are travelled all round the Earth, these vibrations are called earthquakes.
➤ The point of origin of earthquake is called ‘Focus’.
➤ The point on the Earth’s surface vertically above the focus is called ‘Epicentre’.
➤ The waves are recorded by seismograph.
➤ The magnitude of waves is measured on Richter’s scale.
➤ The number on this scale ranges from 0 to 9. Each increase of one unit on the Richter scale represents a tenfold increase in earthquake strength.
➤ No earthquake has been measured above 9 on scale.
➤ The region around the Pacific Ocean is prone to earthquakes.
➤ In India, Himalayan region and Ganga-Brahmaputra valley are prone to earthquakes.
➤ Although earthquakes can’t be predicated, the impact can certainly be minimised if we are prepared before hand.
➤ Around 21% of earthquakes occur in the mid-world mountain belt extending parallel to the
Types of earthquake waves :
There are three types of earthquake waves — Primary waves (P waves) :
➤ These waves are longitudinal waves.
➤ Travel from the point of happening by the displacement of surrounding particles.
➤ They are transmitted through solids, liquids and gases.
➤ These wave travel fastest.
Secondary waves (S waves) :
➤ These waves are transverse waves.
➤ These waves travel through solids only.
➤ Can’t pass through core.
Long waves (L waves) :
➤ These waves are also called surface waves.
➤ Travel on Earth’s surface.
➤ These waves cause maximum destruction.
Effects of Earthquake
➤ Ground shaking
➤ Differential ground settlements
➤ Mud slides
➤ Soil liquefaction
➤ Ground Lunching
➤ Avalanches
➤ Fires
➤ Tsunami
The Earthquake Zones in India
On the basis of intensity of the earthquakes a map of India has been published by the Meteorological Department in collaboration of the Indian Standard Institution. The map shows the five seismic zones based on modified Mercalli scale.
Zone I – Intensity V or below (feeble, slight, moderate rather strong)
Zone II – Intensity VI (strong)
Zone III – Intensity VII (very strong)
Zone IV – Intensity VIII (Destructive Zone)
Zone V – Intensity more than VIII (Disastrous, Catastrophic)
Zone I – No area of India is currently classed. in zone I
Zone II – Includes Southern Punjab and Haryana, certain parts of Plains of Uttar Pradesh, Eastern Rajasthan, Coastal areas of Odisha and Tamilnadu. This is the low damage rests zone.
Zone III: Covers Southern and South Eastern parts of Rajasthan, larger parts of Madhya Pradesh, Maharashtra, Karnataka, Jharkhand and Northern and North-Western parts of Orissa.
Zone IV: Covers Jammu and Kashmir, Himachal Pradesh, Northern parts of Punjab, Haryana, Delhi, Eastern Uttar Pradesh, Tasai and Bhabhat regions, the Himalayan areas of Uttaranchal, Bihar and Sikkim
Zone V: Covers certain parts of Jammu and Kashmir, Himachal Pradesh, Uttranchal, Monghy, and Darbhanga districts of Bihar, Northern part of India and Kutchh region of Gujarat. Some specific areas where the waves are not reported, such zone is called the ‘shadow zone’.
➤ A volcano is an opening in the Earth’s crust.
➤ Crust allows magma to reach the Earth’s surface.
➤ The magma that reaches the Earth’s surface is called lava.
➤ Sometimes magma rises slowly to the surface and spread over a vast area.
➤ Some plateaus and plains have been formed in this way.
➤ The top of cone is usually marked by a funnel shaped depression, which is called crator.
Types of volcanoes : There are three types of volcanoes — Shield volcanoes :
➤ The shield volcanoes are the largest of all the volcanoes on the Earth.
➤ These volcanoes are mostly made up of Basalt.
➤ They become explosive if water gets into the vent.
➤ These are formed by quiet eruption of lava with low silica content.
Active Volcanoes Dormant Volcanoes Extinct Volcanoes

Active VolcanoesDormant VolcanoesExtinct Volcanoes
Mount Etna-Sicily island Stromboli-Lepari island Cotapaxi-Ecuador Mauna loa-Hawaii island Ojos de Salado-Argentina – Chile boundaryFujiyama-Japan Visuvious-Italy Krakatao- IndonesiaKohsultan-Iran Popa-Myanmar Chimborajo- Ecuador Aconcagua-Argentina

Composite Volcanoes
➤ These volcanoes are characterised by eruptions of cooler and more viscous lava than basalt.
➤ These are made of lava and ash.
➤ On the basis of frequency of eruption, volcanoes are classified into active, dormant and extinct volcanoes.
Active Volcanoes : The volcanoes which continues to erupt periodically are called active volcanoes.
Dormant Volcanoes : The volcanoes which are devoid of any activities for a long time but in which there is a possibility of eruption are called dormant volcanoes.
Extinct volcanoes : The volcanoes in which the eruption has completely stopped and is not likely to occur are called extinct volcanoes.
➤ The atmosphere is an insulating blanket protecting the Earth.
➤ It softens the intense light and heat of the Sun.
➤ It’s Ozone layer absorb most of the harmful ultraviolet rays from the Sun.
➤ The atmosphere is composed of various gases and water vapour, and in its uppermost reaches, it is charged with subatomic particles.
➤ Of the many constituents, carbon dioxide, dust particles, water vapour and Ozone are of great importance for the earth’s climatic conditions.
➤ Nitrogen serves mainly as a diluents and its main function is to regulate combustion by diluting oxygen.
➤ Carbondioxide, just 0.03 per cent of the dry air, even though it has great climatic significance. This gas emits about half of the absorbed heat back to the earth and hence a very important function in the heat energy budget.
➤ Ozone, less than 0.0005 per cent by volume, is not uniformly distributed in the atmosphere.
➤ It’s greatest concentration is found between the altitude of 20 and 25 km. It absorbs the ultraviolet solar radiation.
➤ Water vapour content in the air may vary from 0.02 per cent by volume in the cold dry climate to nearly 4 percent in humid tropics.
➤ It absorbs not only the long wave terrestrial radiation, but also a part of incoming solar radiation, thus regulating the energy transfer through the atmosphere.
➤ About 90 per cent of water vapour lies below 6 km of the atmosphere.
Only less than 1 per cent of the total atmospheric moisture in the atmosphere is found above 10 km.
➤ Dust particle include all the solid particles present in the air excepting the gases and water vapour.
➤ They absorb a part of the incoming short-wave solar energy.
➤ A certain percentage of solar radiation energy is reflected back
Rocks And Soils
➤ Rocks are made up of individual substances, which are called minerals found mostly in a solid state.
➤ Each mineral usually contains two or more simple substances called elements of which the whole earth is made.
➤ Out of about 2000 minerals, only twelve are common all over the earth. These twelve minerals are called the rock-formers.
➤ As many as 87 per cent of the minerals in earth’s crust are silicates.
➤ The metal compounds of rocks is known as ‘ores’.
On the basis of the mode of formation, rocks are usually classified into three major types :
(1) Igneous Rocks
➤ Igneous rocks are found mostly in the Earth.
➤ Two-third of the earth’s crust is made of these rocks.
➤ These rocks are called the basic rocks.
➤ These rocks are ancestors of all other rocks and make up 85 per cent or more of the earth’s crust.
➤ Igneous rocks are formed by the cooling, solidification and crystallization of molten earth materials, known as magma and lava.
➤ Igneous rocks are also called as Primary rocks or parent rocks because these were originated first during the formation of crust through the process of cooling of the earth surface.
➤ The igneous rocks are made of silicates (SiO2) and often combine with other oxides of aluminum, iron, magnesium etc. some of the important igneous rocks are granites, rhyolite, pegmatite, syetite, diorite, andestite gabbro, basalt, dolerite and peridotite.
➤ Igneous rocks are generally hard and water percolates through them with great difficulty along the joints.
➤ Since water does not percolate easily, these rocks are less affected by chemical weathering.
➤ These rocks are more prone to mechanical weathering due to their granular structure.
➤ These rocks are non-fossiliferous.
On the basis of mode of occurrence, Igneous rocks are classified into two major groups :
(i) Intrusive Igneous Rocks :
When the rising magma is cooled and solidified below the surface of the earth, they are known as Intrusive Igneous Rocks. These are further sub-divided into :
(a) Plutonic Igneous Rocks :
They result from the cooling of magma very deep inside the earth.
Due to very slow cooling at that great depth, large grains are developed, e.g.-Granite
(b) Hypabyssal Igneous Rocks :
They are formed when magma cools and solidify just beneath the earth surface. They take different shapes and forms depending upon the hollow places in which they solidify.
(ii) Extrusive Igneous Rocks :
These igneous rocks are formed by the cooling and solidification of molten lava on the earth’s surface. Basalt is the most important example of extrusive igneous rocks, others being Gabbro and Obridian. These are generally fine grained or glassy because of quick rate of cooling of lava. The extrusive igneous rocks are divided into two sub-groups :
(a) Explosive Type : Volcanic materials of violent volcanic eruptions include ‘bombs’ (big fragments of rocks), lapilli (pea-sized fragments) and volcanic dusts and ashes.
(b) Quiet Type : In this, lava appear on the surface through cracks and fissures and their continous flow form extensive lava plateaus, e.g., Deccan Plateau, Columbia Plateau.
(2) Sedimentary Rocks
➤ Sedimentary rocks are constituted of sediments, a material from wind or running water that settle down.
➤ About 75% of the surface area of the globe is covered by the sedimentary rocks while rest 25% area is occupied by the igneous and metamorphic rocks.
➤ Though sedimentary rocks cover largest area of the earth’s surface, they constitute only 5% of the composition of the crust while 95% of the crust is composed of igneous and metamorphic rocks.
The fossils are found in the layers of sedimentary rocks. A fossil refers to any part of the once living things preserved in the rock. It may be entire body, a singe bone or a set of foot-prints.
It tells up about the life in past and they help us to date environment.
Fossils also show what kind of animals lived in the past.
➤ The layers of sedimentary rocks hold all reserve of coal, oil and natural gas.
➤ The mechanically formed sedimentary rock contain pieces of other rocks.
➤ Agents like running water, wind and moving ice break them into smaller pieces and deposits them at new sites where they form new sedimentary rocks.
➤ Organically formed sedimentary animals and plants. Limestone, chalk and corals are the most common of this type of sedimentary rocks.
➤ Chemically formed rocks are formed by the direct precipitation of mineral matter from solution.
Rock-salt is an example of such rocks. Gypsum is also formed in a similar manner.
‘Sandstone’ is a common sedimentary rock, is formed mainly of quartz particles cemented together by silica, lime or iron oxide.
➤ ‘Shale’ is most abundant of all sedimentary rocks.
➤ It is compacted silt and clay. Kaolin and clay minerals are abundant in it.
Rock gypsum is a white to reddish in colour. Gypsum and rock salt are formed by the evaporation of sea water and salt lakes.
➤ ‘Chalk’ is a calcareous rock made up of microscopic skeletal elements from a varieties of limes secreting organism.
➤ It is composed of almost pure calcium carbonate.
➤ Most of the sedimentary rocks are permeable and porous but a few of them are also non-porous such as clay.
(3) Metamorphic Rocks
➤ In Greek language the word metamorphic means ‘change of form’.
➤ When the original character of the rocks i.e. their colour, texture and mineral composition is partly or wholly changed, it gives rise to metamorphic rocks, under favourable conditions of heat and pressure.
➤ The formation of metamorphic rock refers that in course of time shale may get changed to slate and schist, limestones to marble, sandstone to quartzite
➤ The metamorphic rocks may be classified into two categories :
(i) The Foliated and
(ii) Non-Foliated.
➤ The foliated rock is characterised by parallel arrangement of slaty minerals such as mica.
➤ In the non-foliated metamorphic rocks, the minerals grains are equi-diamensional e.g. quartzite and marble.
➤ Uranium is found in metamorphic rocks.
➤ Example of metamorphic rocks formed from different rocks— Metamorphic rock Made from
Marble — limestone, chalk, dolomite Quartzite — Sandstone Slate — Shale Graphite and Diamond — Bituminus coal Gneiss — Granite
➤ An uplifted portion of the earth’s surface is called a mountain.
➤ On the basis of their origin or mode of formation, the mountains are classified as structural, residual or dissected and volcanic.
Structural Mountains
➤ Such systems are hundreds of kilometers wide and thousands of kilometer long.
➤ Many of them lie near or parallel to continental coastlines.
➤ Both the fold and the block mountains are included in this type.
Fold Mountains : These mountains have originated due to compressional tectonic forces and have been thrown up to form fold mountains e.g. Himalayas, Andes, Alps etc.
➤ On the basis of age the fold mountains can be grouped into.
(i) New or Young Fold Mountains :
Example : The Alps, the Himalayas, the Circum-Pacific Oceanic Mountains, etc.
➤ The main features of these mountains are the complex folding of the rocks, faulting, volcanic weathering caused by running water, ice, winds, etc.
(ii) Old Fold Mountains : Example : The Caledonion and Hercynian mountains of Central Europe, the Pennines, the Highland of Scotland, etc.
➤ These mountains were folded in very ancient times, then subjected to denudation and uplift; many faults were formed and the layers of the rock were wrapped.
➤ Many mountains exist as relics due to erosion.
Block Mountains : These mountains are formed when great blocks of the earth’s crust may be raised or lowered during the late stages of mountain–building.
➤ When the earth’s crust cracks due to tension or compression, faulting takes place.
➤ A section of the landform may subside or rise above the surrounding level giving rise to Block Mountains or Horst or Rift valley or Graben.
➤ The Block Mountains have a steep slope towards the rift valley but the slope on the other side is long and gentle.
➤ An old fold mountain may also be left as block mountains due to continuous denudation.
➤ The Vosges in France, Black Forest (Rhine valley) in Germany and Salt Range in Pakistan are cited as typical examples of block mountains.
Volcanic Mountains
➤ These mountains are formed by the accumulation of volcanic material.
➤ The slope of the mountains becomes steep and the height increases due to the development of the cones of various types like Cinder cones, Composite cones, Acidic lava cones, Basic lava cones, etc.
➤ Well-known examples of this type are Popocatepetle of Mexico, Mount Raineer of Washington, Lessen Peak of California, the Visuvious of Italy, the Fujiyama in Japan, the Aconcagna in Chile, Kilimanjaro in Africa, Mt. Mayon in Philippines, Mt. Merapi in Sumatra etc.
Residual or Dissected Mountains
➤ They owe their present form due to erosion by different agencies.
➤ They have been worn down from previously existing elevated regions.
➤ This type of Mountains includes, Vindhyachal ranges, Aravallis, Satpura, Eastern and Western Ghats, Nilgiris, Parasnath, Girnar, Rajmahal.
Fold Block Volcanic Residnal

HimalayasVosges-FranceMauna Loa-USAVindhyachal
AlpsVindhya-IndiaHawaii Island (USA) Fujiyama-JapanAravalli
UralSalt Range-PakistanKilimanjaro-TanzaniaWestern Ghat
AppalachiansEastern Ghat
Tian ShanParasnath

Main Plateaus of the World
Arabian Plateau — South-West Asia Deccan Plateau — South India Iran Plateau — Iran Brazil Plateau — Brazil Mexico Plateau — Mexico Alaska Plateau — USA Bolivia Plateau — Bolivia Great Basin Plateau — USA Colorado Plateau — USA Ozark Plateau — USA Pamir Plateau — China Guiana Plateau — Venezuela and Guyana Mongolian Plateau — Mangolia and China Meghalaya Plateau — India Peninsula Plateau — India Plateau
The Soil Profile
➤ The soil profile consists of the weathered material derived from the rock. But the bedrock itself does not form a part of it.
➤ A soil profile generally has three main horizons in it-true soil at the top, subsoil and the bedrock apart from it.
➤ Each horizon is quite distinct from the others by its own physical and chemical composition, and organic contents produced the long process of soil formation.
➤ Each soil type has physical properties like its colour, texture and the structure.
➤ They largely depend upon the nature of the bedrock from which they are derived.
➤ There are five soil forming factors- bedrock, local climate, plant and animal organisms, elevation and the relief.
Soil Classification
➤ The two main zonal groups of soil are the Pedalfers and the Pedocals.
➤ These are further subdivided into twelve major soil types all over the Globe.
➤ Pedalfers are found in humid climates extending from high-latitude coniferous forests, mid-latitude deciduous forest lands to low-latitude tropical forests and grasslands.
➤ Pedocals group of soils are found in the arid, semi-arid and the sub-humid ones in the world.
➤ Pedocals are rich in calcium and mineral salts.
➤ The black earths or chernozomes are one of the richest soils of this group.
Other Important Classification of Soil
Inceptisols : Soil with weakly developed horizons, having minerals capable of further alteration by weathering processes.
Andisols : Soils with weakly developed horizons, having a high proportion of glassy volcanic parent material produced by erupting volcanoes.
Entisols : Soils lacking horizons, usually because their parent material has accumulated only recently.
Oxisols : Very old highly weathered soils of low lattitudes, with a subsurface horizon of accumulation of mineral oxides and very Ultisols : Soils of equatorial, tropical and subtropical latitudinal, zones, with a subsurface horizon of clay accumulation and low base status.
Vertisols : This soil contains high clay content. Vertisols develop deep, wide cracks when dry and the soil blocks formed by cracking move with respect to each other.
Alfisols : Soils of humid and subhumid climates with subsurface horizon of clay accumulation and high base status.
Spodosols : Soils of cold, moist climates, with a well developed B horizon of illuviation and low base status.
Mollisols : Soils of semiarid and subhumid mild midlatitude grasslands, with a dark, humus rich epipedon and very high base status.
Aridisols : Soils of dry climates, low in organic matter, and often having subsurface horizons of accumulation of carbonate minerals or soluble salts.
Histosols : Soils with a thick upper layer very rich in organic matter.

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