Prepare Prelims-2017-Day-36-World Geography

The Earth and The Universe

Exploring the Universe:

  • Earth’s Own Galaxy-Milky Way (Contains 100000 Million Stars)
  • The light from the nearest star travelling at the speed of light e. 186,000 miles per second takes something like 4 yrs to reach us.
  • 8 minutes- ray of light from the sun to reach the earth

The Solar System

  • Comprises Sun and 9 Planets
  • Planets revolve round the sun in elliptical orbits.
  • Mercury the smallest and closest to the sun (Year-88 Days)
  • Venus, twice the distance away from the sun (Next Closest Planet)
  • Earth, has life and all living beings we see around us in earth.
  • Natural Satellite of Earth-Moon
  • 4th Planet-Mars (Dark Patches and have some possibility of some plant life)
  • Jupiter-largest planet, made up of many gases like hydrogen, helium and methane (12 Satellites), Surface very cold.
  • Saturn-Unique planet (3 rings and 9 satellites), 2nd largest after Jupiter
  • 7th Planet-Uranus, another giant planet, 50 times larger than earth, 15 times as heavy (orbits around the sun in a clockwise direction from east to west with five satellites revolving round it.
  • Outermost planet- Neptune and pluto, visible with telescopes, result of mathematical calculations
  • Neptune much colder, pluto smaller than earth.

Earth’s Natural Satellite (Moon)

  • Moon rotates around the earth in an elliptical orbit as earth rotates around the sun
  • Axis of moon is inclined at 58.4 * approx. wrt Plane of ecliptic, as a result  distance of moon from earth keeps on changing
  • Only 59 % of moon’s surface is visible from earth at the max
  • Perigee- Nearest point to the moon’s orbit from Earth
  • Apogee- Farthest point to the moon’s orbit from Earth
  • Sidereal Month- Moon completes 1 rotation in 27 days 7 hrs & 43 min approx. With reference to earth
  • Synoptic Month- Moon completes 1 rotation in 29 days 12 hrs & 44 min approx. With reference to sun

Shape of Earth

  • Oblate spheroid or oblate ellipsoid shape
  • Slightly flattering at poles & slightly bulging at equator
  • Polar radius approx. 21 km shorter than equatorial radius

Axis of Earth

  • Earth rotating around an imaginary line running through North pole & South pole via its center


  • Center most parallel, dividing earth into two equal hemispheres namely Northern & Southern hemisphere
  • Lying at 0* latitude with L = 40000 Km Approx.

Tropic of Cancer

  • Parallels at 23.5* north of equator

Tropic of Capricorn 

  • Parallels at 23.5* south of equator

Arctic Circle

  • Parallels at 66.5* north of Equator

Antarctic circle

  • Parallels at 66.5* south of Equator


Other Features

  • Length of latitudes decreases from equator to poles i.e. max. at equator & 0 at poles
  • As earth’s axis is inclined by 23.5* to its orbital plane, therefore 23.5* is max. latitude upto which sunrays can be perpendicular to any place
  • Means all places between Tropic of cancer & Tropic of Capricorn experience vertical rays of sun twice a year but both the tropics only once
  • Tropic of cancer will get vertical sunrays at summer solstice, when Northern hemisphere of earth is at max. inclination from sunrays
  • Tropic of Capricorn will get vertical sunrays at winter solstice, when southern hemisphere of earth is at max. inclination from sunrays

Evidences of the Earth’s Sphericity:

  • Circum navigation of the earth
  • The circular horizon
  • Ships visibility
  • Sunrise and sunset
  • The Lunar Eclipse
  • Planetary bodies are spherical
  • Driving poles on level ground on a curved earth
  • Aerial Photographs


Rotation of Earth

  • Earth’s rotate around its own axis from west to east viz. anticlockwise direction
  • Earth’s rotate around the sun (Elliptical path) from west to east viz. anticlockwise direction
  • Axis of earth rotation is inclined at 66.5* to its plane of elliptic
  • Axis of earth rotation is inclined at 23.5* to perpendicular to the plane of elliptic
  • Plane of earths equator to plane of elliptic or earth’s axis to axis of revolution is inclined at 23.5 *


  • Velocity of earth’s rotation decreases from equator to poles
  • Correlated with length of the parallels which also decreases from equator to poles
  • velocity at equator & 0 velocity at poles
  • Weight of body is less at equator & greater at poles
  • Because of greater centrifugal force at the equator (mv^2 / r) due to greater velocity at equator
  • Because of higher gravitational force at poles (GMm/r) due to lesser radius of poles than the equator
  • The earth moves in space in two distinct ways viz.
  • It rotates on its own axis from west to east once in every 24 hours causing day & night
  • It revolves around the sun in an orbit every 365(1/4) days causing the seasons & the year
  • Throughout the revolution of earth around the sun, its axis remains tilted in the same direction
  • Its axis continues to point to same spot in heaven known as Polaris / polestar/ parallelism of axis
  • Polestar >> Brightest star in the sky in north direction or northern star


  • Partial or total obstruction of light from a celestial body as it passes through shadow of another celestial body
  • Apparently, eclipse shall occur every month because of revolution of earth around the sun & moon around the earth, but Plane of moon’s orbit around the earth is inclined at 5.9* to the plane of earth’s orbit around the sun


Solar Eclipse

  • When moon comes exactly between earth & sun & obstructs a part or whole of the sun then a partial or total eclipse occur
  • Usually at sunrise or sunset at new moon

Lunar Eclipse

  • When earth comes exactly between Sun & moon
  • Usually occurs at full moon


Dawn & Twilight

  • The brief period between sunrise & full daylight is called dawn & that between sunset & complete darkness is termed as twilight.
  • This is caused by the fact that during the periods of dawn & twilight earth receives diffused or refracted light from the sun while it is still below the horizon.
  • Since the sun rises & sets in vertical path at the equator, the period during which refracted light is received is short.
  • But in temperate latitudes, the rises & sets in oblique path & hence the period of refracted light is longer than that at equator, which is much longer at poles.


  • Angular distance of a place, along meridian on earth’s surface as measured from center
  • Distance between them increases towards the poles
  • Most important lines:The Equator, The tropic of cancer, tropic of Capricorn, arctic circle and Antarctic circle


  • Imaginary lines which joins poles & perpendicular to all parallels
  • Drawn as a semicircle on the globe
  • Also known as meridians
  • Equidistant in nature

Longitude & Concept of time

  • Earth completes 1 rotation in 24 hours means swept 360* in 24 hours >>> 15 * in one hour or 1* in 4 minute
  • Since earth rotates west to east places located in the east gain time while those located in the west loose time
  • Generally, 12: 00 noon at a place is considered when sun’s altitude is highest &  exactly over the meridian at that place
  • Time at all the places located at particular meridian i.e. north & south will be same, however places located at east & west will have different local time

Standard time of a country

  • Standard time of a country is local time of a selected longitude crossing through a place in the country of due importance
  • Standard time of India is local time of longitude passing through Allahabad situated at 82.5* East of Prime meridian i.e. Five & half hours ahead of GMT
  • Calendar date is changed by one day when someone crosses international date line
  • Although line is deviated at some places to mark same date at some countries & islands
  • Thumb Rule Loose 12 hrs west of Prime meridian & gain 12 hrs east of prime meridian

Greenwich Meridian

  • Also known as Prime Meridian or Time Meridian
  • Meridian passing through Royal observatory at Greenwich near London which divides earth in eastern & western hemisphere

International Date Line

  • Exact opposite to Greenwich meridian at longitude of 180*


  • Network of parallels & meridians drawn on the globe
  • Helps to locate a place with given longitudes & latitudes

Great Circles 

  • Imaginary circles which divides the earth into two equal parts & whose center lies at the center of the earth
  • Largest circles that can be drawn on the globe i.e. Equator & all meridians


The altitude of the Sun:


  • When sun is at the greatest distance from the equator
  • Its rays falls vertical either at tropic of Cancer or tropic of Capricorn

Summer Solstice

  • Earth’s axis leans at max 23.5* in northern hemisphere towards the sun
  • Sun’s ray fall vertical at Tropic of cancer around 21/22 June
  • This brings summer season in Northern hemisphere
  • With this duration of days starts decreasing
  • Means June 21/June 22 is longest day  in Northern hemisphere
  • Daylights of 14 hrs at Tropic of Cancer; 12 hrs at Equator; 10 hrs at Tropic of Capricorn
  • Between Arctic Circle & North Pole day lasts for 24 hours & between Antarctic Circle & South Pole same duration of night lasts.
  • At North Pole day last for 6 months & at South Pole night last for 6 month approx.

Winter Solstice

  • Earth’s axis leans at max 23.5* in southern hemisphere towards the sun
  • Sun’s ray falls vertical at Tropic of Capricorn around 21/22 Dec.
  • This brings summer season in Southern hemisphere
  • With this duration of days starts increasing in Northern hemisphere
  • Means Dec 21/June 22 is shortest day  in Northern hemisphere
  • Daylights of 14 hrs at Tropic of Capricorn; 12 hrs at Equator; 10 hrs at Tropic of Cancer
  • Between Antarctic Circle & South Pole day lasts for 24 hours & between Arctic Circle & North Pole same duration of night lasts.
  • At South Pole day last for 6 months & at North Pole night last for 6 month approx.


  • The sun is vertically overhead at the equator on two days of the year usually on 21 March & 21 September
  • Dates changes because a year is not exactly of 365 days
  • These two days are termed as equinox means on these two days all parts of the world have equal days & nights

Seasonal Changes & Their Effect on Temperature

  • In summer, sun is overhead & its sunrays fall almost vertically on the earth, concentrating its heat on a small area;
  • Temperature therefore rises & summers are always warm
  • In winters, the oblique rays of sun come through atmosphere less directly & have their heat absorbed by atmosphere & water vapour;
  • Sun rays fall obliquely & spread over greater area, hence temperature remain low.
  • In addition days are longer than nights in summer & more heat is received over longer daylight duration; Nights are shorter & less heat is lost. Hence, there is net gain in total heat received & temperature rises in summer. Shorter days & longer nights in winters accounts for reverse effect.

The Earth’s Crust

  • Made up of several concentric layers.
  • The outer layer is the earth’s crust-the lithosphere
  • Lithosphere comprises two distinct parts-granite rocks-upper part & lower part-basaltic rocks.

Granite rocks-

SIAL- The Oceanic Crust

  • Si – silicon and Al – aluminum.
  • High density.
  • Iron and calcium are also present.
  • Younger part of crust – 200 million years old.

Basaltic Rocks:

SIMA – The Continental Crust

  • Si – silicon and Ma – Magnesium.
  • Low
  • Aluminum, potassium, and sodium.
  • Older part of the crust – 3600 million years.

Mantle forms the second layer

  • second layer of the interior of the earth.
  • Two sub – layers –1. Upper mantle.
  1. Lower mantle.
  • Thickness varies between 35km – 2900 km.
  • Average density is 4.5 g/cm3
  • Upper portion of mantle and crust together known as

lower mantle – Aesthenosphere.


  • Third layer of the earth.
  • Formed by nickel and iron.
  • Also called  as Nife
  • Nife – Ni – nickel and Fe – ferrous or iron
  • Two parts
  1. 1. Outer
  2. Inner core.
  • Temperature is 11000˚ C.
  • Inner core is in solid state.

CORE – divided in two layers

  1. Inner core
  • Molten in stage
  • Depth varies from 2900km – 5150 km.
  • Density – 10.7 g/cm
  1. Outer core
  • Solid in state
  • High temperature
  • High pressure
  • Depth varies from – 5150 km – 6371 km.

Density – 15 g/cm3

The classification of Rocks

  • Rocks are Igneous Rocks, Sedimentary Rocks, Metamorphic Rocks

Igneous Rocks

  • Formed through cooling & solidification of molten material magma ( Erupted from volcanoes, molten material moves towards surface of earth through crack)
  • Normally crystalline in structure, do not occur in strata & do not contains fossils
  • Can be subdivided on the basis of mineral composition
  • Basic rocks contain high proportion of basic oxides mainly of iron, aluminium & magnesium
  • When they contain high proportion of silica they are said to be acidic, which are less dense & lighter in color than basic rocks for ex. Granite
  • Most igneous rocks are extremely hard & resistant hence are quarried for road making & polished as monuments & grave stones
  • Are parent / primary rocks as all other rocks are derived from it.

In terms of origin they can be divided into mainly 2 classes viz.

Plutonic Rocks §  formed due to solidification of magma (interior of earth)

§  They have cooled & solidified slowly so that large, easily recognized crystals are formed till some depth inside earth crust

§  Examples  Granite, Diorite & Gabbro

§  Exposed at the surface by denudation & erosion

Volcanic Rocks §  formed due to solidification of lava (interior of earth)

§  Molten rocks poured out of volcanoes as lavas

§  They have solidified rapidly on the earth’s surface hence have small crystals

§  For ex. Basalt (which yield Regur or black soil)


Sedimentary Rocks

  • Formed due to deposition of layers of sediment usually along the water bodies over a long period of time
  • Sediment is deposited layer by layer in form of strata hence also known as stratified rocks
  • Process of turning sediments into hard rock layers by pressure is known as lithification
  • Rocks may be fine grained or coarse, soft or hard & material forming them may be brought by streams, glaciers, winds or even animals
  • May be derived from Igneous, Metamorphic or Sedimentary rocks
  • Hence, Sedimentary rocks are most varied in formation of all rocks
  • They are non-crystalline & often contains fossils of animals, plants & other microorganisms

May be classified under 3 categories with respect to their origin & composition viz

Mechanically formed §  Formed from the accumulation of materials derived from other rocks

§  Made from sand grains with tremendously varying texture, composition & colour. For ex : Sandstone

§  Mainly quarried for building purposes or for making grindstones

§  Finer sedimentary materials form clay, widely used for making bricks, shale or mudstone.

§  Sand & gravel may occur in uncemented form

Organically formed §  Formed from the remains of living organisms such as corals or shellfish, whose fleshy part has been decomposed, leaving behind the hard cells.

§  Most common rocks formed due to this process are Calcareous type for ex. Limestone & Chalk

§  Carbonaceous rocks are also organically formed but from vegetative matter – Swaps and forests

§  Pressure of overlying sediments has compressed the plants remains into compact masses of carbon which eventually becomes Peat, Lignite or Coal

Chemically formed §  Formed chemically from solutions of one another

§  For ex. Gypsum (Calcium sulphate) is obtained from evaporation of salt lakes having high salinity

§  Same way Potash & Nitrates may be formed

Metamorphic Rocks

  • Formed when original structure of igneous & sedimentary rocks partially or wholly change under the action of heat & pressure
  • Contains no fossils
  • No stratification
  • For ex. Clay Slate


Igneous to Metamorphic

§  Granite  Gneiss

§  Mica      Schist

§  Gabro    Serpentite

Sedimentary to Metamorphic

§  Limestone              Marble

§  Sandstone              Quartzite

§  Shale                      Schist

§  Coal                        Graphite

§  Bituminous coal     Anthracite coal

Types of Mountains

Fold Mountains

  • Formed by folding of geosyncline sediments under compressible tectonic forces
  • For Ex. Himalaya, Alps, Rockies, Andes, Alapchhian, Ural, Aravalis
  • Since the rock strata have been elevated to great heights, Fold Mountains are also called mountains of elevation
  • Are closely associated with volcanic activities
  • Contains many active volcanoes, especially in circum Pacific fold mountain system
  • Are rich in mineral resources such as Tin, Copper, Gold & Petroleum

Block Mountains

  • Formed due to faults caused by tension or compression forces which lengthen or shorten earth’s crust
  • It causes a section of it to subside or rise above the surrounding level
  • For Ex. Vosges (France), Black Forest (Germany)
  • Faulting results in formation of Block Mountains & their counterparts in rift valleys
  • In general, large scale Block Mountains & rift valleys are due to tension rather than compression

Volcanic Mountains

  • Also known as mountains of accumulation
  • Formed due to accumulation of thick lava as a result of volcanic eruption
  • Common in circum pacific belt
  • For Ex. Fuji Yama (Japan), Mt. Popa (Myanmar), Mt. Mauna loa (Hawai), Mt. Mayon (Phillipines), Mt. Agung (Bali), Mt. Merapi (Sumatra) & Mt. Catopaxi (Euador)

Residual / Dissected / Relict Mountains

  • Formed due to waning of previously existing elevated regions by erosion
  • For Ex. Nilgiris, Parshavnath, Hills of Peninsula India, Mt. Manodnock (USA)
  • Mountains evolved by denudation, where the general level of land have been lowered by agents of denudation; also known as mountains of denudation

Types of Plateaux

  • An elevated area compared to its surroundings, having a large almost flat top area (Also known as tableland)
  • Like all highlands, Plateaus are also subjected to erosional processes, as a result their original characteristics are highly altered
  • According to their mode of formation & their physical appearance, plateaus may be divided into 3 types viz.


Tectonic Plateaus 

  • Formed by earth movements which causes uplift, of a considerable size with fairly uniform altitude
  • For ex. Deccan plateaus, Mesera plateau (tilted of central Iberia) & Harz plateau (Faulted of Germany)


  • When plateaus are surrounded by mountains they are known as inter-montane plateaus for ex. Tibetian plateau, Bolivian Plateau
  • When plateaus are surrounded by sea or plains they are known as Continental Plateaus For ex. Deccan plateau, Greenland plateau, South Africa plateau


Volcanic Plateau

  • Molten lava from the volcanic eruption may solidify to form successive sheets of basalatic lava, known as Lava plateau
  • For ex. Antrim Plateau of Northern island, NW part of Deccan Plateau & Columbia Snake Plateau (Biggest one)


Dissect Plateaus

  • Formed due to continuous weathering & erosion by running water, wind & ice
  • High plateaus worn down & their surface becomes irregular
  • For Ex. Scottish Highland


Generally Plateaus have rich mineral resources & have been actively mined for ex.

  • African plateau yields gold, diamonds, copper, Manganese & Chromium.
  • Brazilian plateau yields iron & Manganese
  • Deccan Plateau Yields Manganese, Iron & Coal
  • Western Australian plateau yields Gold & Iron

Type of Plains


  • Plains usually are the best land of a country & are heavily cultivated & populated
  • Even more at places where rivers transverse the plains
  • For ex: Indo Gangetic Plains, Mississippi Plains & Yang-Tze plain


Some of the most extensive temperate plains are Grasslands like Russian Steppes, North American Prairies & Argentinian Pampas. Plains may be grouped into 3 major types based on their mode of formation viz.

Structural Plains

  • Structurally depressed areas of the world that makes up some of the most extensive natural lowlands on the earth’s surface
  • Rock layers on the earth’s crust are aligned almost horizontally
  • They are formed by horizontally bedded rocks, relatively undisturbed by the crustal movements of the earth
  • Examples include Russian Platforms, Great plains of USA & Central lowlands of Australia

Depositional Plains

  • Plains formed by deposition of materials brought by various agents of transportation
  • Comparatively of equal level but rise gently towards adjacent highlands
  • Depositional work by rivers form extensive alluvial plains, flood plains & deltaic plains; that form most productive agricultural plains of the world
  • For ex. Gangetic plain (for rice & jute), Nile delta of Egypt (for rice & cotton) & Hwang ho plain in China


Glacial Depositional plains

  • Glaciers & ice sheets may deposit fluvio glacial sands & gravels in outwash plains
  • May also drop boulder clay (mixture of various sizes of boulders & clay) to form till plain or drift plain
  • Outwash plains are usually barren lands but boulder clay may be very valuable for farming


Aeolian Depositional plains 

  • Winds may blow Aeolian deposits, very fine particles known as loess, from interior deserts or barren surfaces & deposit them upon hills, valleys or plains forming a loess plateau (ex. in NW China) or a loess plain (Ex. in Pampas of Argentina)
  • The loess help in leveling the undulating plain by filling up groves & depressions
  • Many of the loess covered plains in the world are fertile agricultural regions


Erosional Plains

  • These plains are carved by the agents of erosion (Rain, river, ice, and wind)
  • Such plains of denudation are described as Peneplains, which means almost plains.
  • In glaciated regions, glaciers & ice sheets scours & levels the land forming ice scoured plains
  • However scooped out by the ice are now filled by the lakes for ex. in Northern Europe & Northern Canada.
  • Finland is estimated to have 35000 lakes occupying 10 % of total land surface of the country
  • In arid & semi-arid regions, wind erosion lowers the level of the land which are called Reg in Africa
  • Mechanical weathering in arid & semi-arid areas worns mountain slopes leaving a gentle slope, known as Pediplains or Pediments; with remaining steep hills known as Inselbergs

Vulcanism and Earthquakes


  • A sudden & abrupt explosion in earth crust through which Magma, gases, dust, smoke & solid material burst out
  • Volcanic activity is connected with crustal disturbances, closely related with regions that have been intensely folded or faulted
  • Magma while thrusting its way upto the surface may cool & solidify within crust as Plutonic rocks resulting in intrusive landforms
  • Magmas that reach the surface & solidify, form extrusive landforms

 Sills & Dikes (Common intrusive landforms)

  • When an intrusion of molten magma is made horizontally along the bedding planes of sedimentary rocks, the resultant intrusion is called a Sill.
  • Similar intrusion when injected vertically as narrow walls of igneous rocks within the sedimentary layers are termed as Dikes.
Laccoliths An igneous mound with a dome shaped upper surface & a level base, fed by a pipe like conduit from below
Lopolith An igneous intrusion with a saucer shape
Phacolith A lens shaped mass of igneous rock occupying the crest of an anticline or the bottom of a syncline & being fed by a conduit from beneath
Batholith A large emplacement of igneous intrusive rock, mainly granite, that forms from cooled magma deep in the Earth’s crust

Extrusive Landforms

  • Lava or molten magma ejects at a very high pressure through a pipe known as Volcano’s neck or vent
  • Top portion of volcano is known as crater and a crater lake is formed when rain water gets accumulated in
  • Some volcanos may have greatly enlarged depressions like cauldron known as Calderas
  • Volcanic dust or ash (finer particles) that emerges out of volcano travels round the world & falls as black snow, which can bury house & people.
  • The coarser fragmental rocks are collectively called as Pyroclasts which include cinders, pumice & volcanic bombs.

Types of Volcanoes

Active Volcano

  • Keeps on ejecting volcanic material at frequent intervals
  • Ex – Etna (Italy), Stromboli (Sicily largest island in the Mediterranean Sea, near Italy)
  • Mt Stromboli Lighthouse of the Mediterranean

Dormant Volcano

  • One in which eruption has not occurred for a long time but can occur any time in future
  • Barren Island (Andaman), Versuris (Italy)

Extinct Volcano

  • No eruption has occurred in historic times & possibility of future eruption is also remote
  • Popa (Myanmar). But we can never be thoroughly sure about them.
  • Vesuvius (Bay of Naples near Italy) & Mt. Krakatau (Sunda straits b/w Java & Sumatra) were thought to be extinct & yet both erupted violently

Some Volcanic Eruptions:

  • Vesuvius
  • Krakatau-Krakatau is a small island in the sunda straits midway between Java and Sumatra
  • Pelee-Westindies

Distribution of Volcanoes in the world

  • There are mainly three volcanic belts, besides many volcanoes which are outside these belts
  • Circum-Pacific belt   →  known as Ring of fire & houses around 2/3rd of world’s Volcanoes
  • Mid-Continental belt This belt has various volcanoes of the Alpine mountain chain, Mediterranean Sea (Stromboli, Vesuvius, Etna etc.)
  • Mid-Atlantic belt        → This belt includes the volcanoes of the Mid-Atlantic Ridge


  • A spring characterized by intermittent discharge of water ejected turbulently and accompanied by steam
  • The phenomenon is associated with a volcanic region in which the water below is being heated beyond its boiling point
  • Is often triggered off by the gases seeping out of the heated rocks.
  • Examples include Iceland; New Zealand & Yellowstone park of USA
  • The world’s best known geyser is perhaps old faithful in Yellowstone National park, Wyoming

Hot springs

  • A spring that is produced by the emergence of geothermal heated groundwater from the Earth crust
  • Is more common than geysers
  • The water rises to surface without any explosion
  • Such springs contain dissolved minerals which may be of medical value
  • Examples Include Iceland, Japan & Hawaii

 Weathering, Mass Movement & Ground Water

  1. Weathering
  • The process of wearing away of earth’s surface is commonly known as denudation & is generally carried out in four phases Weathering, Erosion, Transportation & Deposition
  • Warm wet climate promotes rapid chemical weathering while dry climate provide good conditions for physical weathering

Chemical Weathering

  • Extremely slow & gradual decomposition of rocks due to exposer to air & water
  • For example Granite when exposed to weather is found to be rough surfaced because it is mainly made up of Quartz, Feldspar & Mica; Feldspar is more quickly weathered than Quartz hence is worn away, eventually leaving loosened quartz crystals.
  • RegolithWeathered material from the rock or mineral remains of decomposed rocks.
  • When a soil cover on the rock exists, chemical weathering of the rock enhances because the soil absorbs rain water & keeps the underlying rock in contact with this moisture.
  • Rain water absorbs organic acids from the soil & thus become a stronger weathering agent than pure water acting on a bare rock.

Types of Chemical weathering


  • Many minerals are dissolved by water especially with rain water which contains enough carbon dioxide to make it a weak acid
  • For ex. in case of limestone, rain water dissolves calcium carbonate, of which rock is chiefly formed & hence joints & cracks in rock are quickly widened, worning it out easily
  • Rocks are more resistant if they have fewer joints or cracks to harbour moisture
  • All rocks are subjected to solution upto some extent though the process may be much slower depending not only on mineral composition of the rock but also on its structure, density & climatic conditions it faces.


  • Weathering by reaction of oxygen in presence of air & water with minerals present in the rock
  • For example most rocks contain certain amount of iron, which when comes in contact with air is changed in iron oxide & finally into rust, which crumbles easily, loosening the overall structure of the rock


Decomposition by Organic Acids

  • Within the soil which covers most rocks are bacteria which thrive on decaying plant or animal material
  • These bacteria produce acids when dissolved in water, help to speed up weathering of underlying rocks


In some cases, microorganisms & plants like mosses or lichens can live on bare rock damp surface, absorbing chemical elements from the rocks as food & producing organic acids. Hence, they become the agent of both Chemical & Mechanical weathering.

Physical Weathering

  • Also known as Mechanical Weathering
  • Disintegration by Mechanical Process
  • Types of physical weathering→ By insolation, by Frost


By Insolation

Block Disintegration

  • Mainly in dry desert areas, hot at day and cold by night
  • Leads to expansion & contraction of rock setting up stresses in the rock
  • Finally leading to its disintegration

Granular Disintegration

  • Different minerals in rock leads to different rate of expansion & contraction of rock
  • Leads to Fragmentation of rock for ex. Granite


  • Stresses are naturally greatest near the surface & where there are sharp angles in the rock
  • Rectangular blocks are thus gradually rounded by splitting away of sharp corners
  • Finally it leads to peeling off of rock’s outer layer
  • Exfoliation also takes place by repeated wetting & drying of rocks surface as during wetting its outer layer absorbs moisture & expand; when they dry this moisture evaporates & they quickly shrinks, finally leading to peeling of outer layer of the rock


By Frost

  • Mainly at high altitudes & cold climates where during day cracks & joints inside rock fill with water & during night they get frozen
  • This leads to increase in volume of water in rock approx. by 9 %

Biological Weathering

  • By Men, Animals, Insects & Vegetation
  • vegetation grows into crevices of rock cracks or in courtyards or building walls

Mass movement

  • Movement of weathered material down the slope due to gravitational force
  • Movement may be gradual or sudden depending on the gradient of the slope, weight of the weathered debris & presence of a lubricating agent such as water


Soil Creep

  • Slow & gradual but more or less continuous movement of soil down the hill slopes
  • Movement is not very noticeable, especially when slope is fairly gentle or when soil is well covered with grass or other vegetation
  • Most common in damp soils where water act as a lubricant so that individual soil particles move over each other & over the underlying rock
  • Though the movement is slow, the gradual movement tilts trees, fences, posts & so on which are rooted in the soil
  • Soil is also seen to accumulate at the foot of the slope or behind obstacles such as walls, which may burst by weight of the soil accumulated

Soil Flow / Mud Flow (Solifluction)

  • When the soil is completely saturated with water, soil particles easily move over each other & over the underlying rock
  • Soil act as a liquid mixture & soil flow or mud flow occur
  • In Ireland such flows are known as Bog-Burst

Landslide (Slumping or Sliding)

  • Very rapid movements resulting in large mass of soil & rock falling suddenly
  • Landslide usually occurs on steep slopes & by earthquakes & volcanic activities
  • Landslides are often caused by the lubricating action of rain water
  • Slumping is usually common where permeable debris or rock layer overlie impermeable strata such as clay
  • Water sinking through the permeable layer is halted by the clay
  • Damp clay provides a smooth slippery surface over which the upper layers slides easily
  • Man often enhances the possibility of landslide by clearing natural vegetation for agriculture & housing which allows more water to penetrate through soil & rocks
  1. Groundwater
  • When rain falls on earth it is distributed in various ways.
  • Some is immediately evaporated & thus returned to atmosphere as water vapour.
  • Some is absorbed by plants & gradually returned to atmosphere by transpiration from the leaves of the plant.
  • Much of it flow into rivers & streams eventually reaching seas & oceans as run off.
  • A considerable amount of water received from rain or snow, however, percolates downward into the soil & rocks known as groundwater.
  • Groundwater plays an important role in mass movement & weathering and is also important as a mean of natural water storage.
  • It re-enters the hydrological cycle by way of springs.
  • A spring is simply an outlet of stored groundwater, released at a point where water table reaches the surface (a man-made outlet for groundwater is known as well)
  • The amount of water available to form groundwater depends to some extent on climate, nature of the rocks (absorbing power) & seasons of the year.
  • Absorbing power of the rock is determined mainly by its porosity, permeability & its structure.
  • For ex. Sandstone is both porous & permeable, Clay is highly porous but impermeable, Granite is crystalline but pervious

Water table

  • Water which seeps through the ground moves downward until it reaches an impermeable layer of rock through which it can not pass.
  • If there is no ready outlet for the groundwater in form of spring, water accumulates above the impermeable layer & saturate the rock.
  • The permeable rock in which the water is stored is known as aquifer & surface of saturated area is called water table.
  • Depth of water table varies with seasons, relief & type of rocks, as it is far below in hilltops but is close in flat surface areas.



  • The ground water stored in the rock is released onto the surface at points where the water table reaches the surface.
  • A spring is simply an outlet for such water.

Types of Springs:

  1. In areas of tilted strata:
  • Permeable and impermeable rocks alternate, water emerges at the base of the permeable layers.
  1. In Well Jointed Rocks:
  • Water percolate downwards until it reaches joints
  1. Where a dyke or sill or impermeable rock is intruded through permeable rocks
  2. In limestone or chalk escarpments
  3. In karst regions rivers often disappear under ground. Sometimes called a vauclusian spring but is better referred to as a resurgence.


  • Stored water below ground
  • Important type of well-Artesian well, which owing to the nature of its formation is quite distinctive.
  • Where rock layers have been downfolded into a basin shape. Permeable strata such as chalk or limestone may be sandwiched between impermeable layers such as clay.





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