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Fundamental Concepts of Geography
Endogenetic
Exogenetic Forces
Denudation, Weathering and Mass Wasting
UGC NET - GEOGRAPHY
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SAMPLE THEORY
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PAPER - II
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FUNDAMENTAL CONCEPTS OF GEOGRAPHY
The earth’s crust is dynamic. It has moved and moves vertically and horizontally. Of course,
it moved a bit faster in the past than the rate at w hich it is moving now . The differences in
the internal forces operating from w ithin the earth which built up the crust have been
responsible for the variations in the outer surface of the crust.
The earth’s surface is being continuously subjected to external forces induced basically by
energy (sunlight). Of course, the internal forces are still active though w ith different
intensities. That means, the earth’s surface is being cont inuously subjected to by external
forces originating w ithin the earth’s atmosphere and by internal forcers from within the earth.
The external forces are know n as exogenic forces and the internal forces are known as
endogenic forces. The actions of exogenic forces result in wearing down (degradation) of
relief/elevations and f illing up (aggradation) of basins/depressions, on the earth’s surface.
The phenomenon of wearing down of relief variations of the surface of the earth through
erosion is know n as gradation. The endogenic forces continuously elevate or build up parts
of the earth’s surface and hence the exogenic processes fail to even out the relief variations
of the surface of the earth. So, variations remain as long as the opposing actions of
exogenic and endogenic forces continue. In general terms, the endogenic forces are mainly
land build ing forces and the exogenic processes are mainly land w earing forces. The
surface of the earth is sensitive. Humans depend on it for their sustenance and have been
using it extensively and intensively. So, it is essential to understand its nature in order to use
it ef fectively w ithout disturbing its balance and diminishing its potent ial for the future. Almost
all organis ms contribute to sustain the earth’s environment. How ever, humans have caused
over use of resources. Most of the surface of the earth had and has been shaped over very
long periods of time (hundreds and thousands of years) and because of its use and misuse
by humans, its potent ial is being diminished at a fast rate. If the processes which shaped
and are shaping the surface of the earth into varieties of forms (shapes) and the nature of
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mater ials of which it is composed of, are understood, precautions can be taken to minimize
the detrimental ef fects of human use and to preserve it for posterity.
GEOMORPHIC PROCESSES
The endogenic and exogenic forces causing physical stresses and chemical actions on
earth materials and bringing about changes in the conf iguration of the surface of the earth
are know n as geomorphic processes. Diastrophism and volcanis m are endogenic
geomorphic processes. Weathering, mass w asting, erosion and deposition are exogenic
geomorphic processes. Any exogenic element of nature (like water, ice, w ind, etc.,) capable
of acquiring and transporting earth materials can be called a geomorphic agent. When these
elements of nature become mobile due to gradients, they remove the materials and
transport them over slopes and deposit them at low er level. Geomorphic processes and
geomorphic agents especially exogenic, unless stated separately, are one and the same.
A process is a force applied on earth materials affecting the same. An agent is a mobile
medium (like running w ater, moving ice masses, w ind, waves and currents etc.) which
removes, transports and deposits earth mater ials. Running water, groundwater, glaciers,
w ind, w aves and currents, etc., can be called geomorphic agents.
Gravity besides being a directional force activating all dow nslope movements of matter also
causes stresses on the earth’s mater ials. Indirect gravitational stresses activate wave and
tide induced currents and w inds. Without gravity and gradients there w ould be no mobility
and hence no erosion, transportation and deposition are possible. So, gravitat ional stresses
are as important as the other geomorphic processes. Gravity is the force that is keeping us
in contact w ith the surface and it is the force that sw itches on the movement of all surface
mater ial on earth. All the movements either w ithin the earth or on the surface of the earth
occur due to gradients — from higher levels to low er levels, f rom high pressure to low
pressure areas etc.
ENDOGENIC PROCESSES
The energy emanating from within the earth, is the main force behind endogenic
geomorphic processes. This energy is mostly generated by radioactivity, rotational and tidal
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f riction and primordial heat f rom the origin of the earth. This energy due to geothermal
gradients and heat f low from w ithin induces diastrophism and volcanis m in the lithosphere.
Due to variations in geothermal gradients and heat f low from w ithin, crustal thickness and
strength, the action of endogenic forces are not uniform and hence the tectonically
controlled orig inal crustal surface is uneven.
Diastrophism
All processes that move, elevate or build up portions of the earth’s crust come under
diastrophism. They include:
(i) orogenic processes involving mountain building through severe folding and affecting long
and narrow belts of the earth’s crust;
(ii) Epeirogenic processes involving uplif t or w arping of large parts of the earth’s crust;
(iii) earthquakes involving local relat ively minor movements;
(iv) Plate tectonics involving horizontal movements of crustal plates. In the process of
orogeny, the crust is severely deformed into folds. Due to epeirogeny, there may be simple
deformation. Orogeny is a mountain building process w hereas epeirogeny is continental
building process. Through the processes of orogeny, epeirogeny, earthquakes and plate
tectonics, there can be faulting and fracturing of the crust. All these processes cause
pressure, volume and temperature (PVT) changes w hich in turn induce metamorphism of
rocks.
Volcanism
Volcanism includes the movement of molten rock (magma) tow ard the earth’s surface and
also formation of many intrusive and extrusive volcanic forms.
EXOGENIC PROCESSES
The exogenic processes derive their energy from atmosphere determined by the ultimate
energy from the sun and also the gradients created by tectonic factors.
Gravitational force acts upon all earth materials having a sloping surface and tend to
produce movement of matter in dow n slope direction. Force applied per unit area is called
stress. Stress is produced in a solid by pushing or pulling. This induces deformation. Forces
acting along the faces of earth materia ls are shear stresses (separating forces). It is this
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stress that breaks rocks and other earth materia ls. The shear stresses result in angular
displacement or slippage. Besides the gravitat ional stress earth materia ls become subjected
to molecular stresses that may be caused by a number of factors amongst w hich
temperature changes, crystallization and melting are the most common. Chemical
processes normally lead to loosening of bonds betw een grains, dissolving of soluble
minerals or cementing materia ls. Thus, the basic reason that leads to w eathering, mass
movements, erosion and deposition is development of stresses in the body of the earth
mater ials.
As there are different climatic regions on the earth’s surface the exogenic geomorphic
processes vary from region to region. Temperature and precipitation are the tw o important
climatic elements that control various processes.
All the exogenic geomorphic processes are covered under a general term, denudation. The
word ‘denude’ means to strip off or to uncover. Weathering, mass w asting/movements,
erosion and transportation are included in denudat ion. The f low chart (Figure) gives the
denudation processes and their respective driving forces. It should become clear from this
chart that for each process there exists a distinct driving force or energy.
As there are different climatic regions on the earth’s surface ow ing to thermal gradients
created by latitudinal, seasonal and land and w ater spread variations, the exogenic
geomorphic processes vary from region to region. The density, type and distribution of
vegetation which largely depend upon
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Figure : Denudational processes and their driving forces.
precipitation and temperature exert inf luence indirectly on exogenic geomorphic processes.
Within dif ferent climat ic regions there may be local variations of the effects of different
climatic elements due to alt itudinal d if ferences, aspect variations and the variation in the
amount of insolation received by north and south facing slopes as compared to east and
west facing slopes. Further, due to dif ferences in w ind velocities and directions, amount and
kind of precipitation, its intensity, the relation betw een precipitation and evaporation, daily
range of temperature, f reezing and thaw ing frequency, depth of f rost penetration, the
geomorphic processes vary w ithin any climat ic region.
Climat ic factors being equal, the intensity of action of exogenic geomorphic processes
depends upon type and structure of rocks. The term structure includes such aspects of
rocks as folds, faults, orientation and inclination of beds, presence or absence of joints,
bedding planes, hardness or softness of constituent minerals, chemical susceptibility of
mineral constituents; the permeability or impermeability etc. Dif ferent types of rocks w ith
dif ferences in their structure offer varying resistances to various geomorphic processes. A
particular rock may be resistant to one process and nonresistant to another. And, under
varying climatic condit ions, particular rocks may exhibit dif ferent degrees of resistance to
geomorphic processes and hence they operate at dif ferential rates and give rise to
dif ferences in topography. The effects of most of the exogenic geomorphic processes are
small and slow and may be imperceptible in a short time span, but w ill in the long run affect
the rocks severely due to continued fatigue.
Finally, it boils down to one fact that the differences on the surface of the earth, though
originally related to the crustal evolut ion continue to exist in some form or the other due to
dif ferences in the type and structure of earth materia ls, dif ferences in geomorphic processes
and in their rates of operation.
Some of the exogenic geomorphic processes are follow ing :
WEATHERING
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Weathering is an action of elements of weather and climate over earth materia ls. There are
a number of processes w ithin w eathering w hich act either individually or together to affect
the earth materia ls in order to reduce them to fragmental state.
Weathering is def ined as mechanical d isintegration and chemical decomposit ion of rocks
through the actions of various elements of weather and climate.
As very little or no motion of materials takes place in weathering, it is an in-situ or on-site
process.
Weathering processes are conditioned by many complex geological, climatic, topographic
and vegetative factors. Climate is of particular importance. Not only w eathering processes
dif fer from climate to climate, but also the depth of the weathering mantle:-
Figure : Climatic regimes and depth of weathering mantles
Activity
There are three major groups of weathering processes :
(i) chemical
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(ii) physical or mechanical
(iii) bio logical w eathering processes.
Very rarely does any one of these processes ever operate completely by itself , but quite
often a dominance of one process can be seen.
Chemical Weathering Processes
A group of w eathering processes viz; solution, carbonation, hydration, oxidation and
reduction act on the rocks to decompose, dissolve or reduce them to a f ine clastic state
through chemical reactions by oxygen, surface and/or soil w ater and other acids. Water and
air (oxygen and carbon dioxide) along w ith heat must be present to speed up all chemical
reactions. Over and above the carbon dioxide present in the air, decomposition of plants
and animals increases the quantity of carbon dioxide underground. These chemical
reactions on various minerals are very much similar to the chemical reactions in a
laboratory.
Solution
When something is dissolved in water or acids, the water or acid w ith dissolved contents is
called solution. This process involves removal of solids in solution and depends upon
solubility of a mineral in water or w eak acids. On coming in contact w ith water many solids
disintegrate and mix up as suspension in water. Soluble rock forming minerals like nitrates,
sulphates, and potassium etc. are affected by this process. So, these minerals are easily
leached out w ithout leaving any residue in rainy climates and accumulate in dry regions.
Minerals like calcium carbonate and calcium magnesium bicarbonate present in limestones
are soluble in w ater containing carbonic acid (formed w ith the addition of carbon dioxide in
water), and are carried away in w ater as solution. Carbon dioxide produced by decaying
organic matter along w ith soil water greatly aids in th is reaction. Common salt (sodium
chloride) is also a rock forming mineral and is susceptible to th is process of solution.
Carbonation
Carbonat ion is the reaction of carbonate and bicarbonate w ith minerals and is a common
process helping the breaking dow n of feldspars and carbonate minerals. Carbon dioxide
from the atmosphere and soil air is absorbed by water, to form carbonic acid that acts as a
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weak acid. Calcium carbonates and magnesium carbonates are dissolved in carbonic acid
and are removed in a solution w ithout leaving any residue resulting in cave formation.
Hydration
Hydration is the chemical addition of water. Minerals take up w ater and expand; th is
expansion causes an increase in the volume of the material itself or rock. Calcium sulphate
takes in w ater and turns to gypsum, w hich is more unstable than calcium sulphate. This
process is reversible and long, continued repetit ion of this process causes fatigue in the
rocks and may lead to their disintegrat ion. Many clay minerals sw ell and contract during
wetting and drying and a repetition of this process results in cracking of overlying materials.
Salts in pore spaces undergo rapid and repeated hydration and help in rock fracturing. The
volume changes in minerals due to hydration w ill also help in physical w eathering through
exfoliation and granular disintegrat ion.
Oxidation and Reduction
In weathering, oxidation means a combinat ion of a mineral w ith oxygen to form oxides or
hydroxides. Oxidation occurs where there is ready access to the atmosphere and
oxygenated w ater. The minerals most commonly involved in this process are iron,
manganese, sulphur etc. In the process of oxidation rock breakdown occurs due to the
disturbance caused by addition of oxygen. Red colour of iron upon oxidation turns to brown
or yellow . When oxidised minerals are placed in an environment w here oxygen is absent,
reduction takes place. Such conditions exist usually below the water table, in areas of
stagnant w ater and w aterlogged ground. Red colour of iron upon reduction turns to greenish
or bluish grey.
These w eathering processes are interrelated. Hydration, carbonation and oxidation go hand
in hand and hasten the w eathering process.
Physical Weathering Processes
Physical or mechanical w eathering processes depend on some applied forces. The applied
forces could be:
(i) gravitational forces such as overburden pressure, load and shearing stress;
(ii) expansion forces due to temperature changes, crystal growth or animal activity;
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(iii) water pressures controlled by w etting and drying cycles.
Many of these forces are applied both at the surface and within different earth materia ls
leading to rock fracture. Most of the physical weathering processes are caused by thermal
expansion and pressure release. These processes are small and slow but can cause great
damage to the rocks because of continued fatigue the rocks suffer due to repet ition of
contraction and expansion.
Unloading and Expansion
Removal of overlying rock load because of continued erosion causes vertical pressure
release w ith the result that the upper layers of the rock expand producing disintegrat ion of
rock masses. Fractures will develop roughly parallel to the ground surface. In areas of
curved ground surface, arched fractures tend to produce massive sheets or exfoliation slabs
of rock. Exfoliation sheets resulting from expansion due to unloading and pressure release
may measure hundreds or even thousands of meters in horizontal extent. Large, smooth
rounded domes called exfoliat ion domes result due to this process.
Figure : A large exfoliation dome in granite rock near bhongir (Bhuvanagiri) tow n in Andhra
Pradesh
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Temperature Changes and Expansion
Various minerals in rocks possess their ow n limits of expansion and contraction. With r ise in
temperature, every mineral expands and pushes against its neighbour and as temperature
falls, a corresponding contraction takes place. Because of diurnal changes in the
temperatures, this internal movement among the mineral grains of the superf icial layers of
rocks takes place regularly. This process is most effective in dry climates and high
elevations where diurnal temperature changes are drastic. These movements are very small
they make the rocks w eak due to continued fatigue. The surface layers of the rocks tend to
expand more than the rock at depth and this leads to the formation of stress w ithin the rock
resulting in heaving and fracturing parallel to the surface. Due to dif ferential heating and
resulting expansion and contraction of surface layers and their subsequent exfoliation from
the surface results in smooth rounded surfaces in rocks. In rocks like granites, smooth
surfaced and rounded small to big boulders called tors form due to such exfoliation.
Freezing, Thaw ing and Frost Wedging
Frost w eathering occurs due to growth of ice w ithin pores and cracks of rocks during
repeated cycles of f reezing and melting. This process is most effective at high elevations in
mid-latitudes w here freezing and melting is of ten repeated. Glacial areas are subject to frost
wedging daily. In th is process, the rate of f reezing is important. Rapid freezing of w ater
causes its sudden expansion and high pressure. The resulting expansion affects joints,
cracks and small inter granular fractures to become wider and w ider till the rock breaks
apart.
Salt Weathering
Salts in rocks expand due to thermal action, hydration and crystallisation. Many salts like
calcium, sodium, magnesium, potassium and barium have a tendency to expand. Expansion
of these salts depends on temperature and their thermal properties. High temperature
ranges between 30 and 50o C of surface temperatures in deserts favour such salt
expansion. Salt crystals in near-surface pores cause splitting of individual grains w ithin
rocks, w hich eventually fall of f . This process of falling off of individual grains may result in
granular disintegration or granular foliat ion.
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Salt crystallisation is most ef fective of all salt-w eathering processes. In areas w ith alternating
wetting and drying condit ions salt crystal grow th is favoured and the neighbour ing grains are
pushed aside. Sodium chloride and gypsum crystals in desert areas heave up overlying
layers of materials and w ith the result polygonal cracks develop all over the heaved surface.
With salt crystal grow th, chalk breaks down most readily, followed by limestone, sandstone,
shale, gneiss and granite etc.
BIOLOGICAL ACTIVITY AND WEATHERING
Biological weathering is contribution to or removal of minerals and ions from the weathering
environment and physical changes due to grow th or movement of organisms. Burrowing and
wedging by organisms like earthw orms, termites, rodents etc., help in exposing the new
surfaces to chemical attack and assists in the penetration of moisture and air. Human
beings by disturbing vegetation, ploughing and cult ivating soils, also help in mixing and
creating new contacts between air, water and minerals in the earth materia ls.
Decaying plant and animal matter help in the production of humic, carbonic and other acids
which enhance decay and solubility of some elements. Algae utilize mineral nutrients for
growth and help in concentration of iron and manganese oxides. Plant roots exert a
tremendous pressure on the earth materials mechanically breaking them apart.
SOME SPECIAL EFFECTS OF WEATHERING
This has already been explained under physical w eathering processes of unloading, thermal
contraction and expansion and salt w eathering. Exfoliation is a result but not a process.
Flaking off of more or less curved sheets of shells from over rocks or bedrock results in
smooth and rounded surfaces. Exfoliation can occur due to expansion and contraction
induced by temperature changes. Exfoliation domes and tors result due to unloading and
thermal expansion respectively.
SIGNIFICANCE OF WEATHERING
Weathering processes are responsible for breaking dow n the rocks into smaller f ragments
and preparing the way for formation of not only regolith and soils, but also erosion and mass
movements. Biomes and biodiversity is basically a result of forests (vegetation) and forests
depend upon the depth of w eathering mantles. Erosion cannot be signif icant if the rocks are
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not weathered. That means, w eathering aids mass w asting, erosion and reduction of relief
and changes in landforms are a consequence of erosion.
Weathering of rocks and deposits helps in the enrichment and concentrations of certain
valuable ores of iron, manganese, aluminium, copper etc., w hich are of great importance for
the national economy. Weathering is an important process in the formation of soils.
When rocks undergo w eathering, some materials are removed through chemical or physical
leaching by groundw ater and thereby the concentration of remaining (valuable) materia ls
increases. Without such a w eathering taking place, the concentration of the same valuable
mater ial may not be suff icient and economically viable to exploit, process and ref ine. This is
what is called enrichment.
MASS MOVEMENTS
These movements transfer the mass of rock debris dow n the slopes under the direct
inf luence of gravity. That means, air, w ater or ice do not carry debris with them from place to
place but on the other hand the debris may carry w ith it air, w ater or ice. The movements of
mass may range from slow to rapid, affecting shallow to deep columns of materials and
include creep, f low, slide and fall. Gravity exerts its force on all matter, both bedrock and the
products of weathering. So, w eathering is not a pre-requisite for mass movement though it
aids mass movements. Mass movements are very active over w eathered slopes rather than
over unw eathered materials.
Mass movements are aided by gravity and no geomorphic agent like running water, glaciers,
w ind, w aves and currents participate in the process of mass movements. That means mass
movements do not come under erosion though there is a shif t (aided by gravity) of materia ls
from one place to another. Materia ls over the slopes have their ow n resistance to disturbing
forces and w ill yield only when force is greater than the shearing resistance of the materials.
Weak unconsolidated materia ls, thinly bedded rocks, faults, steeply dipping beds, vertical
clif fs or steep slopes, abundant precipitat ion and torrential rains and scarcity of vegetation
etc., favour mass movements.
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Several activating causes precede mass movements. They are :
(i) removal of support f rom below to materials above through natural or artif icial means;
(ii) increase in gradient and height of slopes;
(iii) overloading through addition of materials naturally or by artif icial f illing;
(iv) overloading due to heavy rainfall, saturation and lubrication of slope materials;
(v) removal of material or load from over the original slope surfaces; (vi) occurrence of
earthquakes, explosions or machinery;
(vii) excessive natural seepage;
(viii) heavy draw down of water f rom lakes, reservoirs and rivers leading to slow outf low of
water f rom under the slopes or river banks;
(ix) indiscriminate removal of natural vegetation.
Heave (heaving up of soils due to frost growth and other causes), f low and slide are the
three forms of movements. Figure shows the relat ionships among dif ferent types of mass
movements, their relative rates of movement and moisture limits.
Figure : Relat ionships among dif ferent types of mass movements, their relative rates of
movement and moisture limits.
Mass movements can be grouped under three major classes:
(i) slow movements; (ii) rapid movements; (iii) landslides.
Slow Movements
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Creep is one type under this category which can occur on moderately steep, soil covered
slopes. Movement of mater ials is extremely slow and imperceptible except through
extended observation. Materials involved can be soil or rock debris. Fence posts, telephone
poles lean dow nslope from their vertical position and in their linear alignment are due to the
creep effect. Depending upon the type of materia l involved, several types of creep viz., soil
creep, talus creep, rock creep, rock-glacier creep etc., can be identif ied. Also included in th is
group is solif luction w hich involves slow downslope f low ing soil mass or f ine grained rock
debris saturated or lubricated w ith water. This process is quite common in moist temperate
areas w here surface melt ing of deeply frozen ground and long continued rain respectively,
occur frequently. When the upper portions get saturated and when the low er parts are
impervious to w ater percolation, f low ing occurs in the upper parts.
Rapid Movements
These movements are mostly prevalent in humid climatic regions and occur over gentle to
steep slopes. Movement of w ater-saturated clayey or silty earth materials dow n low-angle
terraces or hillsides is known as earthf low. Quite often, the materials slump making steplike
terraces and leaving arcuate scarps at their heads and an accumulation bulge at the toe.
When slopes are steeper, even the bedrock especially of soft sedimentary rocks like shale
or deeply weathered igneous rock may slide downslope.
Another type in th is category is mudf low . In the absence of vegetation cover and w ith heavy
rainfall, thick layers of weathered materia ls get saturated w ith w ater and either slow ly or
rapidly f low down along def inite channels. It looks like a stream of mud w ithin a valley. When
the mudflows emerge out of channels onto the piedmont or plains, they can be very
destructive engulf ing roads, bridges and houses. Mudf lows occur frequently on the slopes of
erupting or recently erupted volcanoes. Volcanic ash, dust and other fragments turn into
mud due to heavy rains and f low down as tongues or streams of mud causing great
destruction to human habitations.
A third type is the debris avalanche, w hich is more characteristic of humid regions w ith or
w ithout vegetation cover and occurs in narrow tracks on steep slopes. This debris avalanche
can be much faster than the mudflow . Debris avalanche is similar to snow avalanche.
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Page 16
In Andes mountains of South America and the Rockies mountains of North America, there
are a few volcanoes w hich erupted during the last decade and very devastating mudflows
occurred down their slopes during eruption as well as after eruption.
Landslides
These are know n as relatively rapid and perceptible movements. The materials involved are
relatively dry. The size and shape of the detached mass depends on the nature of
discontinuities in the rock, the degree of w eathering and the steepness of the slope.
Depending upon the type of movement of materia ls several types are identif ied in this
category.
Slump is slipping of one or several units of rock debris w ith a backw ard rotation w ith respect
to the slope over which the movement takes place. Rapid rolling or sliding
Figure : Slumping of debris w ith backward rotation
of earth debris w ithout backward rotation of mass is know n as debris slide. Debris fall is
nearly a free fall of earth debris from a vertical or overhanging face. Sliding of individual rock
masses down bedding, joint or fault surfaces is rockslide. Over steep slopes, rock sliding is
very fast and destructive. Slides occur as planar failures along discontinuities like bedding
planes that dip steeply. Rock fall is f ree falling of rock blocks over any steep slope keeping
itself aw ay from the slope. Rock falls occur from the superf icial layers of the rock
In our country, debris avalanche and landslides occur very frequently in the Himalayas.
There are many reasons for this. One, the Himalayas are tectonically active. They are
mostly made up of sedimentary rocks and unconsolidated and semi-consolidated deposits.
The slopes are very steep. Compared to the Himalayas, the Nilg iris bordering Tamilnadu,
Karnataka, Kerala and the Western Ghats along the west coast are relatively tectonically
stable and are mostly made up of very hard rocks; but, still, debris avalanches and
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Page 17
landslides occur though not as frequently as in the Himalayas, in these hills. Many slopes
are steeper w ith almost vertical cliffs and escarpments in the Western Ghats and Nilgiris.
Mechanical w eathering due to temperature changes and ranges is pronounced. They
receive heavy amounts of rainfall over short periods. So, there is almost direct rock fall quite
frequently in these places along w ith landslides and debris avalanches.
EROSION AND DEPOSITION
Erosion involves acquisition and transportation of rock debris. When massive rocks break
into s maller f ragments through weathering and any other process, erosional geomorphic
agents like running w ater, groundw ater, glaciers, w ind and w aves remove and transport it to
other places depending upon the dynamics of each of these agents. Abrasion by rock debris
carried by these geomorphic agents also aids greatly in erosion. By erosion, relief degrades,
i.e., the landscape is w orn dow n. That means, though weathering aids erosion it is not a pre-
condition for erosion to take place. Weathering, mass-wasting and erosion are degradational
processes. It is erosion that is largely responsible for continuous changes that the earth’s
surface is undergoing. Denudational processes like erosion and transportation are controlled
by kinetic energy.
The erosion and transportation of earth materials is brought about by w ind, running w ater,
glaciers, w aves and ground water. Of these the f irst three agents are controlled by climat ic
conditions.
They represent three states of matter — gaseous (wind), liquid (running water) and solid
(glacier) respectively. The erosion can be def ined as “application of the kinetic energy
associated w ith the agent to the surface of the land along w hich it moves”. Kinetic energy is
computed as KE = 1/2 mv2 w here ‘m’ is the mass and ‘v’ is the velocity. Hence the energy
available to perform w ork w ill depend on the mass of the materia l and the velocity w ith
which it is moving. Obviously then you w ill f ind that though the glaciers move at very low
velocities due to tremendous mass are more effective as the agents of erosion and w ind,
being in gaseous state, are less effective.
The w ork of the other tw o agents of erosion waves and ground w ater is not controlled by
climate. In case of w aves it is the location along the interface of litho and hydro sphere —
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Page 18
coastal region — that w ill determine the work of waves, w hereas the work of ground water is
determined more by the lithological character of the region. If the rocks are permeable and
soluble and w ater is available only then karst topography develops.
Deposit ion is a consequence of erosion. The erosional agents loose their velocity and hence
energy on gentler slopes and the materials carried by them start to settle themselves. In
other w ords, deposition is not actually the w ork of any agent. The coarser materials get
deposited first and f iner ones later. By deposition depressions get f illed up. The same
erosional agents viz., running water, glaciers, w ind, waves and groundw ater act as
aggradational or depositional agents also.
SOIL FORMATION
Soil and Soil Contents
A pedologist w ho studies soils def ines soil as a collection of natural bodies on the earth’s
surface containing living matter and supporting or capable of supporting plants.
Soil is a dynamic medium in w hich many chemical, physical and biological activit ies go on
constantly. Soil is a result of decay, it is also the medium for grow th. It is a changing and
developing body. It has many characteristics that f luctuate w ith the seasons. It may be
alternatively cold and w arm or dry and moist. Biological activity is slow ed or stopped if the
soil becomes too cold or too dry. Organic matter increases w hen leaves fall or grasses die.
The soil chemistry, the amount of organic matter, the soil f lora and fauna, the temperature
and the moisture, all change w ith the seasons as well as with more extended periods of
time.
That means, soil becomes adjusted to conditions of climate, landform and vegetation and
w ill change internally w hen these controlling condit ions change.
Process of Soil Form ation
Soil formation or pedogenesis depends f irst on weathering. It is this w eathering mantle
(depth of the w eathered material) w hich is the basic input for soil to form. First, the
weathered materia l or transported deposits are colonised by bacteria and other inferior plant
bodies like mosses and lichens. Also, several minor organis ms may take shelter w ithin the
mantle and deposits. The dead remains of organisms and plants help in humus
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Page 19
accumulat ion. Minor grasses and ferns may grow; later, bushes and trees w ill start grow ing
through seeds brought in by birds and w ind. Plant roots penetrate down, burrowing animals
bring up particles, mass of material becomes porous and spongelike w ith a capacity to
retain w ater and to permit the passage of air and f inally a mature soil, a complex mixture of
mineral and organic products forms.
Soil-form ing Factors
Five basic factors control the formation of soils:
(i) parent material;
(ii) topography;
(iii) climate;
(iv) biological activity;
(v) time.
In fact soil forming factors act in union and affect the action of one another.
Parent Material
Parent material is a passive control factor in soil formation. Parent materia ls can be any in-
situ or on-site w eathered rock debris (residual soils) or transported deposits (transported
soils). Soil formation depends upon the texture (sizes of debris) and structure (disposition of
individual grains/particles of debris) as well as the mineral and chemical composit ion of the
rock debris/deposits.
Nature and rate of w eathering and depth of w eathering mantle are important consideration
under parent materia ls. There may be differences in soil over similar bedrock and dissimilar
bedrocks may have similar soils above them. But w hen soils are very young and have not
matured these show strong links w ith the type of parent rock. Also, in case of some
limestone areas, where the w eathering processes are specif ic and peculiar, soils w ill show
clear relation w ith the parent rock.
Topography
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Topography like parent materials is another passive control factor. The inf luence of
topography is felt through the amount of exposure of a surface covered by parent materia ls
to sunlight and the amount of surface and sub-surface drainage over and through the parent
mater ials. Soils w ill be thin on steep slopes and thick over f lat upland areas. Over gentle
slopes w here erosion is slow and percolation of w ater is good, soil formation is very
favourable. Soils over f lat areas may develop a thick layer of clay w ith good accumulation of
organic matter giving the soil dark colour. In middle latitudes, the south facing slopes
exposed to sunlight have dif ferent conditions of vegetation and soils and the north facing
slopes w ith cool, moist condit ions have some other soils and vegetat ion.
Climate
Climate is an important active factor in soil formation. The climat ic elements involved in soil
development are :
(i) moisture in terms of its intensity, f requency and duration of precipitation - evaporation and
humidity;
(ii) temperature in terms of seasonal and diurnal variations.
Precipitation gives soil its moisture content which makes the chemical and biological
activities possible. Excess of w ater helps in the dow nward transportation of soil components
through the soil (eluviation) and deposits the same down below (illuviation). In climates like
wet equatorial rainy areas w ith high rainfall, not only calcium, sodium, magnesium,
potassium etc. but also a major part of silica is removed from the soil. Removal of silica from
the soil is know n as desilication. In dry climates, because of high temperature, evaporation
exceeds precipitation and hence ground w ater is brought up to the surface by capillary
action and in the process the w ater evaporates leaving behind salts in the soil. Such salts
form into a crust in the soil know n as hardpans. In tropical climates and in areas w ith
intermediate precipitat ion condit ions, calcium carbonate nodules (kanker) are formed.
Temperature acts in tw o ways — increasing or reducing chemical and biological activity.
Chemical activity is increased in higher temperatures, reduced in cooler temperatures (w ith
an exception of carbonation) and stops in freezing conditions. That is why, tropical soils w ith
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higher temperatures show deeper prof iles and in the frozen tundra regions soils contain
largely mechanically broken materia ls.
Biological Activity
The vegetative cover and organisms that occupy the parent materials from the beginning
and also at later stages help in adding organic matter, moisture retent ion, nitrogen etc. Dead
plants provide humus, the f inely divided organic matter of the soil. Some organic acids
which form during humif ication aid in decomposing the minerals of the soil parent materials.
Intensity of bacterial activity shows up differences between soils of cold and warm climates.
Humus accumulates in cold climates as bacterial grow th is slow. With undercomposed
organic matter because of low bacterial activity, layers of peat develop in sub-arctic and
tundra climates. In humid tropical and equatorial climates, bacterial grow th and action is
intense and dead vegetat ion is rapidly oxidised leaving very low humus content in the soil.
Further, bacteria and other soil organisms take gaseous nitrogen from the air and convert it
into a chemical form that can be used by plants. This process is known as nitrogen f ixation.
Rhizobium, a type of bacteria, lives in the root nodules of leguminous plants and f ixes
nitrogen benef icial to the host plant. The inf luence of large animals like ants, termites,
earthw orms, rodents etc., is mechanical, but, it is nevertheless important in soil formation as
they rew ork the soil up and dow n. In case of earthworms, as they feed on soil, the texture
and chemistry of the soil that comes out of their body changes.
Time
Time is the third important controlling factor in soil formation. The length of time the soil
forming processes operate, determines maturation of soils and prof ile development. A soil
becomes mature when all soil-forming processes act for a suff iciently long time developing a
prof ile. Soils developing from recently deposited alluvium or glacial till are considered young
and they exhibit no horizons or only poorly developed horizons. No specif ic length of time in
absolute terms can be f ixed for soils to develop and mature.
DENUDATION, WEATHERING AND MASS WASTING
The collective processes of denudation appear as just tw o facilitating links in the
sedimentary loop of the rock cycle, between the formation of continental crust and the post-
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depositional fate of derived sediments. In practice they form the principal element in any
review of landsurface development and the core of the science of geomorphology.
This approach permits the linkage of geomorphic processes with the time scales and global
patterns of morphotectonic activity, the tectonic machinery providing init ial uplif t and a global
tectonic framew ork for the location of orogeny, cratons and basins w here geomorphic
landsystems develop. Tectonic forcing also provides a major source of general
environmental and geomorphic change.
General processes of denudation, w eathering and mass wasting are introduced here as the
prelude to later chapters where they are show n to operate in more specif ic w ays in particular
geomorphic environments. An outline history of the signif icance of denudation rates and
earlier attempts to def ine denudation chronologies precedes an introduction to opposing
forces in the geomorphic environment – the static force of gravity and the dynamic force of
moving bodies of water, ice and air versus the strength of earth materials.
These are def ined by Mohr–Coulomb failure criteria, w hich are used w idely in applied
geomorphology and geotechnical investigations to summarize the source of principal
strength components and mobilized eroding forces. Weathering is review ed in its own right
w ith classic physical and chemical processes. It is also treated here as a means to an end –
a source of in situ reduction of rock strength facilitating subsequent mass w asting or erosion
– and as a natural extension of geological f ractionation processes.
Mohr–Coulomb criteria are revisited to demonstrate sliding resistance before a review of
principal styles of slope instability and failure, w ith distinctions betw een rock and debris slopes.
The chapter includes a review of debris f low hazard, which appears to be on the increase in
temperate climates through changes in land use and climate.
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Page 1
For IIT-JAM, JNU, GATE, NET, NIMCET and Other Entrance Exams
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INTRODUCTION
VARIOUS STABILITIES AND INSTABILITIES
AIR MASSESS (AM)
TYPES OF AIR MASSES
WEATHER EFFECTIVITIES INDUCED BY
TEST YOUR KNOWLEDGE
UGC NET - GEOGRAPHY
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SAMPLE THEORY
ATMOSPHERIC STABILITY AND
INSTABILITY
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THE AIR MASSES
PAPER - III
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STABILITY AND INSTABILITY
Let's use a balloon to demonstrate stability and instability. In f igure 42 w e have, for three
situations, filled a balloon at sea level w ith air at 31° C-the same as the ambient
temperature. We have carried the balloon to 5,000 feet. In each situation, the air in the
balloon expanded and cooled at the dry adiabat ic rate of 3° C for each 1,000 feet to a
temperature of 16° C at 5,000 feet.
Stability related to temperatures alof t and adiabatic cooling. In each situation, the balloon is
f illed at sea level w ith air at 31° C, carried manually to 5,000 feet, and released. In each
case, air in the balloon expands and cools to 16° C (at the dry adiabatic rate of 3° C per
1,000 feet). But, the temperature of the surrounding air alof t in each situation is different.
The balloon on the lef t w ill rise. Even though it cooled adiabat ically, the balloon remains
warmer and lighter than the surrounding cold air; w hen released, it w ill continue upw ard
spontaneously. The air is unstable; it favors vertical motion. In the center, the surrounding
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Page 3
air is w armer. The cold balloon w ill sink. It resists our forced lif ting and cannot rue
spontaneously. The air is stable-it resists upw ard motion. On the right, surrounding air and
the balloon are at the same temperature. The balloon remains at rest since no density
dif ference exists to displace it vertically. The air is neutrally stable, i.e., it neither favors nor
resists vertical motion. A mass of air in which the temperature decreases rapidly w ith height
favors instability; but, air tends to be stable if the temperature changes little or not at a ll w ith
altitude.
Stable Or Unstable Process
Stability runs the gamut from absolutely stable to absolutely unstable, and the atmosphere
usually is in a delicate balance somew here in betw een. A change in ambient temperature
lapse rate of an air mass can tip this balance. For example, surface heating or cooling alof t
can make the air more unstable; on the other hand, surface cooling or w arming alof t of ten
tips the balance toward greater stability.
Air may be stable or unstable in layers. A stable layer may overlie and cap unstable air; or,
conversely, air near the surface may be stable w ith unstable layers above.
Stratiform Clouds
Since stable air resists convection, clouds in stable air form in horizontal, sheet-like layers or
"strata." Thus, w ithin a stable layer, clouds are stratiform. Adiabatic cooling may be by
upslope f low ; by lif ting over cold, more dense air; or by converging w inds. Cooling by an
underlying cold surface is a stabilizing process and may produce fog. If clouds are to remain
stratiform, the layer must remain stable after condensation occurs.
Cumuliform Clouds
Unstable air favors convection. A "cumulus" cloud, meaning "heap," forms in a convective
updraft and builds upw ard. Thus, w ithin an unstable layer, clouds are cumuliform; and the
vertical extent of the cloud depends on the depth of the unstable layer.
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FIGURE
When stable air (lef t) is forced upward, the air tends to retain horizontal f low , and any
cloudiness is f lat and stratif ied. When unstable air is forced upw ard, the disturbance grows,
and any resulting cloudiness shows extensive vertical development.
We can estimate height of cumuliform cloud bases using surface temperature-dew point
spread. Unsaturated air in a convective current cools at about 5.4° F (3.0° C) per 1,000 feet;
dew point decreases at about 1° F (5/9° C). Thus, in a convective current, temperature and
dew point converge at about 4.4° F (2.5° C) per 1,000 feet as illustrated in f igure 44. We can
get a quick estimate of a convective cloud base in thousands of feet by rounding these
values and dividing into the spread or by mult iplying the spread by their reciprocals. When
using Fahrenheit, divide by 4 or multip ly by .25; when using Celsius, divide by 2.2 or mult ip ly
by .45. This method of estimat ing is reliable only w ith instability clouds and during the
warmer part of the day.
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FIGURE
Cloud base determination. Temperature and dew point in upward moving air converge at a
rate of about 4° F or 2.2° C per 1,000 feet.
Merging Stratiform and Cumuliform
A layer of stratiform clouds may sometimes form in a mildly stable layer w hile a few
ambitious convective clouds penetrate the layer thus merging stratiform w ith cumuliform.
Convective clouds may be almost or entirely embedded in a massive stratiform layer and
pose an unseen threat to instrument f light.
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Temperature distribution of vertically moving air
The term "adiabatic process" simply means w arming by compression, or cooling by
expansion, w ithout a transfer of heat or mass into a system. As air moves up or down w ithin
the atmosphere, it is af fected by this process. This temperature dif ference w ill be 5-1/2
degree decrease per 1,000 feet increase in altitude. This is also termed the dry adiabat ic
lapse rate. The atmosphere may or may not have a temperature distribution that f its the dry
adiabat ic lapse rate. Usually it does not.
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Unstable air encourages vertical movement of air and decreases f ire activity.
The actual lapse rate may be greater or less than the dry adiabatic lapse rate and may
change by levels in the atmosphere. This variation from the dry adiabatic lapse rate is what
determines w hether the air is stable or unstable. If the air is unstable, the vertical movement
of air is encouraged, and this tends to increase f ire activity. If the air is stable, vertical
movement of air is discouraged, and this usually decreases or holds dow n fire activity. The
importance of this atmospheric property w ill become evident by the time you have
completed this unit.
Dry Lapse Rates
The actual temperature lapse rate in a given portion of the atmosphere could range from a
plus 15° per 1,000 feet to a minus 15° per 1,000 feet. These would represent the extremes
of very stable air to very unstable air.
AIR MASSES AND FRONTS
The purpose of this module is to introduce air masses, w here they originate from and how
they are modif ied. Clashing air masses in the middle lat itudes spark interesting weather
events and the boundaries separating these air masses are know n as fronts. This module
examines fronts, with detailed explanations about cold fronts and w arm fronts. Finally,
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Page 8
dif ferent types of advection are introduced; temperature, moisture and voriticity advection.
The Air Masses and Fronts module has been organized into the follow ing sections:
Air Masses
Air masses that commonly inf luence w eather in the United States.
Fronts
Boundaries separating air masses. Includes w arm fronts, cold fronts, occluded and
stationary fronts and dry lines.
Continental Polar Air Masses
cold tem peratures and little moisture
Those w ho live in northern portions of the United States expect cold w eather during the
w inter months. These conditions usually result f rom the invasion of cold arctic air masses
that orig inate from the snow covered regions of northern Canada. Because of the long
w inter nights and strong radiational cooling found in these regions, the overlying air
becomes very cold and very stable. The longer th is process continues, the colder the
developing air mass becomes, until changing weather patterns transport the arctic air mass
southw ard.
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Below is a map of surface observations and the leading edge of a large arctic air mass
blanket ing much of the United States has been highlighted by the blue line. The center of
this air mass is a high pressure center located in northern Montana (indicated by the blue
"H").
From these reports, w e see that most stations in the arctic air mass generally exhibit
relatively colder temperatures, w ith low er dew point temperatures, and w inds generally out
of the north. Notice that on the other side of the blue boundary, outside of this air mass,
surface conditions are much dif ferent, which indicates the presence of an entirely dif ferent
air mass.
Maritime Tropical Air Masses
w arm temperatures and rich in m oisture
Maritime tropical air masses originate over the warm w aters of the tropics and Gulf of
Mexico, w here heat and moisture are transferred to the overlying air from the w aters below.
The northward movement of tropical air masses transports warm moist air into the United
States, increasing the potent ial for precipitation.
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Below is a map of surface observations and the leading edge of a tropical air mass surging
northw ard into the Ohio Valley has been highlighted in red. Southerly w inds behind the
boundary signify the continued northw ard transport of warm moist air.
From these reports, w e see that most stations in the tropical a ir mass generally exhibit
relatively w armer temperatures, w ith higher dew point temperatures, and w inds generally out
of the south. Notice that on the other side of the red boundary, outside of this air mass,
surface conditions are much dif ferent, which indicates the presence of an entirely dif ferent
air mass.
Air masses and their sources
Fahrenheit w hile a short distance behind the front, the temperature decreased to 38
degrees. An abrupt temperature change over a short distance is a good indicator that a front
is located somew here in betw een.
THE HYDROLOGICAL CYCLE
(also know n as the w ater cycle) is the journey water takes as it circulates from the land to
the sky and back again.
The sun's heat provides energy to evaporate water from the earth's surface (oceans, lakes,
etc.). Plants also lose water to the air - this is called transpiration. The w ater vapour
eventually condenses, forming tiny droplets in clouds.
When the clouds meet cool air over land, precipitation (rain, sleet, or snow) is triggered, and
water returns to the land (or sea). Some of the precipitat ion soaks into the ground. Some of
the underground water is trapped betw een rock or clay layers - this is called groundw ater.
But most of the w ater f lows downhill as runoff (above ground or underground), eventually
returning to the seas as slightly salty w ater.
Earth's w ater
Water is the most w idespread substance to be found in the natural environment and it is the
source of all life on earth. Water covers 70% of the earth's surface but it is diff icult to
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Page 11
comprehend the total amount of water when we only see a small portion of it. The
distribution of w ater throughout the earth is not uniform. Some places have far more rainfall
than others.
The Water Cycle
To assess the total w ater storage on the earth reliably is a complicated problem because
water is so very dynamic. It is in permanent motion, constantly changing from liquid to solid
or gaseous phase, and back again. The quantity of water found in the hydrosphere is the
usual w ay of estimating the earth's w ater. This is all the free water existing in liquid, solid or
gaseous state in the atmosphere, on the Earth's surface and in the crust down to a depth of
2000 metres. Current estimates are that the earth's hydrosphere contains a huge amount of
water - about 1386 million cubic kilometres. However, 97.5% of this amount exists as saline
waters and only 2.5% as fresh water.
Hydrological Cycle work
The stages of the cycle are:
• Evaporation
• Transport
• Condensation
• Precipitation
• Groundw ater
• Run-off
Evaporation
Water is transferred from the surface to the atmosphere through evaporation, the process by
which water changes from a liquid to a gas. The sun's heat provides energy to evaporate
water from the earth's surface. Land, lakes, rivers and oceans send up a steady stream of
water vapour and plants also lose w ater to the air (transpiration).
Approximately 80% of all evaporation is f rom the oceans, w ith the remaining 20% coming
from inland water and vegetation.
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Transport
The movement of water through the atmosphere, specif ically f rom over the oceans to over
land, is called transport. Some of the earth's moisture transport is visible as clouds, w hich
themselves consist of ice crystals and/or tiny w ater droplets.
Clouds are propelled from one place to another by either the jet stream, surface-based
circulations like land and sea breezes or other mechanisms. How ever, a typical cloud 1 km
thick contains only enough w ater for a millimetre of rainfall, w hereas the amount of moisture
in the atmosphere is usually 10-50 times greater than this.
Most w ater is transported in the form of water vapour, which is actually the third most
abundant gas in the atmosphere. Water vapour may be invisible to us, but not to satellites
which are capable of collecting data about moisture patterns in the atmosphere.
Condensation
The transported w ater vapour eventually condenses, forming tiny droplets in clouds.
Precipitation
The primary mechanis m for transporting w ater from the atmosphere to the surface of the
earth is precipitation.
When the clouds meet cool air over land, precipitation, in the form of rain, sleet or snow, is
triggered and w ater returns to the land (or sea). A proportion of atmospheric precipitation
evaporates.
Groundwater
Some of the precipitat ion soaks into the ground and this is the main source of the formation
of the w aters found on land - rivers, lakes, groundw ater and glaciers.
Some of the underground water is trapped between rock or clay layers - this is called
groundw ater. Water that inf iltrates the soil f lows dow nward until it encounters impermeable
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Page 13
rock and then travels laterally. The locat ions w here water moves laterally are called
'aquifers'. Groundwater returns to the surface through these aquifers, which empty into
lakes, rivers and the oceans.
Under special circumstances, groundw ater can even f low upw ard in artesian wells. The f low
of groundw ater is much slow er than run-off w ith speeds usually measured in centimetres per
day, metres per year or even centimeters per year.
Run-off
Most of the water which returns to land flows downhill as run-off . Some of it penetrates and
charges groundw ater while the rest, as river f low, returns to the oceans w here it evaporates.
As the amount of groundw ater increases or decreases, the water table rises or falls
accordingly. When the entire area below the ground is saturated, f looding occurs because
all subsequent precipitation is forced to remain on the surface.
Dif ferent surfaces hold different amounts of water and absorb w ater at different rates. As a
surface becomes less permeable, an increasing amount of water remains on the surface,
creating a greater potential for f looding. Flooding is very common during w inter and early
spring because frozen ground has no permeability, causing most rainwater and meltwater to
become run-off .
GLOBAL WARMING
Global w arming has become familiar to many people as one of the most important
environmental issues of our day. This review w ill describe the basic science of global
warming, its likely impacts both on human communit ies and on natural ecosystems and the
actions that can be taken to mitigate or to adapt to it. As commonly understood, global
warming refers to the effect on the climate of human activities, in particular the burning of
fossil fuels (coal, oil and gas) and large-scale deforestation—activities that have grown
enormously since the industrial revolution, and are currently leading to the release of about
7 billion tonnes of carbon as carbon dioxide into the atmosphere each year together w ith
substantial quantit ies of methane, nitrous oxide and chlorofluorocarbons (CFCs). These
gases are know n as greenhouse gases.
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The basic principle of global w arming can be understood by considering the radiation energy
from the sun that warms the Earth’s surface and the thermal radiat ion from the Earth and
the atmosphere that is radiated out to space. On average, these tw o radiation streams must
balance. The greenhouse effect arises because of the presence of greenhouse gases in the
atmosphere that absorb thermal radiat ion emitted by the Earth’s surface and, therefore, act
as a blanket over the surface . It is known as the greenhouse effect because the glass in a
greenhouse possesses similar properties to the greenhouse gases in that it absorbs infrared
radiation while being transparent to radiation in the visib le part of the spectrum. If the
amounts of greenhouse gases increase due to human activit ies, the basic radiat ion balance
is altered.
Because of the long life time in the atmosphere of many greenhouse gases such as carbon
dioxide, their ef fects impact on everyone in the world. Global pollution can only be countered
by global solut ions.
The follow ing sections w ill address the basic science of the greenhouse effect
• climate variability evidenced by past records
• sources and sinks of greenhouse gases
• the concept of radiative forcing and how it is used
• climate models and how well they simulate past and current climate
• projections of climate change over the 21st century
• impacts of climate change especially those on human communit ies
• internat ional policy and action regarding
Figure
A greenhouse has a similar ef fect to the atmosphere on the incoming solar radiat ion and
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Page 15
the emitted thermal radiation.
climate change, including the w ork of the IPCC
• stabilization of climate
• mitigat ion of climate change and implicat ions for technology and
• the future challenge
The enhanced greenhouse effect
After our excursion to Mars andVenus, let us return to Earth! To w hat extent are the
greenhouse gases in the Earth’s atmosphere influenced by human activity? The amount of
water vapour depends mostly on the temperature of the surface of the oceans; most of it
originates through evaporation from the ocean surface and is not inf luenced directly by
human activity. Carbon dioxide is dif ferent. Its amount has increased substantially—by over
30 per cent—since the Industrial Revolution, due to human industry and also because of the
removal of forests . Future projections are that, in the absence of controlling factors, its rate
of increase w ill accelerate and its atmospheric concentration w ill double from its pre-
industrial value w ithin the next hundred years .
This increased CO2 is leading to global warming of the Earth’s surface through its enhanced
greenhouse effect. Let us imagine, for instance, that the amount of CO2 in the atmosphere
suddenly doubled, everything else remaining the same . The solar radiat ion budget w ould
not be affected. But the thermal radiat ion emitted from CO2 in the atmosphere w ill originate
on average from a higher and colder level than before . The thermal radiation budget w ill,
therefore, be reduced, the amount of reduction being about 4Wm-2.To restore the radiat ion
balance the surface and lower atmosphere w ill warm. If nothing changes apart f rom their
temperature—in other w ords, clouds, w ater vapour, ice and snow cover and so on, are all
the same as before—a radiative transfer calculation indicates that the temperature change
would be about 1.2°C.
In reality, of course, many of these other factors w ill change, some of them in w ays that add
to the w arming (positive feedbacks), others in w ays that reduce the w arming (negative
feedbacks). The situation is, therefore, much more complicated than this simple calculat ion;
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it w ill be considered in more detail in section 6. Suff ice it to say here, that the best estimate,
at the present time, of the increased average temperature of the Earth’s surface if CO2
levels w ere to be doubled is about tw ice that of the simple calculation: 2.5°C. As the next
section w ill illustrate, for the global average temperature this is a large change.
Sea-level rise resulting from global w arming w ill, therefore, lag behind temperature change
at the surface. During the follow ing centuries, as the rest of the oceans gradually w arm, sea
level w ill continue to rise at about the same rate, even if the average temperature at the
surface were to be stabilized.
What about the major ice sheets; w ill their contribution continue to be small in the future?
For both ice-sheets there are tw o competing effects. In a w armer w orld, there is more w ater
vapour in the atmosphere that leads to more snowfall. But there is also more ablation
(erosion by melting) of the ice around the boundaries of the ice-sheets and calving of
icebergs during summer months. For Antarctica, the present estimates are that
accumulat ion is greater than ablation, leading to a small net grow th. How ever, it is possible
that larger changes in the ice sheets may begin to occur. The Greenland ice sheet is
probably the more vulnerable; its complete melting w ill cause a sea-level rise of about 7m.
Model studies of the ice sheet show that, w ith a rise in summer temperature in the region of
Greenland of more than 3°C—likely to be realized w ithin a few decades—ablation w ill
signif icantly overtake accumulation and melt down of the ice cap w ill begin. Such melt dow n
is likely to be irreversible. If the temperature continued to rise to say 8°C or more, much of
the melt down would occur during the next 1000 years. Turning to the Antarctic ice-sheet,
the part that is of most concern is in the w est of Antarctica (around 90°W longitude); its
disintegration w ould result in about 6m of sea-level rise. Because a large portion of it is
grounded well below sea level it has been suggested that rapid ice discharge could occur if
the surrounding ice shelves are weakened.
In the absence of such rapid change, about w hich studies at present are inconclusive [89],
the contribution of the West Antarctic Ice Sheet to sea-level rise over the next millennium w ill
be less than 3m.
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A rise in average sea level of 10 cm by 2030 and about half a metre by the end of the 21st
century may not seem a great deal. Many people live suff iciently above the level of high
water not to be directly affected. How ever, half of humanity inhabits the coastal zones
around the w orld. Within these, the lowest lying are some of the most fertile and densely
populated. To people living in these areas, even a fraction of a metre increase in sea level
can add enormously to their problems. Some of the areas that are especially vulnerable are
f irst, large river delta areas, for instance Bangladesh, second, areas very close to sea level
where sea defences are already in place, for instance the Netherlands and third, small low-
lying islands in the Pacific and other oceans. Here, w e just consider the example of
Bangladesh.
Bangladesh is a densely populated country of about 120 million people located in the
complex delta region of the Ganges, Brahmaputra and Meghna Rivers About 10% of the
country’s habitable land (w ith about 6 million populat ion) w ould be lost w ith half a metre of
sea-level rise and about 20% (w ith about 15 million populat ion) w ould be lost w ith a 1m rise.
Estimates of the sea-level rise are of about 1m by 2050 (compounded by 70 cm due to
subsidence because of land movements and removal of groundw ater and 30 cm from the
effects of global w arming) and nearly 2m by 2100 (1.2m due to subsidence and 70 cm from
global w arming)—although there is a large uncertainty in these estimates.
Further exacerbation of the impact w ill arise through the combinat ion of sea-level rise w ith
likely increases in the intensity of storm surges in that region. Further, increased salt w ater
intrusion into ground w ater w ill occur in many low lying regions. Similar situat ions to that in
Bangladesh exist in other parts of south-east Asia, the Nile delta region of Egypt and delta
regions in other parts of Africa and the Americas.
It is not only in places w here there is dense population that there w ill be adverse effects.
Theworld’s wetlands and mangrove swamps currently occupy an area of about a million
square kilometres, equal approximately to tw ice the area of France. They contain much
biodiversity and their biological productivity equals or exceeds that of any other natural or
agricultural system. Over tw o-thirds of the f ish caught for human consumption, as w ell as
many birds and animals, depend on coastal marshes and swamps for
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Land affected in Bangladesh by various amounts of sea-level rise.
part of their life cycles, so they are vital to the total world ecology. These areas could not
adjust to the rapid rate of sea-level rise that is likely and in many cases would be unable to
extend inland. Net loss of wetland area w ill therefore occur.
Fresh w ater resources
The global water cycle is a fundamental component of the climate system. Water is cycled
between the oceans, the atmosphere and the land surface. Water is also an essential
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resource for humans and for ecosytems. During the last 50 years w ater use has grown over
threefold it now amounts to about 10% of the estimated global total of river and groundw ater
f low from land to sea. Increasingly,w ater is used for irrigation. In India about 75% of
availablew ater is so used. Water from major rivers is of ten shared betw een nations; its
growing scarcity is a potentia l source of conf lict. In many areas, groundwater extraction
greatly exceeds its replenishment—a situation that cannot continue indef initely.
With global w arming, there w ill be substantial changes in w ater availability, quality and f low.
On average, some areas w ill become w etter and others drier. Substantial changes in
variations of f low during the year w ill also occur as glaciers and snow cover diminishes
leading to less spring melt. Much of these changes w ill exacerbate the current vulnerability
regarding water availability and use. Especially vulnerable w ill be continental areas w here
decreased summer rainfall and increased temperature result in a substantia l loss in soil
moisture and increased likelihood of drought.
Even greater impact is likely to occur because of increased frequency and intensity of
extremes, especially f loods and droughts. Such disasters are the most damaging disasters
the world experiences; on average they cause more deaths, misery and economic loss than
other disasters. They are especially damaging to developing countries w here, in general,
they are more likely to occur and where there is inadequate infrastructure to cope w ith them.
Impacts of climate change on fresh w ater resources are likely to be exacerbated by other
pressures, e.g. population grow th, land-use change, pollution and economic grow th.
Agriculture and food supply
Climate change w ould affect agriculture and food supply through its impact on crops, soils,
insects, weeds, diseases and livestock. Three factors are particularly important; changes in
water availability changes in temperature and the effect of increased CO2 on plant grow th.
Higher CO2 concentrations stimulate photosynthesis, enabling some plants (e.g. wheat, rice
and soya bean) to fix carbon at a higher rate. This is w hy in glasshouses additional CO2 may
be introduced artif icially to increase productivity. Under ideal conditions it can be a large
effect (for doubled CO2 up to an average of +30% [100]). However, under real conditions on
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the large scale, w here water and nutrient availability are also important factors, increases
tend to be substantially less than what is potentially possible. For instance, for cereal crops
in mid-latitudes, potent ial yields are projected to increase for small increases in temperature
(2–3°C) but decrease for larger temperature increases.
In a w orld inf luenced by global warming, crop patterns w ill change. But the changes w ill be
complex; economic and other factors w ill take their p lace alongside climate change in the
decision-making process. To estimate the effect of climate change on w orld food supply,
elaborate modelling studies have been carried out. These start w ith climate change
scenarios for different locations and times that are inserted into crop models that then
produce projected changes in crop yields. Included also are farm level adaptations (e.g.
planting date shif ts, more climatically adapted variet ies, irrigation and fertilizer application).
These yield changes are then employed as inputs to a world food trade model that includes
assumptions about global parameters, such as population grow th and economic factors.
The outputs from the total process provide information projected up to the 2080s on food
production, food prices and the number of people at risk of hunger.
Ecosystems
About 10% of the w orld’s land area is under cult ivation. The rest is to a greater or lesser
extent unmanaged by humans. Of this about 30% is natural forest. Within this area climate
is the dominant factor determining the distribution of biomes. Large changes in th is
distribution have occurred during the relatively slow climate changes in the past. It is the
very rapid rate of change of climate that w ill cause excessive stress on many systems. How
much it matters depends on the species and the degree of climate change (e.g. temperature
increase or water availability). Two particularly vulnerable types of species are trees and
coral. The viability of some large areas of tropical forests under climate change is of especial
concern. Many corals are already suffering from bleaching caused by increased ocean
temperatures. Further, as large quant ities of extra carbon dioxide are dissolved in the
oceans, their acidity increases posing a serious threat to living systems in the oceans
especially to corals.
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Human health
Human health w ill be affected by many of the impacts described in previous paragraphs
such as deteriorating w ater availability, food shortages and more intense and more frequent
f loods and droughts. Increased spread of insect-borne diseases, such as malaria, is also
likely in a warmer world. The main direct ef fect of climate change on humans themselves w ill
be that of heat stress in the extreme high temperatures that w ill become more frequent and
more w idespread especially in urban populat ions. Studies using data from large cities w here
heat
Distribut ion of average summer temperatures (June, July, August) in Sw itzerland from 1864
to 2003 show ing a f itted Gaussian probability distribut ion—standard deviation 0.94°C . The
2003 value is 5.4 standard deviations from the mean show ing it to be an extremely rare
event. Also shown are return periods calculated from conventional statistics assuming no
warming trend.
waves commonly occur show death rates that can be doubled or trip led during days of
unusually high temperatures. On the positive side, mortality due to per iods of severe cold in
w inter w ill be reduced.
An example of record extreme high temperatures is the heat w ave in Europe during June,
July and August, 2003. At many locations temperatures rose above 40°C. In France, Italy,
the Netherlands, Portugal and Spain over 20 000 additional deaths w ere attributed to the
unrelenting heat. The f igure illustrates the extreme rarity of this event. Studies show that it is
very likely that a large part of its cause is due to the increase in greenhouse gases, that by
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Page 22
2050 such a summer w ould be likely to be the norm and by 2100 would likely be a cool
summer.
Adaptation and mitigation
There are two kinds of action that can be taken—adaptation to reduce the impacts of climate
change as it occurs and mit igation to reduce emissions of greenhouse gases that in turn w ill
reduce the amount of climate change. Some of the impacts of anthropogenic climate change
are already becoming apparent and a degree of adaptation is already a necessity. Many
adaptation options have already been identif ied that can reduce the adverse impacts of
climate change and can also produce ancillary benefits, but they cannot prevent all
damages. Of particular importance is the requirement for adaptation to extreme events and
disasters such as floods, droughts and severe storms . Vulnerability to such events can be
substantially reduced by more adequate preparat ion.
It is associated w ith both the science and the impacts of climate change are considerable
uncertainties—. Politicians and others making decisions are, therefore, faced w ith the need
to w eigh all aspects of uncertainty against the desirability and the cost of the various actions
that can be taken in response to the threat of climate change.
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Page 23
Climate change—an integrat ing framew ork . A complete cycle of cause and effect is show n
beginning w ith economic activity (low er right-hand corner) that results in emissions of
greenhouse gases (of which CO2 is the most important) and aerosols. These emissions lead
to changes in atmospheric composition and hence to changes in climate that impact both
humans and natural ecosystems and affect human livelihood, health and development. An
anticlockw ise arrow represents other effects of development on human communities and
natural systems, for instance changes in land use that lead to deforestation and loss of
biodiversity.
Costing the im pacts
Probably the largest impact of climate change w ill be that of the increased number and
intensity of extreme events. We noted in section 3 the recent increase in extreme events
and the interest of insurance companies w ho have tracked increasing damage from them in
recent decades. Not that insured losses are a good guide to total loss. For instance, the
insured losses for Hurricane Mitch that hit Central A merica in 1998 were small. How ever,
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Page 24
9000 people died and the losses in Honduras and Nicaragua, respectively, amounted to
about 70% and 45% of their annual gross national product (GNP) . China is a country
particularly prone to natural d isasters; f rom 1989 to 1996 they resulted in an average annual
loss equivalent to near ly 4% of GDP.
International policy and action
As observational and modelling tools for studying the climate advanced during the 1970s
and 1980s, the attent ion of scientists became increasingly directed tow ards the effects on
the climate of human activit ies. A scientif ic conference in 1985 organized by the Scientif ic
Committee on Problems of the Environment (SCOPE) a committee of the International
Council of Scientific Unions led to an important publication that described the adverse
effects that could result from continued and increased anthropogenic emissions of CO2.
That in turn led to increasing awareness amongst polit icians of the scale of the potential
problem. Two important international bodies were created, one in 1988 concerned w ith
science (the IPCC) and one in 1992 w ith policy (the Framew ork Convention on Climate
Change (FCCC)). These w ill be introduced brief ly in turn.
The Intergovernmental Panel on Clim ate Change (IPC C)
The IPCC w as formed jointly by tw o United Nat ions bodies, the World Meteorological
Organization (WMO) and the United Nations Environment Programme (UNEP) w ith a remit
to prepare thorough assessments of climate change, its causes and effects. The Panel
established three working groups, one to deal w ith the science of climate change, one w ith
impacts and a third w ith policy responses. The IPCC has produced three main
comprehensive reports, in 1990, 1995 and 2001 together w ith a number of special reports
covering particular issues.