GEOMORPHOLOGY - …. The three concentric ... Earth's interior which in turn, ... Thus the seismic waves have divided the earth's structure as: S.No. Name of Chemical Average

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GEOMORPHOLOGY

INTERIOR STRUCTURE OF THE EARTH

a) Earth's Crust: All of the Earth's landforms(mountains, plains, and plateaus) arecontained within it, along with the oceans,seas, lakes and rivers. There are twodifferent types of crust: thin oceanic crustthat underlies the ocean basins and thickercontinental crust that underlies thecontinents. These two different types ofcrust are made up of different types of rock.The thin oceanic crust is composed ofprimarily of basalt and the thickercontinental crust is composed primarily ofgranite. The low density of the thickcontinental crust allows it to "float" in highrelief on the much higher density mantlebelow. The boundary between the crust andthe mantle is Mohorovicic Discontinuity.

While the crust appears to be solid, it issubjected to repeated movements includingbending, folding, and breaking associatedwith the movement of material in themantle below. The processes of weatheringand erosion are continually wearing thehigh points of crust away. The low pointsare being filled in with the debris generatedby these destructive processes.

The interior of the earth is not composed ofhomogeneous or uniform material. The structureof Earth can be visualized in two ways: chemicallyor by material properties. By comparing materialstrength, the layering of Earth is categorized aslithosphere, asthenosphere, upper mantle, lowermantle, outer core, and the inner core.

Chemically, a tripartite arrangement for theearth has been suggested by Austrian geologist,Suess. The three concentric shells advocated byhim on the basis of their density are outer Siallayer, inner Sima layer and the innermost nickel-iron core of Nife. The scientific study and analysisof various seismic waves of natural and maninduced earthquakes unrevealed the mystery ofearth's interior and divided the earth in threedifferent layers as explained below.

Thus, the earth's interior has three differentlayers; they are (i) the crust (ii) mantle and (iii)the core. These layers are distinguished on thebasis of their (i) Physical and chemical properties,(ii) Thickness, (iii) density (iv) Temperature, (v)Metallic content and (vi) rocks. The geologiccomponent layers of Earth are at the followingdepths below the surface:

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b) Earth's Mantle: It is the thick, dense rockymatter that surrounds the core with a radiusof about 2885 km. The mantle covers themajority of the Earth's volume. This isbasically composed of silicate rock rich iniron and magnesium. The mantle is lessdense than the core but denser than theouter crust layer.

It has different temperatures at differentdepths. The temperature is lowestimmediately beneath the crust and increaseswith depth. The highest temperatures occurwhere the mantle material is in contact withthe heat-producing core. This steadyincrease of temperature with depth isknown as the geothermal gradient.

The geothermal gradient is responsible fordifferent rock behaviors which divide themantle into two different zones. Rocks inthe upper mantle are cool and brittle, whilerocks in the lower mantle are hot and soft(but not molten). Rocks in the upper mantleare brittle enough to break under stress andproduces earthquakes. However, rocks inthe lower mantle are soft and flow whensubjected to forces instead of breaking. Thelower limit of brittle behavior is theboundary between the upper and lowermantle. This layer is separated from the coreby Gutenberg-Wiechert Discontinuity. Theouter and the inner mantle are separatedby another discontinuity named Repettidiscontinuity.

c) Earth's Core: Earth's Core is thought to becomposed mainly of an iron and nickelalloy. This composition is based oncalculations of its density. The core is earth'ssource of internal heat because it containsradioactive materials which release heat asthey break down into more stablesubstances.

The core is divided into two different zones.The outer core is a liquid because thetemperatures there are adequate to melt theiron-nickel alloy. However, the inner coreis a solid even though its temperature ishigher than the outer core. Here,tremendous pressure, produced by theweight of the overlying rocks is strongenough to crowd the atoms tightly togetherand prevents changing it to the liquid state.

The nature and properties of the composition ofthe interior of the earth may be successfullyobtained on the basis of the study of variousaspects of seismic waves mainly the velocity andthe travel paths of these waves while passingthrough a homogenous solid body but thesewaves are reflected and refracted while passingthrough a body having heterogeneouscomposition and varying density zones. If theearth would have been composed of homogenoussolid materials the seismic waves should havereached the core of earth in a straight path butthis is not the case in reality. In fact the recordedseismic waves denote the fact that these wavesseldom follow straight path rather they adoptcurved and refracted paths. Thus it becomesobvious that the earth is not composed ofhomogenous materials. This has helped thescientists in discovering the layered structure ofthe Earth.

TYPES OF SEISMIC WAVES

There are two types of seismic waves, bodywave and surface waves.

• Body waves travel through the interior ofthe Earth. They follow ray paths refractedby the varying density and stiffness of theEarth's interior which in turn, varyaccording to temperature, composition, andphase.

Body waves are divided as

P-WAVES (Primary Waves) are compressionwaves that are longitudinal in nature. Thesewaves can travel through any type of material,and can travel at nearly twice the speed of Swaves. In air, they take the form of sound waves;hence they travel at the speed of sound. Typicalspeeds are 330 m/s in air, 1450 m/s in water andabout 5000 m/s in granite.


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YS-WAVES (Secondary Waves) are shear

waves that are transverse in nature. These wavestypically follow P waves during an earthquakeand displace the ground perpendicular to thedirection of propagation. S waves can travel onlythrough solids, as fluids (liquids and gases) donot support shear stresses. S waves are slowerthan P waves, and speeds are typically around60% of that of P waves in any given material.

• Surface waves are analogous to water wavesand travel along the Earth's surface. Theytravel slower than body waves. Because of theirlow frequency, long duration, and largeamplitude, they can be the most destructive typeof seismic wave.

There are two types of surface waves:

Rayleigh waves, also called ground roll, aresurface waves that travel as ripples with motionsthat are similar to those of waves on the surfaceof water. The existence of these waves waspredicted by John William Strutt, Lord Rayleigh,in 1885. They are slower than body waves.

Love waves are surface waves that cause

circular shearing of the ground. They are namedafter A.E.H. Love, a British mathematician whocreated a mathematical model of the waves in1911. They usually travel slightly faster thanRayleigh waves, about 90% of the S wave velocity,and have the largest amplitude.

Seismic velocities tend to gradually increasewith depth in the mantle due to the increasingpressure, and therefore density, with depth.However, seismic waves recorded at distancescorresponding to depths of around 100 km to 250km arrive later than expected indicating a zoneof low seismic wave velocity. Furthermore, whileboth the P and S waves travel more slowly, the Swaves are attenuated or weakened. This isinterpreted to be a zone that is partially molten.This zone is called the asthenosphere or "weaksphere."

The asthenosphere separates the strong, solidrock of the uppermost mantle and crust abovefrom the remainder of the strong, solid mantlebelow. The combination of uppermost mantleand crust above the asthenosphere is called thelithosphere. The lithosphere is free to move

Thus the seismic waves have divided the earth's structure as:

S.No. Name of Chemical Average Density Physical Propertiesthe layer Composition Thickness (g cm-3)

(km)A.(i) Crust Sial 6 to 45 2.2 to 2.9 Solid part of

lithosphere; partlymolten under thecontinents.

(ii) Inner part of Outer silicate 45 to 100 The solid crust andlithosphere layer, Basaltic upper mantle

B.(i) Asthenosphere 50 to 400 It transmits both S-and P-wave but withreduced velocities.

(ii) Upper Mantle Sima (Peridotite 100 to 1700 3.1 to 4.75 Slightly solid and(mainly under iron-magnesium- slightly plasticoceans) rick silicatre rock) material close to

melting point.

(iii) Lower Mantle Sima (Olivine- 1700 to 2900 4.75 to 5.6 Transition zone ofUltrabasic rocks) mixed metals and

silicate

C.(i) Outer core Nife 2900 to 4980 9.9 - 12.3 Liquid or in a plasticstate. Fe, Ni and Smixture.

(ii) Inner core Barysphere (heavy 4980 to 6400 13.5 Iron and nickel. Solidmetallic rocks) and rigid due to

tremendous overlyingpressure.


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(glide) over the weak asthenosphere. The tectonicplates are, in fact, lithospheric plates.

Seismic waves recorded at increasingdistances from an earthquake indicate thatseismic velocities gradually increase with depthin the mantle (exceptions: at Low Velocity Zoneand at discontinuities). However, at arc distancesof between about 103° and 143° no P waves arerecorded. Furthermore, no S waves are recordbeyond about 103°. Gutenberg (1914) explained

this as the result of a molten core beginning at adepth of around 2900 km. Shear waves could notpenetrate this molten layer and P waves wouldbe severely slowed and refracted (bent).

Between 143° and 180° from an earthquakeanother refraction is recognized (Lehman, 1936)resulting from a sudden increase in P wavevelocities at a depth of 5150 km. This velocityincrease is consistent with a change from a moltenouter core to a solid inner core.

CONTINENTAL DRIFT THEORY

Evidences Supporting the Theory of ContinentalDrift

a) The Fit of Continental Coastlines -Wegener used the jigsaw puzzle fit betweenthe South American and African coastlinesas his first piece of evidence to supportcontinental drift. He didn't believe that these

"pieces" would be so well matched-that is,not unless they had actually once beenconnected.

b) Similar Mountain Ranges and RockSequences - Explorers quickly discoveredthat distant continents contained rock ofsimilar ages and features. These findings

In 1915, the German geologist andmeteorologist Alfred Wegener first proposed thetheory of continental drift, which states that partsof the Earth's crust slowly drift atop a liquid core.

Wegener hypothesized that there was agigantic supercontinent 200 million years ago,

which he named Pangaea, meaning "All-earth".Pangaea started to break up into two smallersupercontinents, called Laurasia andGondwanaland, during the Jurassic period. By theend of the Cretaceous period, the continents wereseparating into land masses that look like ourmodern-day continents.


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seemed to show that the continents maynot have always been separated as they arenow. For example, this is what they foundwhen they looked at the AppalachianMountain Range of North America:

• This range stretches northward fromthe eastern United States up into theAtlantic provinces of eastern Canada.There, it seems to suddenly stop at theisland of Newfoundland.

• Very similar mountain ranges of thesame age and rock-type also appear ineastern Greenland, Ireland, GreatBritain, and Norway. When theselandmasses are placed together, themountains form a single long range.

c) Fossil Evidence - A fossil is any evidenceof ancient life. In the beginning of the 20hCentury, fossil evidence was also found tosupport continental drift. Identical fossilizedplant and animal species have been foundin many different places, on differentcontinents. It seems hard to believe that

such similar organisms would exist so faraway from each other, or that they couldhave swam from one continent to another.It is more likely that these life forms oncelived all together on a single continent, asshown in the following image.

The theory was rejected on following grounds:

a) First, it had been shown that floating masseson a rotating geoid would collect at theequator, and stay there. Thus mountainbuilding process could not be explainedusing this theory.

b) Second, masses floating freely in a fluidsubstratum, like icebergs in the ocean,should be in isostatic equilibrium (wherethe forces of gravity and buoyancy are inbalance). Gravitational measurementsshowed that many areas are not in isostaticequilibrium.

c) Third, there was the problem of why some partsof the Earth's surface (crust) should havesolidified while other parts were still fluid.

SEA-FLOOR SPREADING

The concept of sea floor spreading was firstpropounded by Harry Hess. According to him themid oceanic ridges were situated on the risingthermal convection currents coming up from themantle. The oceanic crust moves in opposite

directions from mid oceanic ridges and thus there iscontinuous upwelling of lavas along the ridge. Thesemolten lavas cool down and solidify to form newcrust along the trailing ends of divergent boundary.Thus there is a continuous creation of new crust.


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Sea floor spreading and magnetic bands

In 1963, Fred Vine, Drummond Matthews,and others found that the crust surrounding themid ocean ridges showed alternating bands --each band magnetized with a polarity oppositeto the surrounding bands. They suggested thatas new sea-floor crust was formed around the riftin the mid ocean ridge, it magnetized differently,depending upon the polarity of the planet at thattime. This supported the theory that Harry Hess

had put forth, that the ocean progressively widensas new sea floor is created along a crack thatfollows the crest of mid-ocean ridges.

Sea Floor spreading is still active in manyparts of the world's oceans. This can be observedfrom under sea vents and trenches. As the seafloor pushes the continents apart the rest of theplates it sink beneath continental plates. Thiscreates deep sea trenches.

PLATE TECTONICS

The theory states that Earth's outermost layer,the lithosphere, is broken into 7 large, rigid piecescalled plates: the African, North American, SouthAmerican, Eurasian, Australian, Antarctic, andPacific plates. Several minor plates also exist,including the Arabian, Nazca, and Philippinesplates.

The plates are all moving in differentdirections and at different speeds (from 2 cm to10 cm per year--about the speed at which ourfingernails grow) in relationship to each other.The place where the two plates meet is called aplate boundary. Boundaries have different namesdepending on how the two plates are moving inrelationship to each other

These plates lie atop a layer of partly moltenrock called the asthenosphere. The plates cancarry both continents and oceans, or exclusivelyone or the other. The Pacific Plate, for example, isentirely oceanic.

Continental plates are composed mainly ofgranite, while oceanic plates are mostly basalt,which is considerably heavier. Essentially, thecontinents are lighter and more buoyant; hence,they float higher on the earth's mantle than theocean's crust does.

A. Convergent boundary: convergentboundaries are the places where plates crashor crunch together. When plates converge,one slips under the other and is said to besubducted. At depths from 185 to 435 milesbeneath the earth's surface, the subductedparts of the plate melt and become part ofthe molten mantle. As new plate materialis being formed continuously, and theexcess is melted into magma, the earth'srocky crust is constantly recycled.

Oceanic Plate vs. Oceanic Plate Convergence

The older of the two plates descends into thesubduction zone when plates of oceaniclithosphere collide along a trench. The descendingplate carries water-filled sediments from theocean floor downward into the mantle. Thepresence of water alters the physical and chemicalconditions necessary for melting and causesmagma to form. The magma rises up through theoverriding oceanic plate, reaching the surface asa volcano. As the volcano grows, it may rise abovesea level to form an island.

Oceanic Plate vs. Continental Plate Convergence

When oceanic lithosphere collides withcontinental lithosphere, the oceanic plate willdescend into the subduction zone. Oceaniclithosphere is denser than continental lithosphereand is therefore consumed preferentially.Continental lithosphere is almost never destroyedin subduction zones. The Nazca Plate dives belowSouth America in a subduction zone that liesalong the western margin of the continent.

Continental Plate vs. Continental PlateConvergence

The tallest mountains in the world Himalayaswere formed (and continue to grow) as a resultof continental collision. Continental lithosphereis relatively light and is deformed adjacent tosubduction zones rather than consumed.

B. Divergent boundary: Places where platesare moving apart are called divergentboundaries also called as spreading centres.When the Earth's brittle surface layer (thelithosphere) is pulled apart, it typicallybreaks along parallel faults that tilt slightlyoutward from each other. As the plates


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separate along the boundary, the blockbetween the faults, cracks and drops downinto the soft, plastic interior (theasthenosphere). The sinking of the blockforms a central valley called a rift. Magma(liquid rock) seeps upward to fill the cracks.In this way, new crust is formed along theboundary. Earthquakes occur along thefaults, and volcanoes are formed where themagma reaches the surface.

Where a divergent boundary crosses theland, the rift valleys get formed which aretypically 30 to 50 kilometers wide. Examplesinclude the East Africa rift in Kenya andEthiopia, and the Rio Grande rift in NewMexico. Oceanic ridges rise a kilometer orso above the ocean floor and form a globalnetwork tens of thousands of miles long.Examples include the Mid-Atlantic ridgeand the East Pacific Rise. Plate separationis a slow process, eg. Divergence along theMid Atlantic ridge causes the AtlanticOcean to widen at only about 2 centimetersper year.

C. Transform boundary: Places where platesslide past each other are called transformboundaries. The plates on either side ofa transform boundary are merely sliding

past each other and not tearing or crunchingeach other.The most famous transform boundary in theworld is the San Andreas Fault. Althoughtransform boundaries are not marked byspectacular surface features, their slidingmotion causes lots of earthquakes. Thestrongest and most famous earthquake alongthe San Andreas Fault hit San Francisco in1906. Many buildings were finished by thequake, and maximum of the city wasdestroyed by the fires that followed.

Salient points:

• The plate boundaries are mainlyrepresented by oceanic ridges and trenches.

• Interactions at plate boundaries causevolcanic activity and earthquakes

• The plates are in motion, moving away fromridges and toward trenches.

• Plates descend into the mantle belowtrenches in subduction zones.

• Plates typically contain both oceanic andcontinental lithosphere.

• Oceanic lithosphere is continually createdand destroyed.

• Continental lithosphere cannot be destroyedbut continents can be subdivided andassembled into supercontinents.


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The earth is composed of various kinds ofelements. These elements are in solid form in theouter layer of the earth and in hot and moltenform in the interior.

About 98 per cent of the total crust of the earthis composed of eight elements like oxygen, silicon,aluminium, iron, calcium, sodium, potassium andmagnesium, and the rest is constituted bytitanium, hydrogen, phosphorous, manganese,sulphur, carbon, nickel and other.

Rocks are naturally formed and are simplycomposed of crystals or particles of one or moreminerals. There are many kinds of rock, and theycan be classified in a number of ways. However,geologists classify rocks based on how the rockswere formed. The three classes are igneous rocks(formed directly from liquid rock), metamorphicrocks (formed by direct alteration of existingrocks), and sedimentary rocks (formed by erodedmaterials from other rocks).

i. Igneous Rocks

Igneous rocks solidify from a liquid magmaas it cools. They are described on two axies: 1)Rocks that are quartz rich (felsic) and magnesiumrich (mafic) and 2) fast cooling (small crystals) andslow cooling (large crystals).

When magma cools rapidly, mineral crystalsdo not have time to grow very large. On the otherhand when magma cools slowly crystals grow toseveral millimeters or more in size. Granite andbasalt are opposites on these axies for thedescription of igneous rocks - granite is a slowcooled quartz rich (felsic) rock, basalt a rapidlycooled magnesium rich (mafic) rock.

Igneous rocks are classified as

a) Extrusive Rocks

Extrusive igneous rocks solidify from moltenmaterial that flows over the earth’s surface (lava).Extrusive igneous rocks typically have a fine-grained texture (individual minerals are notvisible unless magnified) because the lava coolsrapidly when exposed to the atmosphere,preventing crystal growth. Common extrusiverocks are basalt, andesite, and rhyolite.

• Basalt: Basalt is characteristically a dense,black, massive rock, high in calcium and

iron-magnesium- bearing minerals and lowin quartz content. Great examples of basalticlava flows can be found in the deccan regionof India.

• Andesite: Andesite has higher quartzcontent than basalt and is usually lighter incolor. Crystals of the minerals amphibole,biotite, and feldspar are sometimes visiblewithout magnification. It is found innorthwestern portion of Rajmahal traps inIndia.

• Rhyolite: Rhyolite is typically a fine-grained, white, pink, or gray rock, high inquartz and feldspar content with someamphibole and biotite. A well-knownexample is a rhyolite rock exposure in ahillock of 120 metres (394 ft) height,originally called the "Mountain of Birds",forms the foundation for the imposingMehrangarh Fort in Jodhpur.

a) Intrusive Rocks

Intrusive rocks form from molten material(magma) that flows and solidifies underground.These rocks usually have a coarse texture(individual minerals are visible withoutmagnification), because the magma cools slowlyunderground, allowing crystal growth. Commonrock types within the intrusive category aregranite and diorite.

• Granite: Granite is the intrusive equivalentof rhyolite but has a coarser texture.Splendid black and multicolour varieties ofgranite are available in the states ofKarnataka, Andhra Pradesh, Tamil Naduand Uttar Pradesh. Granite deposits are alsowidespread over the provinces of Rajasthan,Bihar, West Bengal, and Gujarat. India isthe largest exporter of granite and graniteproducts in the world.

• Diorite: Diorite has the same texture asgranite but has the mineral composition ofan andesite, which is diorite’s extrusiveequivalent. Diorite is a relatively rare rock;source localities include Sondrio, Italy;Thuringia and Saxony in Germany; Finland;Romania; Northeastern Turkey; centralSweden; Scotland; the Darrans range ofNew Zealand, etc.

ROCKS AND MINERALS


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ii. Sedimentary Rocks

Sedimentary rocks are types of rocks createdfrom deposition of layers upon layers ofsediments over time. These types of rocks areformed on the Earth's surface, as well asunderwater. Wherever sedimentation goes on,sedimentary rocks are formed over time. Thesediments that compose these rocks may be oforganic, chemical or mineral origin.

Although sedimentary rocks constitute only5% of the total crust volume, they extensivelycover most continental surfaces. Most of thenatural energy resources like coal and the fossilfuels are contained within the layers ofsedimentary rock.

Types of Sedimentary Rock

Depending on the nature of sediments thatcreate them, sedimentary rocks can be classifiedinto three prime types:

• Clastic: The clastic sedimentary rocks arecomposed of broken down fragments ofminerals and remnants of weathered rocks.One example of this type of sedimentaryrock is sandstone. They are mostly madeup of quartz, amphiboles, feldspar, clayminerals and other minerals derived fromweathering of igneous and metamorphicrocks.

• Biochemical: The deposition of thebiological remains of various organisms cancreate certain types of sedimentary rock.Some examples are limestone (from the

deposition of calcium carbonate containingremains of corals, foraminifera andmollusks), stromatolites (formed fromdeposition of microorganism biofilms ofblue green algae), oil shale and coal shale.

• Chemical Precipitate: Sediments formedfrom deposition of chemical reactionprecipitates of mineral solutions are calledchemical sedimentary rocks. Chemicalsedimentary rock formation occurs whenwater dissolves many minerals and depositsthem on evaporation. Examples of this typeof sedimentary rock are gypsum, barite,rock salt and sylvite.

Process of Sedimentary Rock Formation

In the process of sedimentary rock formation,there are some basic steps, like weathering,erosion, transportation and deposition.

Weathering

“Weathering is the first step responsible forformation of sedimentary rocks. It involvesmechanical and chemical breakdown of rocks,minerals and soil under the influence ofatmospheric conditions. Elements like water, ice,heat and pressure altogether affect the rates ofweathering. The weathered rock, soil and mineralparticles are then carried away to other places bymeans of erosion.

Erosion

“Erosion is defined as the process of breakingdown larger objects into smaller ones and


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transporting them. Take the example of a newlyformed mountain, which appears tall and steep.After being exposed to the effects of wind, water,glacier and rain for several years, its height isreduced and the shape becomes rounded. This isbrought about by erosion. In the meantime, theeroded rock and mineral particles are carriedaway by natural agents to several places.

Sediment Transportation

The smaller particles generated by erosion arecollectively known as sediments. Ultimately, mostof the broken rocks, minerals and soil particlesare transported by water and wind to other places.Besides these, dead plants and organisms are alsocarried in the flow of wind, water and ice. Theyare also used in the formation of sediment rocks.

Deposition of Sediments

Another step in sedimentary rockformation is deposition of the sediments by waterand other transporting agents. Particles that flowdownstream in water bodies like river, streamsand lakes settle down at the bottom, because ofthe influence of gravity. In due course of time,layers of sediments are piled up one after theother, either in the earth's surface or under waterbodies. This settling down of particles at thebottom of a water body is called sedimentation.

During the process of sedimentation, thebottom layers are pressurized by the above layers,resulting in compaction and formation ofsedimentary rocks. The layers of sediments arecalled strata or beds. During this process, deadplants and animals also get buried in between thebeds, which become fossils afterward.

This is the conclusive stage of sedimentaryrock formation.

Some common sedimentary rocks are shale,sandstone, limestone, and conglomerate.

Shale: Shale is a fine-grained sedimentaryrock that forms from the compaction of silt andclay-size mineral particles that we commonly call"mud". Shales are typically deposited in very slowmoving water and are often found in lakes andlagoonal deposits, in river deltas, on floodplainsand offshore from beach sands. They can also bedeposited on the continental shelf, in relativelydeep, quiet water. This process could have takenmillions of years to complete.

Sandstone: Sandstone is composed ofcemented sand grains. Sandstone reserves inIndia are spread over the states of AndhraPradesh, Assam, Bihar, Gujarat, Haryana,Madhya Pradesh, Meghalaya, Mizoram,Karnataka, Orissa, Punjab, Rajasthan, UttarPradesh, Tamil Nadu and West Bengal. Over 90%of the deposits of sandstone are in Rajasthan,spread over the districts of Bharatpur, Dholpur,Kota, Jodhpur, Sawai-Madhopur, Bundi,Chittorgarh, Bikaner, Jhalawar, Pali, andJaisalmer.

Limestone: Limestone is composed of morethan 50% calcium carbonate (calcite). Theremainder of the rock may contain fine rockfragments, clay, quartz, and seashells. AndhraPradesh has the privilege of possessing about 32%of the country's total reserves of limestone.Limestones are extensively utilised formanufacturing of cement and also buildingstones, particularly flooring and roofing.

Conglomerate: Conglomerate is well-rounded gravel in a matrix of sand, clay, andnatural cementing agents. Two of the manyconglomerates in Utah are the Price RiverFormation visible along Highway 6 betweenThistle and Soldier Summit, Utah County, andthe Shinarump Conglomerate Member of theChinle Formation exposed along the central partof the Burr Trail, east of Boulder, Garfield County.

iii. Metamorphic Rocks

Metamorphic rocks are any rock type that hasbeen altered by heat, pressure, and/or thechemical action of fluids and gases. The meaningof the term 'metamorphic', in fact, is 'changed'.When igneous rocks, or sedimentary rocks, oreven metamorphic rocks get buried very deepunder the earth's surface, a process that takesmillions of years, they get changed into something


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else by the enormous pressure and heat insidethe earth.

“Some examples of metamorphic rocks are:

• Limestone being changed into marble

• Shale turning into slate

• Granite being changed into gneiss

• Sandstone turning into quartzite

Metamorphic rocks are classified by theirstructure and their dominant minerals.Metamorphic rock structure is either foliated (hasa definite planar structure) or non-foliated(massive, without structure).

Foliated Metamorphic Rocks

Common foliated metamorphic rocks areslate, schist, and gneiss.

Slate: This metamorphic rock is fine-grained,containing perfect cleavage, which enables it tobe split into fine sheets. Slate generally containsdark to light brown streaks. Slate is formed bycomparatively low pressures and temperatures,which is referred to as low-grade metamorphism.Not being very hard, slate can be engraved quiteeasily. It has been used in various ways over theyears, for example as grave markers orheadstones. However, one of the problems withslate is the perfect cleavage it has, which resultedin grave stones splitting and cracking along thecleavage lines. It is also commonly used forchalkboards. These days, due to its cracking andsplitting, and its weight, slate is not used much.

Schist: This is a metamorphic rock that isclassified as medium grade, which means it hasbeen formed by more pressure and heat comparedto slate. This rock is coarse grained with theindividual grains of the minerals that it is madeup of being visible to the naked eye. Most of theoriginal minerals are transformed into flakes, andsince it has experienced far more pressure, it isusually found crumpled or folded. Usually,

schists are named according to the main mineralthey have been formed from. For example: talcschist, garnet mica schist, hornblende schist, andbitotite mica schist.

Gneiss: This metamorphic rock is classifiedas high grade, which means that compared toschist it has been subjected to more pressure andheat. Gneiss is distinctly banded and is coarserthan schist. The banding comprises alternatinglayers, which are made up of different minerals.One of the most important minerals that gneissis made up of is feldspar, along with quartz andmica. Gneiss can form from the metamorphosisof sedimentary rock like shale or sandstone, orfrom igneous rock like granite. Gneiss is used asa building stone and for paving.

Non-foliated Metamorphic Rocks

Common non-foliated metamorphic rocksare quartzite and marble.

Quartzite: Quartzite is typically ametamorphosed form of sandstone. Unweatheredquartzite has a “sugary” looking surface.Individual quartz grains are deformed,interlocked, and fused together. When the rockbreaks, it typically breaks through the grains.Some quartzite formations retain their originalbedded (layered) structure such that when brokenthey form flagstones that are commonly used inlandscaping or as veneer for buildings.

Marble: This is formed due to themetamorphosis of dolomite or limestone. Bothdolomite and limestone contain calciumcarbonate in large concentrations. Compared toits parent rock, marble is much harder. Thisenables it to be polished, which is why it is sucha popular material used in buildings, such asbathtubs, floor tiling, sink tops, kitchen countertops, and so on. Artists also use it as a carvingmaterial. Marble is made up of various sizedcrystals and also has many variances in colorbecause of the impurities that are present duringthe formation. Hence, marble can be white, gray,black, red, green, pink, banded and mottled.

THE ROCK CYCLE

The rock cycle consists of a series of constantprocesses through which earth materials changefrom one form to another over time. As withinthe water cycle and the carbon cycle, some

processes in the rock cycle occur over millions ofyears and others occur much more rapidly. Thereis no real beginning or end to the rock cycle, butit is convenient to begin exploring it with magma.


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Magma, or molten rock, forms only at certainlocations within the earth, mostly along plateboundaries. When magma is allowed to cool, itcrystallizes. Rocks that form from cooled magmaare called igneous rocks; intrusive igneous rocksif they cool below the surface (like gabbro),extrusive igneous rocks if they cool above (likebasalt).

Rocks like basalt are immediately exposed tothe atmosphere and weather. Rocks that formbelow the earth's surface, like gabbro, must beuplifted and all of the overlying material must beremoved through erosion in order for them to beexposed. In either case, as soon as rocks areexposed at the earth's surface, the weatheringprocess begins. Physical and chemical reactionscaused by interaction with air, water, andbiological organisms cause the rocks to breakdown. Once rocks are broken down, wind,moving water, and glaciers carry pieces of therocks away through a process called erosion.

Moving water is the most common agent oferosion, all of these rivers carry tons of sedimentweathered and eroded from the mountains oftheir headwaters to the ocean every year. Thesediment carried by these rivers is deposited andcontinually buried in floodplains and deltas.

Under natural conditions, the pressurecreated by the weight of the younger depositscompact the older, buried sediments. Asgroundwater moves through these sediments,minerals like calcite and silica precipitate out ofthe water and coat the sediment grains. Theseprecipitants fill in the pore spaces between grainsand act as cement, gluing individual grainstogether. The compaction and cementation of

sediments creates sedimentary rocks likesandstone and shale.

If sedimentary rocks or intrusive igneous rocksare not brought to the earth's surface by upliftand erosion, they may experience even deeperburial and be exposed to high temperatures andpressures. As a result, the rocks begin to change.Rocks that have changed below the earth's surfacedue to exposure to heat, pressure, and hot fluidsare called metamorphic rocks.

Some of the processes within the rock cycle, likevolcanic eruptions, happen very rapidly, whileothers happen very slowly, like the uplift ofmountain ranges and weathering of igneousrocks. Importantly, there are multiple pathwaysthrough the rock cycle. Any kind of rock can beuplifted and exposed to weathering and erosion;any kind of rock can be buried andmetamorphosed. As Hutton correctly theorized,these processes have been occurring for millionsand billions of years to create the earth as we seeit: a dynamic planet.

ENDO-GENETIC FORCES

The forces coming from within the earth arecalled as endogenetic forces which cause twotypes of movements in the earth, viz, (i)Horizontal movements, and (ii) Verticalmovements.

These movements motored by theendogenetic forces introduce various types ofvertical irregularities which give birth tonumerous varieties of relief features on the earth'ssurface, eg., mountains, plateaus, plains, lakes,faults, folds, etc.

This energy is mostly generated by

radioactivity, rotational and tidal friction andprimordial heat from the origin of the earth. Thisenergy due to geothermal gradients and heat flowfrom within induces diastrophism and volcanismin the lithosphere. Due to variations in geothermalgradients and heat flow from within, crustalthickness and strength, the action of endogenicforces are not uniform and hence the tectonicallycontrolled original crustal surface is uneven.

The endogenetic forces and movements aredivided, on the basis of intensity, into two majorcategories:


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a) Sudden forcesb) Diastrophic forces

Sudden forces are the result of long periodpreparation deep within the earth. Only theircumulative effects on the earth's surface are quickand sudden. Geologically, these sudden forces aretermed as 'constructive forces' because thesecreate certain relief features on the earth's surface.

Diastrophic forces include both vertical andhorizontal movements which are caused due toforces deep within the earth. These diastrophicforces operate very slowly and their effectsbecome discernable after thousands and millionsof years. These forces also termed as constructiveforces, affect larger areas of the globe and Producemeso-level reliefs, for example, mountains,plateau, plains, lakes, big faults, etc.

They include: (i) orogenic processes involvingmountain building through severe folding andaffecting long and narrow belts of the earth'scrust; (ii) epeirogenic processes involving upliftor warping of large parts of the earth's crust; (iii)earthquakes involving local relatively minormovements; (iv) plate tectonics involvinghorizontal movements of crustal plates.

(A) Epeirogenetic movements: Epeirogeneticword consists of two words, viz: 'epiros'(meaning thereby continent) and 'genesis'(meaning thereby original). Epeirogeneticmovement causes upliftment andsubsidences of continental masses throughupward movements are infact, verticalmovements. These forces and resultantmovements affect larger parts of thecontinents. These are further divided intotwo types: upward movement anddownward movement.

(B) Orogenetic movement: The wordorogenetic has been derived from two Greekwords, 'pros' (meaning thereby mountain)and 'genesis' (meaning thereby origin orformation). Orogenetic movement is causeddue to endogenetic forces working inhorizontal movements. Horizontal forcesand movements are also called as tangentialforces.

Orogenetic or horizontal forces work in twoways, namely, (i) in opposite direction, and (ii)towards each other. When it operates in oppositedirections it is called as 'tensional force'. Such

types of force and movement are also called asdivergent forces. Thus, tensional forces createrupture, cracks, fracture and faults in the crustalparts of the earth.

The force that operates face to face is calledcompression force or convergent force.Compressional force causes crustal bendingleading to the formation of folds or crustalwarping leading to local rise or subsidence ofcrustal parts.

Through the processes of orogeny,epeirogeny, earthquakes and plate tectonics, therecan be faulting and fracturing of the crust. Allthese processes cause pressure, volume andtemperature (PVT) changes which in turn inducemetamorphism of rocks.

Folds

A layered rock that exhibits bends is said tobe folded. The layered rock was at one timeuniformly straight but was stressed to develop aseries of arches and troughs. A compressive stresscompact horizontal rock layers and forces themto bend vertically, forming fold patterns. Foldsin rocks vary in size from microscopic crinkles tomountain-sized folds. They occur singly asisolated folds and in extensive fold trains ofdifferent sizes, on a variety of scales.

Folds form under varied conditions of stress,hydrostatic pressure, pore pressure, andtemperature - hydrothermal gradient, asevidenced by their presence in soft sediments, thefull spectrum of metamorphic rocks, and even asprimary flow structures in some igneous rocks.A set of folds distributed on a regional scaleconstitutes a fold belt, a common feature oforogenic zones.

Anticlines and Synclines

An anticline is a fold that is arched upwardto form a ridge; a syncline is a fold that archesdownward to form a trough. Anticlines andsynclines are usually made up of many rock unitsthat are folded in the same pattern. The tip of afold is called the nose. The center axis of a fold iscalled the hinge line and lies in the axial planethat separates the rocks on one side of the foldfrom the rocks on the other side that dip in theopposite direction. Extensive folding isrepresented by a repeated pattern of anticlinesand synclines. Two anticlines are always


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separated by a syncline, and two synclines arealways separated by an anticline. One side of thefold is called the limb; a side-by-side syncline andanticline share a limb. Frequently, an anticline orsyncline can be identified only from thesystematic change in the dips of the sloping rockunits from one direction to the other, identifyingthe hinge line of the fold.

Plunging folds

Plunging folds have been tipped by tectonicforces and have a hinge line not horizontal in theaxial plane. The angle between the horizontal andthe hinge line is called the plunge and, like dip,varies from less than 1 degree to 90 degrees.Plunging folds characteristically show a series ofV patterns on a bedrock surface.

Open, isoclinals, overturned, and recumbent folds

A variety of kinds of folds generally reflectsincreasing amounts of tectonic stress.

Open folds are those in which the anglebetween the two limbs of the fold is more than

900 but less than 1800.

Closed folds are those in which the anglebetween the two limbs of a fold is acute angle.

Symmetrical folds are simple folds in whichboth the limbs incline uniformly.

Monoclinal folds are those in which one limbinclines moderately with regular slope while theother limb inclines steeply at right angle and theslope is almost vertical.

Isoclinal folds are those in which thecompressive forces are so strong that both thelimbs of the fold become parallel but nothorizontal.

Recumbent folds are formed when thecompressive forces are so strong that both thelimbs of the fold become parallel as well ashorizontal.

Overturned folds are those folds in which onelimb of the fold is thrust upon another fold dueto intense compressive forces. Limbs are seldomhorizontal.

Faults

A fault is a fracture or zone of fracturesbetween two blocks of rock. Faults allow theblocks to move relative to each other. Thismovement may occur rapidly, in the form of anearthquake - or may occur slowly, in the form ofcreep. Faults may range in length from a fewmillimeters to thousands of kilometers. Mostfaults produce repeated displacements over


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geologic time. During an earthquake, the rock onone side of the fault suddenly slips with respectto the other. The fault surface can be horizontalor vertical or some arbitrary angle in between.

The parts of a fault are (1) the fault plane, (2)the fault trace, (3) the hanging wall and (4) thefootwall. The fault plane is where the action is. Itis a flat surface that may be vertical or sloping.The line it makes on the Earth's surface is the faulttrace. Where the fault plane is sloping, the upperside is the hanging wall and the lower side is thefootwall. When the fault plane is vertical, there isno hanging wall or footwall.

Any fault plane can be completely describedwith two measurements: its strike and its dip. Thestrike is the direction of the fault trace on theEarth's surface. The dip is the measurement ofhow steeply the fault plane slopes-if you droppeda marble on the fault plane, it would roll exactlydown the direction of dip.

Faults are distinguished on the basis of themovement of the footwall relative to the hangingwall.

� Normal-fault

A normal fault is a dip-slip fault in which theblock above the fault has moved downwardrelative to the block below. This type of faultingoccurs in response to extension and is oftenobserved in the Western United States Basin andRange Province and along oceanic ridge systems.

� Thrust fault

A thrust fault is a dip-slip fault in which theupper block, above the fault plane, moves up andover the lower block. This type of faulting iscommon in areas of compression, such as regionswhere one plate is being subducted under anotheras in Japan and along the Washington coast.

When the dip angle is shallow, a reverse fault isoften described as a thrust fault.

� Strike-slip fault

A strike-slip fault is a fault on which the twoblocks slide past one another. These faults areidentified as either right-lateral or left lateraldepending on whether the displacement of thefar block is to the right or the left when viewedfrom either side. The San Andreas Fault inCalifornia is an example of a right lateral fault.Strike-slip faults are either right-lateral or left-lateral. That means someone standing near thefault trace and looking across it would see the farside move to the right or to the left, respectively.

In reality, many faults show a combination ofdip-slip and strike-slip motion. Geologists usemore sophisticated measurements to analyzethese fault movements.

Landforms formed by faulting

1. Block Mountains

Block mountains form when the layers of theEarth's crust are forced upward near fault lines.

Fault-block mountains have a steep, sharp frontside and a gentler, sloping back. Examples ofblock mountains are the Black Forest of Germany,the Alps in Europe and the Urals. California'sSierra Nevada is a 350-mile long block of granitecontaining Mt. Whitney, the highest mountainpeak in the lower 48 states. This granite mountainwas lifted during the formation of the Basin andRange Province, centered in Nevada andstretching from southern Oregon to west Texas.

2. Rift Valley

When a depression appears between twoblock mountains, that depression is called a rift


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valley, which can be thousands of miles long. Inthe United States, one example of these flat-bottomed valleys is Death Valley in California.Some of the largest rift valleys include the EastAfrican Rift Valley, Russia's Baikal Valley,Germany's Rhine Valley and the Red Sea. Oceanrift valleys occur where tectonic plates on theseafloor spread apart. The largest lakes in theworld are all found in rift valleys.

EXO-GENETIC FORCES

weathering and erosion removes overlying rocks,the pressure is released and the underlying rockexpands. This expansion results in the fracturingof the rock, which weakens it by making itsusceptible to other weathering agents. If cracksdevelop parallel to the surface, sheeting of rocklayers may occur.

� Thermal Expansion (insolation weathering)

This process results from large diurnaltemperature ranges which result in heating andcooling of the rock. When heated, expansion ofthe rock occurs, whilst during cooling the rockcontracts. This expansion and contraction duringcycles of temperature change results in stressesin the rock layers. Outer layers of rock heat andcool quicker than inner layers and over time theupper layers flake /peel off (exfoliation). It shouldbe noted that the effectiveness of this process isheavily debated and some believe that it is onlyreally effective when water is also present.

� Salt Crystallisation

Water passing through crevases and joints inrocks, may be saline (carrying salts in solution).As the water evaporates, the dissolved saltsprecipitate and crystalise froming salt crystals.This may also take place where in rocks such aschalk, the rock is decomposed by solution to formsalt solutions such as sodium carbonate whichwill then crystalise upon evaporation of themosture. The salts may expand up to 3 times theiroriginal size, and therefore the crystals putstresses upon the rock as they grow, resulting ingranular disintegration (gradually breaking offindividual grains of rock).

b) Chemical Weathering

Chemical weathering is where rocks aredecomposed by chemical reaction between

Exogenetic forces also called as denudationalprocesses. These forces are engaged in destructionof the relief features by the process of weathering,erosion etc.

Weathering

It is "the breakdown and decay of rocks in-situ related to elements of the weather (e.g.temperature, rainfall, frost etc.

Weathering is the first stage in the denudationof the landscape. Rocks are weakened andloosened by weathering processes. Thisweakened material is then removed by agents oferosion (e.g. ice, water etc.)

Weathering can be classified as:

a) Physical Weathering

Physical weathering is also known asmechanical weathering and it involves thephysical breakdown of rock - it does not involvechemical change.

� Freeze Thaw

This is the breakdown of rocks due to theexpansion of water during freezing, a processcommon in upland Britain where eveningtemperatures often fluctuate around 0°C. Freezethaw is most effective in jointed rock (e.g. granite).During freezing, water expands by 9% in volume.Water freezing in cracks in rocks, exerts pressure.Alternating freeze-thaw cycles gradually force therock to split or cause rock fragments to break off.Where this process occurs on steep slopes, rockfragments collect at the base of the slope due togravity in the form of a scree slope.

� Pressure Release (also known as dilation)

Rocks such as granite, formed as Igneousintrusions are formed under pressure. When


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elements of the weather and rock minerals,resulting in either the alteration of a rock's internalmineral structure or the formation of newminerals (e.g. feldspar forming Kaolin in theprocess of hydrolysis). Weakened rock or theconsequent deposits are then more easilyremoved by erosion processes. Water plays a keyrole in most chemical reactions and also providesa transport mechanism for other elements thatcarry out weathering. Chemical weathering ismost dominant in hot and humid areas such asequatorial zones and least effective where thereis little rain such as in desert or polar regions(where most water is held as ice). Thesusceptibility of rocks to chemical weathering isdetermined by the types of minerals they containsand their mineral structure. There are a numberof different types of chemical weathering.

� Oxidation

The exposure of rocks to oxygen in air orwater can result in a reaction between the oxygenand iron-based minerals in the rocks. Iron readilyoxidises and during oxidation, blue grey ferrousiron (Fe2+) is transformed to red ferric iron (Fe3+).This causes a weakening of the rock structureenabling them to crumble easily and making themmore susceptible to other weathering processes.

� Carbonation

Rainwater contains dissolved CO2 whichforms a weak carbonic acid (H2O + CO2 = H2CO3).Carbonic acid is able to react with calciumcarbonate (common in rocks such as limestoneand chalk) to form calcium bicarbonate which isthen easily removed in solution in water.Limestone is gradually dissolved in this way asthe calcium carbonate is converted to calciumbicarbonate and carried away in solution byrunning water.

� Solution

Water can act as a solvent by breaking downchemical bonds in minerals causing them todissolve in a process known as solution -carbonation is therefore a form of solutionalthough it is mineral specific in relation tocalcium carbonate. Solution rates tend to increasewith an increased acidity of water.

� Hydrolysis

This is where acidic water reacts with rockforming minerals such as feldspar. This is a

common process in the weathering of granite.Hydrogen ions in the water displace potassiumions in the feldspar. This causes the feldspar tobreak down into a secondary mineral, Kaolin(China Clay). Whilst the feldspar in granitedecomposes, the quarz and mica remain relativelyunaffected but the structure weakened.

� Hydration

This occurs as the addition of water causesminerals in rock to swell (by about 0.5%) due to achemical reaction as the mineral absorbs water('hydrates'), thus involving both chemicaland physical (mechanical weathering). Gypsumis formed when water combines with anhydriteCalcium sulphate and water (CaSO4 (anhydrite)+ 2H2O (water) = CaSO4.2H2O (gypsum)).Gypsum is fairly soluble and can then be fairlyeasily removed by solution.

c) Biological Weathering

Usually consists of a combination of physical(growth of roots into joints in rocks) and chemical(e.g. impact of organic acids) processes.

� Tree Roots

As roots of plants and trees grow downwards,they often enter and exploit cracks joints in rock.As they grow they are able to gradually wedgethe joints further apart, eventually resulting indetatchment of rock fragments (simillar to freeze-thaw)

� Organic Acids

As roots as well as surface litter decayse,organic acids are released into the ground.Percolating rainwater, moves these acides furtherdown and the organic acids may react withminerals in the rock through a process calledchelation. The combination of rainwater andorganic acids combines with aluminium and ironwhich are washed out of the soil.

Respiration of bacteria and tree roots alsoreleases CO2 which when becomes dissolved inwater forms a weak carbonic acid which canincrease the chemical weathering process,carbonation.

� Animal Activity

Burrowing animals help to open up joints inrock and also help to bring rock fragments to thesurface, where they are exposed to further


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weathering. At the coast, animals such as limpetsincrease the rate of chemical weathering throughthe acids secreted as they cling to rock surfaces.

EROSION

Water Erosion

Water is the most important erosional agentand erodes most commonly as running water instreams. However, water in all its forms iserosional. Raindrops (especially in dryenvironments) create splash erosion that movestiny particles of soil. Water collecting on thesurface of the soil collects as it moves towardstiny rivulets and streams and creates sheeterosion.

In streams, water is a very powerful erosionalagent. The faster water moves in streams thelarger objects it can pick up and transport. This isknown as critical erosion velocity. Fine sand canbe moved by streams flowing as slowly as three-quarters of a mile per hour.

Streams erode their banks in three differentways: 1) the hydraulic action of the water itselfmoves the sediments, 2) water acts to corrodesediments by removing ions and dissolving them,and 3) particles in the water strike bedrock anderode it.

The water of streams can erode in threedifferent places: 1) lateral erosion erodes thesediment on the sides of the stream channel, 2)down cutting erodes the stream bed deeper, and3) headward erosion erodes the channel upslope.

Wind Erosion

Erosion by wind is known as aeolian (or eolian)erosion (named after Aeolus, the Greek god ofwinds) and occurs almost always in deserts.Aeolian erosion of sand in the desert is partiallyresponsible for the formation of sand dunes. Thepower of the wind erodes rock and sand.

Ice Erosion

The erosive power of moving ice is actually abit greater than the power of water but since wateris much more common, it is responsible for agreater amount of erosion on the earth's surface.

Glaciers can perform to erosive functions -they pluck and abrade. Plucking takes place bywater entering cracks under the glacier, freezing,and breaking off pieces of rock that are thentransported by the glacier. Abrasion cuts into therock under the glacier, scooping rock up like abulldozer and smoothing and polishing the rocksurface.

Wave Erosion

Waves in oceans and other large bodies ofwater produce coastal erosion. The power ofoceanic waves is awesome, large storm waves canproduce 2000 pounds of pressure per square foot.The pure energy of waves along with the chemicalcontent of the water is what erodes the rock ofthe coastline. Erosion of sand is much easier forthe waves and sometimes, there's an annual cyclewhere sand is removed from a beach during oneseason, only to be returned by waves in another.

PLAINS

Plains are flat and broad land areas on theearth's surface, e.g. prairies, steppes, Great Plainsof India etc. The elevations of these landforms arerelatively low, when measured with reference tothe mean sea level.

Plant life on plains is controlled by the climate.Thick forests usually thrive on plains in humidclimates; grasslands cover fairly dry plains suchas the Great Plains in the United States. Plains areusually well populated because the soil and terrainare good for farming, and roads and railways areeasily built between rural towns and cities.

Origin and development of plains

� Plains are formed primarily by erosion and

the deposition of sediment. Erosion is thegradual wearing away of Earth surfacesthrough the action of wind and water.Sediment is rock debris such as clay, silt,sand, and gravel (or even larger material)that is being carried from its place of originor has already been deposited on Earth'ssurface by wind, water, or ice eg. Ganga-Yamuna plains of India.

� Sometimes coastal lands are submerged underthe sea water because of transgressional phaseof sea. Such coastal lands submerged undershallow water receive sediments regularly andcontinuous sedimentation leads to formationof coastal plains.


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� Deposition of enormous volume of lavasover extensive areas forms volcanic plains.

� Submerged coastal lands emerge as marinecoastal plains due to withdrawl of sea waterduring regressional phase of sea.

Types of plains

1. Erosional Plains

Erosional plains are developments on theEarth's surface caused by natural weathering ofglacier activity, wind movement or water (sea,river & stream) torrent and are subdivided on thebasis of the type of erosional agent. Here we willdiscuss the glacial plain, wind-eroded plain &alluvial plain. Erosional plains are likely todevelop on landscapes that are relatively flat withlow height elevations and shallow depressions.Some plains may develop below sea level but nothigher than the surrounding region; this is a resultof endogenetic factors and diastrophicmovements.

Types of Erosional Plains

a) Alluvial Plain

The alluvial plain is an erosional plain thatoccurs from weathering caused by water currentsin the sea, river or stream. Fluvial (water)movement comes from higher land regions andwear away landmasses to produce low reliefplains. This is what is known as the alluvial plain.These landforms are made up of the depositionof sediment over a long period of time from thefluvial movement to form alluvial soil. An alluvialplain is characterized by its relatively flat andgently sloping landform and is normally formedat the base of a range of hills. Continuous fluvialweathering of these hills is what causes sedimentsto move and spread across lower levels to producethis type of plain. These plains are formed mostlyby slow running rivers, as slower fluvialmovement picks up less sediment off the riverfloor causing more particles to settle and developinto an alluvial plain. Areas where more particlesare dropped off are sometimes referred to as floodplains, and the particles that settle are calledalluvium.

b) Wind-Eroded Plain

Wind-eroded plains develop in dry landexpanses. These formations are created bycontinuous abrasive wind activity wearing down

land formations by a grinding action and asandblasting of wind-borne particles to shape anddisfigure landscape surfaces. Wind-eroded plainsare found in arid regions with large supplies ofunconsolidated sediment. Vegetation is sparse, asa result of such severe wind erosion. These plainsare usually infertile since the most fertile parts ofthe soil are removed by abrasive wind activity.Soil productivity and maintenance is poor, whichimpacts seedling survival and growth andultimately the survival of any type of plant life.

c) Glacial Plain

These are structures formed by the movementof ice from highland areas to low land levels. Slowand continuous glacial movement takes placeover time to cause sizable erosive work to producea glacial plain. There are two types of glacialstructures - true glacial plains, formed of pureglacial material and outwash plains, formed fromdeposition of materials such as sand, marl, gravel,silt & clay after the ablation (melting) of glaciersand ice sheets. Marl deposits from a glacial plainare often used to make cement and are greatfertilizers as well. Outwash glacial plains oftenform an alluvial plain when the ice has melted.As ablation occurs, the water rises and movesaway from the glacial plain and takes with it fine,eroded sediment. As the speed of the waterdecreases, so does its capacity to carry objects insuspension. The water then gradually depositsthe sediment across the landscape and creates analluvial plain.

Location of Erosional Plains

Erosional Alluvial Plain - These are formedall over the globe from the effects of vast watermovement. Prominent alluvial plains are theMississippi Delta, Plain of Lombardi (Italy),Yangtze Plain (China), Indus Plain (Ganga),Amazon Plain (Brazil) and Mekong Plain(Cambodia).

Wind-Eroded Plain - These are veryuncommon and take the longest to form as theyexist in dry regions lacking vegetation. Wind-eroded plains can be found in Libya - TheHamada Red Plain (Sahara Desert) and India -Aravalli Range (Jaisalmer -Thar Desert).

Glacial Erosional Plain - These are found incolder regions with temperatures slightly aboveand below zero degrees Celsius. Actual glacial


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plains exist in Sweden, Finland, Ladakh, ImphalBasin and Canada.

2. Depositional Plains - Deposition is carriedon by rivers, glaciers, winds, sea waves, etc.Plains result as a product of deposition.Types of depositional plains are:

a) Plains of Fluvial Deposition- The Riversstart deposition as a result of the decreasein the speed of rivers and the volume ofwater. An accumulation of sediment alsocontributes to deposition. There are threeareas of deposition-the floor, the mouth andthe valley of the river where the slopesuddenly decreases. The following plainsare described as belonging to this type:

� A flood plain is the floor of a rivervalley beyond the riverbed. A floodplain is formed of mud, sand, and siltthat are left behind when the riveroverflows its banks. These materials arecarried off by the river as it erodes theland upstream. A river in floodconditions can carry a large amount oferoded material, which the overflowwaters deposit onto the flood plain.

� Piedmont alluvial plains are formed atthe foothill zones of the mountains.

� Delta plains are formed by riversthrough gradual deposition of sedimentswhile entering the seas and oceans.

b) Plains of Glacial Deposition - Many plainsare formed due to glacial deposition. Theseplains have great importance. These plainsare found in North America and Europe in

areas, which were affected by glacial action.Till covers mainly the plains. The size ofthe till particles varies from fine particles toboulders. Though a lot of till is local yeterratics are also common. The base rock oftill have been eroded by ice. The surface islightly undulating and has low and broadridges and depressions. Drumlins, Eskers,Moraines, etc., are a common feature ofthese plains.

c) The Desert Plains - The plains where sandaccumulates in large quantity are desertplains and are developed by wind action.The wind produces sand by sand-abrasionof sandstones. The sand does not blowmuch far away from its place butaccumulates in various form. The maincharacteristics of these plains depend uponthe accumulation of sand, the strength ofthe winds, the persistence of the directionof winds and vegetational cover. A fewexamples of such plains are the Sahara ofAfrica, the Koum of Russian Turkistan, thenorth-central Nebraska, etc.

d) Lava plains - These are formed due todeposition of thin sheets of lavas coming outthrough fissure flows. These are found inIceland, USA, Deccan plateau of India etc.

e) Lacustrine plains - These are formed whenthe lakes are filled with sediments eitherdue to filling of lakes by the sedimentsbrought by the rivers or by the upliftmentof the beds of lakes due to diastrophicmovements caused by endogeneticforces.

A plateau is a large highland area of fairlylevel land separated from surrounding land bysteep slopes. Some plateaus, like the plateau ofTibet, i.e. between mountain ranges. Others arehigher than surrounding land. Plateaus arewidespread, and together with enclosed basinsthey cover about 45 percent of the Earth's landsurface. Some plateaus, such as the Deccan ofIndia and the Columbia Plateau of the UnitedStates, are basaltic and were formed as the resultof many lava flows covering hundreds ofthousands of square kilometers that built up the

land surface. Others are the result of upwardfolding; still others have been left elevated by theerosion of nearby lands. Plateaus, like all elevatedregions, are subject to erosion, which removesgreat amounts of the upland surface. Lowplateaus are often farming regions, while highplateaus are usually suitable for livestock grazing.Many of the world's high plateaus are deserts.Other plateaus are the Colorado Plateau of theUnited States, the Bolivian plateau in SouthAmerica, and the plateaus of Anatolia, Arabia,Iran, and the Tibet region of China and the

PLATEAUS


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Canadian Shield or Laurentian Plateau, a U-shaped region of ancient rock, the nucleus ofNorth America, stretching north from the GreatLakes to the Arctic Ocean. Covering more thanhalf of Canada, it also includes most of Greenlandand extends into the United States as theAdirondack Mountains and the SuperiorHighlands. The first part of North America to beelevated above sea level, it has stayed almostwholly untouched by many encroachments of thesea upon the continent.

Classification based upon the methods ofFormation - The plateaus are formed by manymethods. The emission from the earth, waterglacier, wind, etc., form plateaus.

� Plateaus formed by Lava - Lava comes outof the surface of the earth through zones ofweakness. This lava spreads in thesurroundings areas and forms plateau. Thesouthern plateau of India is an extensivelava plateau. Its area is 13 million sq. km.The plateau of Columbia has an area of250,000 sq. km.

� Plateaus formed by Running Water - HighMountains are eroded down by rivers.Plateau is formed as a landform in the lastpart of the cycle of erosion. Endogeneticforce makes peneplain and plateaus. Brazilis one such a Plateau.

� Plateaus formed by Glaciers - Glaciers alsodo erosional and depositional work. Hence,glaciers form mountains in two ways:- (a)by deposition and (b) by erosion. TheRussian plateau and the plateau of Finlandare examples of plateaus formed bydeposition. Among the plateaus formed byerosion are the plateaus of Greenland andAntarctica..

� Plateaus formed by Wind - When windsblow from a desert in a certain direction,fine dust particles along with the windsreach far-off places. For example, the westernwinds coming from the Gobi desert have builtup the loess plateau of China. The Potwarplateau of the district of Rawalpindi in Pakistanwas built up in this way.

The Classification of Plateau according toSituation - The plateaus are also classifiedaccording to their situations. Some plateaus aresurrounded by mountains, others piedmont andcontinental.

� Intermontanne Plateau - These plateausextend along with mountains and areknown as the highest plateaus of the world.The plateaus of Bolivia and Tibet belong tothis type. Other similar examples are theplateaus of Peru and of Mexico.

� Piedmont Plateaus - These plateausextend along with mountains. One sideof the plateaus is mountain while theother side has a plain or a sea. It appearsthat these plateaus rose with themountains under pressure of endogeneticforces. Examples of such plateaus are thoseof Colorado (N. America), Patagonia (SouthAmerica), etc.

� Continental Plateaus - These plateaus areaway from the mountains. They riseabruptly from the plains of seas. Somescholars think that these plateaus are createdby the rise of a plain area under endogeneticforces. Sometimes the emission of lava andits spread close by form these plateaus. TheDeccan Plateau of India, the Arab plateau,the plateaus of Spain and Australia are someof the examples of this type of plateaus.

MOUNTAINS

A mountain is defined as "a natural elevationof the earth surface rising more or less abruptly fromthe surrounding level and attaining an altitudewhich, relative to the adjacent elevation, isimpressive or notable". Mountains can be classifiedon the basis of their structure or their origin.

Mountains may have several forms. Amountain ridge is a system of long, narrow andhigh hills. A mountain range is a system ofmountains and hills having several ridges, peaks,

summits and valleys. A mountain chain consistsof several parallel long and narrow mountains ofdifferent periods. A mountain system consists ofdifferent mountain ranges of the same period. Amountain group consists of several unsystematicpatterns of different mountain systems. Cordilleraconsists of several mountain groups and systems.

The different mountain types are formed indifferent ways, through tectonic plates crunchinginto each other, or sliding past one another, or


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even from magma coming up out of the Earth.The mountains are different in their appearance,and in their formation.

� Fold Mountains

The most common type of mountain in theworld are called fold mountains. When you seevast mountain ranges stretching on for thousandsof kilometers, those are fold mountains. Foldmountains are formed when two of the Earth'stectonic plates collide head on; like two carscrashing together. The edges of each tectonic platecrumple and buckle, and these create themountains. Some examples of fold mountainranges include the Rocky Mountains in NorthAmerica, and the Himalayan Mountains in Asia.

� Fault-Block Mountains

Fault-block mountains (or just "blockmountain") are created when faults or cracksin the Earth's crust force materials upward.So instead of folding, like the plate collisionwe get with fold mountains, block mountainsbreak up into chunks and move up or down.Fault-block mountains usually have a steepfront side and then a sloping back side.Examples of fault-block mountains include theSierra Nevada mountains.

� Dome Mountains

Dome mountains are created when a largeamount of magma pushes up from below theEarth's crust, but it never actually reaches thesurface and erupts. And then, before it can erupt,the source of the magma goes away and thepushed up rock cools and hardens into a domeshape. Since the dome is higher than itssurroundings, erosion works from the topcreating a circular mountain range.

� Volcanic Mountains

Here's a fairly familiar kind of mountain.Volcanic mountains are created when magmafrom beneath the Earth makes its way to thesurface. When does get the surface, themagma erupts as lava, ash, rock and volcanicgases. This material builds up around thevolcanic vent, building up a mountain. Someof the largest mountains in the world werecreated this way, including Mauna Loa andMauna Kea on the Big Island of Hawaii. Other

familiar volcanoes are Mt. Fuji in Japan and Mt.Rainier in the US.

� Plateau Mountains

Plateau mountains are actually formed by theEarth's internal activity; instead, they're revealedby erosion. They're created when running watercarves deep channels into a region, creatingmountains. Over billions of years, the rivers cancut deep into a plateau and make tall mountains.Plateau mountains are usually found near foldedmountains.

Plate tectonics and formation of foldedmountains

When two continents carried on convergingplates ram into each other, they crumple and foldunder the enormous pressure, creating greatmountain ranges.

The highest mountain range in the world, thesnow-capped Himalayas, is an example of acontinent-to-continent collision. This immensemountain range began to form when two largelandmasses, India and Eurasia, driven by tectonicplate movement, collided. Because bothlandmasses have about the same rock density, oneplate could not be subducted under the other. Thepressure of the colliding plates could only berelieved by thrusting skyward. The folding,bending, and twisting of the the collision zoneformed the jagged Himalayan peaks.

About 220 million years ago, India was anisland situated off the Australian coast, andseparated from the Asian continent by a vastocean called the Tethys Sea. When Pangaea brokeapart about 200 million years ago, India began tomove northward. Scientists have been able toreconstruct India's northward journey. When


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India rammed into Asia about 50 million years ago,its northward advance slowed. The collision anddecrease in the rate of plate movement mark thebeginning of the Himalayan uplift.

When two oceanic plates collide thesubducted part melts and the magma rises abovethe oceanic surface volcanic islands are formedin arc form like Aleutian Islands, Kuril Islands,and Ryukyu Islands. Near the arcs trenches are

also formed like Mariana, Mindanao, etc.

When the oceanic lithosphere subductsbeneath the continental plates the down wentplate melts and produces magma. The magmabeing less dense than the surrounding risesslowly and emerges as intrusive igneous rock inthe form of volcanic mountain range on thecontinental crust. And, at the edge of continentalcrust trench is formed.

LAKES

Lakes are inland bodies of water found innatural depressions surrounded by higherground. Lakes are extremely varied in terms oforigin, occurrence, size, shape, depth, waterchemistry, and other features. Lakes can be onlya few hectares in surface area (i.e., less than asquare kilometer), or they can be thousands ofsquare kilometers. Their average depth can rangefrom a few meters to more than a thousandmeters. Lakes can be nearly uniformly round, orthey can be irregularly shaped. Their water canbe highly acidic (as in some caldera lakes), nearlyneutral, or highly alkaline (as in soda lakes). Lakescan be low in nutrients (oligotrophic), moderatelyenriched (mesotrophic), or highly enriched(eutrophic). Lakes may be fresh-water or salt-water (saline).

The different types of lakes are described below:

Glacial Lakes: By far the most importantagents in the formation of lakes are thecatastrophic effects of glacial ice movements thatoccurred 10,000 to 12,000 years ago. Giganticsheets of ice and snow are created in climateswhere snow falls but does not meltAlthough theseglaciers did eventually melt, ten percent of theearth is presently covered with glaciers. Some ofthese glaciers can still be seen in the mountainousareas of the world.

As a glacier moves back and forth across theland, scraping off the tops of hills and bluffs andtaking rocks with it, lakes are formed. Thematerial picked up by the glacier is later droppedoff at other sites. This back and forth and stopand go movement of the glaciers permanentlyalters the landscape. This movement createsseveral important landforms. When the glacierstops, it leaves behind piles of rocks and materialsthat it carried over time, called moraines. These

dam up rivers and smaller streams to form lakes.Sometimes, huge blocks of ice are broken off andcovered by sand and gravel. When the ice melts,the sand and gravel cave in, leaving a large holebehind. These kettles may form large marshes orlakes. As the large mass of ice melts, rivers formbeneath the glaciers.

Solution Lakes: Lakes can form whenunderground deposits of soluble rocks aredissolved by water running through the area,making a depression in the ground. Rockformations made of sodium chloride (salt), orcalcium carbonate (limestone), are most likely tobe dissolved by acidic waters. Once thegroundwater has dissolved the rocks below thesurface, the top of the land caves in, usuallyforming a round-shaped lake, called a solutionlake. Typically, the depressions are deep enoughto extend below the groundwater table and arepermanently filled with water. Solution lakes arecommon in Michigan, Indiana, Kentucky andparticularly in Florida.

Oxbow Lakes: The flow of water from rivershas a great deal of energy and erosive strengththat may create lake basins. As a river winds overthe earth's surface, a greater amount of erosionoccurs on the outer river bend, where the flow ofwater is the fastest. Materials carried by the riverare deposited on the inner portion of the bend,where currents are reduced. As time passes,erosion continues and more materials are left offuntil the U-shaped meander of the river closesin. The main course of the river cuts a new channelto the inner end of the meander. Oxbow lakes areusually shaped like the letter C.

Man-made or Animal-made Lakes: Manysmall lakes in India have been formed by theactivities of the dam construction. Sticks, aquaticplants and mud are used to build dams across


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small streams to form an impoundment of thewater. These ponds are usually very shallow andare rich in nutrients and plant life. Humans haveconstructed artificial lakes (reservoirs) to supplydrinking water to the public, to provide power,to aid in navigation, to provide flood control andfor recreational purposes. These reservoirs areusually well engineered by humans to hold backa certain quantity of water with the use of dams.

Volcanic Lakes: Sometimes, disastrous eventsassociated with volcanic activity form lake basins.The formation of volcanic lakes can occur indifferent ways. As volcanic material, includingmagma, is discharged out of the volcano, emptydepressions or cavities are formed within thevolcano. Some of these depressions cannot drainand become sealed holes on top of the volcano.Rainfall and runoff eventually fill the depressionwith water and a new lake is formed.

Lakes that form in the craters of volcanoes, orcrater lakes, are more common in areas that aresubject to volcanic activity. Lakes formed by thecaving in of a roof of a partially empty magmaticchamber are termed calderas. One of the mostspectacular lakes formed in this way is CraterLake in Oregon. Volcanic basins, like Crater Lake,are usually very round in shape.

Lava flows from volcanic activity can alsoform lakes. The surface lava cools, and becomessolid, while the inside of the lava flow remainshot enough to continue moving. Eventually, thesurface of the hardened lava collapses, forming adepression. These depressions eventually fill withwater to form smaller lakes. Lavastreams also flow into existing rivervalleys and solidify into a dam. Thissolid mass of rock backs up the riverwater into a new lake.

Landslide lakes: Large quantities ofmaterials that fall from the sides of steepvalleys into the floors of stream valleyscan cause dams that create new lakes.Such landslides usually occur as a resultof abnormal meteorological events, suchas excessive rains acting on an unstableslope. Landslide dams may be a resultof rockfalls, mudflows or even iceslides.Lakes that are formed by landslides areusually only temporary because theymay be susceptible to erosion by the flowof the river or stream. If the dam is verylarge, the lake may become permanent.

Tectonic lakes: Tectonic basins aredepressions formed by the movements of theearth's crust deep underground. The major typesof tectonic basins are formed from faulting. Adepression forms when a weak section of theearth's crust separates, resulting in an earthquake.Rainfall and groundwater may collect in thisdepression, forming a lake. This type of basin isreferred to as a graben and is the mode of originof a large number of the most spectacular reliclakes in the world containing a vast number ofnative plant and animal species. The deepest lakein the world, Lake Baikal in Siberia, was formedfrom tectonic activity.

The formation of a lake and the structure andform, or morphology, of the lake basin affects howthe lake functions throughout its life stages.Characteristics like the lake length, width, depth,area and volume are all important to how the lakewater quality may be affected by changes to theland. As humans develop the land surroundingthe lake, they disturb the soils, exchange trees fordriveways or rooftops and replace the naturalvegetation. These changes result in an increasedflow of surface runoff and an increase in theamount of nutrients to the lake. The lake structuredictates how the lake will react to these culturalchanges in the surround lands. Knowing the lakemorphology and how the lake was formed areimportant tools used by scientists to help protectour lakes from pollutants that can deteriorate theirhealth.


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Isostasy is essentially the principle ofhydrostatic equilibrium applied to the Earth. Inits simplest form, it considers rigid blocks of theEarth (usually taken to be the crust) to bebuoyantly supported in an underlying fluidmedium (usually taken as the mantle) and free tomove vertically. These blocks will then move untiltheir weight is exactly balanced by theirbuoyancy, at which point they are said to be 'inisostatic equilibrium'.

The concept was originated by Frenchsurveyors in the eighteenth century workingaround the Andes, who noted that the observedgravitational attraction of the mountains was lessthan that predicted. They inferred the presenceof a low-density 'root' that balances the excessweight of the mountain range and also reducesits gravitational attraction. The theory was furtherdeveloped by the English geodesists Pratt andAiry in the nineteenth century, who has giventheir names to two forms of the theory.

PRATT'S MODEL

Pratt hypothesized that elevation is inverselyproportional to density. Therefore, the higher themountain, the lower is its density (i.e., light rocks"float" higher).

AIRY'S MODEL

Airy hypothesized that mountains have

"roots" which extend down into the mantle.Therefore, elevation is proportional to the depthof the underlying "root".

The main difference between the two modelsis that in the Airy model all blocks have the samedensities but different thicknesses, whereas in thePratt model all blocks have different densities butfloat to the same depth. Generally speakingthough, the Airy model is used for continentaltopography, especially mountain ranges; and thePratt model is used for mid-ocean ridges.Continental mountain ranges have thick crustalroots which are more easily explained using theAiry model. In the Airy model, the elevation isproportional to this root. The higher the elevationis, the thicker the block and root.

At mid ocean ridges the topography issupported by density changes. This is due toincreased temperature at the ridges which causesthe rocks to expand resulting in a lower density.According to the Pratt model, all blocks float atthe same depth. This depth is where theasthenosphere begins. The difference inelevations is due to the density of the rocks.Higher elevations indicate lower density rocksand this higher ground means the lithosphere isthicker. The compensation depth is always thesame in the Pratt model. The height equation isthe same for both models.

Isostatic effects of sedimentation and erosion

Where sedimentation occurs, the weight ofthe sediment may cause the crust below to sink.Similarly, where erosion occurs the crust mayrebound.

Likewise, when ice sheets form the crust maysink. Conversely, when they melt the crust mayrebound, such as what is happening around theBaltic Sea and Hudson Bay area of Canada.Unfortunately, climate change makes thingsworse --- global warming makes the oceanswarmer, which raises the sea level due to thermalexpansion. Furthermore, the oceans are receivingmore and more water from melting glaciers. Bothfactors cause the sea level to rise.

Isostatic forces are thus of major importancein controlling the topography of the Earth'ssurface.

ISOSTASY


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An earthquake is a sudden movement of theEarth, caused by the abrupt release of strain thathas accumulated over a long time. For hundredsof millions of years, the forces of plate tectonicshave shaped the Earth as the huge plates that formthe Earth's surface slowly move over, under, andpast each other. Sometimes the movement isgradual. At other times, the plates are lockedtogether, unable to release the accumulatingenergy. When the accumulated energy growsstrong enough, the plates break free.

The magnitude or intensity of energy releasedby an earthquake is measured on the richter scale.The place of the origin of an earthquake is calledfocus which is hidden inside the earth. The placeon the ground surface which is perpendicular tothe buried focus is called 'epicentre'. Seismicwaves are recorded by an instrument called'seismograph'.

Earthquakes can be caused by a variety ofthings, including meteor impacts and volcaniceruptions, and even sometimes man-made eventslike mine collapses and underground nucleartests. But most naturally occurring earthquakesare caused by movement of pieces of the earth'ssurface, which are called tectonic plates.

Different theories that explain the origin ofearthquakes

a) Faulting and Elastic Rebound Theory:

The horizontal and vertical movementscaused by endogenetic forces result in theformation of faults and fold which in turn causeisostatic disequilibrium in the crustal rocks whichultimately causes earthquakes of varyingmagnitudes depending on the nature andmagnitude of dislocation of rock blocks causedby faulting and folding. In fact, suddendislocation of rock blocks caused by both tensileand compressive forces trigger immediate earthtremor due to sudden maladjustment of rockblocks.

A fault is a fracture in the Earth's crust alongwhich two blocks of the crust have slipped withrespect to each other. Faults are divided into threemain groups, depending on how they move.Normal faults occur in response to pulling or

tension; the overlying block moves down the dipof the fault plane. Thrust (reverse) faults occur inresponse to squeezing or compression; theoverlying block moves up the dip of the faultplane. Strike-slip (lateral) faults occur in responseto either type of stress; the blocks movehorizontally past one another. Most faulting alongspreading zones is normal, along subductionzones is thrust, and along transform faults isstrike-slip.

According to the theory the undergroundrocks are elastic like rubber and expand whenstretched and pulled. The stretching and pullingof crustal rocks due to tensile forces is slowprocess. The rocks continue to be stretched so longas the tensile forces do not exceed the elasticityof the rocks but as the tensile forces exceed therocks elasticity they are broken and broken rockblocks try immediately to occupy their previouspositions so that they may adjust themselves. Allthese processes occur so rapidly that theequilibrium of the concerned crustal surface issuddenly disturbed and hence earth tremors arecaused.

b) Hydrostatic Pressure and AnthropogenicCauses:

Certain human activities such as pumping ofground water and oil deep underground mining,blasting of rocks, nuclear explosion, storage ofhuge volume of water causes tremors. Theintroduction of additional artificial,superincumbent load through the construction oflarge dams and impounding of enormous volumeof water cause disequilibrium of already fragilestructures due to faults and fractures.

c) Volcanic Eruptions

When volcanoes erupt it is because the moltenmagma under the crust of the earth is underenormous pressure and to release that pressureit looks for an opening and exerts pressure on theearth's crust and the plate in turn. A place, whichis the seat of an active volcano, is often prone toearthquakes as well. Earthquakes are also causedafter a volcanic eruption since the eruption alsoleads to a disturbance in the position of plates,which either move further or resettle and canresult into severe or light tremors.

EARTHQUAKE


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d) Plate Tectonic Theory

Finally, in the mid-1960s, researchers in theUnited States and Great Britain came up with atheory that explained why the Earth shook.

The theory, called plate tectonics, is that theEarth's crust, or lithosphere, is comprised of manyplates that slide over a lubricating asthenospherelayer.

Plate tectonics confirms that there are fourtypes of seismic zones. The first follows the lineof mid-ocean ridges. Activity is low, and it occursat very shallow depths. The point is that thelithosphere is very thin and weak at theseboundaries, so the strain cannot build up enoughto cause large earthquakes. Associated with thistype of seismicity is the volcanic activity alongthe axis of the ridges (for example, Iceland,Azores, Tristan da Cunha).

The second type of earthquake associatedwith plate tectonics is the shallow-focus eventunaccompanied by volcanic activity. The SanAndreas fault is a good example of this, so is theAnatolian fault in Northern Turkey. In thesefaults, two mature plates are scraping by oneanother. The friction between the plates can beso great that very large strains can build up beforethey are periodically relieved by largeearthquakes. Nevertheless, activity does notalways occur along the entire length of the faultduring any one earthquake. For instance, the 1906San Francisco event was caused by breakage onlyalong the northern end of the San Andreas fault.

The third type of earthquake is related to thecollision of oceanic and continental plates. Oneplate is thrust or subducted under the other plateso that a deep ocean trench is produced. In thePhilippines, ocean trenches are associated withcurved volcanic island arcs on the landward plate,for example the Java trench. Along the Peru -Chile trench, the Nazca plate is being subductedunder the South American plate which respondsby crumpling to form the Andes. This type ofearthquake can be shallow, intermediate, or deep,according to its location on the downgoinglithospheric slab. Such inclined planes ofearthquakes are known as Benioff zones.

The fourth type of seismic zone occurs alongthe boundaries of continental plates. Typical ofthis is the broad swath of seismicity from Burma

to the Mediterranean, crossing the Himalayas,Iran, Turkey, to Gilbraltar. Within this zone,shallow earthquakes are associated with highmountain ranges where intense compression istaking place. Intermediate- and deep-focusearthquakes also occur and are known in theHimalayas and in the Caucasus. The interiors ofcontinental plates are very complex, much moreso than island arcs. For instance, we do not yetknow the full relationship of the Alps or the EastAfrican rift system to the broad picture of platetectonics.

Effects

� Landslides and damming of the rivers inhighland regions.

� Causes depression forming lakes. May causefaults, thrusts, folds, etc

� Formation of cracks or fissures in theepicenter region and sometimes water, mud,gas are ejected from it.

� Causes the raising or lowering of parts ofthe sea floor e.g. "Sangami bay" in 1923.This causes "tsunamis" or tidal waves.

� May change surface drainage &underground circulation of water like thesudden disappearance of springs in someplaces.

� Rising and lowering of crustal regions forexample in Alaska in 1899-16 m upliftment.

� Devastation of cities, fires, diseases, etc.

Distribution of Earthquakes

The world maps of the distribution of earthquakesprepared by the seismologists on the basis ofcomputer analysis and simulation of 30,000earthquakes have identified the three main zonesof earthquake.

1. Circum Pacific Belt or Ring of Firesurrounding the Pacific Ocean. It is ajunction of continental and oceanic margins;it is a zone of young folded mountains; it isa zone of active volcanoes thus this beltaccounts for the 65 per cent of the totalearthquakes of the world.

2. Mid-Continental Belt representing Alpine-Himalayan chains of Eurasia and northernAfrica and epicenters of east African fault


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zone. This belt represents the collision orsubduction zones of continental plates.About 21 per cent of the total seismic eventsoccur in this belt.

3. Mid-Atlantic Belt representing theearthquakes located along the mid-AtlanticRidge and its offshoots. This belt recordsmoderate and shallow focus earthquakeswhich are essentially caused due to creationof transform faults and fractures because ofdivergent movement of plates.

VOLCANOES

A volcano is an opening or a vent throughwhich heated materials consisting of gases, water,liquid lava and fragments of rocks are discharged.Magma is molten rock within the Earth's crust.When magma erupts through the earth's surfaceit is called lava. Lava can be thick and slow-moving or thin and fast-moving. Rock also comesfrom volcanoes in other forms, including ash(finely powdered rock that looks like dark smokecoming from the volcano), cinders (bits offragmented lava), and pumice (light-weight rockthat is full of air bubbles and is formed inexplosive volcanic eruptions - this type of rockcan float on water).

Although there are several factors triggeringa volcanic eruption, three predominate: thebuoyancy of the magma, the pressure from theexsolved gases in the magma and the injection ofa new batch of magma into an already filledmagma chamber.

As rock inside the earth melts, its massremains the same while its volume increases--producing a melt that is less dense than thesurrounding rock. This lighter magma then risestoward the surface by virtue of its buoyancy. Ifthe density of the magma between the zone of itsgeneration and the surface is less than that of thesurrounding and overlying rocks, the magmareaches the surface and erupts.

Magmas of so-called andesitic and rhyoliticcompositions also contain dissolved volatiles suchas water, sulfur dioxide and carbon dioxide. Forexample, in an andesitic magma saturated withwater and six kilometers below the surface, about5 percent of its weight is dissolved water. As this

magma moves toward the surface, the solubilityof the water in the magma decreases, and so theexcess water separates from the magma in theform of bubbles. As the magma moves closer tothe surface, more and more water exsolves fromthe magma, thereby increasing the gas/magmaratio in the conduit. When the volume of bubblesreaches about 75 percent, the magma disintegratesto pyroclasts (partially molten and solidfragments) and erupts explosively.

The third process that causes volcaniceruptions is an injection of new magma into achamber that is already filled with magma ofsimilar or different composition. This injectionforces some of the magma in the chamber to moveup in the conduit and erupt at the surface.

The nature of this eruption depends mainlyon the gas content and the viscosity of the magmamaterial. Viscosity is just the ability to resist flow-- essentially, it is the opposite of fluidity. If themagma has a high viscosity, meaning it resistsflow very well, the gas bubbles will have a hardtime escaping from the magma, and so will pushmore material up, causing a bigger eruption. Ifthe magma has a lower viscosity, the gas bubbleswill be able to escape from the magma moreeasily, so the lava won't erupt as violently.

Of course, this is balanced with gas content -- if the magma contains more gas bubbles, it willerupt more violently, and if it contains less gas, itwill erupt more calmly. Both factors aredetermined by the composition of the magma.Generally, viscosity is determined by theproportion of silicon in the magma, because ofthe metal's reaction to oxygen, an element found


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in most magmas. Gas content varies dependingon what sort of material melted to form themagma.

As a general rule, the most explosiveeruptions come from magmas that have high gaslevels and high viscosity, while the most subduederuptions come from magmas with low gas levelsand low viscosity. Volcanic eruptions don't oftenfall into easy categories, however. Most eruptionsoccur in several stages, with varying degrees ofdestructiveness.

If the viscosity and the gas pressure are lowenough, lava will flow slowly onto the earth'ssurface when the volcano erupts, with minimalexplosion.

Although volcanologists are well aware ofthese three processes, they cannot yet predict avolcanic eruption. But they have made significantadvances in forecasting volcanic eruptions.

Forecasting involves probable character and timeof an eruption in a monitored volcano. Thecharacter of an eruption is based on the prehistoricand historic record of the volcano in question andits volcanic products. For example, a violentlyerupting volcano that has produced ash fall, ashflow and volcanic mudflows (or lahars) is likelyto do the same in the future. Determining thetiming of an eruption in a monitored volcanodepends on measuring a number of parameters,including, but not limited to, seismic activity atthe volcano (especially depth and frequency ofvolcanic earthquakes), ground deformations(determined using a tiltmeter and/or GPS, andsatellite interferometry), and gas emissions(sampling the amount of sulfur dioxide gasemitted by correlation spectrometer, or COSPEC).

Types of volcanoes

The most common eruption types are:

The most common type of volcanic eruptionoccurs when magma (the term for lava when it isbelow the Earth's surface) is released from avolcanic vent. Eruptions can be effusive, wherelava flows like a thick, sticky liquid, or explosive,where fragmented lava explodes out of a vent. Inexplosive eruptions, the fragmented rock may beaccompanied by ash and gases; in effusiveeruptions, degassing is common but ash is usuallynot.

Volcanologists classify eruptions into severaldifferent types.

a) Hawaiian Eruption

In a Hawaiian eruption, fluid basaltic lava isthrown into the air in jets from a vent or line ofvents (a fissure) at the summit or on the flank of avolcano. The jets can last for hours or even days,a phenomenon known as fire fountaining. Thespatter created by bits of hot lava falling out ofthe fountain can melt together and form lavaflows, or build hills called spatter cones. Lavaflows may also come from vents at the same timeas fountaining occurs, or during periods wherefountaining has paused. Because these flows are


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very fluid, they can travel miles from their sourcebefore they cool and harden.

b) Strombolian Eruption

Strombolian eruptions are distinct bursts offluid lava (usually basalt or basaltic andesite) fromthe mouth of a magma-filled summit conduit. Theexplosions usually occur every few minutes at regularor irregular intervals. The explosions of lava, whichcan reach heights of hundreds of meters, are causedby the bursting of large bubbles of gas, which travelupward in the magma-filled conduit until they reachthe open air.

This kind of eruption can create a variety offorms of eruptive products: spatter, or hardenedglobs of glassy lava; scoria, which are hardenedchunks of bubbly lava; lava bombs, or chunks oflava a few cm to a few min size; ash; and small lavaflows (which form when hot spatter melts together andflows downslope). Products of an explosive eruption areoften collectively called tephra.

Strombolian eruptions are often associatedwith small lava lakes, which can build up in theconduits of volcanoes. They are one of the leastviolent of the explosive eruptions, although theycan still be very dangerous if bombs or lava flowsreach inhabited areas. Strombolian eruptions arenamed for the volcano that makes up the Italianisland of Stromboli, which has several eruptingsummit vents. These eruptions are particularlyspectacular at night, when the lava glows brightly.

c) Vulcanian Eruption

A Vulcanian eruption is a short, violent,relatively small explosion of viscous magma(usually andesite, dacite, or rhyolite). This typeof eruption results from the fragmentation andexplosion of a plug of lava in a volcanic conduit,or from the rupture of a lava dome (viscous lavathat piles up over a vent). Vulcanian eruptionscreate powerful explosions in which material cantravel faster than 350 meters per second (800 mph)and rise several kilometers into the air. Theyproduce tephra, ash clouds, and pyroclasticdensity currents (clouds of hot ash, gas and rockthat flow almost like fluids).

Vulcanian eruptions may be repetitive and goon for days, months, or years, or they may precedeeven larger explosive eruptions. They are namedfor the Italian island of Vulcano, where a smallvolcano that experienced this type of explosive

eruption was thought to be the vent above theforge of the Roman smith god Vulcan.

d) Plinian Eruption

The largest and most violent of all the typesof volcanic eruptions are Plinian eruptions. Theyare caused by the fragmentation of gassy magma,and are usually associated with very viscousmagmas (dacite and rhyolite). They releaseenormous amounts of energy and create eruptioncolumns of gas and ash that can rise up to 50 km(35 miles) high at speeds of hundreds of metersper second. Ash from an eruption column candrift or be blown hundreds or thousands of milesaway from the volcano. The eruption columns areusually shaped like a mushroom (similar to anuclear explosion) or an Italian pine tree; Plinianeruptions are extremely destructive, and can evenobliterate the entire top of a mountain, as occurredat Mount St. Helens in 1980. They can producefalls of ash, scoria and lava bombs miles from thevolcano, and pyroclastic density currents that razeforests, strip soil from bedrock and obliterateanything in their paths. These eruptions are oftenclimactic, and a volcano with a magma chamberemptied by a large Plinian eruption maysubsequently enter a period of inactivity.

e) Pelean Eruptions

Pelean eruptions are explosive eruptionsresulting from the collapse of a lava dome or spireto produce an incadescent pyroclastic flow (NueeArdente). Pelean eruptions are associated withhighly viscous acid and intermediate (rhyolite,dacite and andesite) magmas and generally occurafter the growth of lava domes and spires, butcan also be triggered by landslides. The productsof Pelean eruptions are pyroclastic flows whichcan be block and ash flows or ignimbrites witheutaxitic textures.

f) Lava Domes

Lava domes form when very viscous, rubblylava (usually andesite, dacite or rhyolite) issqueezed out of a vent without exploding. Thelava piles up into a dome, which may grow byinflating from the inside or by squeezing out lobesof lava (something like toothpaste coming out ofa tube). These lava lobes can be short and blobby,long and thin, or even form spikes that rise tensof meters into the air before they fall over. Lavadomes may be rounded, pancake-shaped, orirregular piles of rock, depending on the type oflava they form from.


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Lava domes are not just passive piles of rock;they can sometimes collapse and form pyroclasticdensity currents, extrude lava flows, or experiencesmall and large explosive eruptions (which mayeven destroy the domes!) A dome-buildingeruption may go on for months or years, but theyare usually repetitive (meaning that a volcano willbuild and destroy several domes before theeruption ceases).

Volcanic Belts of the world

1. The Circum-Pacific belt:

This is the most important belt of volcanoes.This is the also called Ring of Fire. The beltextends through the Andes of South America,Central America, Mexico, the Cascade Mountainsof Western United States, the Aleutain Islands,Kamchatka, the Kuril Isles, Japan, the Philippines,Celebes, New Guinea, the Solomon Islands, NewCaledonia and New Zealand.

This belt has 80 active volcanoes. The Circum-Pacific belt meets the mid- continental belt in theEast Indies. This belt is characterised by highvolcanic cones and volcanic mountains. Thevolcanoes of the Aleutian Island, Hawaii Islandand Japan are found in Chains.

Cotapaxi is the highest volcanic mountain(6035m) in the world. Other important volcanoesfound in this belt are Fuziyama, Shasta, Rainierand Hood.

In Alaska there is a Valley of Ten ThousandSmokes. It may be pointed out that in this beltvolcanic eruptions occur because of thesubduction of the Pacific plate below the Asiaticplate.

In Equador, South America, there are about22 volcanoes out of which 15 are more than 4450metres above the sea level. Besides, other highvolcanic mountains are St. Helens (Washington,U.S.A.), Kilauea (Hawaii Island, U.S.A.), Mt. Taal,Pinatubo and Mayon (Philippines). It may bementioned that the volcanoes of Hawaii Islandare situated in the intra- plate region.

2. The Mid-Continental belt:

This belt has various volcanoes of the Alpinemountain chain, Mediterranea Sea (Stromboli,Vesuvius, Etna etc.), Volcanoes of the Aegean Sea.Mt. Ararat, Elburz and Hindukush are alsoincluded in this belt.

It is interesting to note that there are severalvolcanic free zones found along the Alps and theHimalayas. The Rift Valleys of Africa havevolcanoes such as Kilimanjaro, Elgon, Birungaand Rungwe etc.

In the region where the boundaries of Persia,Afghanistan, and Baluchistan meet, there areseveral volcanic cones of large size, and one ortwo of them emit steam and other gases. Thisregion has also a few extinct volcanoes.

3. The Mid- Atlantic belt:

As the name indicates, this belt includes thevolcanoes of the Mid-Atlantic Ridge. Thevolcanoes associated with the Atlantic Ocean arelocated either on swells or ridges rising from thesea floor, or on or near the edge of the continentwhere it slopes abruptly into the deep oceanicbasins. However, in each case, the volcanoes areassociated with zones of crystal movement.

The volcanoes formed along the Mid-AtlanticRidge actually represent the splitting zone of theAmerican plate moving towards west and theEurasian plate moving towards east.

In the splitting zone stated above there isconstant upwelling of magmas. Thus, it is a zoneof crustal weakness. Volcanoes in this belt aregenerally of fissure-eruption type. Volcanoes ofLesser Antilles, Azores, St. Helens etc. areincluded in this belt.

HOT - SPOT VOLCANOES

About five percent of volcanoes are not nearthe margins of tectonic plates. They are overespecially hot places in the Earth's interior calledHOT SPOTS.

HOT SPOTS are created by mantle plumes -hot currents that rise all the way from the corethrough the mantle. When mantle plume comeup under the crust, they burn their way throughto become hot-spot volcanoes.

Famous hot-spot volcanoes include theHawaiian island volcanoes and Réunion Islandin the Indian Ocean.

Hot-spot volcanoes ooze runny lava thatspreads out to create shield volcanoes. Lava fromhot-spot volcanoes also creates plateaux, such asthe Massif Central in France. The geysers, hotsprings and bubbling mud pots of YellowstoneNational Park, USA, indicate a hot spot below.


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Yellowstone has had three huge eruptions inthe past 2 million years. The first produced over2000 times as much lava as the 1980 eruption ofMt St Helens. Hot spots stay in the same placewhile tectonic plates slide over the top of them.Each time the plate moves, the hot spot creates a

Rivers are a sizable stream of freshwaterflowing through a natural channel in the land.Rivers are among the most powerful natural forcesin shaping the earth's surface. In draining the landof surplus water, rivers wear down mountains,plateaus, and other high landforms. In a never-ending process, eroded material is carried by rivers.Some is deposited to form floodplains in thevalleys, some forms deltas at the rivers' mouths,and some is deposited in the sea.

A river passes through development stagesin its lifecycle.

Youthful stage

When streams first form channels they tendto have specific topographic features independentof climate zones and other factors. Thesecharacteristics include: Steep gradients; Nofloodplain; Flat divides; Steep valley sides; "V"shapes profiles; Straight course.

The early stages of stream development areclosely linked to uplifts of the crust, and thereforetectonic activity. In other words, the early stageof stream development is generated by uplift ofthe crust, lowering of sea level, or both. Thisincreases the stream gradient so that the primaryerosional work of the stream is a downcuttingaction. This produces the steep valley sidescharacteristic of an early stage stream valley. Sincethe stream gradient is high and the stream quicklycuts down into bedrock, there is little opportunityfor the stream to meander. Therefore the streamtends to be straight. Because the stream cannotmeander, it also cannot erode laterally to producea floodplain. When observing topographic maps,remember that the contours in the valley of theearly stage stream should be closely spaced downto the edge of the stream indicating the steepvalley walls and "V" shaped profile. No flatfloodplain areas at the base of the stream will be

new volcano.

The movement of the Pacific plate over theHawaiian hot spot created a chain of oldvolcanoes 6000 km long. It starts with THE MEIJISEAMOUNT under the sea north of Japan, andends with THE HAWAIIAN ISLANDS.

FLUVIAL CYCLE OF EROSION


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present. The map should also display a relativelystraight course. Away from the stream valley thedivides will be flat; this is indicated by highelevation values but widely spaced contours.

Some characteristics:

� Tributaries are small and the flow isturbulent.

� Bedload dominates making the water clear.

� Downward erosion predominates creatingdeep, narrow valleys.

� The gradient is steep and many waterfallsand rapids are present.

Mature valley

The mature stage of stream valleydevelopment is marked by the development of afloodplain. As an early stage stream continues todowncut vertically over time the stream channelapproaches the base level. The base level of astream is defined as the lowest level to which astream may erode its channel- usually this is sealevel. When the stream channel elevationapproaches the base level the stream begins toerode laterally producing a series of meanders.The meanders are confined to the floodplain. Thefloodplain itself can be recognized as the very flatarea adjacent to the stream channel. Thefloodplain is usually quite swampy and is oftenmarked with the map symbols for swamp ormarsh terrain. This area of the floodplain that isswampy is known as the backswamp.

As lateral erosion produces meanders, themeanders progressively become morepronounced. This is produced by the tendency ofthe stream to flow at a higher velocity on theoutside of a meander curve where water is deeperand less subject to frictional drag. This increaseserosion of the channel on the outside of themeander. On the other hand, the shallow waterdepth and corresponding higher drag on the insideof the meander bend produces a lower than normalstream velocity. This causes deposition of sandparticles to form a point bar. Point bars are one ofthe most common types of sand bars found inmeandering river systems. As this trend of erosionand deposition occurs the meander bends becomemore and more extreme until finally adjacentbends may "neck down" to form a cutoff.

When a cutoff actually forms, portions of theformer main channel become silted-in isolatingthe meander bend from the rest of the river. Thisleaves an isolated lake termed an oxbow becauseit retains the arcuate shape of the meander bend.Most mature river system floodplains are litteredwith oxbow lakes. In addition to the oxbow lakes,the adjacent point bar deposits that are depositedon the inside of a migrating meander tends tobuild numerous arcuate sandy ridges that areparallel. These features are termed meander scars.

Stream systems are continually in a state offlux. During the spring when most areas of thenorthern hemisphere experience increasedprecipitation, the stream systems may be at orabove flood stage. When the volume of water ina stream surpasses the size of the channel the riverspills over its banks into the adjacent floodplain.Before this event, the velocity of the stream iscommonly very high, therefore, the stream carriesa large sediment load while it is confined to itschannel. When the water leaves the channel,however, the velocity of the water dropsdramatically. This loss of velocity rapidlydecreases the ability of the stream to transportsediment. Directly adjacent to the stream channelthe largest particles - usually coarse sand - aredeposited first. The smaller clay sized particlessettle less rapidly throughout the floodplain. Theannual cycle of flooding builds up deposits ofsand along the banks of a river to produce a linearridge termed a levee.

Following key features are formed in mature stage:

(1) Well-developed floodplain

(2) Rounded instead of flat divides

(3) Oxbow lakes

(4) Levees

(5) Backswamp

(6) Meander scars

(7) Cutoffs

(8) Yazoo tributaries

Old age stage

In this (old) stage, as erosion continues andthe flood plains are widened by continued sloperetreat, finally reach a point where the slope is solow that no net erosion occurs any more, anderosion on the slopes is balanced by depositionon the floodplains. The hills are further reduced


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and the floodplains rise by overbank deposition(flooding), until finally end up with a landscapethat is reduced to a fairly featureless, flat surfacethat gently slopes towards the ocean. This surfaceis also called a peneplain. From now on the riversjust sweep over the floodplain and rework thesediment, creating various floodplain features,such as oxbow lakes, meander scars etc. Isolatedremants of resistant bedrock may rise over thepeneplain surface, the so called monadnocks orinselbergs.

Some characteristics are:

� gradient is gentle, velocity decreases� more deposition than erosion� valley is wide and flat and meanders are

numerous� suspension dominates and laminar flow

occurs� a wide floodplain, levees are present� deltas occur at the river mouth� oxbow lakes occur

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Documents

GEOMORPHOLOGY - …. The three concentric ... Earth's interior which in turn, ... Thus the seismic waves have divided the earth's structure as: S.No. Name of Chemical Average