2. weathering

8/14/2019 2. Weathering

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2. WEATHERINGA) WHAT I5 WEATHERING?

+ The decomposition ond disintegrqtion of rocks ond minerols in silu (temoinoi the ploce or site of occurrence) ot the eorth's surfoce by mechonicol,chemicql ond biologicol processes.

ql Ollier (1969) defined weothering os 'the breokdown ond slterotion ofmoteriols neor the eorih's surfoce lo products thqi qre nore in equilibriumwith newly-imposed physico-chemicol conditions'.

"i" Weqthering brought obout by o rock's direci or indireci contoct withelements of the weother, nomely, precipitotion, heot ond cold.,s Weotherinq is o very slow process ocross time.'{, Weothering does nof involve the removol of the weothered moteriol(regolith) ond thus, it is not equivolent to erosion, which involves ihe rernovol

ond tnovement of rocks by ihe qction of ice, rivers or woves.+ Weothering morks ihe beginning ol denudqtion. Denudotion includes thedisintegrotion of rock on the eqrfh's surfoce ond their removol by erosronor moss movemenl, resulting in the lowering of the lond surfoce.

B) TYPE5 OF WEATHERINo PROCES5E5i) Physiccl weothering processes

.* Physicol weoihering: the physicol disintegrqtion of rock inlo smoller pieces."l fnclude:

7. Fteeze-thow oclion2. Sqlt crystolgrowth3. fnsolotion Weothering / Thermol Frqcturing4 Pressure F.eleose


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1. Freeze-fhdtv Aclion / Frcst wedging,/ Frost ShdlteringDefinition

'* The disiniegrotion of rock by repeded t?eezing ond thowing of icecrystols in the joints ond crevices of rocksCondiiions Conducive for Frost Action

+ Common in tempetate tegions ond high oltitudes (high mountqins)It Most effective in oreos wherel. doily temperotures flucfuqfes obove ond below 0"C. vegelqtion cover is limiied. qdequote supply of moisture from roin or melting of ice. rock contoin loints (eg. Grcnite),bedding plqnes, crevices ond porespocs

Processes ond Resultqnt Lohdforms'L During the doy, melted woter or roinfoll occunulotes in joints ond crevicesof rocks{ At night, os temperqture folls below lreezing point, the woter freezes ond

exponds by obout 9% , exefting ptessute fhot widens the jointsn! fce con exer-f o moximum pressure of 2lQOkg/ cm2 at -22" C..l During the doy, ice lhows when the ternperqture rises obove lreezing Pointond freezing occurs when ternperoture fqlls belbw O"C* Repeoted fteezing ond thqwing qction breoks down lhe rocks into coonse,

ongulor frdgments known os scree or tolus;L These coorse ond ongulor frogjments mqy occumulote ol the boses of steepcliffs to form tolus slopes/cones or scree slope.,{L Another feoture resulting frorn frost-shottering is the cosfellotedoppeoronce of mounldin ridges ond tops where {rost hqs exaggeroted thejoinfing pdttern. Ane.g.isthe ftoliqn Dolomiles where such costellotion ofthe limestone is rcflected in the grophic nomes given to the ridges - the Five

Finger Nqils.


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il Glociol peoks ore commonly shorpened qnd become jogged due to frostoction.nl Areos of lorge pieces or blocks of frost-shottered rock scottered ocrossflotter uplonds ol tempetale regions or ocross londscopes ol highet

lolitudes cqn olso result -forning blockfields or felsenmeer.

_"*"[*ts 1The w.tr fre?es andexpands, putting9re3sore on th rcck

Figure 4- Process of freeze-thow oction

Figure 5. Ftezze-thow oction


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Figure 6. Tolus (scree slope) beneoth frost shottered rocksEuropo, Spoin.

tn the Picos de

2. Solt Crystal Glowth

'l The disintegrotion of rockond pore spoces of rocks due to fhe growth of solt crystols in the iointscsnd'tions conducive fffi

+ Common in deserts ond serni-orid oreos where. evoporotion rofes dre high. soline groundwoter is drown io the rock surfoce by copillory oction underdry conditions+ Common in coostol oreos where there is o constoqt supply of sqlt from the

evoporotion of seqwdler


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Processes ond Resultont Londforms.{ In serni-orid ond qrid oreos, groundwoter is soline, contqining dissolvedsolis such os sodium chloride ond sodiun sulphote..L 6roundwoter is drqwn into the joinfs ond pore spoces of rocks by copillcirydction

copillory oction refers to lhe movement of woter 5etween pore spoces ofthe rocks due to the forces of odhesion, cohesion ond surfqce fension.In coostal oreos, soline seowoter con olso occunulqte in the joints ondcrevices of rocks-When the solt-soturqted wqter evoporotes. tiny solt crystols begin to growin the pore spqces.The growing solf crystdls exei gteot pressure on ihe pore spoces ondjoints. This couses widening of the spoces ond the Prising of the rockconstituents or flokes oport over time.Hollows ond covities ore formed ond they moy be enloeged by prog.essivefloking of the interior surfoces ond gronulor disintegroiion. This explcinsthe smoofh surfoce of fhe inner loyer of the hollows. Wind probobly removesloosened noteriol from the covities.

+ This process contributes to the forrndfion of tofoni, niches, shqllow covesond rock orches."l' Tofoni ore smooth, rounded pifs with diometers, thot con extend to severolmetreS.{ Where solt crystol growth is concentroied ot the bose of o cliff, o niche isformed.

Figure 7. A niche oi the boseof o clitf


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3) fnsolotion Weothering /Definition

iL The disintegrotion ofvolume due to heoting

Themdl Flacfuring

rocks os o result of successive chonges in the rockond cooling cycles

Condilions Conducive for Insolotion Weotherinq+ occurs in low-lotitude, hot deserts where thete is o greot diurnol ronge oftemperoture (difference in the mdximum ond minimum doily temperoture)

Dork-coloured, fine groined rock con obsorb more rodiotionHetetogeneous rocks which comprise minerols with differing coefficients ofexponsion

Processes ond Resultonl Feolures+ Process oppeors fo be more ellective in theory thqn in reality-

fn hot deserts, high dqytime femperaiures thot cqn rise qbove 5o"c ond lownight time tenperotures thoi con drop to O"C con leqd to repeoted heofingond cooling of the surfoce loyer of rocks, cousing the olternofe exponsionond coniroclion of the rock minerqls.Such o process will evenfuolly weoken the rock ond possibly breok it down.Since rock is o poor conductor of heot, the ideo of such rock breokdownseems possible.

;l This meons thqt the heot of the doy is concentroted in the ouler surfoce ofthe rock and this is the regidn whete nighttime rodiqtion most effectivelyreduces tenperoture.

* However, the effecliveness of insololion weothering wos chollenged by someloborotory experirnents where persistenl heoflng ond cooling of rockfrogments produced little weokening, even o'/er simuloted long periods oftilne.


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still, redl wotld e\tidence for the process exists. In hot deserts, ii isthought thot insolotion weothering leods to exfoliotion, which is the peelingoff of loyers from o rock becouse of its poor conduction of heat.It the rcckcomprises q mixture of dork ond light-coloured minerols, gronulordisintegrotion then tokes ploce. This is becquse dork minerols obsorb heotmore ropidly ihon light coloured minerqls, resuhing in differcnt tates ofexponsion ond contrdction of minerols groins in the rock.Such differentiol rotes of exponsion ond contrqction produce stresseswithin ihe rock, eventuolly leoding to its weokening ond disintegrotion.

4) Pressure Peleose/ Unlooding / DilatationDefiniiion

+ The disintegrqlion of rocks by the releose of confining pressure onunderlying rock os overlying r


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Processes ond Resultoht Ldndforms.! Rocks deep beneoth the eqrth's surfqce ore under immense pressure.'L Tectonic forces moy push fhe rocks (for e.9.) o gronite body up.'g As the overlying rock groduolly erodes, pressure on the gronite decreoses,the gtonite then exponds upwords ond outwords, cousing formqtion of crockpqrollel to the surfoce colled sheei jointing.i!' Sheefs mqy detqch themselves loyer by loyer in o process colled exfoliqtion

which is common in gronite, leoding to the formotion of exloliotion dones..! The loyers mqy in turn breok up into individuol boulders (block

dasintegrdtion).

Figure 9. Pressure releose


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ii) Chemicol wecthering Processes+ occurs when oir qnd woter chemicqlly ottock rock, cltering lhe minerol

composition, volume. sirengih ond coherence of the rock.+ Conditions conducive for chenicol weothering include obundont roinfoll, hi(1h'temperotures, permeoble rocks ond rocks contcining less resistont minerolsi! The rote of chemicol weotheting depends on volume of woter Pqssing thesurfoce of o porticle ond solubility of solid being dissolved."$ Able to penettole more deeply into the rock thon physicol weqthering.,+ In the humid tropics, where conditions ore fovourable for chemicolweothering processes penetrote lo a gteal depth, the processes qre

collectively known os deep weothering.

1) Solution{' fs corried out by soil moisture ond groundwqter.+ Affects boih originol rock minerols like rock solt (holite) ond minerols which

hove olreody been weatheredby,fot e.g. corbonotion or hydrolysis.

2) CdtbonotionDefinition

,it Reoction between carhondioxide in woter ond cat'onole rocks io producecolciurn corbonote which is soluble in wqter

condit ions conducive f or corbonotion'! Promoted by high temperoture, high precipitotion ond dense regetationcover

Processes+ Atmospheric corbon dioxide dissolves in rdinwoter, forms weok ocid colled

corbonic ocid which chemicolly ottqcks rocks such qs limestone.


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'[ CO, being olso obunddnt in soils mokes soil wqter ocidjc os well.+ Corbonote rocks (colcite ond dolomite) ore most commonly ottocked in this* CoCOs (calcite) + HrCO3 (corbonic ocid)


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Conditions Conducive f or Hydrotion'l Most octive following successive periods of wet ond dry weother

Processesnl Affects rock minerols lhot odsorb woter.'i!, These minerols undergo increoses in volume or swellihg, setting up Physicqlsfresses within the rock. In some rocks such os shqle, it con swell os much os

60%_

+ As minerols odsorb wqter, besides swelling ond disintegroting physicolly, thechemicol structure is dlfered.$ For e.g. iron oxides ore converted to iron hydroxides.+ Commonly couses surfoce floking of rocks.;L The hydrotion of colciurn sulphote (onhydrite) forms gypsum.

4. HydrolysisDefinition

'L Chemicol decomposition of rocks by the chenicol reoction of rock minerolsond the hydrogen ions in woter.Conditions Conducive f or Hydrolysis

;t The rote of hydrolysis depends upon the qmount of H'(hydrogen) ions'l High temperoture dnd precipitotion fovours fhis processr! Presence of joints ond weck minerols in rocks (especially igneous rocks)

Processes

+ Dominqht chemical weotherin g process which\ is responsib le lor lheformqtion of regolith to o greof depth in the humid tropics (deepwedthering)


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i! Occurs when there is o chemicol reoclion \elween tock minerqls ond woter* fgneous rocks ore porticulorly susceptible to hydrolysis os their silicote

minerols combine reodily wilh wofer.

"! Importont in the weqthering of potossium feldspdr (orthoclqse) in gronite.+ Woter + feldspor - potossium hydroxide + cloy minerols (koolinite) * silico* The potqssium hydroxide reqcis further wi.th corbonic ocid to becomepolqssiun corbonote ond is removed in solution.

5- OxidationDefinition

{L The chemicol reoction \etween rock ninerols (iron) ond oxygen in oir ondmoisture.

Conditions Conducive f or Oxidqtion& Mote effective obove the woter ioble whete the rock con be penetroted viojoint ond pores by otmospheric oxygen.{' Rote of reoction influenced by climote ond rock type (presence of iron

rninerols)Processes

& Occurs when rock minerqls combine with oxygen, usuolly dissolved in woterto produce oxides of iron.,tL Exomple: iron in q ferrous slote is chonged by the oddition of oxyqen into ofettic stote.*, Fe*2 (ferrous iron) --> Fe'3 (ferric ircn) or zFeo + o2 --> Fe2O3+ A common effeci: the'rusting'of the elements ofiron in q rock, os blue-grey

becomes reddish-brown ond ferrous 6e.omes lercic.


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iii) Biologicol Weothering Processes+ They ore o cornbinotion of physicol ond chemicql weqthering. Thus. it is olso

known os biomechonicql qnd biochemicol weothering.

+ Biomechdnicol processesinclude:. Simple breoking of pqrticles, qs by ihe eoting or burrowing of onimols,. Physicol breqkup of jointed rocks by the pressute exefied by iree roots,

especiolly in oreos where bedrock is close fo the surfoce. The action of tree roots open possogewoys for chemicol ottqck

'l Eiochenicol processes include:. Simple chemicol effects, os when solution is enhonced by the corbondicxide produced by respirotion of plonts ond onimols;. Effects of moislure reteniion. These effecls ore partly due 1o thewoter-holding effects of root mosses ond humus, portly by the shodeeffecis of plontsj. The disihtegrotion of rocks lhrough chemicol reociions wilh orgonicqcids, such os humic qcid, which is produced by the decomPosilion of

organic motter. Complex chemicol reoctions such os chelotion. whereby Plonts extroctions (nutrients) from minerqls, cousing the minerols to breok down.


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C) PATTERNS AND PRODUCT5 OF ROCK WEATHERINoi) Block Disintegrotion

+ The breokdown of a rock into blocks olong o system of intersecting jointplones.

& The results of block disintegrotion ore coorse, ongulor cuboidolboulders/blocks which are in turn further broken down by ofher processes.

* Conditions favouring block disintegrotion:. Rock struciure: well-developed recfqngulor joint systen ond bedding

plones which ollow permeqbility by weothering ogents. E9. in igneous ond sedimeniory .ocks with homogeneous structure4 Wedlhe ng prccesses that retult in block dkinfegrafion:

. Freeze-lhaw action. Insolotionweqthering. Biornechonicol weothering - oction of plonf roots. Chemicol weothering ptocesses such os hydrolysis ond corbonoiion olongjoints

Figve 11- Block disintegrotion


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ii) 6ronulor Disintegrotion'iL The breokdown of o rock into its constituent minerols or fine minerol qroinssuch os sond or grovel.iL ln mony sondsiones, the quortz groins ore weokly cemented together withcolcite, which is reodily dissolved ond removed by wotet. Once the cementingmoteriol is renoved or weakened, there is nothing lo hold the groinstogether, ond they fdll aport. or disoggregqte.*, fndividuol ninerol groins simply seporote from one onother olong theirnolurol contocls ond produce sond or grovel. in which eoch porficle hos the

some shope ond size os it did in the originol rocks.+ In gronite, the quaftz grqins qre disoggregqted qs the feldspors oredecomposed into cloy.;i' The soft cloy loosens the intenlocking groins ond the once-solid piece ofgrqnite stqrts to crumble into individuol groins or groups of groins.+ The resuli is thot fhe snoll, iffegulor quortz groins thot once filled the

spqces between feldspor ore now releosed ond become the source of mostsond+ Conditions fayouing granuldr disinlegration :. Coorce-gtdned rocks with heterogeneous rninerols (different colours ond

dif f ercnt coeffictents of exponsion). Rock which ore porous - spoces for solt crystqls to grow& l4eatheing processes that resull in granuld|disinfegtation:

. Solt crystolgrowth

. Insolotionweothering. Frosl oction: ice crystol growth. Chemicol weothering processes such os hydrolysis, oxidotion ondhydrolion


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Figure 12. oronulor disinfegrotioniii) Spheroidol Weothering

"L The breqkdown of on ongulor ond blocky rock into concentric shells ofweothered mqteriols.,{ fn fhe process, o rounded corestone with o rsdidl profile is left behind+ A rounded corestone is produced becouse weatheting oitock on exposedrock from oll sides ot once and, thetefote. decomposition is rnore ropid clongthe corners ond edges of the rock."t As the decomposed moteriol folls off , lhe coriels become rounded ond theblock is reduced to on ellipsoid or o sphere.* The sphere is the geometric form thot hos ihe leqst omounl of surfoce oreoper unit of volume. Once the block oftoins this shope, it simply beconessmoller wifh further weothering.+ Conditions fovouting spheroiddl weolherag:. Rocks with heterogeneous minerqls - differing resistonces to chemicqloitock \* Wedthering processes lhat result in spheroidol uedthering:. Selective chemicol weathering - eg. hydrolysis


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Figure 13. spheroidol weothering

1998 Wadsoorth Publishinq ComFrnv/lTFFiguee 14. Weofhering of ongulor blocks into rounded corestones


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iv) Exfoliction+ The bredkdown of o rock into spolls, loyers or flokes frorn the surfoce of o

rock outcrop or boulderConditions fdvouring exfolialion :. Presence of sheet or curvilineor joints in rocksWeathering processes thot result in exfoliation. fnsolotionweothering. Pressure releose. Chemicql weothering processes such os hydrotion

Figwe 15. Exfoliotion

+

Figure 16- Sheet Joihts focilitqtihg exfoliotion


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D) FACTORS AFFECTINo THE RATE OF WEATHERING PROCESSE5i) oeolooicol foctors

+ Chorocteristics of the rock subjected to wealheting.L Rocks offer o wide ronge of resistonce to weothering, owing to both

mechonicql qnd chernicol foctors.1) Pock Strucfute: Joinls and Bedding Planes

;t Ploy on importont influence on the types of weothering thot will occur.. Joints ore very norrow but horizontolly extensive crocks.. They result from stresses generoted during the cooling ond contrqctionof igneous rocks, the sheoring ond tensionql forces genetoled by eofih

movements in sedimentory rocks, ond in crystolline rocks such os gronileond gneiss, joints ore forrned during pressure releose when overlyingrocks qre renoved by erosion.. A well-\edded ond closely jointed (numerous joints) rock ollows the reodypenetrolion of wofer, ocids ond oxygen, oll ogenfs of decomposition.. Joints effectively increose the sub-surfoce oteo of the rock thol is

exposed to chemicol processes. os well os providing lines of physicolweokness thot con be exploited by processes suah as fteeze-thow. Bycontrost, mqssive rock without bedding plones ond with few or no joints.are fairly resistont. only the surfoce of lhe rock con be ottocked bywealheting ogents, ond liftle or no chenicdl,or physicol breok-down conoccur iniernolly.

- v ,/ 7 r---" 41fl)t1=tt-t,fflflflfl;- -5mrotal surlnce A@ = 12 cmrurfae trs - 6 drFigu.e 77. Joints increose surfoce oreo of rock exposed to weothering


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+ Joints did weoth eting ptocesses in o number of woys:. Chemicol weotheting by ocidulofed roinwoter is concen-troted olong join-is

ond beddinq plones,. rn gronite oreos, lhe rock moy be divided into cuboidol blocks os o resultof chemicql ottock olong verticol controction joints cnd horizontol

pseudo-bedding plones ond the individudl blocks ore in turn subjected tofurther weothering by physicol ond,/or chemicol meons;

. Joints qid the processes of frosi ond ice wedging in periglociol ond glociolclimotes;

. The process of exfolioiion depends on the existence of sheet orcurvilineor jointing, resulting f ron dilototion.

Beddhg planes, these ore the lines of junction seporoting individuol loyersof sedimentory rocks. They constiiute lines olong which the rock reodilyspliis, qnd olso provide routes for the underground movemeni of woter. Fore.9. ih limestohe couniry, underground possoges frequently result fron theopening up by solution of mojor bedding plones.

Figure 18. Orfhogonol Joints


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2) Rock Strenglh and Hardness.1" Rock hordness depends 2 foctors:

Their constituent minerqls, fhe noture of their cementotion (how tightlybonded these minerols qre) ond the degree of compression: for e.9. mosiigneous rock ore hord os o result of their constituent minerols (feldsporond quortz which ore themselves very hord) ond becquse these tninerols,in the process of cooling ond crystollizotion, ore very tightly bondedlogethet.The o9e of the rock: older rocks have often been buried beneoth youngerrocks, the resultqni conpression helped to bind the minerol crysfolstogetlrct.

,tl Rock hordness is not o very importont determinqnl of rote of weothetingsince neorly oll hqrd rocks possess weoknesses (strucfurol or of chemicolcornposition) which qllow ihem to be ottockad by chemicol Processes.

3) Chenicdl composition+ Rock minerols vory in their obility to resisf chemicol chonge ond breokdown.+ Limestone, lor e.g. whose main constituent minerol is colcium cqrbonote, isropidly octed upon by ociduloted roinwoter ond nony types of sqndstones(porticularly those held togelher by colcoreous or ferrous cements) ore olso

reodily ottocked.+ Also, certoin minerqls ore rnore stoble thon others. For e.9. quartz is highlystoble ond resistonce lo chemicol ottqck. Less stoble minerqls: ougite ond

olivine.Light coloured minerols ore more stoble thon dork-coloured tninerols. A dqrkrock such os bqsolt or gobbro will heqt up more ropidly ond so experiencegreqter stroin. thon o light-coloured rock like cholk or limestone, which willreflect the sun's rqys.A rock conposed of voriously coloured minerols is subiected to siroinbecouse the cqpocity of these minerols io obsqrb ihe sun's heot is notuniform, ond thus differentiol exponsion is cqused.


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4) Pock Texture* Refers to the crystolline noture of the tock, whether it is coorse-groined or

f i^e-qained.

+ fhe e{fects of textwe on wedfhering ore highly complex ond dilfer fromone rock type to onother.+ fn some coses, coorse-texiured rocks ore resistont; in other rocks where

sorne of the constituent rninerqls cre chemicolly unstoble, selective chemicolqltock quickly reduces the rock's coherence."1" In mony fine-groined rocks, the finer crystqls dre tightly bonded ondinterlocked, 9ivin9 the rock strength bul there ore more crystol boundorieswhich provide lines of poteniiol weokness, ollowing weothering ogenis tootfock.

ii) Climote+ Climote is the singe most importont foctor influencing wedthering.'{ The mosi importont voriobles ore precipitotion ond temperoture.+ Wqter (from precipitotion) influences the exlent ond notute of weatheting+ Wofer focilitotes mechqnicol weqthering processes such os freeze-thow.+ Woter olso focilitqies chemicol processes such os hydrolysis, solution ond

corbonotion.+ Woter is lhe key ogent of chemicql weothering.* Unlike mechonicol weothering, chemicol weoihering is most effectiv belowthe earth's surfoce since percololing wqter hos goined orgonic ocids. Fromthe soil ond vegetotion.

Acidic woter helps to breok down rock such os chqlk, limestone ond grqnite.The omounl of woter is importont os it determines the depth of weotheringos the woter moves olong the joints/crqcks of rocks.

.{


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+ The importonl ospects of precipitotion orer. Tolol drnount of roinfoll. fntensity of roinfoll. Seosonol vdrio-tions. Infiltrotion. Runoff. Rdte of evoporotion

+ Temperoture is importont in oll ospecfs of weothering. Fot e.g. lhe tate ofchemicol reoctions increoses os temperoture increoses.

-t According to Voh't Hoff's Low, the speed of o chemicol reoction increoses2t/2 times with each rise of fenpe.oture of 100c.d Processes of rock weothering vory from ploce to ploce occording to clinote.

Figure 19. Peltier's diogrom showing(temperoture ond precipitoiion) ond the relofioiship between climotetype ond intensity of wedthe.ihg


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1) Hunid Tropical Clinates+ Regions between lotitudes 100 N ond 5 of the equotor.+ Charocte.ized by high onnuol precipitofion (>17o0mrn), high meon onnuol

tempero.tures (>27"C), high humidity levels (>80%) ond luxuridni vegetotion,which is conducive for chemicol weothering.+ Chernicol weothering is dominonl. However, this does not moh thotphysicol weoihering is totolly obsent.oL Feldspors in granite ond reloted rocks ore oliered to cloys through the

process of hydrolysis.Soluble minerols rnoy be leoched out. Ohly fhe most insoluble minerols remoin(silico, oluminum, iroh) resulting in ihfertile soils.High temperoiures speed up chemicol reoctions + chemicol decompositionof rocks, resulling in o thick regoliih.

+ Frost oction is non-existent excepf on the peoks of high mountoinsgst?I;,lItt;l^

IATGA.POOSOL sEI,'DESER',41VO OSERI

o

l{rt6

Figuee 20. Strokhov's diogrom showirg ihe depthclimotic zones

,1,/

of regolith in different


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2) Seosonally Hunid Tropics Oropicdl Ateds ,yith Wet ond Dry Seosons),rt Regions between ldtitudes 100 - 300 N qnd 5 of the equotor.I Includes the sovonno belt of tropicol Africq ohd .fhe lropicol nonsoon

regions of the fndion sub-continent.{ Gteatq seosonql confrosts of humidity ond tenperotures, with pronouncedwet ond dry seosons.'l High intensity roinfall is experienced mostly in summer, 500 - lOO0 mm/yeor* Vegetotion less luxuriont thon the humid tropicql roinforest, with treesshedding lheir leoves in the dry winter rnonths.+ During the roiny seoson, which decreoses in length iowords the deserifringes, the omount ond iniensity of the doily roinfoll often eguols thot ofthe humid tropics. The conditions fovour chemicol weothering but regoliihsore thinner becquse of the presence of o dry period ond fhe rote of

chemicol weothering is not os high os the hurnid tropics.i! Due to the less dense vegelotion co\/er, surfqce erosion is more ropid duringroin - nore rqpid removol of the upper regolith.+ The onset of the dry seqson reduces chemicol weoihering. IJ the wotettoble remoins high, sub-surfoce weotheting moy still coniinue. Removol ofthe uppet regolith exposes fresh bosql rock to physicol weothering duringthe dry seoson.,il Physical weotheting tokes the form of exfolioiion, which is most octive otthe beginning ond neor the end of the rqins where temperoture chqnges ore

high ond cooling is offected by ftequent rqin showers.,iL Pressure teleose cousing the development of sheet joinis could qlso beexperienced ofte( the roiny seoson os gronitic mosses moy be brough+ to theeqrth's surfqce ofter periodic stripping of the ovetlying loyers by surfocerunoff. This is mqde possible by the ihin \tegetstive cover during the dry

seoson.


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3) Low-lotilude Hol Desefis (Semi-arid / Arid Deserts)+ The lotitude ronge is 15-28" north ond south of the eguotor.'1 Their globol ronge covers obouf l/5 of the eofih, including the world'sgreot

deserts: Sohoro, Sonorq, Thor, Kqlqhori ond the 6reot Austrolidn..+ Characletized by low level of roinfoll ond extreme diurnol ronge of

fenpero-ture& Conducive for the physicol weothering processes of insoldtion weotheringond sqlt cryslol growth fron groundwole.. This is evident from the

presence of fresh ongulor debris on slopes.+ Intense solor heoting in the doy is often followed by ropid nocturnol cooling.Diurnol ronge of femperoture exceeding 50'C is thus common in thisclimotic zone. Such o lorge temperqture chqnge within o doy meons thqt therocks ore subjected to exponsion ond controction within o short spon oflime..[ Chemicol weolhering is minimdl becouse of ihe lock of precipiiotion.

However, chemicol weothering processes ore ptesent ofter Precipitdtion hosoccurred.'{ Due to high evoporotion rotes, solf crystol growth occurs when copillaryoction drows groundwoier into joinls of rocks, which is common in shodyports of desert rock.r! Another source of woter is dew. Dew is experienced in the morning justbefore sunrise when temperotures ore of their lowest for the doy. Woterdroplets collect on rock surfqces qnd reoct with rock minerqls.* Thin loyer of regoli.lh, fresh unqltered bedrock is obundqnf.

4) lUarn Temperale Regions+ Both chemicol ond mechanicol processes operotei chemicol weothering beingmore importont during the summer months whert\ temperotures are highet

ond where there is summer roin. Mechqnical weothering processes such csfreeze-thow occur during lote winter ond eorly sPring whqn ternperoturestend to fluctuote just qbove qnd below zero.


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Soil ond regoliih develop to depths of severol metres.There is sufficient woter present to focilitdte oll the chemicol processesond the temperoture ronge is large enough to occount for the mechonicolprocesses (l5 well.During the cool roiny winters in the Mediterranean zones of Africo.evqporotion rotes ore lowe( ond heqvy frontol roinfolls couse much ledchingor woshing downwords of minerols deeper into the soil.Chemicol weolhering tqkes plqce but is only o froction of the tote of thehumid tropics due to lower temperotures. The presence of o soil monlle ondvegelation cover in certoin ploces fovour the infiltrotion of roin ond theretention of soil solution. The underlying rocks ore olmost continuously moistfor chemicol weqthering to toke ploce.Wiih fhe coming droughts, woter tobles drop ond chemicol weqthering isgreotly reduced.At higher oltitudes, mechonicol frost oction is octive especiolly on theexposed jointed rocks in the higher oreos. Melt woter produced during thedoy seeps into the rock joints ond crocks only to refreeze into ice wedgesduring the cold winter nights.

5) Cool Tenperale Pegions,il, The doninonf weothering processes ore ihe mechonicql processes. The lowtemperoturs during winter often reoching 6elow freezing point, nokingfrost oction o dohirdnt process.

Chemicol weothering is insignificonf despite the presence of woler. This isbecquse for os long os 6 months of the yeor, ihe woter thql is present ot orneqr the ground surfoce is trozen as ice ond is therefore unqvciloble forchemicol processes to occur.However, chemicol weotheting is not fotqlly obseni. During the thow periodeoch yeor which occurs in ihe summer seoson, melt woter issuing frombeneoih snow patches do engoge in some chemiqol weoihering Processes,such ds cdrbonotion.


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6) Polar Clinatesil' Polor cfimotes ore chorocterized by ice sheels, ice cops ond volley glociers -these profect lond surfoces from weothering.+ Lorgely mechonicol weotherihg in ihe form of treeze-thow during the few

months when there is sunlighi.+ Summer roin conloins dissolved COz ond Oz + some chemicol weolhering

though rqies of reqction ore slow bcause of the low iemperotures.

iii) Topoqrophic factors1) Elevalion

* The higher the elevoiion, the bettet chonce thete is for octive woiermovement through the rock. In low-lying oreos, the droinoge moy be poor.Moreover, the wdter tends to become soturoted with dissolved ncteriol ond,os o resuli, becomes incqpoble of undertoking further solution.

2) Slope'* High ond especiolly steep slops leqd to the downslope woshrhg dnd creep of

wealhered moteriol. There is q tendency for roinwoter to flow off thesurfoce rother thon lo percolofe downwqrds io undertoke weothering.3) Aspect

+ Slopes exposed to sun ond roin will be more prone to weoihering thonsheltered slopes."l slopes focing the sun in high qltitudes will be more subject to fteeze-lhowoction thon those thof ore olwoys cold ond sunless.

iv) Biotic fqctors \+ Plqnts moy qct os d protecfive cover ond so prevenf the wdsting (removol) ofsoil.+ Their roots moy qssist fhe penetrafion of woter ond so help solution.


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Pldnt roots moy exert q wedging elfect where they Penettote crocks in therock. Burrowing onimcls moy loosen the soil ond 5o mcke it eosier for otherfoctors (like woter) io do their work.Pldnts moy olso extroct certqin minerols from the soil cnd so help to breokthem down.

v) Time foctor^1" The length o+ time a rock surfoce hos been exposed to weoihering willallect lhe degree to which o porticulor type of rock is weothered ond the

thickness of the weothered moter;ol.

Figste 21. Ove"|view ol foctors offecting rotes of weqthering

2. weathering

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