托福阅读――地质地貌类(九)

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1、智课网托福备考资料托福阅读――地质地貌类（九）DesertFormationThedeserts,whichalreadyoccupyapproximatelyafourthoftheEarth'slandsurface,haveinrecentdecadesbeenincreasingatanalarmingpace.Theexpansionofdesert-likeconditionsintoareaswheretheydidnotpreviouslyexistiscalleddesertification.Ithasbeenestimatedthatana

2、dditionalone-fourthoftheEarth'slandsurfaceisthreatenedbythisprocess.Desertificationisaccomplishedprimarilythroughthelossofstabilizingnaturalvegetationandthesubsequentacceleratederosionofthesoilbywindandwater.Insomecasestheloosesoilisblowncompletelyaway,leavingastonysurface.Inothercas

3、es,thefinerparticlesmayberemoved,whilethesand-sizedparticlesareaccumulatedtoformmobilehillsorridgesofsand.Evenintheareasthatretainasoilcover,thereductionofvegetationtypicallyresultsinthelossofthesoil'sabilitytoabsorbsubstantialquantitiesofwater.Theimpactofraindropsontheloosesoiltends

4、totransferfineclayparticlesintothetiniestsoilspaces,sealingthemandproducingasurfacethatallowsverylittlewaterpenetration.Waterabsorptionisgreatlyreduced;consequentlyrunoffisincreased,resultinginacceleratederosionrates.Thegradualdryingofthesoilcausedbyitsdiminishedabilitytoabsorbwaterr

5、esultsinthefurtherlossofvegetation,sothatacycleofprogressivesurfacedeteriorationisestablished.Insomeregions,theincreaseindesertareasisoccurringlargelyastheresultofatrendtowarddrierclimaticconditions.Continuedgradualglobalwarminghasproducedanincreaseinaridityforsomeareasoverthepastfew

6、thousandyears.Theprocessmaybeacceleratedinsubsequentdecadesifglobalwarmingresultingfromairpollutionseriouslyincreases.Thereislittledoubt,however,thatdesertificationinmostareasresultsprimarilyfromhumanactivitiesratherthannaturalprocesses.Thesemiaridlandsborderingthedesertsexistinadeli

7、cateecologicalbalanceandarelimitedintheirpotentialtoadjusttoincreasedenvironmentalpressures.Expandingpopulationsaresubjectingthelandtoincreasingpressurestoprovidethemwithfoodandfuel.Inwetperiods,thelandmaybeabletorespondtothesestresses.Duringthedryperiodsthatarecommonphenomenaalongth

8、edesertmargi