with thE DEvElopmEnt oF sCiEnCE AnD tEChnology, wE know muCh oF glACiErs. For All thEir grEAt DivErsity oF shApEs AnD sizEs, glACiErs Consist oF two EssEntiAl typEs: vAllEy glACiErs, whiCh Flow Downhill From mountAins AnD ArE shApED By thE ConstrAints oF topogrAphy, AnD iCE shEEts, whiCh Flow outwArD in All DirECtions From DomElikE CEntErs oF ACCumulAtED iCE to CovEr vAst ExpAnsEs oF tErrAin. whAtEvEr thEir typEs, most glACiErs ArE rEmnAnts oF grEAt shrouDs oF iCE thAt CovErED thE EArth Eons Ago. in A FEw oF thEsE glACiErs thE olDEst iCE is vEry AnCiEnt inDEED: thE AgE oF pArts oF tilEAntArCtiC shEEt mAy BE ovEr 500,000 yEArs.glACiErs ArE Born in roCky womBs ABovE thE snow linE, whErE thErE is suFFiCiEnt wintEr snowFAll AnD summEr ColD For snow to survivE thE AnnuAl mElting. thE long gEstAtion pErioD oF A glACiEr BEgins with thE ACCumulAtion AnD grADuAl trAnsFormAtion oF snowFlAkEs. soon AFtEr thEy ComE to thE grounD, ComplEx snowFlAkEs ArE rEDuCED to CompACt, roughly sphEriCAl iCE CrystAls, thE BAsiC ComponEnts oF A glACiEr.As nEw lAyEr oF snow AnD FErn, snow thAt survivEs thE mElting oF thE prEvious summEr, ACCumulAtE, thEy squEEzE out most oF thE Air BuBBlEs trAppED within AnD BEtwEEn thE CrystAls BElow. this proCEss oF rECrystAllizAtion ContinuEs throughout thE liFE oF thE glACiEr.thE lEngth oF timE rEquirED For thE FormAtion oF glACiEr iCE DEpEnDs mAinly upon thE tEmpErAturE AnD thE rAtE oF snowFAll. in iCElAnD, whErE snowFAll is hEAvy AnD summEr tEmpErAturE is high Enough to proDuCE plEnty oF mElt wAtEr, glACiEr iCE mAy ComE into BEing in A rElAtivEly short timE ― For ExAmplE, tEn yEArs. in pArts oFAntArCtiCA, whErE snowFAll is sCAnt AnD thE iCE rEmAins wEll BElow its mElting tEmpErAturE yEAr-rounD, thE proCEss mAy rEquirE hunDrEDs oF yEArs.thE iCE DoEs not BEComE A glACiEr until it movEs unDEr its own wEight, AnD it CAn not movE signiFiCAntly until it rEAChEs A CritiCAl thiCknEss ― thE point At whiCh thE wEight oF thE pilED-up lAyErs ovErComEs thE intErnAl strEngth oF thE iCE AnD thE FriCtion BEtwEEn thE iCE AnD thE grounD、this CritiCAl thiCknEss is ABout 60 FEEt. thE FAstEst moving glACiErs hAvE BEEn gAugED At not muCh morE thAn two AnD A hAlF-milE pEr yEAr, AnD somE CovEr lEss thAn 1/100 inCh in thAt sAmE Amount oF timE.But no mAttEr how inFinitEly smAll thE Flow, movEmEnt is whAt DistinguishEs A glACiEr From A mErE mAss oF iCE.whiCh will BE thE BEst titlE For thE pAssAgE
A、how glACiErsComE intoBEing.
B、how glACiErs movEArounD、
C．thEClAssiFiCAtion oF glACiErs.
D．thE volumE AnD shApE oF glACiErs.