托福閱讀長(zhǎng)難句21天訓(xùn)練營(yíng)Day

長(zhǎng)難句解析21天訓(xùn)練營(yíng)Day19Continuedsedimentation—theprocessofdeposits’settlingontheseabottom—buriestheorganicmatterandsubjectsittohighertemperaturesandpressures,whichconverttheorganicmattertooilandgas.Asmuddysedimentsarepressedtogether,thegasandsmalldropletsofoilmaybesqueezedoutofthemudandmaymoveintosandylayersnearby.Bothoilandgasarelessdensethanwater,sotheygenerallytendtoriseupwardthroughwater‐saturatedrockandsediment.Whenthewellreachesapool,oilusuallyrisesupthewellbecauseofitsdensitydifferencewithwaterbeneathitorbecauseofthepressureofexpandinggastrappedaboveit.Waterorsteammaybepumpeddownadjacentwellstohelppushtheoilout.ThedevelopmentoftheoilfieldontheNorthSlopeofAlaskaandtheconstructionoftheAlaskapipelineareexamplesofthegreatexpenseanddifficultyinvolvedinnewoildiscoveries.Offshoredrillingplatformsextendthesearchforoiltotheocean’scontinentalshelves—thosegentlyslopingsubmarineregionsattheedgesofthecontinents.Itmaybeinapooltoosmallortoofarfromapotentialmarkettojustifytheexpenseofdrilling.Offshoreplatformsmayalsoloseoil,creatingoilslicksthatdriftashoreandfoulthebeaches,harmingtheenvironment.TheWilmingtonfieldnearLongBeach,California,hassubsidedninemetersin50years;protectivebarriershavehadtobebuilttopreventseawaterfromfloodingthearea.Whilethereareadozenormoremassextinctionsinthegeologicalrecord,theCretaceousmassextinctionhasalwaysintriguedpaleontologistsbecauseitmarkstheendoftheageofthedinosaurs.Fortensofmillionsofyears,thosegreatcreatureshadflourished.Then,suddenly,theydisappeared.ThebodythatimpactedEarthattheendoftheCretaceousperiodwasameteoritewithamassofmorethanatrilliontonsandadiameterofatleast10kilometers.長(zhǎng)難句解析21天訓(xùn)練營(yíng)Day20Scientistsfirstidentifiedthisimpactin1980fromtheworldwidelayerofsedimentdepositedfromthedustcloudthatenvelopedtheplanetaftertheimpact.ThissedimentlayerisenrichedintheraremetaliridiumandotherelementsthatarerelativelyabundantinameteoritebutveryrareinthecrustofEarth.Evendilutedbytheterrestrialmaterialexcavatedfromthecrater,thiscomponentofmeteoritesiseasilyidentified.Thisimpactreleasedanenormousamountofenergy,excavatingacraterabouttwiceaslargeasthelunarcraterTycho.Theexplosionliftedabout100trilliontonsofdustintotheatmosphere,ascanbedeterminedbymeasuringthethicknessofthesedimentlayerformedwhenthisdustsettledtothesurface.Suchaquantityofmaterialwouldhaveblockedthesunlightcompletelyfromreachingthesurface,plungingEarthintoaperiodofcoldanddarknessthatlastedatleastseveralmonths.TheexplosionisalsocalculatedtohaveproducedvastquantitiesofnitricacidandmeltedrockthatsprayedoutovermuchofEarth,startingwidespreadfiresthatmusthaveconsumedmostterrestrialforestsandgrassland.Followingtheeachmassextinction,thereisasuddenevolutionaryburstasnewspeciesdeveloptofilltheecologicalnichesopenedbytheevent.Earthisatargetinacosmicshootinggallery,subjecttorandomviolenteventsthatwereunsuspectedafewdecadesago.Inmodernagriculture,mineraldepletionofsoilsisamajorconcern,sinceharvestingcropsinterruptstherecyclingofnutrientsbacktothesoil.Mineraldeficienciescanoftenbedetectedbyspecificsymptomssuchaschlorosis(lossofchlorophyllresultinginyelloworwhiteleafissue),necrosis(isolateddeadpatches),anthocyaninformation(developmentofdeepredpigmentationofleavesorstem),stuntedgrowth,anddevelopmentofwoodytissueinanherbaceousplant.Phosphorus‐deficientplantsareoftenstunted,withleavesturningacharacteristicdarkgreen,oftenwiththeaccumulationofanthocyanin.長(zhǎng)難句解析21天訓(xùn)練營(yíng)Day21Muchoftheresearchonnutrientdeficienciesisbasedongrowingplantshydroponically,thatis,insoillessliquidnutrientsolutions.Thistechniqueallowsresearcherstocreatesolutionsthatselectivelyomitcertainnutrientsandthenobservetheresultingeffectsontheplants.Acroponics,atechniqueinwhichplantsaresuspendedandtherootsmistedwithanutrientsolution,isanothermethodforgrowingplantswithoutsoil.Asurveyofknownhyperaccumulatorsidentifiedthat75percentofthemamassednickel,cobalt,copper,zinc,manganese,lead,andcadmiumareothermineralsofchoice.Manyarefoundintropicalandsubtropicalareasofthemetalsmayaffordsomeprotectionagainstplant‐eatinginsectsandmicrobialpathogens.Onlyrecentlyhaveinvestigatorsconsideredusingtheseplantstocleanupsoilandwastesitesthathavebeencontaminatedbytoxiclevelsofheavymetals–anenvironmentallyfriendlyapproachknownasphytoremediation.Thisscenariobeginswiththe