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Teacher Instruction Sheet ModelingSeafloorSpreading AdaptedfromalessondevelopedbySanLorenzoUSDTeachers:JulieRamirez,VeenuSoni,MarilynStewart,and LawrenceYano(2012) TeacherBackground Theprocessofseafloorspreadingcreatedtheseaflooroftheoceans.Forexample,intheAtlantic Ocean,NorthAmericaandSouthAmericamovedawayfromEuropeandAfricaandtheresultingcrack wasfilledbymantlematerial,whichcooledandformednewlithosphere.Theprocesscontinuestoday. Moltenmantlematerialscontinuallyrisetofillthecracksformedastheplatesmoveslowlyapartfrom eachother.Thisprocesscreatesanunderwatermountainchain,knownasamid-oceanridge,alongthe zoneofnewlyformingseafloor. Moltenrockeruptsalongamid-oceanridge,thencoolsandfreezestobecomesolidrock.Thedirection ofthemagneticfieldoftheEarthatthetimetherockcoolsis"frozen"inplace.Thishappensbecause magneticmineralsinthemoltenrockarefreetorotatesothattheyarealignedwiththeEarth's magneticfield.Afterthemoltenrockcoolstoasolidrock,thesemineralscannolongerrotatefreely.At irregularintervals,averagingabout200-thousandyears,theEarth'smagneticfieldreverses.Theendofa compassneedlethattodaypointstothenorthwillinsteadpointtothesouthafterthenextreversal. Theoceanicplatesactasagianttaperecorder,preservingintheirmagneticmineralstheorientationof themagneticfieldpresentatthetimeoftheircreation.Geologistscallthecurrentorientation"normal" andtheoppositeorientation"reversed." USGS Teacher Instruction Sheet Inthefigureabove,twoplatesaremovingapart.Amid-oceanridgemarksthelocationwheremolten magmaandlavearemovingup,cooling,andformingnewoceanfloor.Thezonesofnormal magnetizationareindicatedby//////shadingoftheoceaniccrust. Thefigurebelowshowstheobservedmagneticpatternalongthemid-AtlanticRidgesouthofIceland. Mattox(1992) Basedonthepatternandspacingoftheoceanicmagneticstripesandtheinferredmotionoftheplates, theageoftheoceanfloorcanbedetermined.Inthefigurebelow,theageoftheoceanfloorisdepicted. Müllerandothers(1997) Teacher Instruction Sheet Suggestedtimetocompletethislessonistwo50-minuteperiodsbutmaybeadjustedforalonger classperiod. DAY1 • ShowavideoaboutHaroldHessandhisdiscoveries. https://www.youtube.com/watch?v=GyMLlLxbfa4 • AskstudentstotalktotheirelbowpartneranddiscusswitheachotherwhoWegenerwas.Have studentscalloutwhattheyrememberabouthisdiscoveries. • AskstudentshowWegenermusthavefeltwhenpeopledidnotsupporthisideaabout ContinentalDrift? • WhydidthescientificcommunityinitiallydismissWegner’sideaaboutContinentalDrift? • Whatdidotherscientistshaveissueswith? • HowdoHess’discoveriesrelatetowhatWegener’sContinentalDrift? DAY2 • Studentsmayneedclarificationaboutthemagneticpolereversals. • Usea“StericDemonstratorofMagneticLine–of-Force”modeltoshowtheexistenceofthe magneticfieldsandthespatialdistributionofmagneticlinesofforce. • “YesterdaywetalkedaboutHarryHessandhistheoryofseafloorspreading.Whatimportant discoveriesdidHessandhisfellowscientistsmake?“ ExpectedStudentResponses • Continuouschainofunderseamountainsknownasthemid-oceanridge • Symmetricalpatternsofmagneticreversalsshownbymetalcrystalsembeddedinrocks • Rocksthatwerefartherawayfromridgewereolder(ageofrocks) QuickWrite AskstudentswhymodelsarenecessaryforstudyingtheEarth?Studentsmayrecordtheirthoughts onthestudentdatasheet. ManyEarthprocessesoccuroververylargedistancesandoveraverylongperiodsoftime(millions ofyears).ThescaleofEarthprocessesmakesitdifficulttodirectlymeasuretheseprocesses.We havelearnedthatscientistsoftenusemodelstoexplaintheirthinking.Wehaveusedglobes,maps, diagrams;chartsetc.whensizeorsafetyissueskeepusfromviewingtherealthing.Sotodayweare goingtoconsiderhowwemightmakeamodelthatwouldexplainordemonstrateseafloor spreading. ThinkPairShare Turnandtalktoyourelbowpartner:Ifyouweremakingamodelofsea-floorspreading,whatarethe importantideasthatareassociatedwithseafloorspreading?Shareout. Expectedstudentanswers: •Themid-oceanridge •Symmetricalmagneticpatterns Teacher Instruction Sheet •Ageofrocks Studentsshouldusethestudentdatasheettorecordtheirthoughts. Youhavecomeupwithsomeimportantideasthatneedtobeincluded.(Ifanypartsaremissing,ask “Whatabout_______________doyouthinkthisshouldbeincluded?”) “TodaywearegoingtocreateamodelthatIhopewillincorporateallofyourimportantpoints.” 1.Studentswillmaketheirownmodelsofseafloorspreadingandthemagneticstripsthatoccurin theseafloor.Havestudentsworkingroupsoffour. 2.Havestudentspickupmaterialsneededfortheactivity. Materials: 60cmlongpaperstrip 1boxofmarkers 1barmagnet 1compass 1cardboardboxlid MeterStickorruler 3.Assignjobs: • StudentA:PaperPuller • StudentB:CompassReader • StudentC:Marker • StudentD:MagnetManager Thefollowingdirectionshavemanysteps.Sincetheyarepulling,marking,flippingetc.itis recommendedthattheteacherdemonstratehowtocreatethemodelbyshowingatleast2 examplesofmagneticreversals. 4.PaperPuller:Measureandcut60cmofthepaperstrip.Foldthestripinhalflengthwisesoitis approximately30cmlong.Pushtheendsofthestripthroughtheslitinthelidofacardboardbox sothatitisstickingoutabout5cmonthetopofthelid.Leavethefoldedendofthestrip underneaththebox. 5.Marker:Drawalineinthecenterofthepaper(whereitiscomingoutofthebox)makingsureto markbothsidesofthepaper. 6.CompassReader:Placethecompassnexttothepaperwhereitiscomingoutofthebox. 7.MagnetManager:Placethemagnetontheoppositesideofthepaperfromthecompass. 8.CompassReader:ObservewhichdirectionisNorthandtelltheMarker. 9.Marker:DrawanarrowpointingNorthonbothsidesofthepaper.Labelthefirstsectiononboth sideswithan“A”. 10.PaperPuller:pulloutsomemorepaper.Marker:drawalineinthecenterofthepaperwhereit cameoutofthebox.MagnetManager:Flipthemagnetintheoppositedirection.Compass Reader:Readthecompass.TelltheMarkertodrawanarrowonpointingtothenewNorthon bothsidesofthepaper.Labelthesecondsectiononbothsidesofthepaperwitha“B”. 11.Repeatstep10untilyoureachtheendofthepaper.MakesurethattheMagnetManagerflips themagneteachtime. Teacher Instruction Sheet LookingatPartsoftheModel Havestudentsrecordtheirideasonthestudentworksheet. 1. Whatdoesthepaperstripusedinthemodelrepresentinreallife?Paperstriprepresents seafloor 2. Whenthestripsofpaperarepulledout,whatprocessdoesthisrepresent?Pullingoutthestrips ofpaperrepresentstheprocessofseafloorspreading 3. Whatdoesthemagnetrepresent?ThemagnetrepresentstheEarth’smagneticfield 4. Whatdothesectionswitharrowsrepresent?(Whyaretheypointingindifferentdirections)? ArrowsrepresentthedifferentorientationsoftheEarth’smagneticfieldwhentheseafloor formed. 5. Whatdothedifferentsectionsofthepaperstrip(A,B,C,etc.)represent?Thedifferentsections representthemagneticstripesintheseafloor 6. Inthemodel,whatdoesflippingthemagnetrepresent?Flippingthemagnetrepresentsthe Earth’smagneticreversals 7. Didweincludealltheimportantelementsneededtoexplainseafloorspreading?Ifyes,whatare they?Ifno,whatismissing? 8. Canyouthinkofawaythatwecouldadd/delete/changeitemsthatwouldmakethismodelmore effective? 9. CanyouthinkofawaythatwecouldusethismodeltoexplainWegener’sideaofContinental Drift? ExitTicketInthespacebelow,drawadiagramofseafloorspreading.Includeasmuchdetailasyoucan. Vocabularywords–keyvocabularywordsthataretargetedortaughtaspartofthelesson. (Understandingthesewordsisessentialforstudentstounderstandthekeyconceptsofthislesson.) Seafloor MagnetMagneticField Mid-oceanRidge PotentialPitfalls • Studentsshouldunderstandthattherearenotvisiblestripesontheseafloor.Magneticstripesin diagramsportraytheorientationofthemagneticfieldthatismeasuredbyscientificinstruments thatarepreservedintherocksandarenotvisible. • Spreadingatmid-oceanridgesisnotacontinuousprocess(likeaconveyerbelt)butoccursin episodes.