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Teacher Instruction Sheet
ModelingSeafloorSpreadingAdaptedfromalessondevelopedbySanLorenzoUSDTeachers:JulieRamirez,VeenuSoni,MarilynStewart,andLawrenceYano(2012)TeacherBackgroundTheprocessofseafloorspreadingcreatedtheseaflooroftheoceans.Forexample,intheAtlanticOcean,NorthAmericaandSouthAmericamovedawayfromEuropeandAfricaandtheresultingcrackwasfilledbymantlematerial,whichcooledandformednewlithosphere.Theprocesscontinuestoday.Moltenmantlematerialscontinuallyrisetofillthecracksformedastheplatesmoveslowlyapartfromeachother.Thisprocesscreatesanunderwatermountainchain,knownasamid-oceanridge,alongthezoneofnewlyformingseafloor.Moltenrockeruptsalongamid-oceanridge,thencoolsandfreezestobecomesolidrock.ThedirectionofthemagneticfieldoftheEarthatthetimetherockcoolsis"frozen"inplace.ThishappensbecausemagneticmineralsinthemoltenrockarefreetorotatesothattheyarealignedwiththeEarth'smagneticfield.Afterthemoltenrockcoolstoasolidrock,thesemineralscannolongerrotatefreely.Atirregularintervals,averagingabout200-thousandyears,theEarth'smagneticfieldreverses.Theendofacompassneedlethattodaypointstothenorthwillinsteadpointtothesouthafterthenextreversal.Theoceanicplatesactasagianttaperecorder,preservingintheirmagneticmineralstheorientationofthemagneticfieldpresentatthetimeoftheircreation.Geologistscallthecurrentorientation"normal"andtheoppositeorientation"reversed."
USGS
Teacher Instruction Sheet
Inthefigureabove,twoplatesaremovingapart.Amid-oceanridgemarksthelocationwheremoltenmagmaandlavearemovingup,cooling,andformingnewoceanfloor.Thezonesofnormalmagnetizationareindicatedby//////shadingoftheoceaniccrust.Thefigurebelowshowstheobservedmagneticpatternalongthemid-AtlanticRidgesouthofIceland.
Mattox(1992)
Basedonthepatternandspacingoftheoceanicmagneticstripesandtheinferredmotionoftheplates,theageoftheoceanfloorcanbedetermined.Inthefigurebelow,theageoftheoceanfloorisdepicted.
Müllerandothers(1997)
Teacher Instruction Sheet
Suggestedtimetocompletethislessonistwo50-minuteperiodsbutmaybeadjustedforalongerclassperiod.
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)QuickWriteAskstudentswhymodelsarenecessaryforstudyingtheEarth?Studentsmayrecordtheirthoughtsonthestudentdatasheet.ManyEarthprocessesoccuroververylargedistancesandoveraverylongperiodsoftime(millionsofyears).ThescaleofEarthprocessesmakesitdifficulttodirectlymeasuretheseprocesses.Wehavelearnedthatscientistsoftenusemodelstoexplaintheirthinking.Wehaveusedglobes,maps,diagrams;chartsetc.whensizeorsafetyissueskeepusfromviewingtherealthing.Sotodaywearegoingtoconsiderhowwemightmakeamodelthatwouldexplainordemonstrateseafloorspreading.ThinkPairShareTurnandtalktoyourelbowpartner:Ifyouweremakingamodelofsea-floorspreading,whataretheimportantideasthatareassociatedwithseafloorspreading?Shareout.Expectedstudentanswers:
•Themid-oceanridge•Symmetricalmagneticpatterns
Teacher Instruction Sheet
•AgeofrocksStudentsshouldusethestudentdatasheettorecordtheirthoughts.Youhavecomeupwithsomeimportantideasthatneedtobeincluded.(Ifanypartsaremissing,ask“Whatabout_______________doyouthinkthisshouldbeincluded?”)“TodaywearegoingtocreateamodelthatIhopewillincorporateallofyourimportantpoints.”1.Studentswillmaketheirownmodelsofseafloorspreadingandthemagneticstripsthatoccurin
theseafloor.Havestudentsworkingroupsoffour.
2.Havestudentspickupmaterialsneededfortheactivity.
Materials:
60cmlongpaperstrip 1compass1boxofmarkers 1cardboardboxlid1barmagnet MeterStickorruler
3.Assignjobs:
• StudentA:PaperPuller• StudentB:CompassReader• StudentC:Marker• StudentD:MagnetManager
Thefollowingdirectionshavemanysteps.Sincetheyarepulling,marking,flippingetc.itisrecommendedthattheteacherdemonstratehowtocreatethemodelbyshowingatleast2examplesofmagneticreversals.
4.PaperPuller:Measureandcut60cmofthepaperstrip.Foldthestripinhalflengthwisesoitis
approximately30cmlong.Pushtheendsofthestripthroughtheslitinthelidofacardboardboxsothatitisstickingoutabout5cmonthetopofthelid.Leavethefoldedendofthestripunderneaththebox.
5.Marker:Drawalineinthecenterofthepaper(whereitiscomingoutofthebox)makingsuretomarkbothsidesofthepaper.6.CompassReader:Placethecompassnexttothepaperwhereitiscomingoutofthebox.7.MagnetManager:Placethemagnetontheoppositesideofthepaperfromthecompass.8.CompassReader:ObservewhichdirectionisNorthandtelltheMarker.9.Marker:DrawanarrowpointingNorthonbothsidesofthepaper.Labelthefirstsectiononboth
sideswithan“A”.10.PaperPuller:pulloutsomemorepaper.Marker:drawalineinthecenterofthepaperwhereit
cameoutofthebox.MagnetManager:Flipthemagnetintheoppositedirection.CompassReader:Readthecompass.TelltheMarkertodrawanarrowonpointingtothenewNorthonbothsidesofthepaper.Labelthesecondsectiononbothsidesofthepaperwitha“B”.
11.Repeatstep10untilyoureachtheendofthepaper.MakesurethattheMagnetManagerflipsthemagneteachtime.
Teacher Instruction Sheet
LookingatPartsoftheModelHavestudentsrecordtheirideasonthestudentworksheet.1. Whatdoesthepaperstripusedinthemodelrepresentinreallife?Paperstriprepresents
seafloor2. Whenthestripsofpaperarepulledout,whatprocessdoesthisrepresent?Pullingoutthestrips
ofpaperrepresentstheprocessofseafloorspreading3. Whatdoesthemagnetrepresent?ThemagnetrepresentstheEarth’smagneticfield4. Whatdothesectionswitharrowsrepresent?(Whyaretheypointingindifferentdirections)?
ArrowsrepresentthedifferentorientationsoftheEarth’smagneticfieldwhentheseafloorformed.
5. Whatdothedifferentsectionsofthepaperstrip(A,B,C,etc.)represent?Thedifferentsectionsrepresentthemagneticstripesintheseafloor
6. Inthemodel,whatdoesflippingthemagnetrepresent?FlippingthemagnetrepresentstheEarth’smagneticreversals
7. Didweincludealltheimportantelementsneededtoexplainseafloorspreading?Ifyes,whatarethey?Ifno,whatismissing?
8. Canyouthinkofawaythatwecouldadd/delete/changeitemsthatwouldmakethismodelmoreeffective?
9. CanyouthinkofawaythatwecouldusethismodeltoexplainWegener’sideaofContinentalDrift?
ExitTicketInthespacebelow,drawadiagramofseafloorspreading.Includeasmuchdetailasyoucan. Vocabularywords–keyvocabularywordsthataretargetedortaughtaspartofthelesson.(Understandingthesewordsisessentialforstudentstounderstandthekeyconceptsofthislesson.)Seafloor MagnetMagneticField Mid-oceanRidge PotentialPitfalls• Studentsshouldunderstandthattherearenotvisiblestripesontheseafloor.Magneticstripesin
diagramsportraytheorientationofthemagneticfieldthatismeasuredbyscientificinstrumentsthatarepreservedintherocksandarenotvisible.
• Spreadingatmid-oceanridgesisnotacontinuousprocess(likeaconveyerbelt)butoccursinepisodes.
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