modeling seafloor spreading · modeling seafloor spreading adapted from a lesson developed by san...

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.