Download Modeling Seafloor Spreading

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.