Ch02_2e_Ans_to_Problems.pdf
CH02_2e_IM.pdf
Chapter_02_Answers_to_Quick_Review.pdf
Chapter 2: Earth Structure and Plate Tectonics
Answers to Study Problems
1) Estimatethethicknessofoceaniclithospherethatisa)5million,b)10 million,c)20million,andd)50millionyearsold.
Solution:
Thickness(km)=10xsqrt(ageinmillionsofyears)
a) Thickness=10xsqrt(5)=22.4km
b) Thickness=10xsqrt(10)=31.6km
c) Thickness=10xsqrt(20)=44.7km
d) Thickness=10xsqrt(50)=70.7km
2) Giventheresultsyouobtainedinthepreviousproblem,howwouldyou describeaplotofoceaniclithospherethicknessasafunctionofageofthe lithosphere?
Solution:
Theplotisnotlinear. Thethicknessincreasesmorerapidlyatyoungerages thanatolderages.
3) NeartheHawaiianIslands,thePacificPlateismovingtothenorthwestata speedofabout7cm(~2.76in)peryear. HowfarwilltheIslandofOahu movein30millionyears?Giveyouranswerinkmandmiles. Comparethis withthedistancebetweenLosAngeles,CAandNewYork,NY.
Solution:
Distance(km)=(7cm/yr)X(30,000,000yr)x(1km/100,000cm) =2100km
Distance(mi)=(2100km)x(1mi/1.609km)=1305mi
ThedistancebetweenLosAngelesandNewYorkisabout3944kmor2451 mi. ThisisroughlytwicethedistancethatOahuwouldmove.
Chapter 2: Earth Structure and Plate Tectonics
Quick Review Questions
2.1 Earth’s Interior
1. P-wavestravelthroughsolids,liquids,andgases. S-wavestravelthroughsolids.
2. Continentalcrustiscomposedlargelyofgranitewithanaveragedensityof2.7 g/cm3 andanaveragethicknessof40km. Oceaniccrustiscomposedlargelyof basaltwithanaveragedensityof2.9g/cm3 andanaveragethicknessof7km.
3. TheMohoistheboundarybetweenthecrustandthemantle. Itisdeeperunderthe continentsbecausecontinentalcrustisgenerallythickerthanoceaniccrust.
4. (a)Thecrustandmantlearelayersthatdifferfromoneanotherinchemical composition.(b)Thelithosphere,asthenosphere,andmesospherearelayersthat differfromoneanotherinrigidity.
5. Becauseofisostaticequilibrium.
6. Theoutercoreaninnercorearesimilarbecausetheyhavethesamechemistry. The outercoreandinnercorearedifferentbecausetheyareindifferentphysicalstates. Theoutercorebehaveslikealiquidandtheinnercoreissolid.
2.2 History of a Theory: Continental Drift
1. Wegenerusedavarietyofdifferentkindsofevidenceinproposinghistheoryof driftingcontinents. Theseincludedthegeographicfitofcontinents,andseveral differentfeaturesthatcanbefoundondifferentcontinentsnowbutthatfittogether whenthecontinentsarejoinedsuchas:patternsoffossilplantsandanimals; mountainrangesofsimilarage,structure,andcomposition;unusualsequencesof rocksandrockunits;andpatternsofglaciation.
2. Pangaeaincludedallmodernlandmasses.
3. LaurasiaincludedNorthAmericaandEurasia. GondwanalandincludedAfrica, SouthAmerica,India,Australia,andAntarctica.
4. Theprimaryobjectionwasthelackofaplausibledrivingmechanism.
2.3 Evidence for a New Theory: Seafloor Spreading
1. Mantleconvectioncellsarecausedbyheatingofthemantlefrombelow.
2. IntheHessmodel,theplatesarecarriedontopofmantleconvectioncells.
3. Earthquakesarenotuniformlydistributedaroundtheglobe. Theyconcentrate alongtheedgesofplates. Theyaregenerallyshallowalongdivergentandtransform boundariesandrangefromshallowtodeepalongconvergentboundariesinvolving platesubduction. Heatflowisveryhighalongdivergentboundariesanddecreases withincreasingdistancefromthem. TheageofEarth’scrustincreaseswith increasingdistancefromdivergentboundaries. Thethicknessofseafloorsediments increaseswithincreasingdistancefromdivergentboundaries.
4. MagneticreversalsoccurwhenthepolarityofEarth’smagneticfieldreverses. The polarityofEarth’smagneticfieldisrecordedinseafloorrocksastheycool. Whena magneticreversaloccurs,theeventisrecordedintherocks.
5. Thepatternsofmagneticreversalsinseafloorrocksarecharacterizedbyroughly linearbandsofhighandlowmagneticintensityintherocksthatareparalleltothe spreadingcenter.
2.4 Plate Tectonics
1. Thisisamechanicalexerciseforthestudents.
2. Divergentboundarieswhereplatesmoveawayfromeachother. Convergent boundarieswhereplatesmovetowardeachother. Transformboundarieswhere platesslidepasteachother.
3. Mostdivergentplateboundariesarefoundintheoceanbasins.
4. Atransformfaultistheactiveportionofafracturezone. Transformfaultsare locatedbetweensegmentsofridgecrest. Transformfaultsareplateboundaries.
5. Mosttransformfaultsjointwosegmentsofdivergentboundaries.
6. AWadati-BenioffzoneisazoneofearthquakeactivitythatdescendsintoEarth’s interior. Theyareassociatedwithoceantrenchesandsubductingoceanic lithosphere.
7. Oceaniclithosphereisdenserandheavierthancontinentallithosphere.
8. Apassivecontinentalmarginisnotaplateboundary. Anactivecontinentalmargin isaplateboundary. Activecontinentalmarginsoftenhaveoceanictrenchesare transformfaults.
2.5 Motion of the Plates
1. Theaveragerateofseafloorspreadingisabout5cm(2in)peryear.
2. Intheconvectionmodelofplatemotiontheplatesrideontopoflargeconvection cellsinthemantle. Thismodelisdrivenbyheatandgravity.
3. Intheridge-push,slab-pullmodelofplatemotiontheplatesslidedownthesidesof mid-oceanridges(pushingontherestoftheplate). Astheymoveawayfromthe ridgestheycool,thicken,andincreaseindensity. Eventuallytheyarepulledintothe
mantleatoceantrenchesbytheweightoftheirleadingedges. Thismodelisdriven bygravity.
4. Ahotspotisapersistentrisingplumeofhotmantlematerialthatremainsstationary forlongperiodsoftime.
5. HotspotscreatevolcanoesatEarth’ssurface. Asaplatemovesoverthehotspot,a chainofextinctvolcanoeswillbecreatedmarkingboththedirectionofplate movementandthespeedoftheplate.
2.6 History of the Continents
1. ThePanthalassaOceanwasamassiveoceanthatincludedmostofEarth’sseawater. TheTethysSeawasamuchsmallerbodyofwateroccupyinganindentationin Pangaeabetweenwhatwouldeventuallybecomepresent-dayAustraliaandAsia.
2. Panthalassaeventuallybecamethepresent-dayPacificOcean.
3. TheWilsoncycleisadescriptionofthestagesinthecreationanddestructionofan oceanbasin. Aninitialcontinentisbrokenapartbyrifting,creatingayoung, shallow,oceanbasin. Continuedriftingwidenstheoceanbasinandseafloor eventuallybecomesoldenoughtosubduct. Subductionhastensclosingoftheocean basinandeventuallycontinentalcollisioneradicatestheoceanentirely.
Chapter 2: Earth Structure and Plate Tectonics
Chapter 2: Quick Review Questions
2.1 Earth’s Interior
1. P-wavestravelthroughsolids,liquids,andgases. S-wavestravelthroughsolids.
2. Continentalcrustiscomposedlargelyofgranitewithanaveragedensityof2.7 g/cm3 andanaveragethicknessof40km. Oceaniccrustiscomposedlargelyof basaltwithanaveragedensityof2.9g/cm3 andanaveragethicknessof7km.
3. TheMohoistheboundarybetweenthecrustandthemantle. Itisdeeperunderthe continentsbecausecontinentalcrustisgenerallythickerthanoceaniccrust.
4. (a)Thecrustandmantlearelayersthatdifferfromoneanotherinchemical composition.(b)Thelithosphere,asthenosphere,andmesospherearelayersthat differfromoneanotherinrigidity.
5. Becauseofisostaticequilibrium.
6. Theoutercoreaninnercorearesimilarbecausetheyhavethesamechemistry. The outercoreandinnercorearedifferentbecausetheyareindifferentphysicalstates. Theoutercorebehaveslikealiquidandtheinnercoreissolid.
2.2 History of a Theory: Continental Drift
1. Wegenerusedavarietyofdifferentkindsofevidenceinproposinghistheoryof driftingcontinents. Theseincludedthegeographicfitofcontinents,andseveral differentfeaturesthatcanbefoundondifferentcontinentsnowbutthatfittogether whenthecontinentsarejoinedsuchas:patternsoffossilplantsandanimals; mountainrangesofsimilarage,structure,andcomposition;unusualsequencesof rocksandrockunits;andpatternsofglaciation.
2. Pangaeaincludedallmodernlandmasses.
3. LaurasiaincludedNorthAmericaandEurasia. GondwanalandincludedAfrica, SouthAmerica,India,Australia,andAntarctica.
4. Theprimaryobjectionwasthelackofaplausibledrivingmechanism.
2.3 Evidence for a New Theory: Seafloor Spreading
1. Mantleconvectioncellsarecausedbyheatingofthemantlefrombelow.
2. IntheHessmodel,theplatesarecarriedontopofmantleconvectioncells.
3. Earthquakesarenotuniformlydistributedaroundtheglobe. Theyconcentrate alongtheedgesofplates. Theyaregenerallyshallowalongdivergentandtransform boundariesandrangefromshallowtodeepalongconvergentboundariesinvolving platesubduction. Heatflowisveryhighalongdivergentboundariesanddecreases withincreasingdistancefromthem. TheageofEarth’scrustincreaseswith increasingdistancefromdivergentboundaries. Thethicknessofseafloorsediments increaseswithincreasingdistancefromdivergentboundaries.
4. MagneticreversalsoccurwhenthepolarityofEarth’smagneticfieldreverses. The polarityofEarth’smagneticfieldisrecordedinseafloorrocksastheycool. Whena magneticreversaloccurs,theeventisrecordedintherocks.
5. Thepatternsofmagneticreversalsinseafloorrocksarecharacterizedbyroughly linearbandsofhighandlowmagneticintensityintherocksthatareparalleltothe spreadingcenter.
2.4 Plate Tectonics
1. Thisisamechanicalexerciseforthestudents.
2. Divergentboundarieswhereplatesmoveawayfromeachother. Convergent boundarieswhereplatesmovetowardeachother. Transformboundarieswhere platesslidepasteachother.
3. Mostdivergentplateboundariesarefoundintheoceanbasins.
4. Atransformfaultistheactiveportionofafracturezone. Transformfaultsare locatedbetweensegmentsofridgecrest. Transformfaultsareplateboundaries.
5. Mosttransformfaultsjointwosegmentsofdivergentboundaries.
6. AWadati-BenioffzoneisazoneofearthquakeactivitythatdescendsintoEarth’s interior. Theyareassociatedwithoceantrenchesandsubductingoceanic lithosphere.
7. Oceaniclithosphereisdenserandheavierthancontinentallithosphere.
8. Apassivecontinentalmarginisnotaplateboundary. Anactivecontinentalmargin isaplateboundary. Activecontinentalmarginsoftenhaveoceanictrenchesare transformfaults.
2.5 Motion of the Plates
1. Theaveragerateofseafloorspreadingisabout5cm(2in)peryear.
2. Intheconvectionmodelofplatemotiontheplatesrideontopoflargeconvection cellsinthemantle. Thismodelisdrivenbyheatandgravity.
3. Intheridge-push,slab-pullmodelofplatemotiontheplatesslidedownthesidesof mid-oceanridges(pushingontherestoftheplate). Astheymoveawayfromthe ridgestheycool,thicken,andincreaseindensity. Eventuallytheyarepulledintothe
Copyright ©McGraw-Hill Education. Permission required for reproduction or display.
mantleatoceantrenchesbytheweightoftheirleadingedges. Thismodelisdriven bygravity.
4. Ahotspotisapersistentrisingplumeofhotmantlematerialthatremainsstationary forlongperiodsoftime.
5. HotspotscreatevolcanoesatEarth’ssurface. Asaplatemovesoverthehotspot,a chainofextinctvolcanoeswillbecreatedmarkingboththedirectionofplate movementandthespeedoftheplate.
2.6 History of the Continents
1. ThePanthalassaOceanwasamassiveoceanthatincludedmostofEarth’sseawater. TheTethysSeawasamuchsmallerbodyofwateroccupyinganindentationin Pangaeabetweenwhatwouldeventuallybecomepresent-dayAustraliaandAsia.
2. Panthalassaeventuallybecamethepresent-dayPacificOcean.
3. TheWilsoncycleisadescriptionofthestagesinthecreationanddestructionofan oceanbasin. Aninitialcontinentisbrokenapartbyrifting,creatingayoung, shallow,oceanbasin. Continuedriftingwidenstheoceanbasinandseafloor eventuallybecomesoldenoughtosubduct. Subductionhastensclosingoftheocean basinandeventuallycontinentalcollisioneradicatestheoceanentirely.
Copyright ©McGraw-Hill Education. Permission required for reproduction or display.