The$Way$the$Earth$Works:$Plate$Tectonics$Plate&Tectonics&The¶digm&of&“how&the&Earth&works”&• Earth’s&outer&shell&is&broken&into&rigid&plates&that&move&&A&case&study&of&scientific&revolution&• It&allows&scientists&to&predict&geologic&events,&and…&• Reconstruct&the&geologic&past&Tectonic&theory&evolved&in&the&1960s&Previous&research&provided&a&strong&foundation&• Wegener&(1915)&–evidence&supporting&continental&drift&• Hess/Dietz&(1968)&–the&seaQfloor&spreading&hypothesis&• By&1968,&evidence&for&tectonics&was&overwhelming&&Sea&floor&spreading&+&continental&drift&=&plate&tectonics&Plate&tectonic&theory&provides&a&unified&mechanism&explaining:&• Igneous&sedimentary,&and&metamorphic&rocks&• The&distribution&of&earthquakes&and&volcanoes&• The&origin&of&continents&and&ocean&basins&• The&distributions&of&fossil&plants&and&animals&• The&genesis&and&destruction&of&mountain&chains&• Continental&drift&Lithosphere&Tectonic&plates&are&fragments&of&lithosphere&• Lithosphere&is&made&of&BOTH&crust&and&the&upper&mantle&• The&lithosphere&is&in&motion&over&the&asthenosphere&&Lithosphere&bends&elastically&when&loaded&Asthenosphere&flows&plastically&when&loaded&Buoyancy&• First&described&by&Archimedes&more&than&2.2&Ka&• Floating&solids&displace&water&equal&to&their&mass&• An&iceberg&“sinks”&until&the&mass&of&water&it&displaces&is&equal&to&the&total&mass&of&the&iceberg&• This&concept&applies&to&lithospheric&plates&2&types&of&lithosphere&Continental&~150km&thick&• Granitic&crust&• 35Q40&km&thick&• Lighter&(less&dense)&• More&buoyant&–floats&higher&Oceanic&~7&to&100&km&thick&• Basaltic&crust&• 7Q10&km&thick&• Heavier&(more&dense)&&• Less&buoyant&–sinks&lower&Plate&boundaries&• Lithosphere&is&fragmented&into&~20&tectonic&plates&• Plates&move&continuously&at&a&rate&of&1&to&15&cm/yr&• Plates&interact&along&their&boundaries’&Global&earthquakes&• Outline&lithospheric&plate&• Specific&types&of&earthquakes&occur&at&different&types&of&plate&boundaries&Plate&boundaries&Location&on&Earth&where&tectonic&plates&meet&• Identified&by&concentrations&of&earthquakes&• Associated&with&many&other&dynamic&phenomena&Plate&interiors&are&almost&earthquakeQfree&Continental&margins&• Where&land&meets&the&ocean&• Margins&near&plate&boundaries&are&“active”&• Margins&far&from&plate&boundaries&are&“passive”&• Passive&margin&continental&crust&thins&seaward&• Transitions&into&oceanic&crust&&• Traps&eroded&sediment&• Develops&into&the&continental&shelf&Plate&boundaries:&3&types&• Divergent&–normal&faults&(e.g.&midQocean&ridge)&• TransformQ&strikeQslips&faults&(e.g.&San&Andreas&fault)&• Convergent&–reverse&or&thrust&faults&(e.g.&the&Andes)&Divergent&–Tectonic&plates&move&apart&(also&called&spreading&boundary)&Convergent&–tectonic&plates&move&together&(also&called&convergent&margin&subduction&zone&consuming&boundary&trench)&Transform&–tectonic&plates&slide&sideways&(also&called&transform&fault)&Divergent&boundaries&• Sea&floor&spreading&causes&plates&to&move&apart&• Magma&wells&up&to&fill&the&gap&• Magma&cools,&adding&material&to&each&plate&• Example&–&the&Red&Sea&MidQOcean&Ridges&• Linear&mountain&ranges&in&Earth’s&ocean&basin&• ExampleQ&the&Mid&Atlantic&Ridge&• Sea&floor&spreading&opens&the&axial&rift&valley&• Rising&asthenosphere&melts,&forming&mafic&magma&• Pooled&magma&solidifies&into&oceanic&crustal&rock&• Pillow&basalt&–magma&quenched&at&the&sea&floor&• Dikes&–preserved&magma&conduits&• GabbroQ&deeper&magma&Ocean&Crustal&Age&• New&crust&is&closer&to&the&ridge;&older&crust&farther& a way&• Oldest&oceanic&crust&is&found&at&the&far&edge&of&the&basin&Oceanic&lithosphere&• The&hot&asthenosphere&is&at&the&base&of&the&MOR&• Aging&ocean&crust&moves&away&from&this&heat…&• Cooling,&increasing&in&density,&and&sinking&• Accumulating&increasing&thicknesses&of&sediment&Convergent&boundaries&• Lithospheric&plates&move&toward&one&another&• One&plate&dives&back&into&the&mantle&(Subduction)&• Subduction&recycles&oceanic&lithosphere&• Subduction&is&balanced&by&sea&floor&spreading&• Earth&maintains&a&constant&circumference&&&&Subduction&• Old&oceanic&lithosphere&is&more&dense&than&mantle&&• Bent&down,&the&leading&edge&sinks&like&an&anchor&Fate&of&subducted&plates?&• The&lower&the&mantle&may&be&a&“plate&graveyard”&Subduction&Features&• Subduction&is&associated&with&unique&features:&• Accretionary&prisms&&• Volcanic&arcs&• Back&arc&basins&Accretionary&prisms&–deformed&sediment&wedges&• Sediments&scraped&off&subducting&plates&are&smeared&and&welded&onto&the&overriding&plates&&• These&contorted&sediments&can&be&pushed&above&sea&Volcanic&arcQ&volcanic&belt&on&an&overriding&plate&• The&descending&plate&partially&melts&at&~150km&depth&• Magmas&burn&upward,&fueling&volcanic&eruptions&• A&curved&earth&&volcanic&belts&are&curved&&• Arc&Type:&• Continental&crustQ&continental&arc&• Oceanic&–island&arc&Back&arc&basinQ&a&marginal&sea&behind&an&arc&• Forms&between&an&island&arc&and&a&continent&• Offshore&subduction&traps&a&piece&of&oceanic&curst&or…&• Stretching&creates&a&new&spreading&ridge&Types&of&convergent&boundaries&• OceanQOcean&convergence&• OceanQcontinent&convergence&&• ContinentQContinent&convergence&Transforms&boundaries&• Lithospheric&slides&past;¬&created&or&destroyed&• Characterized&by..&• Earthquakes&&• Absence&of&volcanism&Oceanic&transforms&• The&MOR&axis&is&offset&by&transform&faults&• Transforms&bear&strong&evidence&of&sea&floor&spreading&&• Abundant&earthquakes&common&between&ridge&segments&&• Earthquakes&vanish&past&ridge&segment&overlaps&Lithospheric&plates&slide&past&each&other&• Many&transform&faults&offset&spreading&ridge&segments&• Some&transforms&cut&through&continental&crust&Continental&transforms&–chop&continental&crust&•
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