plate tectonics ii: how it works continental crust versus oceanic crust mantle convection the...
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Plate Tectonics II: How it works
Continental Crust versus Oceanic CrustMantle ConvectionThe AsthenosphereBoundaries
ConvergentDivergentTransform
Upwelling Magmanew crust added
felsic and low density“light and fluffy”
mafic and high density“dark and dense”
Mid-Ocean Ridge
Continental Crust Oceanic Crust
Subducting Slabold crust destroyed Mantle
Aesthenosphere
thick and buoyantdoes not subduct
thin and sinksdoes subduct
Tectonic plates “float” on the higher density mantle.
The plates “float” on the asthenosphere, which is a thin boundary layer incorporating parts of the uppermost mantle and the base of the crust.
The asthenosphere is “mooshy” (can be sheared relatively easily) allowing the plates above the slide along.
The driving mechanism of plate tectonics is mantle convection. Hot mantle material rises at ridges and cooler mantle material sinks at subduction zones.
There are several large tectonic plates and a number of much smaller plates. The Earth’s continents sit on plates composed of both oceanic and continental crust.
The huge Pacific plate is composed almost entirely of oceanic crust, and is being subducted around almost its entire western boundary.
PacificPacific
NorthAmerican
NorthAmerican
NazcaNazcaSouth
AmericanSouth
American
AntarcticAntarctic
AfricanAfrican
EurasianEurasian
Australian-IndianAustralian-Indian
PhillipinePhillipineCaribbeanCaribbean
PacificPacific
Major Tectonic Plates
Plate Boundaries
convergent
divergent
transform
The contacts along which plates meet are called boundaries.Individual plates move relative to each other in one of three ways:
Convergent - moving toward each other. Divergent - moving away from each otherTransform - moving past each other
convergentdivergent
transform
The different kinds of boundaries have different properties:
Convergent – old oceanic crust destroyed through subduction Divergent – new oceanic crust accreted at mid-ocean ridgesTransform – crust neither created or destroyed
Convergent Boundaries
It is impossible for plates to move toward each other unless crust is “moved out of the way” - usually by
subduction and destruction of oceanic crust at trenches.
Convergent Boundaries
It is impossible for plates to move toward each other unless crust is “moved out of the way” - usually by subduction and destruction of oceanic crust at trenches.
Convergent Boundaries
Trenches form where oceanic crust is subducted down into the mantle.
Benioff Zones - zones of earthquake foci that increase in depth with distance from the trench in the direction of subduction
Western Pacific
http://wwwneic.cr.usgs.gov/
Benioff Zone
Convergent Boundaries
http://wwwneic.cr.usgs.gov/
The western Pacific Ocean is surrounded almost completely by trenches, which subduct the giant Pacific Plate and several smaller plates. The volcanic activity along the Pacific Rim inspired the nickname “Ring of Fire.”
Oceanic-Continental
Convergent Boundaries
Subduction leads to orogeny
http://geo.lsa.umich.edu/~crlb/COURSES/270/
A sinking slab of oceanic crust will partially melt as it heats up, creating pockets of magma that rise through the crust, forming volcanoes.
Convergent Boundaries
A sinking slab of oceanic crust will partially melt as it heats up, creating pockets of magma that rise through the crust, forming volcanoes.
http://geo.lsa.umich.edu/~crlb/COURSES/270/
Edge of North American Plate
Convergent Boundaries
When oceanic crust converges with oceanic crust, the denser plate (usually the oldest and coldest) generally subducts. An arc of volcanic islands forms from upwelling magma.
Oceanic-Oceanic
http://geo.lsa.umich.edu/~crlb/COURSES/270/
Convergent Boundaries
Island Arcs
http://www.geology.pomona.edu/PETRO/IAMAP.GIF
Lesser Antilles
http://www.sabapark.com/research_activities/volcano_monitoring/volcanic_info.htm
Convergent Boundaries
Montserrat is a very small island with a very
big problem.
© Montserrat Volcano Observatory
Video Interlude
http://www.volcano.si.edu/
Uninhabitable zone.
Convergent Boundaries
Since continental crust is buoyant, it does not subduct easily. While the edge of a plate can be dragged under by the weight of attached oceanic crust, it does not go far.
Continental-Continental
http://geo.lsa.umich.edu/~crlb/COURSES/270/
Convergent Boundaries
The massive compressive stresses caused by the collision causes huge low-angle reverse faults to form, stacking crust on crust. The crust also thickenes through ductile compression.
http://pubs.usgs.gov http://www.high-altitude-medicine.com/photos-mountain.html
The Himalayas
The Himalayas
Divergent Boundaries
When plates move away from each other, mantle material wells up into the space (called a rift), generally creating new oceanic crust. The great oceanic mid-ocean ridges are formed of this upwelling, basaltic magma.
Divergent Boundaries
Mid-ocean ridges are long mountains formed parallel to oceanic rifts.
The plates on either side of the ridge grow as new crustal material is added at the spreading center.
Rifts are characterized by relatively shallow earthquake foci along the length of the rift.
http://wwwneic.cr.usgs.gov/
Mid-Ocean Ridges
Divergent Boundaries
Pillow Basalts - lava extruded into ice-cold ocean water forms weird-looking structures that resemble pillows.
http://geo.lsa.umich.edu/~crlb/COURSES/270/
Divergent Boundaries
Spreading Centers
http://geo.lsa.umich.edu/~crlb/COURSES/270/
upwelling magma causes bulge
Divergent Boundaries
Spreading Centers
upwelling magma causes bulge rift valley forms as plates pull apart
http://geo.lsa.umich.edu/~crlb/COURSES/270/
Divergent Boundaries
Spreading Centers
upwelling magma causes bulge rift valley forms as plates pull apart oceanic crust accretes to continental crust as ridge forms
http://geo.lsa.umich.edu/~crlb/COURSES/270/
Divergent Boundaries
Spreading Centers
upwelling magma causes bulge rift valley forms as plates pull apart oceanic crust accretes to continental crust as ridge forms continents continue to move apart, opening ocean
http://geo.lsa.umich.edu/~crlb/COURSES/270/
Transform Boundaries
Most transform boundaries are associated with mid-ocean ridges (they form perpendicular to the rifts). One of the few major continental crust-continental crust transform boundaries runs up the west coast of North America.
Transform Boundaries
Most transform boundaries are associated with mid-ocean ridges (they form perpendicular to the rifts). This movement resolves stresses caused by different rates of spreading along the divergent boundary.
Transform Boundaries
The San Andreas fault system is part of a system of strike-slip faults caused by the relative motion of the North American and Pacific plates.
http://pubs.usgs.gov http://wwwneic.cr.usgs.gov/http://www.ingv.it/~roma/cultura/ingescuola/terremotopagina/SanAndreas.html