AndrijaMohorovicic´
The first velocity discontinuity discovered was The first velocity discontinuity discovered was named for its discoverer. Most people now use named for its discoverer. Most people now use
a shortened form of the name and call this a shortened form of the name and call this boundary the Moho.boundary the Moho.
discontinuitiesdiscontinuities
Each discontinuity marks the Each discontinuity marks the boundary between two layers boundary between two layers with different physiochemical with different physiochemical conditions.conditions.
http://pubs.usgs.gov/gip/interior/
Thickness(km)
Density (g/cm3)
silica rocksandesite, basalt at base
peridodite, eclogite, olivine, spinel, garnet, pryoxene
magnesium and silicon oxides
iron+oxygen, sulfur, nickel alloy (liquid)
iron+oxygen, sulfur, nickel alloy (solid)
TypicalRocks
CrustCrust 30 2.22.9
Upper MantleUpper Mantle 720 3.4
Lower MantleLower Mantle 2,171 4.4
Outer CoreOuter Core 2,259 9.9
Inner CoreInner Core 1,221 12.8
CenterCenter 13.1
http://www.glossary.oilfield.slb.com
Moho DiscontinuityMoho Discontinuity
zone including zone including uppermost mantle and uppermost mantle and
lowermost crust is lowermost crust is called the called the
aesthenosphereaesthenosphere
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.
PacificPacificPacificPacific
NorthNorthAmericanAmerican
NorthNorthAmericanAmerican
NazcaNazcaNazcaNazcaSouthSouth
AmericanAmericanSouthSouth
AmericanAmerican
AntarcticAntarcticAntarcticAntarctic
AfricanAfricanAfricanAfrican
EurasianEurasianEurasianEurasian
Australian-IndianAustralian-IndianAustralian-IndianAustralian-Indian
PhillipinePhillipinePhillipinePhillipineCaribbeanCaribbeanCaribbeanCaribbean
PacificPacificPacificPacific
Major Tectonic PlatesMajor Tectonic Plates
The plates “float” on the astenosphere, which is a thin boundary zone where the mantle and crust meet.
The asthenosphere is “mooshy” (can be sheared relatively easily) allowing the plates above the slide along.
Upwelling MagmaUpwelling Magmanew crust added
felsic and low densityfelsic and low density“light and fluffy”
mafic and high densitymafic and high density“dark and dense”
Mid-Ocean RidgeMid-Ocean Ridge
Continental CrustContinental Crust Oceanic CrustOceanic Crust
Subducting SlabSubducting Slabold crust destroyed MantleMantle
AesthenosphereAesthenosphere
thick and buoyantthick and buoyantdoes not subduct
thin and sinksthin and sinksdoes subduct
Tectonic plates “float” on the higher density mantle. Tectonic plates “float” on the higher density mantle.
Fig. 19.12
The driving mechanism of plate tectonics is mantle mantle convectionconvection. Hot mantle material rises at ridges and cooler mantle material sinks at subduction zones.
Plate Plate BoundariesBoundaries
convergent
divergent
transform
The contacts along which plates meet are called boundariesboundaries.Individual plates move relative to each other in one of three ways:
ConvergentConvergent - moving toward each other. DivergentDivergent - moving away from each otherTransformTransform - moving past each other
convergentdivergent
transform
The different kinds of boundaries have different properties:
ConvergentConvergent – old oceanic crust destroyed through subduction DivergentDivergent – new oceanic crust created at mid-ocean ridgesTransformTransform – crust neither created or destroyed
Convergent BoundariesConvergent Boundaries
It is impossible for plates to move toward each other unless crust is “moved out of the way” - usually by subductionsubduction and destruction of oceanic crust at trenchestrenches.
Oceanic-ContinentalOceanic-Continental
Convergent BoundariesConvergent Boundaries
Subduction leads to orogenySubduction leads to orogeny
http://geo.lsa.umich.edu/~crlb/COURSES/270/
A sinking slab of oceanic crust will partially meltpartially melt as it heats up, creating pockets of magma that rise through the crust, forming volcanoes.
Convergent BoundariesConvergent 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-OceanicOceanic-Oceanic
http://geo.lsa.umich.edu/~crlb/COURSES/270/
Convergent BoundariesConvergent Boundaries
Island ArcsIsland Arcs
http://www.geology.pomona.edu/PETRO/IAMAP.GIF
Lesser Lesser AntillesAntilles
http://www.sabapark.com/research_activities/volcano_monitoring/volcanic_info.htm
Convergent BoundariesConvergent 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 BoundariesConvergent 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
The Himalayas
The Himalayas
Divergent BoundariesDivergent 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 centerspreading center..
Rifts are characterized by relatively shallow earthquake foci along the length of the rift.
http://wwwneic.cr.usgs.gov/
Mid-Ocean RidgesMid-Ocean Ridges
Transform BoundariesTransform Boundaries
The San Andreas fault systemSan 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
Boundary TypeBoundary Type
ConvergentConvergent
DivergentDivergent
TransformTransform
VolcanoesVolcanoes
yesyes
yesyes
nono
MountainsMountains
yesyes
yesyes
nono
EarthquakesEarthquakes
shallow-deepshallow-deep
shallowshallow
variesvaries