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