Plate Tectonics II: How it works Continental Crust versus Oceanic Crust Mantle Convection The Asthenosphere Boundaries Convergent Divergent Transform.

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.

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.

Plate Movement

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.


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.


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


Descending Slab

http://wwwrses.anu.edu.au/

Western Pacific



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


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.


A sinking slab of oceanic crust will partially melt as it heats up, creating pockets of magma that rise through the crust, forming volcanoes.


Edge of North American Plate

October 1, 2004

http://vulcan.wr.usgs.gov/

October 1, 2004


October 4, 2004

Volcanologists install new equipment in the crater.


October 4, 2004

Mount St. Helens lets out some more steam


Mount St. Helens


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




Oce

anic

-Oce

anic


Island Arcs

http://www.geology.pomona.edu/PETRO/IAMAP.GIF

Lesser Antilles

http://www.sabapark.com/research_activities/volcano_monitoring/volcanic_info.htm


Montserrat is a very small island with a very

big problem.

© Montserrat Volcano Observatory

Video Interlude

http://www.volcano.si.edu/

Uninhabitable zone.


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



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.



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.


Mid-Ocean Ridges


Pillow Basalts - lava extruded into ice-cold ocean water forms weird-looking structures that resemble pillows.



Spreading Centers


upwelling magma causes bulge


Spreading Centers

upwelling magma causes bulge rift valley forms as plates pull apart



Spreading Centers

upwelling magma causes bulge rift valley forms as plates pull apart oceanic crust accretes to continental crust as ridge forms



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://pubs.usgs.gov

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.


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.



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

Boundary Type

Convergent

Divergent

Transform

Volcanoes

yes

yes

no

Mountains

yes

yes

no

Earthquakes

shallow-deep

shallow

varies

Plate Tectonics II: How it works Continental Crust versus Oceanic Crust Mantle Convection The Asthenosphere Boundaries Convergent Divergent Transform.

Documents

transform crust

destruction of oceanic

convergent boundaries

western pacific slide

plate movement slide

plates meet

smaller plates

individual plates