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Plate Tectonics II: How it works
Continental Crust versus Oceanic Crust
Mantle Convection
The Asthenosphere
Boundaries
Convergent
Divergent
Transform
Continental Crust
Oceanic Crust
felsic and low density
“light and fluffy”
thick and buoyant
does not subduct
mafic and high density
“dark and dense”
thin and sinks
does subduct
Mid-Ocean Ridge
Aesthenosphere
Subducting Slab
old crust destroyed
Mantle
Upwelling Magma
new crust added
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
Major Tectonic Plates
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.
Eurasian
North
American
Pacific
Phillipine
Caribbean
Pacific
African
Nazca
South
American
Antarctic
Australian-Indian
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 other
Transform - moving past each other
Plate
Boundaries
convergent
divergent
transform
The different kinds of boundaries have different properties:
Convergent – old oceanic crust destroyed through subduction
Divergent – new oceanic crust accreted at mid-ocean ridges
Transform – crust neither created or destroyed
convergent
divergent
transform
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
Benioff Zone
Western Pacific
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
http://wwwneic.cr.usgs.gov/
Convergent Boundaries
Descending Slab
Western Pacific
http://wwwrses.anu.edu.au/
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.”
Convergent Boundaries
Oceanic-Continental
http://geo.lsa.umich.edu/~crlb/COURSES/270/
Subduction leads to orogeny
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
Edge of North American Plate
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.
October 1, 2004
http://vulcan.wr.usgs.gov/
October 1, 2004
http://vulcan.wr.usgs.gov/
October 4, 2004
Volcanologists install new
equipment in the crater.
http://vulcan.wr.usgs.gov/
October 4, 2004
Mount St. Helens lets out some more steam
http://vulcan.wr.usgs.gov/
Mount St. Helens
Convergent Boundaries
Oceanic-Oceanic
http://geo.lsa.umich.edu/~crlb/COURSES/270/
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
Convergent Boundaries
http://geo.lsa.umich.edu/~crlb/COURSES/270/
Convergent Boundaries
Island Arcs
Lesser
Antilles
http://www.geology.pomona.edu/PETRO/IAMAP.GIF
http://www.sabapark.com/research_activities/volcano_monitoring/volcanic_info.htm
Convergent Boundaries
Video Interlude
© Montserrat Volcano Observatory
Montserrat is a very
small island with a very
big problem.
Uninhabitable zone.
http://www.volcano.si.edu/
Convergent Boundaries
Continental-Continental
http://geo.lsa.umich.edu/~crlb/COURSES/270/
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.
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
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
Divergent Boundaries
Mid-Ocean Ridges
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/
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
http://geo.lsa.umich.edu/~crlb/COURSES/270/
upwelling magma causes bulge
rift valley forms as plates pull apart
Divergent Boundaries
Spreading Centers
http://geo.lsa.umich.edu/~crlb/COURSES/270/
upwelling magma causes bulge
rift valley forms as plates pull apart
oceanic crust accretes to continental crust as ridge forms
Divergent Boundaries
Spreading Centers
http://geo.lsa.umich.edu/~crlb/COURSES/270/
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
Divergent Boundaries
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 crustcontinental 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
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://www.ingv.it/~roma/cultura/ingescuola/terremotopagina/SanAndreas.html
http://wwwneic.cr.usgs.gov/
Boundary Type Volcanoes Mountains Earthquakes
Convergent
yes
yes
shallow-deep
Divergent
yes
yes
shallow
Transform
no
no
varies