Download tectonics2a

The first velocity discontinuity discovered was
named for its discoverer. Most people now use
a shortened form of the name and call this
boundary the Moho.
Andrija
Mohorovicic´
discontinuities
Each discontinuity marks the
boundary between two layers
with different physiochemical
conditions.
Moho Discontinuity
Thickness Density
(km)
(g/cm3)
Crust
30
2.2
2.9
Upper Mantle
720
3.4
http://www.glossary.oilfield.slb.com
zone including
uppermost mantle and
lowermost crust is
called the
aesthenosphere
Lower Mantle
2,171
4.4
Outer Core
2,259
9.9
Inner Core
1,221
12.8
Typical
Rocks
silica rocks
andesite, 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)
Center
13.1
http://pubs.usgs.gov/gip/interior/
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
African
Nazca
South
American
Antarctic
Pacific
Australian-Indian
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.
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.
Fig. 19.12
The driving mechanism of
plate tectonics is mantle
convection. Hot mantle
material rises at ridges and
cooler mantle material
sinks at subduction zones.
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 created 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
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
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.
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
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
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/
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