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Brain Bender
 I am the layer of the earth that is responsible
for moving the tectonic plates. The
temperature differential between the top and
bottom of my layer causes convection.
 Which layer of the earth am I?
Plate Tectonics
What is causing plates
to move?
Plate Tectonics
Plate Tectonics: A model of global
tectonics that suggests that the outer
layer of the Earth, the lithosphere, is
composed of several rigid, large plates
that move relative to one another by
sliding on a weak layer, the
asthenosphere in the upper mantle;
continents and ocean basins are
passive riders on these plates.
This is a unifying concept of geology
developed after WWII and only
accepted during the 1960s!
Some Specifics on Plates
 Plates are composed of fragments of the
lithosphere (crust and upper mantle)
 Plates may include both continental and
oceanic crust
 Plates move in relation to each other at
varied rates
 No major tectonic movements within
plates
 Dynamic actions concentrated along plate
boundaries
Plate Boundaries
Plate Boundaries
Three types of plate boundaries:
 Convergent: plates collide, resulting in subduction and
mountain building
 Divergent: plates moving apart, resulting in new lithosphere
produced at mid-oceanic ridge
 Transform: two plates slide past one another
Remember: Earth’s interior convection is mechanism for
plate tectonics
Convection
in the Earth
Convergent Boundaries
 Plates move together (converge)
 One plate may be forced beneath another
Subduction: Process in which one lithospheric
plate descends beneath another
Subduction Zone: Convergence of tectonic plates
where one plate dives beneath another and is
consumed in the mantle
 3 possible scenarios:
 Ocean to Ocean
 Ocean to Continent
 Continent to Continent
Convergent Boundaries
Ocean-Ocean Convergence
 Oceanic plate meets oceanic plate
 Denser plate forced under (Subduction)
 Trench forms
 Volcanism occurs in “Island Arc”
 Wide earthquake zones
 Examples: Japan, Philippines
Oceanic-Oceanic Convergence
Ocean-Continent Convergence
 Dense oceanic plate subducts beneath continent =




subduction zone
Melting produces magma = volcanism
Arc of volcanoes forms on continent (volcanic
mountain belts)
Oceanic trench forms parallel to subduction zone
Earthquakes
 Examples: Cascades, Andes
Oceanic-Continental Convergence
Name this specific type of boundary.
Continent-Continent Convergence
 2 continental plates meet
 Both are low density
 Neither can subduct (too buoyant)
 So-they build upward (as mountains)
 Shallow Earthquakes
 Example: Himalayas (India vs. Asia)
Himalaya Mountains:
India-Eurasia Collision
 Subduction of
oceanic crust under
Eurasia led to the
collision of India with
Eurasia, creating the
Himalaya Mountains
Continental-Continental Convergence
Divergent Plate Boundaries
 Plates moving apart (extension)
 Occur in 2 places:
 Oceanic
(Mid Ocean Ridges)
 Terrestrial (Continental Rift Valleys)
 Results in
 Creation of new seafloors
 Extensional stress and shallow
earthquakes
 Basaltic volcanism
Divergent
Boundaries
 Continental Rift


Continental
divergent boundary.
Example: East
African Rift Zone
 Mid Ocean Ridge


Oceanic divergent
boundary.
Example: MidAtlantic Ridge
Transform Boundary
 Oceanic or continental crust
 Results in a shear zone between the
plates.
 Minimal mountain building
 Minimal volcanic activity
 Lots of seismic activity!
 Example: San Andreas Fault,
California
Transform Boundaries
 Strike-slip faults (no vertical motion)
 Lateral motion (shear)
 Plates slide alongside one another
 Occur in 2 realms:
 Continents (such as the San Andreas fault)
 Mid ocean ridges where they offset ridge
segments (fractures)
San Andreas Fault
Plate Boundaries
 Divergent plate
boundary = midocean ridges
 Convergent plate
boundaries =
subduction zones
 Transform plate
boundaries, San
Andreas fault
Plate Motion
 Plates move at variable speeds
 Max
Velocity ~16-17 cm/yr
 Min Velocity ~ .1 cm/yr
 Average Velocity ~ 2.5 cm/yr
Hot Spots
 Volcanic centers that originate from rising plumes of
magma from the deep mantle, called mantle plumes
Mantle Plumes:
Long-lived structures in the mantle
 Originate at great depth, perhaps near the
core-mantle boundary

Results in
 Intra-plate (middle of plate) volcanism
 A chain of volcanoes forms as tectonic plates move over
a stationary hot spot
 The bend of a seamount chain over a hot spot records
the change of plate motion
Mantle
 Mantle can behave as a solid or a
fluid.
 Temperature, pressure, and stress
are factors in how the mantle
behaves
 Rapid stress causes the mantle to
act as a solid
 Solid mantle can fracture and break
Mantle
 Upper mantle is less fluid because it is
cooler.
 Crust and upper mantle form the
fractured lithosphere
 When plates move strain accumulates
 Too much strain causes the plates to
rebound like a snapped elastic
 These movements are earthquakes
Importance of Plate Tectonics
The theory provides explanations for:
 Earth’s major surface processes
 The distribution of
 Quakes
 Volcanoes
 Mountains