Transcript plate tectonics pp

Theory of
Plate
Tectonics
The theory states that
the Earth’s crust is
broken up into plates
that are in motion,
driven by convection
currents in the mantle.
Three types of plate boundary
• Divergent
• Convergent
• Transform
Divergent Boundaries
• Spreading ridges
– As plates move apart new material is erupted to fill the
gap
Ocean – Ocean Divergence
P.112
• Rising convection currents in
the asthenosphere push the
plates apart forming a midocean ridge in the oceanic
crust.
• Magma erupts out of the midocean ridge
p. 112
Mid-Ocean Ridges
• Areas of high heat flow and
elevation.
• Forms new crust where the
plates separate.
• Sea floor spreading.
• Black smokers or vents form
near the ridge.
• Pillow
lavas
form as
the lava
cools
quickly
in the
water.
P. 112 Iceland is an
O-O divergent boundary
• The Mid-Atlantic
Ridge is going through
the country
• The country is
spreading 1 inch a
year
• The country is above
sea level
• More active volcanoes
than anywhere else in
the world..
Iceland: An example of continental rifting
• Iceland has a divergent plate
boundary running through its
middle
C-C Divergent
p. 113
• Rift Valleys form
where the two
continental plates are
separating.
P. 113
The Great
Rift Valley
of East
Africa
East African Rift System
Convergent Boundaries
• There are three styles of convergent plate
boundaries
– Continent-continent collision
– Continent-oceanic crust collision
– Ocean-ocean collision
C-C Convergent
p. 114
• Plates of similar density
are converging creating
folded mountains.
• No volcanoes are
created.
Continent-Continent Collision
• European
Alps
• Himalayas
Himalayas
p. 114 The Appalachian Mtns. in
the eastern U.S. is a folded
mountain range.
O-C Convergent
p. 115
• An oceanic and
continental plate are
colliding, the denser
oceanic plate subducts
(dives) beneath the less
dense continental plate.
Continent-Oceanic Crust Collision
• SUBDUCTION
p. 115 Trenches
• Trenches form deep
valleys at the point
where the plate dives
down. These are the
deepest points on Earth.
Subduction
• Oceanic lithosphere subducts
underneath the continental
lithosphere
• Oceanic lithosphere heats and
dehydrates as it subsides
• The melt rises forming
volcanism
• E.g. The Andes
p. 115 Subduction
• As the plate subducts,
friction generates heat
which melts the subducting
plate, magma rises to create
volcanic mtns that parallel
the boundary.
–Cascade Mtns (Wash,
Oregon)
–Andes Mtns. (S. America)
p. 115 Subduction
• Crust is destroyed in
the process of
subduction.
• Creates the world’s
most explosive
volcanic eruptions.
O-O Convergence
p. 116
•The denser older of
the two oceanic
plates subducts.
•Marianas Trench –
33,035 feet below
sea level.
p. 116 Subduction
• Trenches form at the
boundary.
• Volcanic islands (called
island arcs) form at the
surface where the magma
rises.
– Philippines
– Japan
– Aleutian Islands (off Alaska)
– Indonesian Islands
Ocean-Ocean Plate Collision
• When two oceanic plates collide, one runs over the other
which causes it to sink into the mantle forming a
subduction zone.
• The subducting plate is bent downward to form a very
deep depression in the ocean floor called a trench.
• The worlds deepest parts of the ocean are found along
trenches.
– E.g. The Mariana Trench is 11 km deep!
p. 117 Transform Boundaries
• Plates slide past each other
• Shearing : Stress that pushes a mass of rock
in opposite directions.
Above: View of the San Andreas
transform fault
What will happen in the
future?
• The Atlantic Ocean will continue to
expand.
• The Pacific Ocean will close.
• The Mediterranean will disappear
connecting Africa with Europe.
• India will continue to push into
Eurasia pushing the Himalaya Mtns.
higher.
• Los Angeles will continue to the
northwest joining with San
Francisco.
Age The age of the ocean crust.
Courtesy of www.ngdc.noaa.gov