Transcript PLATE TECTONICS - Los Angeles Mission College

PLATE TECTONICS
Theory of Lithospheric Plate
Motion
(c) Vicki Drake, 2011
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Continental Drift Theory
► Proposed
by Alfred Wegener, early 1900s,
Germany
► Suggested that Earth’s continents have split
and rejoined many times during Earth’s
history.
► Last joining was approximately 200 million
years ago with the formation of “Pangaea”
(all land) with Europe, North America, Africa
and South America joined together.
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PANGAEA
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Evidence for Plate Tectonics
►
“Fit” of continents, especially along edges of continental shelves
►
Similar fossils found on continents separated by oceans
 Dinosaurs of similar species found widely separated continents
► Indicates that continents were joined
 Mammals differentiated on separate continents
► Indicates that continents were separated
►
“Belts” of mountain ranges with similar geology and ages
 Mountains in North and South America connect to mountains in Africa
►
Age of ocean floor and magnetic reversals
 Moving from mid-ocean ridges on the sea floor, ocean sediments grow
older (youngest closest to the ridge)
 Oldest age of any ocean floor dates only to approximately 180 million
years ago
 Moving from mid-ocean ridges on sea floor, the magnetism of specific
rocks making up the deep ocean floor, indicate reversals of normal
magnetism (i.e, north and south magnetic poles ‘flip’ positions)
►
Glacial ‘scour’ as large glaciers moved across continents, ‘scratching’
the surface.
 Pattern discernible when continents pushed back to Pangaea formation
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Magnetic Reversals and Age of
Ocean Crust
Mid-Ocean Ridge
Edges of Continents
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What is the Theory of Plate
Tectonics?
► The
Earth’s crust and upper mantle are bonded
together into the Lithosphere
 Lithosphere composed of Earth’s crust and upper rigid
Mantle bonded together
► Lithosphere
is broken into many moving
slabs/sections (‘plates’).
► Plates are sliding on softer, hotter layer of mantle
just below Lithosphere: Asthenosphere
► Theory of Plate Tectonics explains the motion and
interaction of lithospheric plates
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Map of tectonic plates and relative motions
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What’s Causing the Plates to Move?
►
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Many hypotheses have been presented to explain the
motion of the lithosphere/tectonic plates
Wegener thought some kind of centrifugal forces broke up
Pangaea and drove the continents apart
Scientists today postulate that strong convection currents
within the mantle are moving the lithosphere around
 Shallow Cell Convection: only the upper mantle (the
asthenosphere) is fluid enough to move the plates
 Deep Cell Convection: The entire mantle (from the outer core
boundary to the asthenosphere) is hot enough to produce strong
convection cells
 Two-tiered Convection: The asthenosphere has its own convection
cells, driven in part by larger convection cells in the lower mantle
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Deep Convection cell hypothesis to explain movement of lithospheric plates
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Three Basic Types of Plate Boundaries
► Convergent
Boundaries:
 two lithospheric/tectonic plates are on a collision
course
► Divergent
Boundaries:
 two lithospheric/tectonic plates are splitting into
two or more pieces and are moving away from
each other in opposite directions
► Transform
Boundaries:
 two lithospheric/tectonic plates are sliding past
each other, horizontally, in opposite directions
(neither colliding nor splitting apart)
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TYPES OF PLATE BOUNDARIES
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CONVERGENT BOUNDARIES
► When
two lithospheric plates are moving
towards one another – they form a
“Convergent Boundary”
► One of two interactions are the result of a
Convergent Boundary
 Subduction Zone
 Collision Zone
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Subduction Zone: Convergent
Boundary
► In
a subduction zone, one lithospheric plate will be
pushed beneath another (‘subducted’).
► The non-subducted plate will ride over the
subducted plate.
 This is a ‘destructive’ boundary, as lithosphere is
destroyed and consumed back into the mantle
► Subducted
plate subjected to extreme heat with
melting and formation of magma occurring
 Volcanic arcs (either on land or the ocean floor)
 Earthquakes
 Deep trenches on ocean floor
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Subduction Zone: Oceanic to
Oceanic
► When
two oceanic plates enter a subduction
zone, the older, colder and more dense of
the two will be subducted.
► A chain of volcanic islands on the ocean
floor will develop.
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Philippines
Japan
Indonesia
Marianas
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Convergent Plate Boundaries: oceanic-oceanic subduction zone
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Subduction Zone: Oceanic to
Continental
► When
an oceanic plate and continental plate
enter a subduction zone, the oceanic plate
will be subducted.
► Chains of volcanic mountains will form on
the continental plate.
 Cascade Range (CA to WA), North America
 Andes, S. America
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Pacific Plate subducting beneath
North American Plate; producing
Cascade Volcanoes extending
between northern California
through Washington
Pacific Plate subducting
beneath South American
Plate; producing volcanoes
in the Andes Mountains
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Pacific Ring of Fire: volcanoes and earthquakes found
along the subducting edges of the Pacific Plate
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Collision Zone: Continental to
Continental
► When
two continental plates collide, neither will be
subducted.
 Continental plates are too thick and buoyant for
subduction.
► Great
folded and faulted mountains will result
from the collision, along with earthquakes
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Himalaya Mtns, Asia
Swiss Alps, Europe
Appalachians, North America
Atlas Mtns, NW Africa
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COLLISION ZONE: EUROPE
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INDIA COLLIDING WITH ASIA
India’s collision with Asia pushed up the Himalaya
Mountains and the Tibetan Plateau
India is still pushing up the Himalaya Mountains
even today!
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Earthquakes and Collision Zones
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DIVERGENT BOUNDARY
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When a lithospheric plate is split into two or more pieces, a
Divergent Boundary is formed
“Sea-floor spreading” is a type of divergent boundary in
which new ocean floor is formed
 This is a ‘constructive’ boundary where new lithosphere is created
 First identified by Dr. Harry Hess, Princeton
►
Rift Valleys are another type of divergent boundary where
the plate has not completely split apart.
 East African Rift Valley
► Region
of Eastern Africa extending from Mozambique to Ethiopia is
being separated from the rest of continental Africa
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MID-ATLANTIC RIDGE: Sea floor spreading zone
South
America
Africa
Atlantic Ocean Basin
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East African
Rift Valley
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TRANSFORM BOUNDARY
► When
two lithospheric plates slide past each other
in opposite directions, this is a Transform
Boundary (also known as a Strike-slip boundary)
 This is a ‘conservative’ boundary – crust is neither
created or destroyed
 San Andreas Fault, North America
► Plate
plate
boundary between the Pacific Plate and North American
► There
will be no subduction, no volcanoes
► There will be earthquakes and horizontal
displacement of surface features
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Transform Boundary – two tectonic
plates slip past each other, parallel,
but in opposite relative directions
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San Andreas Fault, western North
America
San Andreas Fault through Carrizo Plain
(west of Bakersfield)
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