Topic 5 Plate TectonicsPlate Dynamics

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Topic 5Plate TectonicsPlate Dynamics

• GEOL 2503
• Introduction to Oceanography

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Theory of Plate Tectonics

• Continental drift and seafloor spreading
  combined
• Lithosphere composed of individual plates moving
  independently
• Seven major and five minor plates

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Mantle Convection Cells
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Types of Plate Boundaries

• Divergent
• Convergent
• Transform Fault

Plate boundaries are where most of the fun stuff
happens
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Divergent Plate Boundaries

• Plates diverge, or move apart
• New rock (ocean crust) is created from cooling
  magma
• Best example is the mid-ocean ridge

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The North Atlantic Ocean is dominated by the
Mid-Atlantic Ridge
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The Mid-Atlantic Ridge is part of the mid-ocean
ridge system. The ridge is a long volcanic
mountain chain. Iceland is one place where the
ridge has actually built up above sea level.
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Hydrothermal Vents

• Plumes of hot water and gases rising along rifts
  (cracks in crust)
• Submersible Alvin found mounds and chimney
  shaped vents called black smokers
• Large communities of animals exist at these vents

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Hydrothermal vents are common along mid-ocean
ridges
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Sulfur-fixing bacteria form the base of a
deep-sea food chain that supports exotic life
along hydrothermal vents
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A deep-sea vent, commonly called a black smoker
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Project Famous(French-American Mid-Ocean
Undersea Study)

• First submersible descent into the Rift Valley
  of the mid-ocean ridge
• 1973 Southwest of Azores
• Took pictures, rock samples, and water samples

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The Red Sea us a divergent plate boundary grown
to a narrow new ocean basin
The East Africa Rift System is an example of the
beginning of the breakup of a continent
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Convergent Plate Boundaries

• Plates converge or move towards each other
• Old crust sinks back into Earth at subduction
  zones
• Three forms based on the types of crust involved
• Ocean-ocean convergence
• Ocean-continent convergence
• Continent-continent convergence

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Ocean-Ocean Convergence

• A lithospheric plate containing oceanic crust
  collides with another lithospheric plate
  containing oceanic crust
• One plate subducts, oceanic trench forms
• Partial remelting of crust leads to volcanism,
  island arc forms
• Examples Aleutian Islands, Lesser Antilles

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Ocean-Ocean Convergent Plates
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Ocean-Continent Convergence

• A lithospheric plate containing oceanic crust
  collides with a lithospheric plate containing
  continental crust
• Ocean crust is denser, so it subducts, oceanic
  trench forms
• Snowplow effect builds mountains
• Partial remelting of crust leads to volcanism,
  volcanoes form in mountains
• Andesite lava named after Andes Mountains
• Examples Andes Mountains, Sierra Mountains

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Mt. St. Helens, WAvolcanism associated with
subduction
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Convergent boundaries are responsible for oceanic
trenches
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Mariana Trench
The Mariana Trench is the deepest spot on Earth
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Remember hypsography
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Continent-Continent Convergence

• A lithospheric plate containing continental crust
  collides with another lithospheric plate
  containing continental crust
• Intervening oceanic crust subducts then
  continental masses collide, form high mountains
  from doubling up of continental crust
• Partial remelting of crust leads to volcanism
• Examples Himalayas, ancient Appalachian
  Mountains during formation of Pangea

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Continent-continent convergence gives the worlds
highest mountainsthe Himalayas
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Himalayan foothills
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Transform Fault Boundaries

• Plates side past each other
• Produces sharp, vertical displacements called
  Escarpments
• ExampleSan Andreas Fault

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The famous San Andreas fault is only part of the
transform boundary between the North American
Plate and the Pacific Plate
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The San Andreas fault looks like a linear gash in
the Earths surface in this view from southern
California
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Continental Margins

• The edges (margins) of the continents are not
  always right at plate boundaries
• Active margin (also called leading edge) is the
  edge near a convergent boundary
• Passive margin (or trailing edge) is the edge
  toward a divergent boundary

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The U.S. Atlantic continental margin is a
passive, trailing edge, with gentle slope and
shallow offshore.
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The Pacific coast of the United States is a an
active, leading edge, with steep slopes and deep
water nearshore.
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Motion of Plates

• 2 Theories
• Formation of new crust moving sideways at ridges
  from convection cells into Earth
• Slab pull theory suggests fracturing and
  sinking of old ocean plates can be the driving
  force
• Rate of motion of plates average about 6 cm/yr.

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Hot Spots

• Intraplate volcanism (not at plate boundaries)
• Mantle source of heat and molten rock
• Produce volcanoes
• Occur in oceans and on land

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Lithosphere moves over hot spot fixed in one
location
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The Hawaiian Island chain formed as the Pacific
Plate has moved over a hot spot
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Emperor Seamounts and Hawaiian Islandsall from
the same hot spot
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Hawaiian Hot Spot

• What can you tell about the history of movement
  of the Pacific Plate?
• Look at the trend of the Hawaiian Island chain
  and the Emperor Seamount chain
• Break it down into movement from 70-about 40
  million years and from 40 million years to the
  present

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Summary slide of plate tectonic activity
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History of the Continents

• About 200 million years ago Pangea began to break
  apart
• Older continents have assembled and been broken
  apart many times before Pangea
• Movement powered by heat from radioactive decay
  in the Earths interior
• Pattern may have existed throughout Earths
  history

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Continental Features

• Cratons
• Older, stable portions of continents
• Have not been involved in mountain building or
  been deformed since the Precambrian (about 600
  million years ago)
• Terranes
• Smaller crustal fragments
• Associated with margins of continents
• Added to continents by collisions and accretion

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Continents grow through time by terrane accretion
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Earthquakes are concentrated along (1) the
mid-ocean ridge, (2) subduction zones, and (3)
Mediterranean-Himalayan belt.
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