Unit 3 Plate Tectonics and Volcanoes

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Unit 3Plate Tectonics and Volcanoes
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Hypothesis of Continental Drift

• This idea preceded the theory of Plate Tectonics
• Proposed by German scientist Alfred Wegener
• All continents were at one time joined in one
  mega continent Pangea
• Pangea then broke up and the pieces drifted apart

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Wegeners evidence for Continental Drift

• Fit of the continents

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The shape of the continent we see is determined
by sea level, which changes over the
centuries.Continental Shelves are deeper and
wider than that shape. They are the true edge
of the continent.
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Wegener found an excellent fit when he used the
continental shelves to match up the continents.
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Wegeners evidence for Continental Drift

• Fit of the continents
• Fossils that match up

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Wegeners evidence for Continental Drift

• Fit of the continents
• Fossils that match up
• Evidence of ancient glaciers match up
• Evidence of tropical climates form a band across
  Pangea

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Wegeners evidence for Continental Drift

• Fit of the continents
• Fossils that match up
• Evidence of ancient glaciers form a clump like an
  ice cap
• Evidence of tropical climates form a band across
  Pangea
• Rock types match up
• Structures like faults match up

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Wegeners theory was not accepted in his day (he
died in 1930).

• What was missing A plausible mechanism for the
  continents to move.
• He proposed that the continents drifted on top of
  the ocean crust.
• This explanation was the best science could offer
  until we learned some critical information about
  the ocean floor.

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The Ocean Floor Revealed

• In the 1960s scientists used sonar to map the
  irregular shape of the ocean floor
• The images revealed prominent mid-ocean ridges
• The mid-ocean ridge lines exactly follow the
  shape of the continental margins

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Seafloor Spreading

• The continents split apart and new ocean floor
  was formed in gap
• As each new spreading event occurs, new lava
  forms at the mid-ocean ridge
• The older ocean crust and the continents are
  pushed ever farther away from the spreading center

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Plate Tectonics is born

• Seafloor spreading provided the mechanism that
  Wegeners Continental Drift theory lacked.
• The Supercontinent of Pangea and how it fits
  together are vindicated
• We also learned important things about the deep
  Earth. The Earth was revealed to be covered with
  a crust that is broken into many plates
• Dating has shown that Pangea began to break up
  235 million years ago

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Parts of the Earth

• Outer crust, very thin
• Lithosphere includes thecrust and the rigid
  upper-most part of the mantle
• The tectonic plates aremade of lithosphere
• Lithosphere floats on the mantle

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Parts of the Earth

• Mantle is a solid but it flows (like a
  marsh-mallow does) mantle has the consistency
  of asphalt
• Outer core, liquid
• Inner core, solid

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The Crust

• A single plate may have both continental crust
  and ocean crust on it
• Ocean crust is thin and dense, made of basalt
  (lava rock)
• Older ocean crust is denser than newer ocean
  crust because it cools and contracts as it ages
• Continental Crust is thicker and less dense
• The crust is part of the lithosphere

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Plate Tectonics is the movement of the plates.
Plate tectonics is driven by mantle convection.
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Convection

• Recall that convection is the transfer of heat by
  the movement of currents

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Mantle Convection

• The mantle is heated by the outer core, making it
  less dense
• The hotter mantle material rises up to the
  tectonic plates
• When it gets to the top of the mantle it spreads
  out
• This rising and spreading out of the mantle
  pushes the plates above it apart
• The result is Seafloor Spreading

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Mantle Convection

• So close to the surface, the mantle cools and
  becomes less dense
• The cooled part of the mantle becomes dense
  enough to sink
• As the cooled mantle sinks, it drags down the
  plate that was on top of it
• The result is subduction
• Subduction is when a plate sinks back into the
  mantle

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Seafloor spreading

• The newest ocean crust is lighter and sits
  higher. The result is a Mid-ocean ridge.
• As spreading continues, the rock that was at the
  ridgeline is moved away like on a conveyor belt.

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Plate Boundaries

• A plate boundary is the meeting place between the
  edges of two tectonic plates.

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Three types of Plate Boundaries

• Divergent
• Convergent
• Transform

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Divergent Plate Boundaries

• Divergent means separating
• Two plates diverge, or move apart from each other

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Divergent Plate BoundariesOcean-Ocean

• This is known as seafloor spreading
• The surface expression is a mid-ocean ridge
• New ocean floor is created at these boundaries
• The birthplace of oceans
• Examples Mid-Atlantic Ridge and East Pacific Rise

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Divergent Plate BoundariesThe earliest stage
Continental Rifting

• When a single plate begins to break into two,
  this is called continental rifting.
• Continent-continent divergent boundary

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Stages of Continental Rifting

• 1- The continental crust begins to pull apart
• 2- The continental crust warps upward in the
  place where it is pulling apart and the mantle
  gets closer to the surface
• 3- The crust thins in the rifting area, forming a
  rift valley
• 4- Volcanoes rise up
• 5- The plates succeed in separating

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Stages of Continental Rifting

• 6- As the two plates separate, a gap forms that
  is filled by magma (liquid rock from the mantle)
• 7- Ocean crust (basalt lava rock) forms in the
  gap
• 8- A skinny new strip of ocean floor develops

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Stages of Continental Rifting

• 10- As the spreading continues the ocean floor
  widens, separating the edge of continents from
  each other more and more
• Eventually the seafloor spreading is significant
  enough that it is no longer considered
  continental rifting, just plain seafloor
  spreading

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Continental Rifting

• Example 1 North America has a failed rift
• Long ago the continent of North America started
  to break apart, from the Great Lakes down to
  Kansas.
• It got to the point that the crust was thinned
  where the plate was pulling apart.
• Then it stopped.
• The Great Lakes now fill the low spots created by
  the thinning of the crust at the failed rift.

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Continental Rifting

• Example 2 The East African Rift Valley
• The continent of Africa is also ripping in two,
  creating a rift valley where the crust is
  thinning
• In places magma is able to well up, creating
  volcanoes such as Mt. Kilimanjaro

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Continental Rifting

• Example 3 The Red Sea
• The Red Sea is a skinny strip of brand new ocean
  floor being created where Saudi Arabia is pulling
  away from Africa

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Divergent Plate BoundariesTwo plates separate

• Continental Rifting, one plate breaking into two
• Seafloor Spreading new ocean floor is created

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Convergent Plate BoundariesTwo plates collide

• The denser plate subducts
• Put these in order from most dense to least
  dense continental crust old ocean crust new
  ocean crust

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Convergent Plate BoundariesTwo plates collide

• The denser plate subducts
• In order from most dense to least dense old
  ocean crust (most dense) new ocean crust
  continental crust (most buoyant)

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Convergent Plate BoundariesTwo plates collide

• The denser plate subducts
• Old ocean crust will subduct under new ocean
  crust
• Ocean crust will subduct under continental
  crust

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Convergent Plate Boundary1-Ocean-Ocean

• Older ocean crust subducts under newer ocean
  crust
• Along the edge is an ocean trench, a deep area
  where the subducting plate bows down

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Convergent Plate Boundary1-Ocean-Ocean

• The subducting plate melts
• Magma rises up
• Creates a chain of volcanoes on the overriding
  plate
• The volcanoes are called Island Arcs

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Convergent Plate Boundary1-Ocean-OceanExample
The Caribbean Islands
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Convergent Plate Boundary2-Ocean-Continent

• Ocean crust subducts under continental crust
• Along the edge is an ocean trench, where the
  subducting plate bows down

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Convergent Plate Boundary2-Ocean-Continent

• The subducting plate melts
• Magma rises up
• Creates a chain of volcanoes on the overriding
  plate, along the edge of the continent
• The volcanoes make a mountain chain

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Convergent Plate Boundary2-Ocean-ContinentExamp
le 1 The Peruvian Trench and the Andes
Mountains, where the Nazca Plate (in the
Pacific Ocean) is subducting under the South
American Plate
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Convergent Plate Boundary2-Ocean-ContinentExamp
le 2 The Marianas Trench near Japan is the
deepest part on the surface of the Earth.
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Convergent Plate Boundary2-Ocean-ContinentExamp
le 3 The Cascade Range in the NW USA. Mount
St. Helens is one of the volcanoes in the
chain.
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Convergent Plate Boundary3-Continent-Continent

• Continental crust cannot subduct
• No subduction or rising magma
• Crust of Earth is squeezed together and pushed
  upwards

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Convergent Plate Boundary3-Continent-Continent
Example The Indian sub-continent is pushing
into Eurasia. The result is the Tibetan
Plateau and the Himalayas Mt Everest
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Convergent Plate Boundary3-Continent-Continent
Example The Indian sub-continent is pushing
into Eurasia. The result is the Tibetan
Plateau and the Himalayas Mt Everest
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All three types of plate boundaries cause
earthquakes

• Divergent
• Convergent
• Transform

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Only two of the plate boundaries cause volcanoes

• Divergent
• Convergent

Transform boundaries do not cause volcanoes.
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Transform Plate Boundaries

• Two plates slide past each other
• Transform plate boundaries create earthquakes and
  can deform the crust into small hills, but no
  volcanoes or big mountains.

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Transform Plate Boundaries1-Ocean-Ocean

• Major offset perpendicular to the Mid-Ocean Ridge
  axis
• Examples Offset of East Pacific Rise in South
  Pacific Offset of Mid-Atlantic Ridge

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Transform Plate Boundaries2-Ocean-Continentor
Continent-continent

• Earthquakes

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Transform Plate Boundaries2-Ocean-Continentor
Continent-continent

• Example San Andreas Fault in California. The
  Pacific Plate is moving North, while the North
  American Plate is moving South

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Quiz Three kinds of Plate Boundaries

• Transform
• Divergent
• Convergent

• Seafloor is destroyed
• Seafloor is created
• Volcanic chain
• Plates separating
• Plates sliding past each other
• Mid-ocean ridge
• Plates colliding

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Answers Three kinds of Plate Boundaries

• Transform Plates sliding past each other
• Convergent -Seafloor is destroyed -Plates
  colliding -Volcanic chain

• Divergent -Seafloor is created-Plates
  separating-Mid-ocean ridge

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Quiz Three kinds of Plate Boundaries

• Transform
• Convergent
• Divergent

• What are the map symbols?

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Quiz

• Which will be overridingold ocean crust or new
  ocean crust?
• Which will subductcontinental crust or ocean
  crust?
• What happens when continental crust collides with
  continental crust?

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Answers

• Young ocean crust will override old ocean crust
• Ocean crust will subduct under continental crust
• When continental crust collides with continental
  crust no subduction occurs the crust is squeezed
  together and pushed upwards