Plate Tectonics

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Plate Tectonics

• Earth Science Ch 9

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• An idea proposed by Alfred Wegener that the
  continents had once been joined to form a single
  supercontinent, Pangaea.
• Initially the idea was proposed when Wegener
  noticed that S. America and Africas coastlines
  matched like puzzle pieces
• More evidence includes fossils that are found in
  eastern S. America and southern Africa.

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• Pangaea

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• This is how Africa and S. America joined together

Notice how some areas have experienced erosion
and dont quite touch.
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• Glossopteris

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• Mesosaurus found in these areas that once
  adjoined. This dinosaur couldnt swim!

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• Rock evidence exists in the form of several
  mountain belts that end on one coastline and
  reappear on a landmass across the ocean.
• Glacial deposits showed that ice sheets covered
  southern Africa and South America. Wegener
  proposed that these areas were once near the
  South Pole.

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• Mountain chains that are seemingly unrelated, but
  of similar age are found on different land masses.

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• Areas of glacial deposits

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• Areas that have evidence of tropical swamps in
  the Northern Hemisphere suggest that these areas
  were once near the equator.
• Wegener did not know the mechanism of moving
  continents across the globe.
• By 1968 new technology and data on earthquake
  activity and new information about earths
  magnetic field became available

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• Evidence of shifting magnetic poles

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• According to the plate tectonics theory, the
  uppermost mantle along with the overlying crust
  behaves as a rigid layer. The layer is know as
  the lithosphere.

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• Three types of boundaries
• Divergent where two plates move apart, example
  includes the mid-Atlantic rift and the East
  African Rift valley. Think of these as
  constructive plate margins. This is where new
  oceanic lithosphere is generated. As the plates
  move away from each other, fractures are created.
  The fractures fill with molten rock. The magma
  cools to produce new sea floor.

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• Iceland straddles the mid Atlantic rift. There
  are many volcanoes there. Remember this is where
  so much energy comes from geothermal sources.

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• New oceanic lithosphere is generated at oceanic
  ridges. The ocean ridges stretch more than
  70,000 km. The ridge can be 1000 to 4000 km
  wide. Rift valleys can develop on land too. One
  of these is in Africa, the East African Rift.
  Volcanoes like Kilimanjaro and Mount Kenya are
  there. Another is the Rhine River Valley.

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Mt. Kenya
Mt. Kilimanjaro
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• It is thought that the reasons continents break
  up is because there is a rising plume of hot
  magma and the crust is weakened.

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• Seafloor spreading rates are about 5 cm per year.
  All of Earths oceans could have been generated
  within the last 200 million years. None of the
  oceans floor is older than 180 million years old.

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• Convergent boundary where two plates move
  together. Examples include the coast of S.
  America. The Pacific plate is subducting under
  the S. American plate.

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• Lithosphere is destroyed at convergent
  boundaries. These are also called destructive
  plate margins. Convergent boundaries can form
  between two oceanic plates, between one oceanic
  plate and one continental plate or between two
  continental plates.

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• Oceanic Continental Oceanic subducts under the
  continental. Typical features include a trench
  offshore and a mountain range along the shore.
  The Andes

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• Oceanic-Oceanic one oceanic crust subducts
  beneath the other. Volcanoes form on the ocean
  floor and build chains of islands. The Aleutian
  Islands is an example. Volcanic island Arcs.
  Next to the Aleutians is the Aleutian trench.

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• Continental-continental. When two continental
  plates collide the result is complex mountains
  such as the Himalayas. As one plate rams the
  other, sea floor between them folds like it was
  in a vise. The new mountain range is composed of
  deformed and metamorphosed sedimentary rocks and
  fragments of the volcanic arc,

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• Transform Fault boundaries this is where two
  plates grind past each other without destroying
  the lithosphere. The San Andreas Fault is a
  transform fault boundary. Other transform fault
  boundaries occur beneath the ocean where segments
  of mid ocean ridges have broken.

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• Each of the plates contains a combination of
  these three types of boundaries. Shrinking or
  growing of continents depends on the locations of
  convergent and divergent boundaries. The
  Antarctic plate is growing larger. The
  Philippine plate is becoming smaller by
  descending into the mantle.

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• Evidence for Plate tectonics.
• Magnetic minerals in ancient rocks show evidence
  of where the magnetic poles were at the time of
  their formation. These rocks possess
  Paleomagnetism.

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• There is much evidence that Earths magnetic
  field periodically reverses polarity. The north
  pole becomes the south pole and vice versa.

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• Ships towing instruments across the ocean floor
  have revealed alternating strips of high and low
  intensity magnetism that ran parallel to the
  ridge.

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• Earthquake Patterns there is a link between
  deep focus earthquakes and ocean trenches.

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• Ocean drilling confirmed what the seafloor
  spreading hypothesis predicted. The youngest
  oceanic crust is at the ridge crest and the
  oldest at the continental margins. No oceanic
  crust is older than180 million years. Some
  continental crust is 4 billion years old.

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• Hot Spots rising plume of mantle material. As
  the pacific plate moved over the hot spot
  volcanic mountains have been created. There are
  remains of Hawaiian islands under the Pacific.
  There is a new undersea volcano called Loihi that
  hasnt broken the surface yet.

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• The Causes of Plate Motion
• Convection that occurs in the mantle is the
  driving force for plate movement. This movement
  of matter is called convective flow. It occurs
  because there are areas that arent heated
  equally within Earth. The heat is generated by
  the radioactive decay of elements such as uranium.

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• Slab pull the old cold oceanic crust is dense
  and sinks into the asthenosphere and pulls the
  trailing lithosphere along.
• Ridge push causes oceanic lithosphere to slide
  down the sides of the oceanic ridge.

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Ridge push
Slab pull
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Before the Precambrian Era--total conjecture
(because no evidence exists) about 1100 million
years ago, the supercontinent of Rodinia was
assembled though its exact size and
configuration are not known, it appears that
North America formed the core of this
supercontinent at that time, the east coast of
North America was adjacent to western South
America and the west coast of North America lay
next to Australia and Antarctica Rodinia split
into 2 halves approximately 750 million years
ago, opening the Panthalassic Ocean North
America rotated southwards towards the
ice-covered South Pole the northern half of
Rodinia, composed primarily of Antarctica,
Australia, India, Arabia, and the continental
fragments that would one day become China,
rotated counterclockwise, northwards across the
frigid, North Pole between the two halves of
Rodinia lay a third continent - the Congo craton,
made up of much of north-central Africa