Some%20questions%20we%20will%20answer%20today:

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• Some questions we will answer today
• How is the earth always changing?
• What forces inside the earth create and change
  landforms on the surface?
• What is the theory of plate tectonics and how
  does it work?
• What two theories help make up the theory of
  plate tectonics?
• What is continental drift and sea floor
  spreading?
• What happens when the plates crash together, pull
  apart, and slide against each other?

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Different theories about phenomena on earths
surface

• Most of them contradict each other
• Not reliable
• Eg.
• Roman Church
• Copernicus
• Aristotle
• Galileo

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Bases of studies of every discipline
Physics Chemistry Biology Earth Science
Gravity Atom Cell Plate tectonic
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The Plate Tectonic Theory

• One single theory
• explains almost all phenomena prevalent in the
  lithosphere
• volcanism,
• mid-oceanic ridges,
• deep sea trenches,
• earthquakes,
• mineralization,
• mountain-building and many more.
• Paradigm Shift Yet tip of the iceberg

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• To really understand how the earth became to look
  as it does today, and the theory of plate
  tectonics, you also need to become familiar with
  two other ideas
• Continental Drift
• and
• Seafloor Spreading.

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The idea of moving continents
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Continental Drift Theory

• In the early 1900s a German explorer and
  scientist proposed the continental drift theory.
  He proposed that there was once a single
  supercontinent called Pangaea.

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Continental drift An idea before its time

• Alfred Wegener
• Proposed hypothesis in 1915
• Published The Origin of
• Continents and Oceans
• Continental drift hypothesis
• Supercontinent Pangaea began breaking apart about
  200 million years ago

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CONTINENTAL DRIFT THEORY

• Alfred Wagener- 1912, revised in 1924.
• He was a meteorologist studying global climatic
  changes in the geologic past.
• jigsaw fit of continents
• Brazil- Gulf of Guinea
• North American coastline- Scandinavian coastline
• Ethiopia- Western India and Pakistan
• Australia- Bay of Bengal

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THE THEORY

• Super continent- Pangaea
• Covered by huge water body- Panthalassa
• Pangaea- Laurasia Gondwanaland
• Seperated by a narrow sea- Tethys Sea
• 250-300 mya, both started coming closer, Indian
  peninsula started drifting north-eastwards,
  Tethys Sea became smaller
• Himalayas and Alps were formed
• 180 mya, Americas started drifting westwards,
  forming Rockies and Andes

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• Evidence used in support of continental drift
  hypothesis
• Fit of the continents
• Fossil evidence
• Matching rock type and mountain belts
• Paleoclimatic evidence

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Evidence for Continental Drift

• Jigsaw Puzzle fit of continents

Alfred Wegener during Greenland expedition
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Glacial Deposits
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More evidence

• Matching geologic structures including
• Mountain chains
• Ore deposits
• Same rocks of same age

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The Evidence for Continental Drift

• Fossil Evidence
• Glossopteris
• Cynognathus
• Mesosaurus
• Lystrosaurus

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Wegener not believed

• Why? -
• What could possibly force the continents to move
  across the ocean floor in this way. They would be
  crushed.
• He was a meteorologist, not a geologist

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Sea-Floor Spreading

• Sea-floor spreading The process by which molten
  material adds new oceanic crust to the ocean floor

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Seafloor Spreading

• Emerged from the study of the ocean floor.
• Series of mountains that extend around the world,
  stretching more than 64 thousand kilometers (40
  thousand miles).

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Developments 50s and 60s

• World war 2 submarines found mountains under the
  oceans the mid-ocean ridges
• Theory of seafloor spreading suggested by
  Princeton professor Dr. Harry Hess

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Seafloor spreading
First look at the earths layers as shown here.
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Evidence for Sea-Floor Spreading
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Trend of the Mid-Atlantic Ridge

• Similar to the coastlines of adjacent continents
• Europe and Africa to the east
• North and South America to the west

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Oceanic Crust is thin

• Both sides of the ridge, oceanic crust is hardly
  4 to 5 km thick
• Rest all oceans between 10 to 20 km

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Volcanoes

• Continuous chain of volcanoes all along the
  mid-Atlantic ridge
• What would be the effect of this at the ridge?

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Age of rocks

• Youngest near the ridge
• As we move away from the ridge, rocks are older
  in age
• No rock in the Atlantic ocean is more than 200
  million years old ( The crust is not more than
  200 million years old)

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How are these matching stripes evidence of
sea-floor spreading?

• The pattern of stripes is the same on both sides
  of the ridge, indicating that the sea floor has
  spread from the mid-ocean ridge

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What happens to the rock along the ridge when new
molten material erupts?

• The spreading molten material pushes the older
  rock to both sides of the ridge

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Where is the oldest part of the ocean floor?

• The oldest part of the ocean floor is farthest
  from the mid-ocean ridge

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Pangaea revisited

• By piecing together this information, we can see
  how the continents have moved over the past 200
  million years, due to seafloor spreading

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If new crust develops from mid-oceanic ridges.

• Where does the old crust go?

• Why the amount of lava coming out and forming
  crust does not reduce?

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Look at the observations made by

• Hugo Benioff Vine and Mathews- American
  GEOLOGISTS

• They observed certain phenomena in the eastern
  coast of Eurasia

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Deep sea coast

• The sea near the east coast of China was very
  deep.
• Yellow Sea
• South China Sea
• East China Sea

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Ring of Fire

• Chain of volcanic islands running parallel to the
  east coast of Eurasia
• Aleutian
• Japan
• Phillipines

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Volcanic activity

• Intense
• Explosive.
• Lava different than that of the Mid-Atlantic ridge

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Frequent earthquakes

• Japan gets at least 2 earthquakes every day

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Conclusion

• As Eurasia is pushed from Mid-Atlantic Ridge,
• It pressed against and subsided under the crust
  which make up the floor of the Pacific Ocean
• Constant rubbing of both the plates melts some
  rocks, magma forms and erupts out forming the
  islands.
• Process known as??????????????????????????

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Subduction

• Subduction The process by which oceanic crust
  sinks through a deep-ocean trench and back into
  the mantle a convergent plate boundary

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Subduction at Deep-Ocean Trenches
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Deep-Ocean Trenches

• Deep-Ocean Trenches A deep valley along the
  ocean floor through which oceanic crust slowly
  sinks towards the mantle

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What happens to the ocean floor at deep ocean
trenches?

• At deep-ocean trenches, subduction allows part of
  the ocean floor to sink back into the mantle,
  over tens of millions of years

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Plate Tectonics

• Unifying concept
• Sea floor spreading
• Continental Drift
• Earthquakes
• Volcanoes
• Mountains
• Sea islands

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• According to the theory of plate tectonics, the
  earths outer shell is not one solid piece of
  rock. Instead the earths crust is broken into a
  number of moving plates. The plates vary in size
  and thickness. They keep moving in different
  directions

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2 Types of Plates

• Ocean plates - plates below the oceans
• Continental plates - plates below the continents

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What is the Asthenoshere?

• The plastic layer below the lithosphere
  asthenosphere
• The plates of the lithosphere float on the
  asthenosphere

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Drifting of continents

• When the tectonic plates under the continents and
  oceans move, they carry the continents and oceans
  with them.

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Sea-Floor Spreading

• Mid oceanic ridges
• Magma comes out
• Forms new crust
• Pushes existing crust on both sides
• Drifting continents

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Subduction

• Heavier plate subsides under lighter plate.
• Magma erupts over the thinner plate
• Sometimes magma may pile over on the ocean to
  form islands

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Plate Tectonic Theory

• Plates of rigid lithosphere (oceanic and
  continental) move from the energy of heat
  transfer below
• Their interactions define divergent, convergent,
  and transform boundaries and control many surface
  processes

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Plate tectonics

• Sea floor spreading provides the driving
  mechanism for movement
• However, it is not the continents that are
  moving, but the plates of lithosphere
  floating in effect on the asthenosphere
• The lithosphere is made up of about 20 plates
  which move relative to each other in several ways
• Lets look at a generalized sketch

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The Plates
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Types of Plate Margins
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• Plate boundaries
• Interactions among individual plates occur along
  their boundaries
• Types of plate boundaries
• Divergent plate boundaries
• Convergent plate boundaries
• Transform fault boundaries

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DIVERGENT PLATE BOUNDARY

• At a divergent plate boundary lithospheric plates
  move away from each other.
• The mid-Atlantic Ridge, a topographically high
  area near the middle of the Atlantic Ocean, is an
  example of a divergent plate boundary.
• New crustal material

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Features of Divergent Boundaries

• Mid-ocean ridges
• rift valleys
• fissure volcanoes

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Theyre Pulling Apart!

• When plates pull away from one another they form
  a diverging plate boundary, or spreading zone.

Thingvellir, the spreading zone in Iceland
between the North American (left side) and
Eurasian (right side) tectonic plates. January
2003.
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Divergent Atlantic Ridge
LAVA FOUNTAINS KRAFLA VOLCANO ICELAND
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Divergent boundaries in Continents

• Continental rifts
• Splits landmasses into two or more smaller
  segments

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The East African Rift
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CONVERGENT PLATE BOUNDARY

• At a convergent plate boundary, lithospheric
  plates move toward each other.
• The west margin of the South American continent,
  where the oceanic Nazca Plate is pushed toward
  and beneath the continental portion of the South
  American Plate, is an example of a convergent
  plate boundary

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Convergent plate boundaries

• Types of convergent boundaries
• Oceanic-oceanic convergence
• When two oceanic slabs converge, one descends
  beneath the other
• Often forms volcanoes on the ocean floor
• If the volcanoes emerge as islands, a volcanic
  island arc is formed (Japan, Aleutian islands,
  Tonga islands)
• Subducting plate bends downward forming an
  oceanic trench

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Oceanic-oceanic Collision
Figure 12.16
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Oceanic-Continental Collision
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ANDES

• The Andes Mountain Range spans the entire length
  of South America, along the western coast. During
  this subduction some Nazca crust is scraped off
  along base of the Andes, adding height to the
  entire range.

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RING OF FIRE
OCEANIC-CONTINENTAL COLLISONS SUSTAIN
MOST VOLCANIC ACTIVITY AROUND THE PACIFIC OCEAN
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Convergent plate boundaries

• Types of convergent boundaries
• Continental-continental convergence
• Continued subduction brings continents together
• Less dense, buoyant continental lithosphere does
  not subduct
• Result is a collision between two continental
  blocks
• Process produces mountains (Himalayas, Alps,
  Appalachians)

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The collision of India and Asia produced the
Himalayas
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Transform fault boundaries

• Third type of plate boundary
• Plates slide past one another and no new
  lithosphere is created or destroyed

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Transform Boundaries

• Transform faults mark fractures in the crust
  where plates slide laterally past each other
• The San Andreas fault separates the Pacific plate
  from the North American plate

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• These areas are likely to have a rift valley,
  earthquake, and volcanic action.

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San Andreas Fault, CA
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Hot spots

• Caused by rising plumes of mantle material
• Volcanoes form over them (Hawaiian Island chain)
• Mantle plumes are long-lived structures and
  originate at great depth, perhaps at core-mantle
  boundary

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The Hawaiian Islands form over stationary hot spot
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OTHER HOTSPOTS
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Plate motions also can be looked at into the
future, and we can have a stab at what the
geography of the planet will be like. Perhaps in
250 million years time there will be a new
supercontinent.