Plate tectonics

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• Chapter 18
• Plate tectonics

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• History of plate tectonics
• The earths surface is divided into several major
  and minor plates and the interaction between
  these plates is known as plate tectonics.

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• There are 8 major and about 30 minor plates.
• N. American
• S. American
• African
• Eurasian
• Indian
• Pacific
• Australian
• Antarctic

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• Intense geologic activity occurs at the plate
  boundaries where plates
• Collide with one another
• Move away from one another
• Slide past one another

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• The concept of plate tectonics was developed in
  1960s by combining two theories
• Continental Drift
• Sea floor spreading

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• 1. Continental Drift proposed by Alfred Wagener
  in 1912.
• He found similarities in S. America, Africa,
  India, Antarctica and Australia.

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• On this basis he proposed
• All the continental landmasses were once joined
  together in one supercontinentPangea. All the
  oceans formed one super oceanTetheys.
• The Pangea then split into a northern Laurasia
  and a southern Gondwanaland.

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• Revival of continental drift Wegeners mechanism
  of continental drift was not very convincing and
  his theory remained discarded till 1960s.
• It was revived with the advent of
  paleomagnetism, which confirmed that the
  continents have moved relative to one another.

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• Additional evidence for cont. drift
• The continents fit like a jigsaw puzzle
• Rock similarity
• Fossils and age similarity

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• 2. Sea floor spreading proposed by Harry Hess in
  1962.
• He proposed that the sea floor moves away from
  the crest of a mid ocean ridge and finally
  disappears beneath a continent or an island arc
  (subduction).

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• Mantle convection is responsible for ocean
  spreading.

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• Plates and plate boundaries
• Rigid lithospheric plates move over plastic
  asthenosphere.

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• Plate boundaries 3 types
• Divergent plate boundary plates move away from
  one another. Also known as Constructive P.B. or
  spreading center.

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• 2. Convergent P.B plates move towards one
  another. Also known as Destructive P.B.
• 3. Transform P.B plates move horizontally past
  one another. Also known as Conservative P.B.

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• Do plates really move?
• Plate motion is very slow but predictable. 1-10
  cm/year.
• Movement is measured by satellites, lasers and
  GPS.

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• Do plates really move?
• The magnetic anomalies at the sea floor and
  movement along a transform fault indicate plate
  motion.

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• Divergent Plate Boundaries
• Two plates move away from one another.
• Can occur within a continent or an ocean. If it
  occurs in a continentrifting.

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• Divergent Plate Boundaries
• Generally basaltic magma erupts and spreads on
  either side of the fractures causing the plates
  to push away from one another.

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• Examples
• Red Sea.
• East African Rift

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• A passive continental margin forms when one of
  the moving portions of the plate are covered by
  sediments.

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• A mid ocean ridge is formed when divergent P.B.
  occurs in the middle of an oceanmid Atlantic
  ridge.

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• Convergent P.B.
• Two plates move towards each other and collide
  forming mountains.

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• Convergent P.B.
• Three types of convergent p.b
• Ocean-ocean convergence
• Ocean-continent convergence
• Continent-continent convergence

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• Ocean-ocean convergence one plate subducts under
  the other, a trench and a volcanic island arc are
  formed.

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• Ocean-continent convergence oceanic plate
  subducts beneath the continent and an active
  continental margin is formed.

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• Continent-continent convergence neither plate
  subducts and a collisional mountain chain is
  formed.

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• 3. Transform P.B plates slide past one another
  an no significant material is created or
  destroyed.
• Transform faults generally connect two divergent
  p.b. or two trenches.

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• Why plates move?
• Mantle convection.
• Ridge Push
• Trench Pull

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• Mantle plumes and Hot Spots
• Mantle convection moves towards the earth surface
  in the form of a plume and appear on the surface
  as a Hot Spot.

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• Plate tectonics and ore deposits
• Valuable metallic ores are associated with
  divergent boundaries and volcanism on the sea
  floor.

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