Competitive Exams: Plate Tectonics Geography Notes on Constructive and Destructive Margins in Plate Tectonics

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Plate Tectonics

Because all the plates move with different velocities in different directions and manner, therefore they are in dynamic action with respect to each other as well as along the Plate Boundaries. There are three Types of Plate boundaries:

Constructive Margin or Divergent Plate Boundary

  • Zones of tension, where the lithosphere splits and moves apart.
  • Hot magma comes up through cracks and forms new crusts; therefore it is constructive in nature.
  • Thus rifting of continents and formation of ocean basins take place.
  • This involves a series of stages:
  • Intracontinental Rifting - Rift is formed due to tension and magma rises into it.
  • Interplate thinning - Hot magma rises and melts the lithosphere thereby de opening the rift. If rift deepens sufficiently, sea water may enter (e. g. Red Sea) .
  • Ocean ridge formation- Finally magma wells into the rift at an increasing rate and the land-mass is gradually separated into two parts forming a ridge (e. g. Mid -Atlantic Ridge and the Carlsberg ridge in the Indian Ocean) .
  • Characterized by volcanic activity with fissure eruptions; formation of new crust, submarine mountains, ridges and rises and occurrence of shallow foci earthquakes.

Destructive Margin or Convergent Plate Boundary

When the plates collide, the leading edge of one (the plate having higher density) is bent downward, allowing it to descend beneath the other. Upon entering the hot asthenosphere, the plunging plate is heated, melted and is completely assimilated in the upper mantle. Since one of the plates is destroyed here, this boundary is known as convergent destructive margin, 0cean -There may be different Collision Rising: types of collisions depending upon whether the crust of the plate is continental or oceanic:

  • Oeean -ocean collision (e. g. collision of Pacific and European beneath the N. American plate) .
  • Characterized by mountain building, rock deformation, metamorphism, earthquakes and volcanic activity.
  • Slabs of oceanic crust along with sediments are scraped off by the over-riding continental material and are incorporated in a mass of complex mixture of rocks called a melange. Within The melange, distinctive assemblage of deep-sea sediments, submarine lavas, peri-dotite and gabbro all together formanophiolite suite.
  • Continent -continent collision
    • When two plates carrying continental crust collides (e. g. Indo-Tibetan collision) Characterised by mountain building, ophiolite suite, earthquakes and remnants of past volcanic activity.
    • Two continental plates approach each other and their oceanic crusts get subducted one below the other . After oceanic crust of one is completely consumed; the oceanic crust of the other is consumed into the mantle. Now, continental crusts of each collide. These are low density crusts and therefore do not subduct, rather because of the convergence and simultaneous buoyancy effect upliftment is generated. (E. g. Tibetan Plateau)
    • Zone where the two continental crusts are plastered is known as the SUTURE Zone (e. g. Indus - Tsangpo suture zone) .

Conservative Margin (Parallel or Transform Fault Boundary)

  • At conservative margin the plates slide past each other without the formation of new crust. It is generally formed at diverging boundaries (e. g. MOR) where different parts of plates move with different velocity resulting in formation of faults known as Transform fault (e. g. San Andreas fault) .