Theory of Plate Tectonics

Plate tectonic theory states that the dynamics of the earth’s outer shell, the lithosphere are separated into plates that move over the asthenosphere, the upper molten portion of the mantle. Oceanic and continental plates come together, spread apart, and interact at boundaries all over the planet

  • The Plate Tectonic theory was accepted after seafloor spreading was validated around 1960.
  • The earth’s crust is made up of seven major and many minor rigid, continental and oceanic tectonic plates. These plates are constantly moving.
Name of seven major plates:
  • Pacific plate
  • North American Plate
  • Eurasian Plate
  • African Plate
  • Antarctic Plate
  • Indo-Australian plate
  • South American Plate

Some Minor Plates:

  • Nazca plate- Between South America and Pacific plate
  • Cocos plate- Between Central America and Pacific plate
  • Philippine Plate- Between the Asiatic and Pacific plate
  • Caroline plate- Between Philippine and Indian Plate.

Plate tectonics

 

Plate Boundaries:

Plate boundaries are the area between the plates where most tectonic activities take place. Three types of boundaries exist and a fourth i.e. mixed type is characterized.  Each type of plate boundary generates distinct geologic processes and landforms.

  • Convergent Boundaries or destructive- Subduction
  • Divergent Boundaries (Constructive)
  • Transform boundaries (Conservative)
  • Plate boundary zones

Convergent Boundaries (Destructive or active margins)- Subduction:

At convergent boundaries, the plates move and collide with each other. The collision buckles the edge of one or both plates, creating a mountain range or subduction of one of the plates under the other, creating a deep seafloor trench.

  • At the  Zone of Ocean to Continent subduction: the dense oceanic lithosphere plunges beneath the less dense continent. example: The Andes mountain range in South America, and the Cascade Mountains in the Western United States.
    • It leads to volcanism.
  • At the  Zone of Ocean to Ocean subduction: the older, cooler, denser crust slips beneath the less dense crust. Example: the Aleutian Islands, Mariana Islands, and the Japanese island arc
    • The earthquake caused by this motion, deep trench form an arch shape.
    • The upper mantle of the subducted plate then heats and magma rises to form curving chains of volcanic islands.
    • Deep marine trenches are typically associated with subduction zones, and the basins that develop along the active boundary are often called “foreland basins”.
  • Continent to Continent: Collision between masses of granite continental lithosphere subduction. For example- the Alps were formed when the African plate collides with the Eurasian plate. The Himalayas, also an example of continent-to-continent boundaries.
Divergent Boundaries or constructive:

At divergent boundaries, the plates slide apart from each other forming a narrow rift valley. These are constructive in nature. Here, geysers spurt super-heated water, and magma, or molten rock, rises from the mantle and solidifies into basalt, forming a new crust. Oceanic ridges are constructed due to these plates.

  • At the zone of the ocean to ocean rifting: divergent boundaries form by seafloor spreading, formation of a new ocean basin.
  • At the zone of the continent to continent rifting: Divergent boundaries causes new ocean basin to form continental split, spreads. Example- Africa’s East African rift and valley of the rift sea.
  • Active zones of mid-oceanic ridge– Example- The Mid Atlantic ridge produces geysers and volcanos, and East pacific rise.
    • The Eyjafjoell volcano of Iceland erupted on 17.4.2010 was on the divergent plate of the Atlantic Ocean.

Transform boundaries (conservative- Shearing or neutral plate margins): The transform boundaries occur where two lithospheric plates slide past each other along transform faults, they neither construct nor destroy.  The transform faults occur across a spreading center.

  • In this type of fault, strong earthquakes can occur.
  • Example of transform boundary- The San Andreas Fault in California.

Plate boundary zone: It occurs where the effects of the interactions are unclear, and the boundaries, usually occurring along a broad belt, are not well defined and may show various types of movements in different episodes.

subducting-plate

Movement of Plates:

The tectonic plates are not in a fixed position as they move horizontally over the Asthenosphere. These plates collide, move apart, or slide next to each other that leads to earthquakes or Volcanic Eruption.

Rate of Movement of tectonic plates: The rates vary as follows:

  • Slowest rate (less than 2.5 cm/yr.) – The Arctic Ridge
  • fastest rate (more than 15 cm/yr.)- East Pacific Rise- (mid-oceanic ridge, a divergent tectonic plate boundary)
Forces responsible for the movement of tectonic plates:
  • The tectonic plates are able to move due to the relative density of the oceanic lithosphere and the relative weakness of the asthenosphere.
  • Dissipation of heat from the mantle is acknowledged to be the original source of the energy required to drive plate tectonics through convection or large-scale upwelling and doming.
  • The excess density of the oceanic lithosphere sinking in subduction zones is a powerful source to generate plate motion.
  • The weakness of the asthenosphere is also responsible for the easy movement of tectonic plates.
  • Although subduction is thought to be the strongest force driving plate motions.
  • These deriving forces of three categories: mantle dynamics related, gravity related (main driving force accepted nowadays), and earth rotation related.

 

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