Geography; Tectonics, Margins, Drift theory and Earthquakes

Plate tectonics theory –
- Describe the large scale motion of lithosphere.
- States that lithosphere is divided into several rigid segments called plates, it includes both oceanic and and continental crusts.
- Plates moves on asthenosphere. (solid which flows under stress)
- About 20 such plates are identified.
- Seven major plates are:
  - Eurasia.
  - Antarctica.
  - North America.
  - South America.
  - Pacific.
  - African.
  - And Indian plate.
- Various intermediate size plates such as:
  - China.
  - Philippine.
  - Arabian.
  - Iran.
  - Nazaca.
  - Cocos.
  - Caribbean.
  - And Scotia plates.
- Depending upon the type of movement, plate margins are –
  - Divergent margin. (constructive)
    - Plate moves apart with upwelling from mantle.
    - Formation of mid oceanic ridges.
    - Example: Surtsey Island along with mid Atlantic ridges.
  - Convergent margin. (destructive)
    - Two plates collide, one plate bending downward and subducting below the other.
    - Formation of deep oceanic trench and Fold Mountains.
    - Example: Himalayan boundary fault, Pacific ring of fire etc.
  - Parallel plate margin. (constructive/tansform boundary)
    - Plates slides past one another without the creation or destruction of crust.
    - Example: San Andreas Fault along western cost of USA (Mexico) is a famous transform fault.
Continental drift theory –
- By Alfred Wagener in 1915.
- About 250 million year ago, only one continent (Pangea) and one mass of water body (Panthalassa) was there.
- Breaking process started about 200 million year ago.
- Northern rift cuts Pangaea from east to west creating Eurasia in the north and Gondwanaland in south. A shallow sea called Tethys was situated between.
Earthquakes –
- Caused by endogenetic forces.
- Magnitude or intensity of energy released is measured by Richter scale.
- Damage caused is measured by modified Mercalli intensity scale.
- Study is known as seismology.
- Seismic waves are of three types:
  - Primary waves.(P waves)
    - Shorter wavelength and higher frequency.
    - Longitudinal waves.
    - Can travel through solid, liquid and gas.
  - Secondary waves.(S waves)
    - Shorter wavelength and higher frequency.
    - Transverse wave.
    - Travels through all solid particles.
  - Surface waves or long waves.(L waves)
    - Waves of long wavelength, confined to the skin of the earth crust.
    - It causes most of the structural damage.
- Homoseismal lines: Imaginary lines where waves arrives at the same time.
Eurasian plate is static while Indian plate is moving south to north at 5 cm/year.
- Due to this earthquake occurs in Himalayan region, and height of Himalayas is increasing at the speed of 1 cm/year.
Most of the world earthquakes occur in –
- The zone of young fold mountain.
- The zone of folding and faulting.
- Zones of junction of continental and oceanic margin.
- Zones of active volcanoes.
- Along different plate boundaries.
Traditional zones of earthquakes –
- Circum Pacific belt.
- Mid Continental belt.
- Mid Atlantic belt.