Plate Tectonics Theory - UPSC Geography Notes

Plate Tectonics Theory

Plate tectonics is the theory which studies plates and various tectonic activities which take place along with plate margin. This theory was given in 1960s by Tuzo Wilson who coined the term Plate, Dan McKenzie, W. Jason Morgan, Arthur Holmes, Harry Hess. According to this theory, earth's outer rigid layer called lithosphere is broken into several large and small slabs which are called plates. These plates float and move slowly over the semi-molten layer beneath them called the asthenosphere. Due to this plate movement, several plate margins are formed along with various tectonic activities like earthquakes, volcanism, mountain building, ocean trenches, and mid-oceanic ridges.

Types of Plates

• On the basis of their constituents
  1. Continental Plate
  2. Oceanic Plate
  3. Oceanic cum Continental Plate
• On the basis of their size
  1. 7 Major Plates are
     • North American Plate
     • South American Plate
     • African Plate
     • European Plate
     • Indo-Australian Plate
     • Antarctic Plate
     • Pacific Plate
  2. 20+ Minor Plates
     • Juan de Fuca
     • Cocos Plate
     • Caribbean Plate
     • Nazca Plate
     • Scotia Plate
     • Somalian Plate
     • Arabian Plate
     • Turkish Plate
     • Burma Plate
     • Philippine Plate
     • Bismarck Plate

Distribution of Plates

Plate Margin

There are three types of plate margins

1. Constructive Plate Margin It is formed when two plates move apart due to divergence. Two convection currents operating beneath the lithosphere in the asthenosphere; due to this: 1. Fissure eruption of volcano occurs 2. Mid oceanic ridges are formed e.g. MOR of Atlantic 3. Earthquakes of mild magnitude occur

   

2. Conservative Plate Margin Conservative plate margins are those where two plates neither converge nor diverge. The two plates simply slide past each other, which is why there is no construction or destruction of crust. It only develops:
   1. Transform faults
   2. High Magnitude earthquakes
   3. No volcanism e.g. San Andreas fault of North America.

      

3. Destructive/Convergent Plate Margin A destructive plate margin or convergent plate boundary develops when two plates move towards each other; in this case, there are three possibilities:
   1. Continental and Oceanic Plate Convergence In this case
      • central eruption of volcanoes develop, e.g., Mt. Stromboli of Italy.
      • folded mountains are formed e.g. the Andes mountains were formed due to plate convergence between the South American plate and the Nazca plate.
      • high-magnitude earthquakess

        

   2. Oceanic and Oceanic Plate Convergence When two oceanic plates converge, the relatively heavier plate subducts under the lighter plate; due to this:
      • Island arcs are formed and sometimes festoons also.
      • Central eruption of volcanoes, e.g., Mt. Fuji of Japan, Mt. Mayon and Mount Taal in the Philippines, Mt. Krakatoa of Indonesia.
      • High Magnitude earthquakes and Tsunami e.g. the 2004 tsunami was near Indonesia, 2011 tsunami was due to Japan
   3. Continental and Continental Plate Convergence When two continental plates converge, the relatively heavier plate subducts beneath the lighter plate due to this
      • High mountain ranges are formed, e.g., the Himalayas.
      • No volcanic eruption because
        • Thickness of the lithosphere becomes almost double due to subduction / under thrusting
        • Due to lack of water steam support to rising magma is very weak.
        • Rising magma is very viscous.
      • High-magnitude earthquakes.

        

An island arc is Curved chain of volcanic islands formed at a convergent plate boundary where one oceanic subducts beneath another oceanic plate. The subducted plate melts in the mantle magma rises through cracks. Repeated volcanic eruptions form a chain of islands parallel to the ocean trenches; due to Earth's curvature and plate movement, the chain generally develops a crescent or arc shape. e.g. Japan, Philippines, Indonesia, etc. Festoons are highly curved, chain-like curved arcs that represent the most pronounced form of island arcs, resembling a garland or necklace. They are formed by intense subduction and strong compressional forces. The differential plate movement causes the arc to bend more sharply. These are often associated with deep ocean trenches and active volcanoes. Indonesia and the Philippines are examples of island arcs and festoons.

• Indonesia

• Indonesia is located where several plates meet such as the Indo-Australian Plate, Eurasian plate, the Pacific Plate and the Philippine Plate.

• Formation Process of Indonesia

  Stage 1- Subduction of Indo-Australian Plate: The oceanic part of the Indo-Australian plate moves northward. It subducts beneath the Eurasian plate Stage 2 - Formation of Ocean Trenches e.g. Java Trench and Sunda Trench Stage 3 - Magma generation: Subduction of the plate in the asthenosphere leads to its melting and formation of magma Stage 4 - Development of Volcanic arc: Magma rose to the surface and formed Volcanic islands like Sumatra, Java, Bali, Lombok, etc. Indonesia is the world's largest archipelago. It has about 17,000+ islands, over 120 active volcanoes and experiences frequent earthquakes and tsunami due to active subduction.

• Formation Process of Philippines

  Philippines lies at the junction of Philippine plate, Eurasian plate and Sunda plate Stage 1 - Oceanic plate convergence: The The Philippine Sea plate and parts of the Eurasian plate moved toward each other. One heavier oceanic plate began subducting beneath the other. Stage 2 - Formation of deep Trenches: Subduction created deep ocean trenches such as Mariana trench, Mindanao trench. Stage 3 - Magma Generation: The descending plate released water into mantle partial melting occurred. Magma rose through volcanoes. Stage 4 - Volcanic Islands are formed: These islands merge and grew into larger islands such as Luzon. Philippines has more than 7000 island and numerous active volcanoeses.

Force responsible for plate Tectonics

According to geoscientists, the most important force responsible for plate movement is the flow of convection currents operating in the asthenosphere. These convection currents are formed due to the decay of radioactive elements, which produces a huge amount of heat and melts the surrounding rock. These molten rocks form convection currents. When two convection cells converge or diverge, the overlying plates converge or diverge, respectively. There are two secondary forces also responsible for plate movement:

1. Ridge push force: This force is applied by the magma rising through the divergent plate boundary
2. Slab Pull: When a subducting plate enters into asthenosphere it pulls rest of the plate inside.

Evidence of Plate Tectonics Theory

Evidence of Plate Tectonics Theory include:

1. Sea Floor spreading
2. Palaeomagnetism
3. Hot spots (Volcanos)

Sea Floor Spreading and its support to Plate Tectonics Theory

The idea of sea floor spreading refers to the process by which new oceanic crust is formed at mid oceanic ridges and gradually moves away on either side. The concept was proposed by Harry Hess in the 1960s and became a crucial foundation for the theory of plate tectonics. According to this theory, molten magma rises from the mantle through fissures at divergent plate boundaries, especially along mid oceanic ridges such as mid Atlantic ridge. As the magma cools and solidifies, it forms new basaltic crust. Continuous addition of new material pushes the older crust away from the ridge, causing the ocean floor to spread.

Mechanism of sea floor spreading

• Upwelling of magma occurs due to convection currents in the mantle.
• New oceanic crust is created at the mid oceanic ridge.
• Older crust moves outward symmetrically.
• Oceanic crust is eventually destroyed at subduction zones or ocean trenches.

Evidence supporting Sea floor Spreading.

1. Age of Oceanic Crust:- Rocks near the mid-oceanic ridges are younger while those farther away are older.

   

   It clearly suggests that gradual spreading of the sea floor is a reality.
2. Thickness of Sediments:- Marine sediments are thinner near mid-oceanic ridge and thicker away from them because older crust has had more time to accumulate sediments.

   

3. Palaeomagnetism:- Alternating magnetic bands on either side of ocean ridges record periodic reversals in earth's magnetic field. The symmetrical arrangement proves continuous creation and outward movement of oceanic crust.