Plate Tectonics

The theory of Plate Tectonics is the foundation for understanding most of the dynamic processes of the Earth’s surface. It explains the large-scale movements of lithospheric plates over the semi-fluid asthenosphere and has revolutionized our understanding of mountain building, earthquakes, volcanic activity, and the distribution of continents and oceans.

Origin of Plate Tectonics Theory

• The theory of Plate Tectonics evolved by combining two earlier theories:
  • Continental Drift Theory by Alfred Wegener, which proposed that continents drifted over time.
  • Seafloor Spreading Theory, proposed by Harry Hess, which suggested that new ocean floor is created at mid-ocean ridges and destroyed at trenches.
• The theory gained scientific acceptance in the 1960s and is now a central concept in geology.

Seafloor Spreading Theory

• Describes the process by which new oceanic crust is formed at mid-ocean ridges and gradually moves away.
• Proposed by Harry Hess, this theory explained how molten material rises from the mantle at divergent boundaries and solidifies, pushing older crust away from the ridge.

Evidence Supporting Seafloor Spreading:

• Molten material rising at ridges
• Magnetic striping of rocks on the seafloor indicating past magnetic reversals
• Age of ocean floor increases with distance from ridges
• Drilling samples show newer crust near the ridge and older crust further away
• Presence of deep ocean trenches where old crust is subducted

Plate Tectonics Theory

• Earth’s lithosphere is broken into several large and small tectonic plates that float over the semi-fluid asthenosphere.
• These plates move due to convection currents in the mantle, as proposed by Arthur Holmes.
• Each plate acts as a rigid unit and may consist of oceanic crust (denser, basaltic) or continental crust (less dense, granitic).
• Plate boundaries are zones of intense geological activity like earthquakes, volcanoes, and mountain formation.

Major and Minor Lithospheric Plates

Major Plates:

• Pacific Plate – Largest, mostly oceanic
• North American Plate
• South American Plate
• Eurasian Plate
• African Plate
• Indo-Australian Plate (sometimes divided into Indian and Australian plates)
• Antarctic Plate

Minor Plates:

• Nazca Plate – West of South America
• Cocos Plate – West of Central America
• Arabian Plate
• Caribbean Plate
• Philippine Plate
• Caroline Plate
• Juan de Fuca Plate
• Fuji Plate

Types of Plate Boundaries

1. Divergent (Constructive) Boundaries

• Plates move away from each other.
• Found mainly at mid-ocean ridges, like the Mid-Atlantic Ridge.
• Magma rises to fill the gap, forming new crust.
• Features: Mid-ocean ridges, rift valleys, fissure volcanoes.

2. Convergent (Destructive) Boundaries

• Plates move toward each other.
• The denser plate is subducted beneath the lighter plate, leading to crustal destruction.

Types of Convergent Boundaries:

• Oceanic–Continental:
  • Oceanic plate subducts beneath the continental plate.
  • Forms volcanic arcs, trenches, and earthquakes.
  • Example: Andes Mountains
• Oceanic–Oceanic:
  • One oceanic plate subducts under another.
  • Results in island arcs and oceanic trenches.
  • Example: Mariana Trench, Japan Arc
• Continental–Continental:
  • Neither plate subducts; intense compression leads to fold mountain formation.
  • Example: Himalayas

3. Transform (Conservative) Boundaries

• Plates slide past each other horizontally.
• No creation or destruction of crust.
• Movement causes fault lines and earthquakes.
• Example: San Andreas Fault (USA), North Anatolian Fault (Turkey)

Significance of Plate Tectonics

• Explains major geological phenomena such as earthquakes, volcanic eruptions, and mountain formation.
• Helps in mineral exploration as tectonic activity brings mineral-rich material to the surface (e.g., copper, uranium).
• Predictive value: By tracking plate movements, future continental arrangements and seismic risks can be estimated.
• Helps understand the distribution of natural resources, oceanic ridges, and sedimentary basins.
• Offers insight into the past configurations of the Earth through concepts like Pangaea, Laurasia, and Gondwana.

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Subject: Geography

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