Plate Tectonic Theory

Plate tectonics is a unifying, integrated and holistic theory that deals with the whole mechanism of evolution, nature and motion of the plates and their resultant reactions. This theory describes and explains

• Evolution and development, decay and death of continents and oceans.

• Origin of endogenetic forces and related landforms.

• Origin and distribution of earthquakes, volcanism and fold mountains.

Movements of Plates

Thermal convective current originating from mantle-core boundary is the source for the movement of the plates. Rising and laterally spreading outflowing thermal convective current causes the plates to diverge, whereas laterally inflowing and sinking thermal convective current causes the plates to converge.

Plate Margins and Plate Boundaries

Outer most fringes of the plates are known as plate margins. A zone of interaction and related tectonic processes and landforms originating due to the movement of plate margins is called plate boundaries. Plate boundaries are classified into three categories. They are −

• Convergent plate boundary or Zone of Subduction or Destructive plate boundary

• Divergent plate boundary or Zone of Rifting or Constructive plate boundary

• Transform plate boundary or Zone of Shearing or Conservative plate boundary

• Convergent plate boundary or Zone of Subduction or Destructive plate boundary

  Convergent plate boundary or Zone of Subduction or Destructive plate boundary

Interaction between Oceanic and Continental plate boundaries

When the oceanic plate (high density) and the continental plate (less density) gets collided, the denser oceanic plate undergoes subduction under the influence of gravity and the failing limb of the thermal convective current. The subducted plate due to the intense temperature, gets melted to form new magma. Cracks that are formed during nucleation process (sudden release of energy in convergent plate boundary due to breaking of rocks by accumulation of pressure) will penetrate deep into the zone of subduction also known as Wadati-Benioff zone.

Ocean Water penetrated into the crack, comes out as water vapor due to high temperature prevailing at the zone of subduction. Now the newly formed magma finds its way out through the vents created during this process as volcanic eruption on either continental or oceanic plate margin.

Due to collision of the two plates, fold mountains are formed on the continental boundary and trenches on the oceanic boundary. Here, the newly formed volcanoes, trenches, coast line and the fold mountains lie parallel to each other. For example, the North American and the South American plate converges with Pacific plate and the Nazca minor plate forms Rocky Cordillera and Andes Cordillera respectively. The Pacific Ring of Fire is one of the significant resultant features of this convergence.

Interaction between two oceanic plate boundaries

When the two oceanic plate boundaries get collided with each other, the relatively denser oceanic plate undergoes subduction under the influence of gravity and the failing limb of the thermal convective current. Similar to the oceanic and continental plate boundary collision, subducted plate gets melted thereby forming volcanic system on either side of the plate boundaries. Due to collision of the two plates, Islands and ridges on the lighter plate boundary. For example, Oceanic plate boundaries of Indo-Australian plate and Eurasian plate converges to form Islands of Andaman & Nicobar and Indonesia.

Interaction between two continental plate boundaries

When the two continental plate boundaries which are having almost same density gets collided, as there is no room for subduction, both the plate boundaries rise to form high altitude fold mountain system. The Great Himalayan fold mountain system is the only known of its kind.

Divergent plate boundary or Zone of Rifting or Constructive plate boundary

Tectonic processes and landforms associated with divergent plate boundaries are classified into following types.

• Intra Plate Rifting

• Inter Plate Rifting

• Sea Floor Spreading

Let’s discuss each one them separately in brief −

Intra Plate Rifting

Intra plate rifting occurs in the continental plate which is thick and lighter, and placed above the rising or laterally spreading thermal convective currents. As the rocks of continental plate are thermally heated, there is an expansion in the volume of the rocks. This thermal heating and expansion of continental rocks is the source for dooming effect, lateral stretching and formation of long narrow, deep and steep fault inside the continental plate (i.e., normal faulting). The Great Rift valley of Africa is formed due to this phenomenon.

Inter Plate Rifting

When two continental plate margins move apart under the influence of the raising and laterally spreading thermal convective currents, a long narrow and deep sea in opened up between the plate margins. This tectonic process is described as inter plate drifting or opening of the sea. This phenomenon is observed between the African and Arabian minor plate resulting in the opening of Red Sea.

Sea Floor Spreading

When intra plate or inter plate drifting occurs in oceanic plate margin(s) under the influence of the raising and laterally spreading thermal convective currents, undergoes divergence. The basaltic magma is upwelling and outflows laterally. On the process of cooling and solidification of the magma on the either side of the rift forms new oceanic crust. Mid Oceanic Ridges and Rises are the landforms that are associated with the phenomenon of Sea Floor Spreading. Slow moving Divergent plate boundaries result in the formation of Mid Oceanic Ridges (Narrow and Steep) whereas fast moving Divergent Plate boundaries result in the formation of Mid Oceanic Rises (broad and Gentle). Mid Atlantic Oceanic Ridge is the longest and most extensive Mid Oceanic Ridge in the world.

Transform plate boundary or Zone of Shearing or Conservative plate boundary

Transform plate boundary occurs when two plate margins slide past each other (opposite direction) horizontally forming the shearing zone between them. Nucleation occurs in the shearing zone as a result of release of built-up pressure which is followed by intense seismicity. Due to the non-influence of thermal convection current and absence of subduction of plate margin, no volcanicity is found in the transform plate boundary. The unique feature of fault formation in the transform plate boundary is found notably in the San Andreas fault, where the Pacific and North American plates slide past each other.

Conclusion

Understanding the past and present movements of the plates, helps in the prediction of future movement of them and their associated landforms. The study of plate tectonics also helps in mitigation of catastrophic events such as Earthquake and volcanic eruption.

Frequently Asked Questions

What is Plate tectonic theory?

Plate tectonics is the theory that describes how the earth’s outer shell called ‘Plates’, glides over the mantle and the formation of their resultant physical relief features.

What is difference between Plate margins and Plate boundaries?

Outer most fringes of the plates are known as plate margins. A zone of interaction and related tectonic processes and landforms originating due to the movement of plate margins is called plate boundaries.

What is the Process of Inter plate rifting?

When two continental plate margins move apart under the influence of the raising and laterally spreading thermal convective currents, a long narrow and deep sea in opened up between the plate margins. This tectonic process is described as inter plate drifting or opening of the sea. This phenomenon is observed between the African and Arabian minor plate resulting in the opening of Red Sea