Mechanism of Plate Movement, Plate Tectonics Vs Earthquakes and Volcanoes Part-3

Download PDF of This Page (Size: 309K)

Mechanism of Plate Movement

Arthur Holmes, a British geologist, in 1928 – 1929, proposed that convectional currents exist underneath the lithosphere. The centre of the convectional current is not exactly known, but it is believed that it has an average depth of about 100-250 km below the surface. The inception of the current is initiated by heat generation due to radioactive minerals.

Due to integration and disintegration of the atomic minerals heat is produced and hence the melting of surrounding rocks occurs. In this way, the currents start operating. These convectional currents are classified into rising and falling with divergence and convergence activities, respectively.

Thermal Convection and Plate Movement

Thermal Convection and Plate Movement

Thermal Convection and Plate Movement

With rising convectional current, the transport of hot and viscous matter takes place upwardly. After reaching about 100 kms below the surface, the current gets diverged leading to split into the upper part. The molten material penetrates into the split, thus creation of the new surface and the draft of the mammoth plate in opposition direction took place.

It occurs below the mid-oceanic ridge. On the other hand, two sets of diverging thermal convectional currents brings two plates together and it is known as the convergent boundary where subduction takes place. The plates of the lithosphere are constantly in motion because of convectional currents. Their relative motion depends upon the force operating over them.

Plate boundaries are very important and significant structural features. Boundaries are very distinct and easy to identify. They are associated with newly formed mountain systems, oceanic ridges and trenches. Plates are moving continuously and have relative direction of movement. Based on the direction of movement three types of plate boundaries can, easily, be identified:

  1. Divergent Boundary

  2. Convergent Boundary

  3. Fracture or Transform Boundary Fault

The convectional currents are caused due to radio-activity. These currents get diverted on approaching the crust layer. Diverging currents produce tension at the contact zone of the crust leading to fracture. Magnetic materials penetrate into the fractures and gets solidified. This continuous process pushes the blocks in opposite direction and creates a new zone, known as the zone of construction.

At the convergent boundary, two adjacent plates come closer to each other and collide. When both the sides are of continental in nature, a mountain formation is evident. When one of the two is continental and the other is oceanic, again mountain comes into being along the boundary. In this case, continental plate overrides the oceanic plate.

When both the plates are oceanic, both of them break, subduct and penetrate below, hence trenches are formed. Along this boundary earthquakes and volcanic activities are prominent. In all these three situations, the surface area is reduced. Therefore, this is also known as the zone of destruction.

Transform fault occurs when two adjacent plates slide past each other. Direction of the movement may be along or against the plates but they move parallel to each other. Therefore, neither there is any construction of new area nor it has any destruction. Hence, it is known as the zone of preservation.

Plates are not a permanent feature but they vary in size and shape. Plates can split or get welded with adjoining plates. Almost all tectonic activities occur along the plate boundaries.

Prior to the advent of plate tectonic theory, the continental drift theory which was proposed by Wegener was criticized, particularly about the forces. In fact, it was out rightly rejected in spite of the apparent evidences. But further researches about the material of sea floor and Palaeomagnetism supported the theory but the proposition of plate tectonics theory in 1960’s has solved the problems regarding the understanding of the mechanism of movement.

Plate Tectonics vs Earthquakes and Volcanoes

The distribution of earthquakes and volcanoes over the globe clearly reveals that they are strongly associated with the boundaries of plates. Plate boundaries are the zones where every sort of tectonic activity does take place. The release of energy is created because of the movement of plates is manifested in this zone in the form of earthquakes and volcanic eruption.

World Disribution of Earthquake Activity

World Disribution of Earthquake Activity

World Disribution of Earthquake Activity

In Brief, It Can Be Said That:

The surface of the earth is dynamic. This dynamism is due to the forces operating from inside the surface known as endogenetic forces as well as on the surface or atmospheric forces known as exogenetic forces. The surface of the earth is irregular. Hence, a sort of dynamic equilibrium is always in operation which is termed as isostasy.

Airy propounded the idea of uniform density of all rocks on the surface but has its roots depending upon the height of the column. A greater root will be found beneath the higher and lofty body of mountains and having smaller root under lower columns like plateau or plain. On the other hand, Pratt accepted that the rocks found on the earth have different densities. At a particular depth, the weight of all columns of varying height will be compensated. Hence, higher column of mass will have lower density and lower column will have higher density. Therefore, both of them are explaining the isostatic balance, but with different perspectives.

The distribution of land and water on earth surface is not static. It has changed, it is changing and it will change in the future too. This changed position is said to be continental drift in crude way which was conceived by Wegener, but the mechanism explained by him was not scientific. Therefore, his ideas of continental drift were denounced in spite of his strong and testifying evidences.

With the concept of convectional current theory of Holmes and proposition of the plate tectonics, a new thinking came in understanding the surface of the earth. Study on Palaeomagnetism as well as sea floor spreading have supported the plate tectonics theory. According to this theory, the earth surface is made up of several broken blocks of enormous size with great depth considered to be a plate. There are seven bigger size plates and twenty-seven smaller size plates.

As per the concept of convectional current, their movement takes place in three possible ways. Firstly, two adjacent plates move away on divergent boundary where a new zone is constructed. Secondly, two adjacent plates come closer on convergent boundary and get subducted where a zone is destroyed. Finally, in which two adjacent plates slide past each other on fracture or transform boundary fault where the margins of both plates are preserved. Because of these different tectonic activities, earthquakes and volcanoes are associated with the plate margins.

Developed by: