Plate tectonics is a theory which gave a new dimension to the knowledge of continental drift, ocean floor spreading, volcanic activity, seismic waves, structural features etc. This idea came out of the work of Harry Hess, who propounded the theory of ocean floor spreading. This theory gave some basic points over which the concept of plate tectonics was first given by WJ Moran (1967). The theory was further supported by McKenzie by and Parker by their individual works.
This theory is based on the assumption that the crustal structure of the earth is divided into a number of plates, and these plates are continuously in motion both with respect to each other and to the earth’s axis of rotation. Every plate is supported by plate boundary which is the surface zone of motion between two plates. The general theory of lithospheic plates with their relative motions and boundary interactions is plate tectonics. Tectonic is a noun mean the study of tectonic activity. Tectonic activity, in turn, refers to all forms of breaking and bending of the entire lithosphere.
The global system of lithospheric plates
The global system of lithospheric plates consists of seven major and six important minor plates. The major plates are named as the Pacific plate, the Indian plate, the North American plate, the South American plate, the African plate, the Eurasian plate, and the Antarctic plate. The minor plates are the Arabian plate, the Philippines plate, the Cocos plate, the Caribbean plate, the Nasca plate and the Scotia plate.
Geologists have identified and named several other smaller plates within the above-mentioned plates. The average thickness of the continental plate is 60-70 kms. However, some oceanic plates are only 5-11 km deep and the fold mountains is about 100 km thick.
Plates may be continental, oceanic and continental-cum-oceanic. The all plates rest over a viscous and plastic asthenosphere, also called the low velocity zone (LVZ). They not only rest but also make relative movement over this slippery upper mantle (asthenosphere).
As all the plates form part of the crust of a spherical earth and all the plate boundaries adjoin one another without leaving gap on any scale, it follows that tall plate motions are relative and depend upon one another.
All such movements, and construction and destruction of plates must be within the limiting factor so that the surface of the earth remains practically unchanged. It is proven by the fact that the radius of the earth has increased only about 5% during the last 600 million years.
Plate boundaries and margins, and the nature of movement
The edges of the plates are designated as boundaries and margins. When movement occurs, the edges are called boundaries. Margins refer only to the peripheries of the plates. Generally, plates spread away from the mid-oceanic ridges and is consumed along thee oceanic trench. Three types of movement occur along the plate margins. They are known as:
i. Constructive movement
ii. Destructive movement
iii. Conservative movement
Constructive movement occurs along the upwelling zone. Here, mid-oceanic ridges are formed and plates move apart. Destructive movement occurs along the oceanic trench where the moving plate bends down and plunges into the soft asthenosphere. The process of down-plunging of one plate beneath another is called subduction.
The leading edge of the descending plate is cooler than the surrounding asthenosphere. Its rocks are also more brittle and denser than the surrounding asthenosphere. Consequently, once subduction has begun, the slab sinks under its own weight. Gradually, however, the slab is heated by the surrounding asthenosphere and is ultimately converted into magma at a depth called the Benioff Zone.
This magma ascends like hot air balloons through the overlying continental lithosphere. By the time this magma reaches the earth’s surface, its quantity is huge enough to build a distinct structure called volcanoes, which tend to form a volcano chain parallel to the deep oceanic trench that marks the line of oceanic plate. That is, the meeting lines of the two plates beneath the ocean are littered with volcanoes.
It is clear that a single lithospheric plate is simultaneously undergoing accretion (growth by addition) and consumption (by softing and molting) so that the plate might conceivably maintain its size. A third type of movement is known as conservative or passive movement. Here, plate margins make just shearing or friction. One plate merely slides past the other with no head-on motion. The plane along which motion occurs is a nearly vertical fracture extending down the entire lithosphere. It is called a transform fault.
A fault is a plane of rock fracture along which there is motion of the rock mass on one side with respect to that on the other. Drawing below shows the top view of a single rectangular lithospheric plate with two transform boundaries. The rectangular plate, as assumed, is surrounded by a stationary plate.
Mechanics of plate movement
Plate movement is now a hundred per cent reality but not a single plate tectonic theory is able to explain satisfactorily the mechanism of plate movement. It is, however, believed that the low velocity zone keeps the basic clue of the movement of plates. Its low velocity and high temperature make rocks of shear strength. It becomes a plastic and slippery layer, and plates move over it withous resistance.
There are also other views. It is believed that plates may be pushed apart by mantle upwelling, pulled apart by sinkers, asthenosphere or simply slide under the influence of gravity. Moreover, stresses set up between and within plates may in part determine their motions. This may be particularly relevant in the case of smaller plates. It seems probable that plate movements are not solely attributable to any one of these effects, but result from a combination of all acting in varying degrees on individual plates.
Objections
Though this theory has received global appreciation but it is criticised on the following points:
i. Spreading boundaries are longer than the converging boundaries. Then how the balance is maintained.
ii. Subduction zone is not found in all the oceans. It is limited to pacific coasts.
iii. Some studies have revealed that same plates record constrasting movements which should not take place.
iv. Actual number of plates is not known.
v. Benioff Zone is also not found in the lower parts of all subduction zones.
vi. This theory does not explain the origin of some old mountains. Eastern Highlands of Australia, Drakensberg of South Africa and Siena De la mar of Brazil are not explained.
The theory, however, is able to answer the following structural aspects:
i. It explains the causes of the occurrence of the Pacific trenches, the Andes Mountains, and volcanic intrusions.
ii. It explains the causes of the origin of all tertiary mountains.
iii. This theory explains the causes of the origin of transform fault and mid-oceanic ridges.
iv. It gives more scientific explanation of the causes and the extent of volcanic and earthquake zones of the world.
v. It gives a completely new dimension to the idea of continental drift. It is this theory which put forward the idea of the movement of plates and not of continents. It further gave more specified explanation of the nature continental drift.
vi. This theory explains the symmetrical geology of the formation of mid-oceanic ridge, ocean floor, ocean deeps, coastal ranges and volcanic intrusion. No other theory is so much unifying than the plate tectonics theory. It is like a geological wizard explaining most of the complicated structures.
Though there are some objections to this theory, it is a revolutionary and comprehensive approach towards the complex systems of the geology of the earth. It is now almost accepted as the most powerful and fundamental theory explaining and correlating so many major geological features of the earth. To many, limited objections might be due to our limited knowledge about the mantle and the core of the earth. The theory has to its credit to give a new dimension to the geological and geographical investigations, and the apprehensions raised by some critics may disappear in due course.
(Based on RB Singh's lecture)
