Introduction
It is known that the crust of the earth consists essentially of about 35 km thick layer of solid rock matter, which varies in thickness from about 5 km in the oceanic areas to even such thickness as 70 to 80 kms in the mountainous regions of the Alps and the Himalayas. That the crust is not fully rigid, but has been repeatedly deformed in the geologic past and is subjected to movements even now is proved by earthquakes frequent on the oceanfloors and rarer in continents, volcanism, folding and faulting of large expenses of rock strata and recent elevation and depression of coastal areas.There are now tangible evidences chiefly from palaeomagnetic studies that the crust of the earth (oceanic and continental) together with the upper portion of the mantle, which overlies the asthenosphere (the low-velocity zone approximately at 100 to 150 kms depth) constitute the lithosphere, which is disjointed into plates or blocks extensively by faults or thrusts. Thus the lithosphere is made up of lithospheric plates.
Plate tectonics provides a modern view of the rock cycle. It involves a world-wide net-work of moving plates of lithosphere.
Important Features of Plate Tectonics
(i) It is assumed that the earth is composed of 20 lithospheric plates.
(ii) They have the thickness from 0 to 10 kms at the ridges to 100-150 kms elsewhere.
(iii) The plates may contain continental as well as oceanic surfaces.
(iv) These plates are continuously in motion both with respect to each other and to the Earth’s axis of rotation.
(v) Virtually all seismicity, volcanicity and tectonic activity is localized around plate margins and is associated with differential motion between adjacent plates.
(a) Plate boundary It is the surface trace of the zone of motion between two plates.
(b) Plate margie The marginal part of a particular plate.
Two plate margins meet at a common plate boundary.
(vi) These plates are small and large, separated by faults and thrusts, lying mostly across ridges or parallel to the continental borders (trenches).
(vii) They move with velocities ranging from 1 to 6 cm per year.
(viii) Where two plates diverge, we find extensional features, typically the oceanic ridges, symmetrical about the vertical axis.
(ix) Where two plates slide past each other, we find transcurrent faults, i.e., the large strike slip faults joining segments of ocean ridges or arcs.
(x) Where two plates converge, and one is thrust beneath the other we find the island arcs, the huge assymmetric features that are the sites of greatest earthquakes, explosive volcanism, great topographic relief and many other distinctive features.
Type of plate margin
There are three types of plate margin
(a) Constructive,
(b) Destructive, and
(c) Conservative.
(a) Constructive
In this case new crust is created by the upwelling of materials from the mantle. The lithospheric plates diverge at the crest of the mid oceanic ridges where new surface is created. Thus a ridge represents a zone along which two plates are in motion away from each other, yet they do not separate because new material is continuously added to the rear of each. Boundaries at which the net effect of motion is to generate surface area are termed as ‘Sources’.
In case of constructive plate boundaries, the greatest principal stress is vertical and the plate boundary will consist ofa set of normal faults dipping about 60° from the horizontal.
(b) Destructive
It represents the zone of convergent plate boundaries, along which two lithospheric plates are coming together and one plate is forced to plunge down into the mantle, In this case the plate boundary will be a reverse fault dipping at angles of 30° from the horizontal. This overriding of one plate on another gives rise to trenches and island arcs. Plate boundaries at which the net effect of the motion is to destroy surface area are called 'Sinks’.
(c) Conservative margins
When the lithospheric plates can slide past one another and that the plates neither gain nor lose. surface areas, there results a transcurrent or transform fault, which marks the conservative plate boundaries.
Although plates may comprise either continental or oceanic crust or both, it seems that only those parts of plates which are capped by oceanic crust can participate in the main processes of plate growth and destruction.
Ealer’s theorem
It is a geometrical theorem which shows that every displacement of a plate from one position to another on the surface of a sphere can be regarded as a simple rotation of the plate about a suitably chosen axis passing through the centre of the sphere. All points on the plate travel along small circular paths about the chosen axis in passing from their initial to final position. It follows that any plate boundary which is conservative must be parallel to a small circle, the axis of which is the axis of rotation for the relative motion of the plate on either side. Conversely, any plate boundaries which are not parallel to such small circles must be either constructive or destructive.
physical characters
To understand the mechanisms of the movement of the plates, it is necessary to know the physical characters of the lithosphere:
(a) Thermal property. The lithosphere strongly modifies the stress and temperature fields as it transmits them from the asthenosphere to the earth’s surface. As the hot newly created, lithospheric plate moves away from the accreting plate boundary, it progressively cocls according to an exponential law through flow of heat at its Surface.
(b) Elasticity. The lithospheric plate can be considered to be a thin elastic sheet which floats over a fluid substratum and bends under super crustal load.
(c) Mechanical properties. The newly created oceanic lithoshpere near the mid oceanic ridges is hot and very thin and should be much weaker than the normal ocean bearing or continent bearing lithospheric plate. However at equality of thickness, continent bearing plates are easier to deform than ocean bearing plates.
(d) Lithosphere as a stress guide. The zone of deep and intermediate earthquakes often called Benioff zones, correspond to stresses occurring within a lithospheric plate which sinks into the asthenosphere and not to faulting between the asthenosphere and the lithospheric plates.
The tectonically active zones are now referrable to boundaries of six major and about twice as many minor, more or less rigid inter plate.
(e) Kinematics of relative movements. According to Wilson the lines of creation of surtace produce surface symmetrically, while lines of destruction of surface destroy surface asymmetrically. These Lines could end abruptly against what he called a transform fault, along which the movement was pure strike slip.
Causes of Plate Motion
1. Oceanic crust formation. As the crustal formation at the mid oceanic ridges is a continuous process, it begins to spread ate rate of l - 6 cm/year and this may cause the motion of the plates.
2. Rates of motion. Since spreading occurs at ridges, at rates ranging from 1 - 6 cm/year but crust is consumed at a rate of 5 to 15 cm/year at oceanic trenches, the plates are to move.
3. Oceanic topography. The mechanism must be consistent with the development of topographic ridges at centres of spreading ridges rises 2 to 4 kms above the level of the ocean floor and near the axes slope away more or less symmetrically from the crest.
4. Gravity. Ridges are close to isostatic equilibrium but sinks are characterised by topographic trenches which shows the largest negative gravity anomalies. The gravitational difference may cause plate motion.
5. Thermal. With increasing distance from ridge crest the scatter of heat flow valucs diminish and the mean heat flow falls until it teaches average level for the oceans. Oceanic trenches have abnormally low heat flow but a short distance away in the adjacent island arcs, the flow is high. The difference between these heat flow values seem to be responsible for plate motion.
6. Convection current condition in the mantle zone seems to be responsible for plate motion, as the diverging current drags the lithospheric plates along the direction of their flow.
7. Strength of the lithosphere. Even though lithospheric plates appear to be able to move great distances without undergoing significant internal deformation, in some instances the plates are 20 times as long as they are thick. With such a length to thickness ratio neither compressional nor tensional stresses could be transmitted from one end of the plate to the other, unless the frictional resistance beneath the plates are very small. As the plates are above the viscous melt, it suffers no friction and can move.
Importance of plate-tectonics
The theory of plate tectonics is useful in explaining the phenomena like ;
(i) Continental drift.
(ii) Mountain building (where two continental plates collide with each other).
(iii) Island arcs (where an oceanic plate undergoes subduction beneath a continental plate).
(iv) Oceanic trenches.
(v) Ocean floor spreading.
(vi) Mid oceanic ridges (where plates diverge).
Source
A Text book of Geology, By G.B Mohapatra