The theory of Plate Tectonics developed by geoscientists during early 1960s is often described as a most revolutionary concept in the history of GEOLOGY as a science. It is now widely accepted that most complex geological riddles, past and present, are solved conveniently by the concept of plate tectonics.
According to this concept, supported of course by convincing data and observations, the earth’s outer brittle layer extending down to a depth of 150-200 km and called lithosphere (stony zone), is actually divided into several blocks or slabs or rigid plates. These lithospheric plates have been and are even now in a process of gradual shifting (or drifting or wandering) with respect to each other.
In the words of J.T. Wilson, “the Earth instead of appearing as an inert statue is a living mobile thing.” What are these lithospheric plates, how are they related to each other, why are they in a state of perpetual drift and what are major geological effects of their shifting are some of the major questions that have been convincingly explained and widely accepted during last thirty years or so. Finer details of many answers are being constantly worked out. As on today, the concept of Plate Tectonics has come to stay as a widely acclaimed theory.
The Lithospheric Plates:
According to this concept, the upper rigid part of the Earth, from surface down to a depth 150- 200 km (and broadly called Lithosphere; litho = stoney) is actually divided into several slabs or blocks or plates, which are conveniently called Lithospheric Plates. About twelve of these plates have been so far distinguished and studied and their boundaries almost deciphered.
Some of these plates encompass continents, others encompass oceans and still others are large enough to be made of both continental and oceanic parts. The most important thing attributed to these plates is that they are supported from below on a hot-plastic flexible part of upper mantle zone, called Asthenosphere on which these are capable of shifting or drifting with respect to each other especially along and at the plate boundaries.
These plate movements are neither random nor occasional but regular though very slow phenomena. Latest technological tools (e.g. GPS Global Positioning System) have been used to determine actual rate of movement of some of these plates.
Before proceeding further, it shall be proper to name the twelve major and minor lithospheric plates under focus of study of geoscientists.
1. The Pacific Plate
2. The North American Plate
3. The South American Plate
4. The African Plate
5. The Antarctic Plate
6. The Indian (Australian) Plate
7. The European Plate
8. The Arabian Plate
9. The Nazea Plate
10. The Caribbean Plate
11. The Scotia Plate
12. The Philippine Plate
Movement of Plates:
The crux of the Plate Tectonics theory is in the type of drift they undergo with respect to the adjoining plates along the plate boundaries.
Three principal types of boundaries are:
(i) The diverging boundaries,
(ii) The converging boundaries, and
(iii) The transform boundaries
(i) The Divergent Boundaries:
These are places/narrow zones where two adjoining plates are in the process of pulling away from each other. It is best seen in the case of North American Plate versus Eurasian Plate. The first plate is drifting towards south west whereas Eurasian plate is moving away towards south east.
(ii) The Convergent Boundaries:
These are narrow zones where two adjoining plates are closing on each other. Example is provided by the South American plate and the Nazca plate.
(iii) The Transform Boundaries:
Zones where the adjoining boundaries just slide apart each other in a horizontal direction are grouped in this category. The surface example is provided by the San Andres Fault.
Global Effects of Plate Movements:
(a) Divergent Boundaries:
The best way to understand the mechanism of plate movements is through the examples of effects produced by these drifts in different parts of the earth during the recent past.
The North American plate is believed to be drifting away in a westward direction with respect to Eurasian Plate which is moving in an easterly direction. It is, therefore, classic example of ‘diverging plate boundaries’.
It is believed that this divergent movement started some 200 million years ago and resulted pouring out or emergence of huge volumes of molten material from, below that has all built up the mid-Atlantic ridge which forms a major part of the sub oceanic ridge encircling the globe from Arctic Ocean to South Africa. This type of process is sometimes also referred as sea floor spreading. This particular case of divergence is believed to be taking place at a rate of 2.5 cm/year or 25 km/million years.
It may be interesting to note that the volcanic country ICELAND is an island located on the MID ATLANTIC RIDGE and has received much importance as a natural laboratory of recording movements connected to the divergent plate movements during last fifty years. Many cracks have been recorded appearing and other widening around KRAFLA VOLCANO in northern Iceland. The displacements of about 7 m during the period of 1975-1984 in this region are often cited as examples of divergence.
Some protagonists of plate tectonics have postulated development of new spreading centers under the African Plate along the EAST AFRICAN RIFT ZONE. According to them if such a spreading happens and continues, then the earth will have its next major ocean along the EAST AFRICA. It would take, of course, a geologic time (millions of years) to happen.
(b) Convergent Boundaries:
The other significant movement of lithospheric plates is of convergent type in which two adjoining plates come close to each other from opposite directions and COLLIDE.
Three possibilities have been suggested (and supported by observations):
Of the two converging plates, one is a continental plate made up primarily of lighter and brittle rocks and the other is an oceanic plate made up of harder and denser material.
Both the converging plates are primarily oceanic plates of heavier and denser type.
Both the converging plates are primarily of continental type made of rigid and brittle rocks along the boundaries.
Convergence of such combination of plates leads to entirely different results which are verified by live examples on a global scale.
When an oceanic plate is pushed on to a continental plate, being heavier, harder and denser, it (the oceanic plate) goes or SUBDUCTS under the continental plate. The movement is of a continuous, smoothly sinking type, the plate sub-ducting deep into the interior of the earth for as much depth as the mutual dimensions of the converging plates allow.
The net result of this type of subduction is just reverse, in geological terms, than that of diverging boundaries: subduction involves destruction of the crust. In fact it results into long, narrow and deep trenches along the sub-ducting boundaries. The Peru-Chile Trench developed off the South American coast line is described as a typical example of subduction.
What happens to the over-riding continental plate? It is naturally lifted up at the boundaries and results in mountain ranges and belts. The South American continental plate supporting the high range of Andes mountains is considered a proof enough of this particular case.
When both the converging plates are of oceanic type, that is, they are located at the ocean floor at great depths, subduction remains the principal movement. The result is again primarily development of trenches, rather deeper and the deepest trenches.
The boundary of subducting plate may subduct far quite deep into the asthenosphere. The Challenger Deep, the deepest trench recorded plunges almost 11,000 meters down into the earth’s interior. It is located at the southern subduction end of the Marine Trench caused by convergence of Pacific Plate (Oceanic type).
In third type of convergence where both the plates are made up of continental rocks, there is no subduction as such but collision of almost head-on type; such colliding often results in buckling (folding), fracturing (jointing) and displacement (faulting) at different locations depending on nature of rocks i.e. if they are soft, brittle or flexible in nature respectively. Buckling also results in creating a heaving up at the plate boundaries.
The origin and evolution (in terms of height) of the Himalayas is considered as one of the best examples supporting collision of two continental plates—the Indian plate and the Eurasian plate.
The process is believed to have started more than 50 million years ago with the Eurasian plate advancing southward towards Indian plate that was itself drifting northwards. The slow continuous convergence of these two large continental plates continued over millions of years and resulted in pushing up the Himalayan belt and the intervening Tibetan plateau to their present heights mostly during the last ten million years.
The Himalayas, standing as the tallest and the youngest of continental mountain ranges of the world are cited as the most dramatic example by the proponents of the Plate Tectonics. Their growth is believed to be a continuous process even at present. Hence the Himalayan Systems are considered tectonically very active and location of many major earthquakes of the past and of many potential earth quakes of future as well.
(c) The Transform Boundaries:
Third type of plates have boundaries where there is neither subduction nor is any collision. The adjoining plates just slide ahead in their own directions along the boundary regions in a horizontal direction. These are called transform boundaries or simply transform faults and are very common in oceanic floors.
In fact, it is the study of large number of oceanic type transform faults (the strike slip faults) that is thought by some as preceding idea for the concept of plate tectonics. The transform faults may occur in the vicinity of diverging boundaries or the converging boundaries intersecting or interrupting those boundaries effecting the sub oceanic ridges and trenches as well. These are considered the cause of many shallow earthquakes recorded globally.
The San Andres fault zone (1,300 km long and at places 10 km wide) of California is considered as a classic example of transform fault recorded on the land surface. This fault connects the East Pacific rise in the South and the Explorer ridge in the north. Both are divergent boundaries.
It is believed that along this fault, the Pacific Plate has been grinding horizontally past the North American Plate for ten million years at an average rate of about 5 cm/yr. The land on the west side of the fault zone (on pacific plate) is moving in a north western direction relative to the land on east side of the fault zone i.e. North American plate.
(d) The Driving Force:
The concept of plate tectonics has now been universally accepted.
The facts accepted are:
(i) The upper solid (lithospheric part) is broken into blocks as plates which are in a state of persistent wandering in different directions at different rates, converging here, diverging there and moving apart each other elsewhere;
(ii) These plates are drifting on a hot-plastic upper zone of the mantle (asthenosphere) extending to a depth of 150-200 km below the surface, that is, much below the crustal boundaries under continents (60 km) and oceans (about 10 km).
(iii) Most of major superficial geological features of the planet earth such as continental mountains, sub oceanic ridges and trenches, and oceanic bodies as also most of recurring geological processes such as volcanism, earthquakes, tsunamis, folding, faulting and jointing are easily explained as directly or indirectly related to plate movements (i.e. plate tectonics).
There is, however, yet no broad agreement about the forces that have been (and are) driving these lithospheric plates over the asthenosphere, the top-soft-hot region of the mantle part of the planet.
1. Are these forces due to convection currents resulting from flow of heat due to rise of temperature with increasing depth?
2. Are these the gravitational forces acting on subducting plates that give rise to drag effect that play a role in displacing other plates?
3. Has the rate of heat loss from the interior of the earth to play any part in plate dynamics?
These and other many more questions are presently receiving the attention of the scientists. But there is no dispute now on the basic truth that the lithospheric plates have been moving in the past and are wandering even at present that can be determined with technological sophistication made available to the man during the close of twentieth century.