The rigid lithospheric slabs are called plates. The study of the whole mechanism of evolution, nature and motions of plates, deformation within plates and interactions of plate margins with each other is collectively called as plate tectonics. Plate tectonic theory, a significant scientific advancement of the decade 1960’s is based on two major scientific concepts e.g. (1) the continental drift theory and (2) the concept of seafloor spreading.
The rigid lithospheric plates floating over the underlying plastic asthenosphere. A plate includes not only continents but oceans as well, hence, there are oceanic and continental plates.
There are seven major plates, an equal number of intermediate-sized plates and perhaps a dozen smaller plates are recognized. Many of the smaller plates are remnants of once larger plates that are not being subducted.
The major plates are as follows:
- Antarctica and the surrounding oceanic plate
- North American (with western Atlantic floor separated from the South American plate along the Caribbean islands) plate
- South American (with western Atlantic floor separated from the South American plate along the Caribbean islands) plate
- Pacific Plate
- India-Australia-New Zealand plate
- Africa (with the eastern Atlantic floor) plate
- Eurasia and the adjacent plate.
The source of heat that drives plate tectonics is the primordial heat leftover from the planets formation as well as the radioactive decay of Uranium, Thorium, and potassium in the Earth’s crust and mantle.
The driving mechanism for plate tectonics is thought to be convection within Earth’s mantle. A very sluggish thermal convection system appears to be operating within the plate, bringing deep-seated heated rocks slowly to the surface.
Plates may be pushed away from mid-ocean ridges to a certain extent, but it appears that much of the motion is a result of the plates being pulled along by the subduction of dense oceanic lithosphere down into the asthenosphere.
Three types of plate boundaries and margins are identified which define three fundamental kinds of geological activity.
(1) Divergent Plate Boundaries:
Divergent Plate Boundaries also called as Constructive Plate Margins, represent zones of divergence where there is continuous upwelling off molten material (Lava) and thus new oceanic crust is continuously formed.
Oceanic plates split apart along the mid-oceanic ridges and move in opposite directions. Divergent plate margins are constructive in the sense that there is the continuous formation of new crust along these margins because of cooling and solidification of basaltic lava which comes up as magma due to rifting of plates along the mid-oceanic ridges.
The Mid-Atlantic Ridge, the East Pacific Ridge, and the Chagos – Laccadives Ridge in the Indian Ocean and other oceanic ridges have come into being in this manner.
Divergent plate boundaries can also develop within continent resulting in a continental rift Valley search as the east African rift Valley that extends from Ethiopia southward through Mozambique. The Red Sea is also an outcome of spreading taking place within the continent.
Divergent movement of plates results in:
- The volcanic activity of the fissure flow of basaltic magma.
- Creation of new oceanic crust
- Formation of submarine mountain ridges
- Creation of transform faults.
- The occurrence of shallow focus earthquake.
- Drifting of oceanic plates etc.
(2) Convergent Plate Boundaries:
Convergent Plate Boundaries also called as Destructive or Consuming Plate Margins, because two plates converge along a line and collide wherein leading edge of one plate (of relatively lighter material) overrides the other plate (of relatively denser material) and the overridden plate is subducted or thrust into upper-mantle and thus a part of the crust (plate) is lost in the mantle.
Such collision boundaries are known as Subduction Zones or Benioff Zones. In a subduction zone, many things happen-
- The occurrence of the explosive type of volcanic eruptions.
- Deep focus earthquakes and
- The formation of folded mountains, island arcs, and festoons, oceanic trenches, etc.
Based on Plate tectonics, there are three types of Plate Collision.
(A) Oceanic – Continental Convergence: Because oceanic lithosphere includes dense basaltic crust, it is denser than continental lithosphere, and so oceanic lithosphere always underrides Continental lithosphere when the two collide. The dance oceanic plate slowly and inexorably sinks into the asthenosphere in the process of subduction.
The subducting slab pulls on the rest of the plate – such ‘slab pull’ is probably the main cause of most plate movement. Whenever such an oceanic- continental convergent boundary exists, a mountain range is formed on land (the Andes range of South America is one notable example, the Cascades in Northwest North America is another) and a parallel oceanic trench develops as the seafloor is pulled down by the subducting plate.
As the subducting plate descends into the asthenosphere The earthquakes become progressively deeper, with some subduction zones generating earthquakes as deep as 600 km below the surface.Volcanoes develop from magma generated in the subduction zone. Early researches thought that a subducted plate would melt when pushed down into the hot asthenosphere. However, more recent research indicates that such a result is unlikely. Oceanic crust is relatively cold when it approaches a subduction zone and would take a long time to become hot enough to melt.
A more likely explanation is that beginning at a depth of hundred kilometers water is driven off from the oceanic crust as it is subducted and this water lowers the melting point of the mental rock above, causing it to melt.
This magma Rises through the overriding plate producing both extrusive and intrusive igneous rocks. The chain of volcanoes that develops in an association with an oceanic-continental plate subduction zone is sometimes referred to as a continental volcanic arc such volcanoes frequently erupt explosively.
Metamorphic rocks often develop in association with subduction zones. The margin of a subducting oceanic plate is subjected to increasing pressure, although relatively modest heating, as it begins to descend – this can lead to the formation of high pressure, low-temperature metamorphic rocks such as Blue Schist.
Besides, the Magma generated in the subduction zone may cause contact metamorphism as it rises through the overlaying continental rocks.
(B) Oceanic – Oceanic Convergence: involves the collision of two oceanic plates. As one the oceanic plates having relatively denser material subducts beneath the other, an oceanic trench is formed, shallow and deep-focus earthquakes occur, and volcanic activity is initiated with volcanoes forming on the ocean floor.
With time, a volcanic island arc ( such as the Aleutian Island and the Mariana Islands) develops; such an Arc may eventually become a more mature Island arc system (such as Japan and the island of Sumatra and Java in Indonesia are today).
(C) Continental – Continental Convergence: Where there is a convergent boundary between two continental plates, no subduction takes place because continental crust is too buoyant to subduct. Instead, huge fold mountain systems such as the Himalayas, the Alps are built up. Under the condition of continental collision, volcanoes are rare, but shallow focus earthquakes and regional metamorphism are common.
(3) Transform Boundaries:
Transform Boundaries also called as Conservative Plate Margins. Where two plates pass or slide past each other along transform faults. These are called conservative because the crust is neither created nor destroyed. The significant tectonic expression of such a situation is the creation of
Transform Faults, which move on an average parallel to the direction of plate motion. Transform faults are associated with a great deal of seismic activity, commonly producing shallow-focus earthquakes.
Transform fault offset mid-oceanic ridges. Besides oceanic transform faults, there are also continental transform faults e.g. San Andreas fault (California, USA), Alpine fault(Africa), etc.
The other manifestations of conservative plate margins include no volcanic activity, seismic events, creation of ridge and valley, fracture zone, etc.
Plate tectonics has enabled scientists to explain the problem of the origin of folded mountains, continental drifts, vulcanicity, and earthquakes.