How do the global patterns of occurrence of seismicity,volcanism and orogeny may be explained by the theory of the plate tectonics

Plate tectonics theory deals with the mechanism of the movement,evolution and destruction of the plates at the subduction zones.A the mid-oceanic ridges there is crustal tension while at the trenches there is compression(horizontal).

The conventional mechanisms determining plate dynamics and Kynamatics can be grouped under the following:-

a)Basal effects driving from convection cells in the mantle;

b)Resistive or retarding forces;

c)Driving mechanisms associated with ridge push and plate pull mechanism.

The horizontal compressive strain (S_h) occurring to the plates at the subduction zones is caused due to the vertical stress resulting from the gravitational overloading is generated by the relationship-

S_h=S₂/(m-1) Where,m=Poison’s number which for the basaltic rocks has the value of 4.0;

However,it must be said that the basic rule of the pate tectonics which stated that all plate activity occurred at the plate margins has been proved wrong.There are 2 types of earthquakes associated with the plate tectonics:-

a)Intra-plate earthquakes(i.e.,within the plate boundaries);

b)Inter-plate earthquakes(i.e.,along the edges of the interacting plates)

Converging plate boundaries contribute to 90% of the world’s release of seismic energy for the shallow,intermediate and deep focus earthquakes,while 10% of the earthquakes occur at the mid-oceanic ridges.when straining of the plates occur the weakest rocks or the point of the greatest strain break.

Straining of the rocks in the vertical direction is also common.The elastic rebound occurs along the dipping fault surfaces ,causing vertical displacement or disruption in the level lines at the surface and the fault scarps.ridge crests were never continuous but consisted from the beginning segments offset by transform faults.As the crust is carried away from each segment,it moves in the same direction on both the sides of the fault and there is’nt any earthquakes.

However,between the crest segments the pieces move in opposite direction with a sense i.e.,the reversal of the strike slip faults and the earthquakes occur there.volcanic earthquakes occur in the vicinity of the volcanoes before and after or during the eruption’s.

They are caused by the elastic rebound of the ground surface when the magma reservoir beneath the vent is filling causing the swelling of the ground.Release of the super heated steam and the gas disturbs the unstable equilibrium of the magma below the vent,thereby producing further flow of the magma in the subterranean tubes stimulating local volcanic earthquakes .

When Pangaea came under the stress,domes with triple rifts may have formed so close together that their branches connected to become continuous rift valleys,fore-runners of new oceans.these dome which were found in rows aided lava outflows known as plateau basalts.Hawaiian chain of islands and new evidences confirm that most if not all hot spots have fixed position an that they are uncommon on the continents nowdays.

These hot spots are clustered in an east-west zone across the pacific and the north-south zone across the Atlantic,Africa and Europe.this partly explains the cause of the intra-plate volcanism.The rift valleys found at the mid-oceanic ridges and the rifting caused due to break-up of a continent(probably due to local hotspot beneath the lithosphere) is lined with volcanoes.

In the latter case rifting is followed by drifting of the continental plates in opposite direction and the intrusion of the oceanic crust along with invasion of the sea in the rift valleys.The margins cool as the distance to the mid-oceanic ridges increases,the relief of the continent reverses and the major continental drainage now enters the sea.

In the mid-oceanic ridges volcanic cones are active as long they are near the source of the magma but since they are placed on continuously spreading sea floor on the either sides of the ridges.Hence,after sometimes they move out of the reach of magma supply and become dormant and finally reaches the extinction phase.

In the convergent plate margins where one plate is subducted under the other plate.This causes faulting which allows the magma to escape through the gap created by faulting.The portion of the plate which has been subducted displaces magma equal to it’s own weight/volume as per archemedis’s principle.

 

Active margins/Pacific margin type plate margins are subdivided into:-

a)Steady state margins;

b)Collision margins.

 

Steady state margins give rise to the continental margin orogen which is formed when oceanic lithosphere is subducted beneath continental lithosphere.Ex.-Andes of the south America.Out of the 4 kinds of collision margins,2 kinds yield orogens,they are:-

a)Inter-continental collision orogen:-Formed when 2 continental plates collide.Ex.-Himalayas.One of the greatest evidence against the geosynlinal origin of the himalaya and in favour of that of the plate tectonic theory is the fossiliferous nature of the major mass of the Himalaya,particularly lesser Himalaya.

b)modified continental margin orogen:-is formed where an intra-oceanic island arc moves into a subduction zone bound by the continental crust.

In case of the Transform margin,transpression gives the rise to transverse orogens.

 In conclusion,it can be said that plate tectonics theory joins these 3 separate tectonic processes into one bond and helps us in understanding the interrelationships of these 3 processes.