Plate Boundaries
Plate boundaries are the surface trace of the zone of motion between two plates. Depending upon the movement of tectonic plates, plate boundaries are classified into 3 types constructive, destructive, and conservative plate boundaries, respectively.
Constructive plate boundaries
These are also known as diverging plate boundaries, where two plates move away from each other as a result, a fissure develops, allowing hot molten magma to come up from the mantle and to form a new plate material. The fissure represents the zone of spreading since the upwelling of magma creates the new crust. The divergence of plates may take place in the middle of an ocean or continent.
Divergent boundaries are the sites of seafloor spreading and rift valleys.
At divergent plate boundaries in the ocean floor, magma rises upward-pushes/diverges two or more plates apart, and marks the formation of mid-oceanic ridges.
The best-known example of divergent plate boundary is the Mid-Atlantic Ridge where the American plates are separated from the Eurasian and African plates.
A rift valley is a lowland region, formed by the rifting of earth's tectonic plates. Rift valleys are found both on land and at the bottom of the ocean and are created by the process of seafloor spreading.
Rift valleys differ from river valleys and glacial valleys in that they are created by tectonic activity and not the process of erosion.
The Great Rift Valley system which stretches from the Middle East in the north to Mozambique in the south is a geologically active area.
Rift valleys feature volcanos, hot springs, geysers, and frequent earthquakes.
Destructive plate boundaries
These are also known as convergent plate boundaries, where, two plates move towards each other. The character of boundaries and geological activities associated with them depends on the type of plate involved in the convergence.
Oceanic-oceanic convergence:
In this collision, two oceanic plates converge and one plate bends downward beneath the other, this phenomenon is called Subduction.
Subduction of the plate forming the ocean crust is the cause of trench which is formed immediately over the zone of subduction of the descending plate. At the depth between 150-200 kms melting of the descending plate takes place due to frictional heat and higher geothermal heat.
Initially, tholeiitic lava (rich in K, lack of Al) comes out of oceanic crust melts, later on, Andesitic lava predominates, perhaps due to magmatic differentiation.
Along with the downward plate, part of the trench sediments and some water is also carried down into the hotter environment. With the rise in temperature and pressure in higher depths, this water gets released and rises into the overlying plate causing partial melting of rock by lowering its melting point. Even if the original magma isn’t Andesitic, a part of it may differentiate into Andesite.
Being less dense than the surrounding rock, the magma bodies rise slowly to the surface forming a curved line of volcanos that forms a string of volcanic islands (parallel to the already formed oceanic trench) known as Island arc.
Between the Island arc and the continent lies a marginal ocean basin. The eruption of magma takes place, when the top of the downbent lithospheric plate comes in contact with the asthenosphere.
Accordingly, the horizontal distance between the island arc and the ocean trench varies. The angle of subduction determines the distance. The steeper the subduction angle shorter the distance between the arc and the trench. Similarly, when the subduction angle is gentle the arc-trench spacing also becomes increase.
