Plate Tectonic Is a Theory of Geology Which describes the large scale motion of earth's lithosphere. The theory builds on the older theory of continental drift from the first half of the 20th century by Alfred Wagoner and the concept of seafloor spreading developed in the 1960s.
The earth is separated into layers on the basis of mechanical properties and its composition. The topmost layer is the lithosphere comprising the crust and solid uppermost part of the mantle. The lithosphere is hotter and flows like a liquid on geological time scale.
According this theory of plate tectonic, the lithosphere is divided into many plates that are called tectonic plates. In case of earth, there are seven major tectonic plates and many smaller ones (fig. 5.1 (a) and (b)).
These tectonic plates are able to move because lithosphere has a higher strength and low density than the underlying asthenosphere. These plates move in different directions speeds in relation to one another. The location where two tectonic plates meet is called a plate boundary. Plate boundaries are generally associated with geological event such as earthquake and the creation of topographic features as mountains, volcanoes, trenches and ocean ridges. As the plates move relative to each other three types of plate boundaries are created which are associated with different types of surface phenomena. Three different types of plate boundaries are convergent or collision boundaries, divergent boundaries and transform boundaries
(i) Convergent boundaries: convergent boundaries occur when two plates move towards each other and collide. Formations of mountains are the example of convergent boundaries.
(ii) Divergent boundaries: divergent boundaries occur where two plates move away from each-other. Mid-ocean ridges are the examples of this boundary rift.
(iii) Transform boundaries: transform boundaries occur when two plates move side-by -side along the same direction or in opposite direction and faults are created.
The relative movement of these plate boundaries varies across the earth. The lateral movement of the plates is typically at a speed of 0.66 to 8.50 centimetres per year.