Theory of Plate Tectonics

The theory of plate tectonics explains the relative movement of the plates present in the Earth's outer layer. It would probably be impossible to recognize the Earth if we had seen it 225 million years ago, because at that time all the major continents were just one giant super continent called the Pangaea. Due to the heat built up underneath the continent, Pangaea began to rift and split apart. The area between these new sub-continents was filled by oceans. They continued to drift apart and finally reached the positions they currently occupy at present. Till today, these continents are still moving apart.

The outermost part of the Earth's interior consists of two layers: the lithosphere and the asthenosphere. The asthenosphere lies below the lithosphere, having comparatively low viscosity, shear strength and can flow like a liquid on the geographical times scales. The lithosphere consists of the crust and the rigid uppermost part of the mantle. It is broken up into tectonic plates which move in relation to one another. They move at one of the three types of plate boundaries: transform boundaries, divergent boundaries and convergent boundaries. The lateral movement of the plates is mainly at the speeds of 50-100 mm annually. Earthquakes, volcanoes, mountain and oceanic trench formation normally occur along these boundaries.

Types of Plate Boundaries

The three types of plate boundaries that exist are mainly characterized by the relative motion of the plates with respect to each other. These plates are also related to different types of surface phenomena. The three types of plate boundaries are as follows:

1. Transform Boundaries: Transform boundaries also known as conservative boundaries, occurs when the plates slide or the grind past each other along the transform faults. This motion is either sinistral (left side toward the observer) or dextral (right side toward the observer). According to the scientist John Tuzo Wilson, the plates can't glide past each other due to friction, rather a stress is built up between the plates which gradually reaches a level that exceeds the strain. Depending upon the rheology of the rock, the strain can be accumulative or instantaneous in nature. Earthquakes are a common phenomenon along the transform boundaries, and are caused due to the energy released by the instantaneous strain release. The San Andreas Fault in the western coast of North America is a good example of transform boundaries. Here, the Pacific and the North American plates move relative to each other in a pattern in which the Pacific plate is moving northwest with respect to the North America plate. Other examples of transform boundaries are the Alpine Fault in New Zealand, the North Anatolian Fault in Turkey and the Mendocino Fracture Zone offshore northern California.
2. Divergent Boundaries: Divergent or constructive boundaries occur where two plates slide apart from each other and the space created is filled with new crustal material sourced from molten magma that forms below. The origin of these boundaries at triple junctions is linked with the phenomenon known as hotspots. According to this phenomenon, large convective cells bring huge quantities of hot asthenospheric material near the surface, and the kinetic energy generated is sufficient to break apart the lithosphere. The hotspot which may have started the Mid-Atlantic Ridge system recently underlies Iceland which is extending at a rate of a few centimeters per year. The divergent boundaries are symbolized in the oceanic lithosphere by the rifts of the oceanic ridge system, including the East Pacific Rise, the Mid-Atlantic Ridge, and in the continental lithosphere by rift valleys such as the Great Rift Valley in East Africa.
3. Convergent Boundaries: Convergent boundaries, also known as active margins, occur where two plates slide towards each other forming either a subduction zone (if one plate moves underneath the other) or a continental collision (if the two plates contain continental crust). Deep marine trenches are mainly related with subduction zones. The subducting slab consists of many hydrous minerals which release water on heating, causing the mantle to melt and producing volcanism. The Japanese islands, the Andes mountain range in South America and the Aleutian islands in Alaska are good examples of convergent boundaries.

Until now scientists are unable to determine what drives the plate tectonics, though they have come up with many theories, like the convection below the Earth's mantle pushes the plates apart, in a similar way the air heated by our body rises upward and gets deflected sideways when it reaches the ceiling, or the Earth's gravity pulls the older, colder, and thus heavier ocean floor with more force than the newer, lighter sea floor.