TECTONIC PLATES

 TECTONIC PLATES

The solid lithosphere is believed to be made up of several crustal plates or tectonic plates (which are sixteen to twenty great slabs of rock) floating on the part of the mantle called the asthenosphere. They move in response to convection currents in the upper mantle. Magma rises from the core and lower mantle towards the surface and spreads out at mid-oceanic ridges. The plates are hot at mid-oceanic ridges but over a period of a million years or so, as these plates moved apart and cooled, the colder plate descended at the trenches dragging the surface plate with it.

The asthenosphere showed at a subduction boundary

A crustal plate is an area of continental and oceanic crust along with the upper mantle. These plates have been in constant motion and over millions of years they have formed continents and ocean basins and their irregularities. The shape of the continents is believed to have evolved and changed over millions of years due to crustal plate movements.

In 1915, Alfred Wegener, a German meteorologist, had put forward the theory of Continental Drift based on the jigsaw fit of different continents. According to this theory, a supercontinent called Pangaea, set in a huge ocean called Panthalassa Pangaea, set in the huge ocean called Panthalassa broke up 200 million years ago, and gradually the continents drifted to their present locations.

Separation of  continents by plate tectonics: The present continents formed from the breakup of a supercontinent, Pangaea

THE THEORY OF PLATE TECTONICS

This theory suggested that the earth's crust is made up of rocky plates which are moving constantly and continue to do so at a very slow rate of 2 to 5 cm in a year. The ocean floors are continually moving, spreading from the center, sinking at the edges, and then be regenerated. It proposes that convection currents beneath the plates move the crustal plates in different directions. This theory explains the formation, movement, and subduction of the earth's plates. It clarifies why and how mountains ranges, volcanoes, and earthquakes occur on the earth's surface and how the continents were separated from each other. 

Structure of a mid-ocean ridge and various boundaries

The basic assumption of the plate tectonic theory is that the crustal plates are rigid. Therefore, the distance between two places on different plates may change on a continuous basis (a few centimeters at a time). Also since each plate moves at a distinct unit, all the major interactions between the plates occur along the plate boundaries. Most of the earth's volcanic activities, earthquakes, and mountain-building activities take place along the plate boundaries. They are the lines of weakness in the earth's crust through which the restless interior of the earth releases magma. 

TECTONIC ACTIVITIES

Tectonic activities are earth movements and processes which tend to build up various features of the earth's crust. They are very powerful. The two types of tectonic movements are vertical movements and horizontal movements. They derive their energy from the high pressure and heat inside the earth.

• Vertical movements raise and sink extensive areas of the crust or wrap it but do not fracture or fold it. They form various raised platforms. They are also called continental building movements or epeirogenic movements.
• Horizontal movements occur due to compressional or tensional forces. They are responsible for folding and mountain formation or orogeny. At times, the crustal rocks get fractured or crack along the lines of weakness. These lines of cracks are called faults.

All tectonic activities are accompanied by numerous earthquakes and volcanic activity.