PLATE TECTONICS

A. Wegener, in 1915, first gave the term 'continental drift', which implied that parts of the crust are capable of horizontal movements round the globe, causing the continents to slowly change their positions in relation to one another. The body of theory now called plate tectonics, put forward by A. Holmes and others, embodies concepts which explain- the distribution and origin of many relief features.

The basic principle of plate tectonics is simple-the lithosphere of the earth is considered to be divided into lithospheric plates, of which there are six major plates and many smaller blocks within or between each major plate. Each plate is capable of moving over the asthenosphere, carrying oceanic and continental crust alike. These plates move as single independent bodies-they move past each other or collide. At plate boundaries, major tectonic landforms are created.

The areas of collision may be made up of three elements: deep trenches in the ocean floor; arc-like rows of volcanic iSlands; and mountain ranges. Through geological time, as these plates drifted apart, vast areas of ocean basins came into existence, with thin basaltic crust. At the same time the processes of sedimentation, orogeny and igneous intrusion acting over the subduction zones (plate descended in the mantle) have gradually created thick continental crust with its granitic upper layer. Thus, the continents have evolved and increased in size over a 3-4 million period.

The process of drifting, however, continues, which is observable by phenomena like continued upthrust of the Andes' mountains, the widening of the Atlantic Ocean and the north­ward shift of coastal California.