ABSTRACT
Today it is accepted that the Earth is covered by some rigid plates which move
across its surface, over and on a partially molten internal layer. Using geological
terms, the plates form the lithosphere, which is the Earth’s solid rock. The rigid
lithosphere can be considered to float on the ductile asthenosphere, which flows.
So, the lithosphere (surface of the Earth) is broken up into what are called tectonic
plates; Plate tectonics (from the Greek tecton, meaning one who constructs and
destroys) being the theory of geology developed to explain the phenomenon of
continental drift. This theory defines the tectonic plates and their boundaries. In
function of their surface, the tectonic plates are divided into major and minor
plates. Tectonic plates can include continental crust, oceanic crust or both. The
distinction between continental crust and oceanic crust is based on the density of
constituent materials. The continental crust is composed primarily of granite, so it
is relatively light. An average thickness may by around 30-40 km, while the
thickest part is about 70 km in the zone of the Himalayas and Tibet. The oceanic
crust is denser than the continental one, being composed of basalt. It is relatively
thin, about 7 km thick. As a result, the oceanic crust generally lies below the sea
level, while the continental crust is situated above the sea level. There are 52
important tectonic plates (USGS, nd, Wikipedia, nd). The 14 major tectonic plates
are shown in Figure 5.1. Among them, the seven most important plates are:
