The Earth Contents

2. Plate Tectonics

The crust is made up of a series of plates which have been in continual motion throughout geological time

As the terminology implies, the continental and oceanic crusts are found to correspond closely with areas of land and sea on the Earth's surface, which are called plates. Plates can be exceptionally large (up to several thousand kilometres across) and may be about 100 kilometres thick. Relative to this thickness, the surface differences in rock types which we observe around us become quantitatively insignificant, and the plate composition approximates that of the underlying rock, either granite or basalt. At this global scale we can also map out the boundaries of the plates on the Earth's surface to show a jigsaw-like pattern of irregularly-shaped and different-sized regions of either granite or basalt. These plates have been in continual motion throughout geological time, and have greatly changed their positions relative to each other, the poles and equator.

Plate composition approximates that of the underlying rock; which two types of rock are the most common?