From Academic Kids
The plates are around 100 km (60 miles) thick and consist of two principal types of material: oceanic crust (also called sima) and continental crust (sial). Under both lies a relatively plastic layer of the Earth's mantle called the asthenosphere, which is in constant motion. This is in turn underlaid by a solid layer of mantle.
The composition of the two types of crust differs markedly. Oceanic crust consists largely of basaltic rocks, while the continental crust consists principally of lower density granitic rocks rich in aluminium and silica. The two types of crust also differ in thickness, with continental crusts considerably thicker than oceanic.
The churning of the asthenosphere carries the plates along in a process known as continental drift, which is explained by the theory of plate tectonics. Interaction between the plates creates mountains and volcanoes, as well as giving rise to earthquakes and other geological phenomena.
The boundaries of the plates do not coincide with those of the continents. For instance, the North American Plate covers not only North America but also Greenland, far eastern Siberia and northern Japan.
As far as is known, the Earth is the only planet in the Solar System to possess tectonic plates, although there have been suggestions that Mars may also have possessed plates in the past before the planet's crust froze in place.
- Picture of tectonic plates (http://snobear.colorado.edu/Markw/Mountains/03/week3.html)