|Þis gewrit hæfþ wordcwide on Nīwenglisce.|
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.
Þæs þe is gewiten, is sēo Eorðe se āna planēta in þǣre Sunnlican Endebyrdnesse tō hæbbenne foldlice clūtas, þēah þe man sægde þæt Mars mihton geāgen in þǣre forþgewitenan tīde, ǣr þǣm þe sēo planētanhrūse frēas on stæle.
- Bird, P. (2003) An updated digital model of plate boundaries also available as a large (13 mb) PDF file 
- Landcarte eorþclūta