The reasons for the fractured nature of the earth's crust are not known. Theories include the impact of an asteroid fracturing the surface. In other theories the cooling of the earth following its formation caused the surface to fracture, Universe Today reports. Lithospheric plates are roughly divided into two types: continental and oceanic. Basaltic rock forms oceanic plates; the rock that forms continental plates is predominantly granite. Basaltic rock is composed of heavier minerals, such as magnesium and aluminum, with the granite-like rocks of the continental plates being constructed from lighter minerals, quartz and feldspar.
As both the continental and oceanic plates float on the mantle beneath the surface of the earth, the lithospheric plates are continually moving. The movement of oceanic plates can be increased by a process known as sea spreading: magma forced through plate boundaries cools to rock in sea water and pushes the plates apart. Subduction is the process of the heavier oceanic plates sliding beneath the lighter continental plates.
Each lithospheric plate is irregularly shaped and can be of widths ranging from a few hundred miles to thousands of miles. The depth of each tectonic plate varies from a few miles in deep oceanic plates to hundreds of miles in thickness for continental plates. Some of the deepest plates are the continental plates of North and South America. Amongst the largest lithospheric plates are the Pacific and Antarctic.
Violent collisions of the plates cause earthquakes. Along fault lines the movement of plates causes the formation of landforms, including volcanoes, mountains and dangerous fault systems, Universe Today reports. Thecontinental plates last longer than oceanic plates, which are more often subject to the process of subduction.
