The basic premise of the plate tectonic theory is that the Earths surface is like a cracked eggshell. Each piece being called plates. These are large slabs of solid rock composed of the lithospheric materials. Lithosphere is composed of the solid crust and the upper part of the mantle. There are two types of crust on the Earths surface Continental crust Oceanic crust A Plate This crust is mainly composed of granite. Density is approximately 2.7 grams/cm 3 Waters density is 1g/cc Average thickness is 30-50km Continental Crust The dominate rock is basalt The average density is 2.9 g/cm3 Thickness is only 5-7km The two crust are part of the lithosphere, which floats on the asthenosphere. The Oceanic crust being denser will sink lower into the asthenosphere. Being at a lower elevation, oceanic crust become filled with water due to gravity. Water flows down hill from higher elevations. Oceanic Crust Why the large contrast in crust thickness is due to a principle of balance or support called isostasy. Think of the crust like floating icebergs. The higher elevated object requires a larger base of support. Differences in Crust A plate will vary in thickness because the Earths crusts varies in thickness. Under the continents, the plate can be up to 250km Under the oceans they can be up to 100km thick. The plates also vary in size and shape Lithospheric Plates There are seven large plates and seven smaller plates. They fit together like pieces of a jigsaw puzzle. The plates vary in composition as well. Most plates contain a combination of continental and oceanic crust Some do compose almost entirely of oceanic crust or continental crust. The difference between Wegners continental drift theory and plate tectonic, is that Wegner saw continents as the only moving components of the earth. The lithosphere floats on the semi-molten asthenosphere. The nature of the asthenosphere allows for the movement, much like ice slabs moving on a lake. The rate of movement is quite slow and variable, ranging from 1 to 19 cm per year. The direction of motion is also variable. Plate Movement Lithosphere As plates move, their borders interact. It is at the boarders where Mountains Trenches Oceanic ridges, Earthquakes activity, and Volcanism are found The interior of the plates are generally geologically quiet, but maybe affected by edge activity. There are three possible interactions between lithospheric plates. Plates can: Move away form each other Move toward each other Slide past each other Plate Interaction Where two plates move away form one another, a divergent plate boundary is created. Where two plates move toward each other, a convergent plate boundary is created. Sliding of one plate past another creates a transform plate boundary mlml Links of Interest