  The basic premise of the plate tectonic theory is that the Earth’s 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 Earth’s surface  Continental crust  Oceanic crust A Plate

  This crust is mainly composed of granite.  Density is approximately 2.7 grams/cm 3  Water’s 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 Earth’s 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

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  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 Wegner’s 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

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