What are plates?

Tectonic plates, or lithospheric plates, are rigid structures about 70 km thick (under oceans) to 100-150 km thick (under continents) and a few hundred to a few thousand kilometers wide that move on the Earth's surface.  A plate includes all of the Earth's crust and that part of the upper mantle that is rigid.  Below this lithospheric plate is the aesthenosphere, a portion of the mantle which is partially melted and deforms plasticly, allowing it to flow over geological time periods.  The plates move on this weaker, viscous aesthenosphere.  There are about 14 major lithospheric plates on Earth today.  The plates have changed size, geometry, and orientation through geological time.
 

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