tectonic plates

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Earth's tectonic plate

Courtesy  NASA Goddard Space Flight Center 
Used with permission.

Tectonic plates are the plates that comprise the fractured surface of the earth. Tectonic plates are of two types: continental plates comprised mostly of granitic-type rock rich in silica and aluminum (also called "sial") and oceanic plates comprised mostly of basaltic-type rocks rich in magnesium and aluminum (also called "sima"). Earthquake and volcanic activity is common along the boundaries of lithospheric plates.