1 A segment of the lithosphere, which has little volcanic or seismic activity but is bounded by almost continuous belts (known as plate margins) of earthquakes and, in most cases, by volcanic activity and young subsea or subaerial mountain chains. Most Earth scientists consider there are currently seven large, major plates (the African, Antarctic, Eurasian, Indo-Australian or Indian, North American, Pacific, and South American Plates). There are also several smaller plates (e.g. the Arabian, Caribbean, Cocos, Nazca, and Philippine Plates) and an increasingly long list of microplates (e.g. the Gorda, Hellenic, and Juan de Fuca Plates). The positions of the boundaries of some present-day plates are disputed, particularly within and adjacent to collision zones, e.g. the Alpine–Himalayan belt, so it is not surprising that very little agreement has been reached about the histories of plates in the geologic past.
2 A general term applied to plane pieces of skeletal material usually formed from calcium carbonate. Plates occur in groups of several types, e.g. the delthyrium in some brachiopods (Brachiopoda) is closed by a pair of deltidial plates.
3 The outer covering of a crinoid (Crinoidea) body, which consists of a series of rows of plates.
4 The bony covering, often fused to the ribs, on the upper and lower surfaces of the body of a turtle. The upper surface is the ‘carapace’, the lower is the ‘plastron’.