According to the U.S. Geological Survey website, tectonic plates are massive, irregular-shaped slabs of solid rock, composed of oceanic and continental lithosphere. The continental crust is made up of lightweight minerals like feldspar and quartz, while the oceanic crust is made of heavier and denser basaltic rocks.
The sizes of these plates vary greatly. The Antarctic and Pacific Plates have been recorded as some of the largest. The boundaries between tectonic plates are usually unseen as they lie beneath the ocean. However, the boundaries for oceanic plates are mapped out accurately by measurements known as GEOSAT satellites. Volcanic activity and earthquakes occur close to these boundaries.