The outer parts of the solid earth, termed lithosphere, are in relative motion. However, the zones within which relative motion is achieved are relatively narrow, defining large tracts of relatively stable, rigid earth's surface. These are the tectonic plates. The evidence for motion between plates is felt continuously - as earthquakes. So a map of earthquakes tells us where the boundaries of the plates are at the surface. You can see these on a map, exploring the shape and relative motions at the boundaries, by clicking here. Plate boundaries can be classified depending on what's happening along them. Parts of tectonic plates are represented by the continents, others by parts of the ocean floor. Most plates contain both. To see these distinctions how the base of plates can be defined, click here .