The name of the theory that describes the motion of tectonic plates is plate tectonics. This theory explains how the Earth's outer shell, or lithosphere, is divided into large plates that float on the semi-fluid mantle beneath them. The interactions between these plates, such as divergent and convergent boundaries, lead to geological phenomena such as earthquakes, volcanic activity, and the formation of mountain ranges. Plate tectonics is essential for understanding the dynamic nature of Earth's surface.