But plate tectonics is the theory which suggests that the Earth's crust is made up of solid plates moving relative to each other. Plate tectonics and the plate tectonics theory aren't really two separate things. However plates are pieces of the Earth's crust which are moving.
Plate tectonics is a theory and not a law because there is no way that we can observe how the plates move, therefore it remains a theory. Plate tectonics is an explanation that is accepted as to why Earth's continents are where they are today.
In the past years and centuries earthquakes and volcanic eruptions are theory of plate tectonics. <--- IDK if this is right ...(no not right) The theory of Plate tectonics describes the large scale motions of Earth's lithosphere.