Why is the theory of plate tectonics not a law?
Plate tectonics explains the evidence of volcanos, earth quakes and mountain building.
A law usually gives a precise prediction of what will happen. Newtons law of gravitation gives a precise mathematical formula for calculating the force of attraction between two masses. But Newton's law is not a theory of gravitation because it does not explain why this force of attraction exists.
The theory of plate tectonics does not give a precise prediction of the rate or direction of plate movements. The plate movements first proposed under the theory of continental drift hypothesis rapid and sudden massive movements of the continents in the past. The plate movements observed today are very slow but the theory does not specify how fast the plates may have moved in the past or may move in the future. So the theory of plate tectonics can not function as a law.
A theory explains why and how something happens. Einstein's general theory of relativity provides an explanation of why two masses create a force of attraction. Good theories make good predictions. Einsteins theory precisely predicted the bending of light in a solar eclipse due to the gravitational force bending space.
Einstein's theory is accepted because it explains accurately what is observed and predicts what will happen ( Einstein's theory not only explains why it makes precise prediction so some say it should now be called a law as well as a theory)
Plate Tectonics explains the mid Atlantic Ridge, The great rift valley of Africa, the subduction zones, earthquakes, the location of volcanos, and Mountain ranges. Plate Tectonic explained the earth movements much better than the early geosyncline theories.
Because Plate Tectonics explains things it is a theory, Because it does not provide a means of calculation precisely when and where the plates will move it is not a law.