Theory of Plate Tectonics- In the 1960's, scientists developed a new theory that combined continental drift and seafloor spreading. According to the theory of plate tectonics, Earth's crust and part of the upper mantle are broken into sections. The plates can be thought of as rafts that float and move on this layer. Plates are made of the crust and a part of the upper mantle. These two parts combined are the lithosphere. This rigid layer is about 100km thick and generally is less densethan material underneath. the plastic like layer below the lithosphere is called asthenosphere. the rigid plates of the lithosphere float and move around on the asthenosphere.