Describe the forces that cause the tectonic plates to move. Describe the different kinds of plate tectonic boundaries and what happens at each type of plate boundary. The expanding Earth hypothesis was first formulated in the 1950s by S. Warren Carey, an Australian geologist. Though Professor Carey recognized sea floor spreading and the idea that the continents had once been assembled into the supercontinent of Pangaea, he did not recognize an important element of the modern day plate tectonic paradigm. What was that element and how does it weaken the expanding Earth hypothesis? Think about the entire Plate Tectonics process.