Final answer:
Temperature and density are the two variables responsible for the movement of Earth's mantle, which causes tectonic plates to move through the process of convection. This natural heat transfer mechanism drives the shifting of plates, a fundamental aspect of plate tectonics.
Step-by-step explanation:
The two variables responsible for the movement of the mantle within the Earth, which in turn causes tectonic plates to move, are temperature and density. This process is known as convection in the mantle. Mantle convection involves the upward flow of warmer, less dense material and the sinking of cooler, denser material. This mode of heat transfer is instrumental in driving plate tectonics, causing the gradual shifting of the continental and oceanic plates that shape the Earth's surface.
Mantle convection is similar to other natural processes where heat transfer occurs, such as in astronomical bodies like stars, and in everyday occurrences like boiling water. It's an effective cooling system for the planet, allowing heat from the Earth's interior to move towards the surface and eventually dissipate into space.