At about 7th grade we learn that the center of the earth is molten iron, and we learn about the movement of tectonic plates. That movement causes earth quakes, volcanos, mountain formations etc. All that activity expends energy, and it is all caused by the gravitational attraction of earth's mass. A naive understanding would say this will continue forever, but that sounds like perpetual motion. How is the paradox resolved?
The initial heat from the formation of Earth is slowly escaping into space. The interior of the Earth heated up by a combination of two mechanisms:
- Losing gravitational potential energy from parts that went from a high-energy configuration (cloud of dust) to a low-energy one (clump of matter)
- Decay of radioactive elements
This is ultimately the source of the energy for the movement of the plates. When it cools enough to make the interior solid (what happened e.g. with Mars), the plates will stop moving.
Since the interior of Earth is liquid, its cooling results in chaotic movement of the liquid (convection). The liquid interior of the Earth catches on the tectonic plates by friction and causes them to move.