Observations show that, in general, larger planets and moons have had geological activity more recently than smaller bodies have. All of the planets show past signs of volcanism, tectonics, and/or erosion, but only the larger ones are still geologically active today. Why is this? One reason has to do with the planet’s interior. Most of the heat inside of Earth is left over from the accretion process during Earth’s formation and radioactive decay. What would you expect to have happened as the Earth (or any other planet) cooled after its formation?

The hard crust thickened, closing off volcanic vents and the mantle began to solidify, slowing the motion that causes volcanism and tectonics.

Explanation:

The amount of tectonic activity on a planet is controlled by the amount of heat stored in the planets interior after formation. The larger the amount of heat, the more energy stored that is transferred to the surface in the form of geological activity. A build up of pressure in the earth is released, by things such as a plate movement which forces molten rock to exploded into the air causing a volcanic eruption.