Why does only earth have tectonic plates?

Models have shown that for plate tectonics to get going a planet has to be just the right size: too small and its lithosphere – the solid part of the crust and upper mantle – will be too thick. read more

Earth is not the only planet with tectonic activity; both Venus and Mars exhibit forms of tectonic activity, although some of that activity appears more prevalent in the past (in the case of Mars). read more

The main reason appears to be the collision, 4.5 billion years ago, of Earth with a Mars-sized planet (which astronomers have named Theia). read more