planet

Stagnant lid

The commonest tectonic regime among rocky bodies in the Solar System: the mantle still convects below, but the stress it delivers to the crust is not enough to break it, so the planet is encased in a single unbroken rigid shell. Heat escapes by slow conduction through that lid, plus localized volcanism. Venus is the case worth dwelling on, because it is nearly Earth's size with nearly Earth's heat budget — but its greenhouse baked the water out of its crust, and dry rock is strong enough that convection cannot break it.