Answer:
The hard crust thickened, closing off volcanic vents and the mantle began to solidify, slowing the motion that causes volcanism and tectonics.
Step-by-step 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.