The earth's gravity creates pressure, with the highest pressure in the core. This creates a perpetual pressure gradient between the core and the surface. In weather, if we have high and low pressure systems, there is movement of air between them. If higher pressure moves toward lower pressure; volcano, this reduces the pressure gradient. But since gravity continues to act, it squashes the matter restoring the pressure gradient for another cycle. The problem with this perpetual motion scenario, is gravity creates a downward force that will point in the opposite direction of the force that is pushing from high to low pressure. These will cancel. We can't place a hose between the top and bottom of a lake and use the pressure on the bottom to pump water into the air, because gravity cancels this upward force. There is a work-around. This can be done using materials of various densities. The light materials will try to float upward as the heavies sink. With water and oil, for example, the lighter oil will follow the pressure gradient upward, while water will follow the gravity gradient downward. These balance out. Another way to do this is with heat since most materials get less dense when heated and more dense when they cool. The hotter lighter materials will follow the pressure gradient while the cooler denser materials will follow gravity and sink, with a balanced forming based on heating and cooling. Radioactive decay combines both of these to make heat and make small less dense atoms from heavier atoms.