This movement is called convection. It can be found in liquids, gasses and rheids - solids that can also experience movement ( on a big time scale), such as the earth mantle. One of the types of convection is diffusion: net movement of gasses from higher to lower concentration areas.
Uranium-lead radiometric dating is most often performed on the mineral zircon.
Middle America refers to the states to the south of the US and specifically Mexico, the states of Central America and Islands near Central America. Venezuela does not belong to those states; It is part of South America and it is quite far from the other areas, relative to the distance between them.
