S (the synodic period) = 360 / (360/ P - 360/365.256) days. S (the synodic period) = 360 / (360/365.256 - 360/ P) days. A diagram showing the relationship between orbital size and synodic periods. As the planets move around the Sun they change their positions relative to each other.
Some geologists consider that mantle plumes are generated in the lower levels of the asthenosphere by the decay of concentrations of radioactive isotopes. However, many believe that they are generated at much deeper levels in the lower mantle, perhaps even at the boundary of the mantle with the outer core.
A mantle plume is a mechanism proposed in 1971 to explain volcanic regions of the Earth that were not thought to be explicable by the then-new theory of plate tectonics. Some such volcanic regions lie far from tectonic plate boundaries, for example, Hawaii.