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.
Some scientists think that plate tectonics cools the mantle, and mantle plumes cool the core. Two of the most well known locations that fit the mantle plume theory are Hawaii and Iceland as both have volcanic activity. Note: The above text is excerpted from the Wikipedia article Mantle plume, which has been released under the GNU Free Documentation License.