A synodic year is the time it takes for a planet-sun alignment to reoccur. For the case of the sun, it is the time it takes the sun to come to the same place on the ecliptic (equinox to equinox) and is called a Tropical Year. A tropical year is 365.242 mean solar days (366.242 sidereal days). It is just over 20 minutes shorter than a sidereal year (again, the effect of precession). Because a tropical year is 365.242 mean solar days long, the vernal equinox would be later and later every year if our calendar year were strictly 365 days long.
Let's review. Mantle plumes are hypothesized columns of hot mantle rocks that rise through the mantle from the mantle-core boundary to the base of the lithosphere. The lower lithosphere rocks are heated by the plumes and melt to form hot spots.
Varying between 0 to 100mm per year, the movement of a plate is driven by convection in the underlying hot and viscous mantle. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries in zones that may be anything from a few kilometres to a few hundred kilometres wide. To watch a simulated fly-by along New Zealand's plate boundary check out this video.