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.
Mantle Plumes. The estimated dates of the younger continental flood basalts compiled from recent sources are shown in Table 1. Several lines of evidence suggest that in most instances the greatest number of individual eruptions and the largest volumes of lava probably occurred within a million years or less.