It would make sense that as the Moon causes the oceans to rise gravity may also effect the magma which is more liquid that solid.
One could imagine that the tidal effect may be little extra energy required to push or pull the magma further up the lava column.
It would be interesting to see the incidence of eruptions of volcanoes when related to a "king tide" which evidences a greater gravitational effect
.
Even when considering tidal effect upon the plates one would suspect that the additional tidal energy provided around the full Moon may be the point when the action starts
. The straw that breaks the camels back as it were.
mmm a beard eh is the man to be trusted
alex