The spreading ridge (ELSC in the north, and the Valu Fa Ridge in the south), is farther from the subducting plate in the north, and gradually gets closer to the subducting plate to the south. See image below.
Several other geological feature covary with this trend: the depth of the seafloor shoals from ~3000 m to ~1700 m, spreading rate decreases, lavas become more felsic, and there is a magma chamber reflector present only south of 20deg 30'S.
Thus, in the Lau Basin we can study how geophysical, petrologic, hydrothermal and biological factors along the ELSC and VFR differ as a function of distance from the subducting plate and as functions of all of the other differences that covary with distance from the subducting plate (including depth and lava chemistry).
Summarized in the Lau ISS Workshop Report, July 2006