The relationship between the spatial density of hydrothermal venting (ph) and spreading rate (us) is undefined along back-arc ridges. Here we report a continuous survey of hydrothermal plumes along the ~400-km-long Eastern Lau Spreading Center. The mean ph/us ratio for each of three tectonic segments followed the global midocean ridge trend, with ph doubling as us increased northward from 39 to 96 mm/yr. However, we found little correlation between ph and the previously imaged distribution of axial melt lenses, which are near-continuous along the southern two segments but wholly absent along the northernmost segment. We infer that venting on the southern segments is melt driven, whereas venting on the northern ELSC is entirely powered with heat extracted by deep circulation through hot rock. This segment may be a rare example of an entire fast-spreading segment where hydrothermal convection cells are powered by downward-migrating cracking fronts.