On 3 April 2001, a 20 kg point source of fluorescein dye was released 30 m above the bottom of the active summit caldera of Vailulu'u submarine volcano, Samoa. Vailulu'u crater is 2000 m wide and at water depths of 600–1000 m, with the bottom 200 m completely enclosed; it thus provides an ideal site to study the hydrodynamics of an active hydrothermal system. The magmatically driven hydrothermal system in the crater is currently exporting massive amounts of particulates, manganese, and helium. The dispersal of the dye was tracked for 4 days with a fluorimeter in tow-yo mode from the U.S. Coast Guard icebreaker Polar Sea. Lateral dispersion of the dye ranged from 80 to 500 m d⁻¹; vertical dispersion had two components: a diapycnal diffusivity component averaging 21 cm² s⁻¹, and an advective component averaging 0.025 cm s⁻¹. These measurements constrain the mass export of water from the crater during this period to be 8_−1.3 ^+4.6 × 10⁷ m³ d⁻¹, which leads to a “turnover” time for water in the crater of ∼3.2 days. Coupled with temperature data from CTD profiles and Mn analyses of water samples, the power output from the crater is 610₋₁₀₀ ⁺³⁵⁰ MW, and the manganese export flux is ∼240 kg d⁻¹. The Mn/Heat ratio of 4.7 ng J⁻¹ is significantly lower than ratios characteristic of hot smokers and diffuse hydrothermal flows on mid-ocean ridges and points to phase separation processes in this relatively shallow hydrothermal system.