Deep-sea hydrothermal vents are conduits of heat and chemicals from the lithosphere to the hydrosphere. Their plumes rise hundreds of meters from the seafloor into the water column; during their ascent, they entrain ambient water and are subject to cross flows. Source fluxes can vary in time, partly in response to seismic and magmatic events. Long-term measurements of physical properties of hydrothermal plumes provide windows to conditions within Earth’s interior. This article provides examples of long-term measurements of acoustic scattering recorded along Endeavour Segment of the Juan de Fuca Ridge. Acoustic backscatter data from particles and temperature fluctuations provide information on width, shape, and vertical velocity in the plumes from which entrainment and upward transport are estimated. Acoustic forward scatter by turbulence within the plume gives time series of the plume’s path-averaged vertical velocity and temperature fluctuations and exhibits variability that is dependent on the ambient horizontal flow. At several vents, direct measurements of heat flux using in situ devices and video imagery have yielded an integrated heat output for various sulfide structures. In conjunction with these measurements, plume models that incorporate ambient stratification and horizontal tidal flows are yielding insights into the vertical and azimuthal dependence of entrainment, rise-height variability, and plume bending.

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