The entry of lava into the ocean at Kilauea Volcano, Hawaii has produced extensive sea surface hydrothermal plumes. Studies at this site over 1989-1991 showed that particle-rich, chemically altered thermal plumes of 1--3 m thickness can be mapped for distances up to 2 km from the entries using an array of physical and chemical tracers. Particle concentrations in the plumes were as high as 65 mg L-1, and produced light attenuation anomalies exceeding 11 m-1; the maximum observed seawater temperature was 69 ¿C. The following extremes in plume chemical markers were measured: H2, 2.1 μM; dissolved Mn, 320 nM; dissolved Si, 55 μM; ∑CO2, 1.6 mM; alkalinity, 1.5 meq L-1; and pH, 7.1. Dissolved H2 linearly increased, and ∑CO2, pH, and total alkalinity decreased with increasing seawater temperature. Dissolved Mn and Si also increased with increasing temperature but were better correlated with each other, indicating that heat is not conserved in the surface plumes. Heat and mass inputs to the ocean are comparable to those from mid-ocean ridge crest vent fields. ¿ American Geophysical Union 1995