There is more than just a sizzle when red-hot lava meets the sea. The plumes seen in this picture consist not only of steam produced by the evaporation of water, but also of aerosols and gases that stem from the reaction between the lava and salt water.

M. Edmonds and T. M. Gerlach have investigated the composition of such plumes produced by lava from Kilauea Volcano, Hawaii (Earth Planet. Sci. Lett. doi: 10.1016/j.epsl.2006.02.005). Their main tool was open-path Fourier transform infrared spectroscopy, which allowed remote sensing of the plumes and estimation of the amounts of various components — water, carbon dioxide, nitrogen dioxide, sulphur dioxide and hydrogen chloride. The most notable of Edmonds and Gerlach's conclusions stem from their analyses of this last species, HCl.

First, from thermodynamic considerations they calculate that the HCl gas is created following the hydrolysis of magnesium chloride salts (and not of sodium chloride, as an alternative explanation has it). Second, given that conclusion, they estimate how much HCl is produced by the lava–seawater interaction. The outcome depends on various assumptions and factors, including the type, extent and duration of the lava flow.

Edmonds and Gerlach estimate that a lava flow of 1 m3 s−1 could in principle produce 3.7 kg s−1 of HCl, or 300 tonnes daily. For various reasons that they discuss, this number is likely to be much lower (3–30 tonnes). Figures of this latter order of magnitude produce only localized high concentrations of HCl gas and acid rain. But the authors point out that in the past the story must have had a more serious edge. Eruptions of Hawaiian volcanoes in 1840, 1919 and 1950 produced massive lava flows, with sustained lava fluxes entering the sea. The result was an estimated HCl output of 200–2,200 tonnes per day over several weeks, a much more serious environmental hazard.