The chemistry of lava-seawater interactions: The generation of acidity

High concentrations of acid were found to arise from the interaction betwee n molten rock and seawater at the shoreline of Kilauea Volcano, Hawaii. A s eries of field samplings and experiments show that the acid was derived fro m two sources: the release of magmatic volatiles and water-rock reactions. Although the bulk of the magmatic volatiles (CO2, H2O, and SO2) are vented at Puu Oo cinder cone before the lava's transit downslope to the ocean, a p ortion of the sulfur (S) and fluoride (F) gasses are retained by the lava a nd then are released partially when the lava is quenched by seawater. The p rimary water-rock reaction responsible for acid formation appears to be Na- metasomatism, which is much different from the predominant acid-forming rea ction found in submarine hydrothermal systems, Mg-metasomatism. Analyses of surface seawater and of precipitation (rain) deposited at the shore show t hat similar to 30% of the acid comes from magmatic gasses with the balance from reactions between the rock and the salts found in seawater. Experiment al results show that similar to 4 +/- 1.5 mEq of acid are formed per kilogr am of lava entering the ocean, and of this 1 +/- 0.5 mEq/kg of lava came fr om S and F, with the balance coming from water-rock reactions. On the basis of lava extrusion rates, similar to 200-720 x 10(6) Eq/yr of acid are bein g formed at this site. The deposition of the acid results in the alteration of subaerial lava hows along the coast, and the lowering of the pH of the adjacent surface ocean waters by more than 1 unit. The ejection of this aci d into the atmosphere contributes to the formation of an extensive haze dow nwind of the lava entries. Copyright (C) 1999 Elsevier Science Ltd.