The chemistry of lava–seawater interactions: the generation of acidity

Abstract

High concentrations of acid were found to arise from the interaction between molten rock and seawater at the shoreline of Kilauea Volcano, Hawaii. A series of field samplings and experiments show that the acid was derived from 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 portion of the sulfur (S) and fluoride (F) gasses are retained by the lava and then are released partially when the lava is quenched by seawater. The primary water–rock reaction responsible for acid formation appears to be Na-metasomatism, which is much different from the predominant acid-forming reaction found in submarine hydrothermal systems, Mg-metasomatism. Analyses of surface seawater and of precipitation (rain) deposited at the shore show that ∼30% of the acid comes from magmatic gasses with the balance from reactions between the rock and the salts found in seawater. Experimental results show that ∼4 ± 1.5 mEq of acid are formed per kilogram of lava entering the ocean, and of this 1 ± 0.5 mEq/kg of lava came from S and F, with the balance coming from water–rock reactions. On the basis of lava extrusion rates, ∼200–720 × 106 Eq/yr of acid are being formed at this site. The deposition of the acid results in the alteration of subaerial lava flows along the coast, and the lowering of the pH of the adjacent surface ocean waters by more than 1 unit. The ejection of this acid into the atmosphere contributes to the formation of an extensive haze downwind of the lava entries.