Chemical composition of Mount St. Helens volcanic ash

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Volcanic ash samples from the May 18, 1980, Mount St. Helens eruption were analyzed for major, minor, and trace composition by a variety of analytical techniques. Results indicate that the basic composition of the ash consists of approximately 65% SiO2, 18% Al2O3, 5% FetO3, 2% MgO, 4% CaO, 4% Na2O, and 0.1% S. Thirty seven trace metals are reported including Ba, Cu, Mn, Sr, V, Zn, and Zr. A change in the chemical composition of the ash as a function of distance from the volcano is related to a similar change in physical characteristics of the ash. Water soluble components were also determined after column leaching experiments were performed. Concentration levels of soluble salts were found to be moderately high (1500‐2000 µg/g) with molar ratios suggesting the presence of NaCl, KCl, CaSO4, and MgSO4. Heavy metals such as Cu, Co, Mn, and Zn were found at appreciable concentrations (10‐1000 µg/g). Unexpectedly high concentration levels of ammonium (45 µg/g) and nitrate (100 µg/g) ions as well as dissolved organic carbon (130 µg/g) were observed in several ash leachates. Results for fluoride and boron show low average levels of ∼5 and ∼ 0.5µg/g, respectively.