Freshlyâ€erupted volcanic ash commonly has a surface coating of soluble salts and acids. This coating is formed when primary volcanic gases (such as the strong acids HCl and HF, and SO2 (which rapidly converts to sulphate aerosol and sulphuric acid) condense onto particles as the volcanic plume cools in the atmosphere. The acidic condensate can then interact with ash particle surfaces and release elements such as calcium and magnesium, which may then form a range of secondary salts.
Ash surfaces may be strongly acidic (have a low pH) which may cause damage on direct contact, for example, acidic irritation of skin, eyes and mucous membranes and acidic damage to crops.
A wide range of readily water soluble elements have been reported in fresh volcanic ash. The major species are calcium, magnesium, sodium, potassium, aluminium, chloride, fluoride and sulphate. In many cases, these elements are present as simple salts such as sodium chloride (NaCl) or calcium sulphate, or gypsum (CaSO4). The release of these elements may be beneficial to the environment (e.g. if soil nutrients such as sulphur are added) or harmful (if potentially toxic elements such as fluorine are ingested by livestock grazing on ash-covered feed).
The table below shows ash leachate compositions for a range of eruptions.
Fuego, Guatemala | Pacaya, Guatemala | Santiaguito, Guatemala | Mt. St. Helens, USA | Ruapehu, New Zealand | ||||||||||
Element | Mean | Max | Mean | Max | Mean | Max | Mean | Max | Median | Max | ||||
Al | 5.2 | 26.8 | 12.8 | 21.2 | 5.2 | 19.6 | - | - | 42.8 | 1160 | ||||
B | 0.088 | 0.044 | 0.06 | 0.108 | 1.08 | 3.92 | - | - | - | - | ||||
Ba | 0.296 | 1 | 0.68 | 1.12 | 0.132 | 0.348 | 0.152 | 0.24 | - | - | ||||
Br | - | - | - | - | - | - | - | - | 1.6 | 13.6 | ||||
Ca | 400 | 1040 | 196 | 304 | 600 | 2240 | 440 | 800 | 1890 | 6760 | ||||
Cd | 0.008 | 0.04 | 0.056 | 0.128 | 0.056 | 0.256 | 0.0052 | 0.0252 | - | - | ||||
Cl | 124 | 232 | 204 | 840 | 440 | 1400 | 392 | 668 | 248 | 2020 | ||||
Co | 0.0036 | 0.0328 | 0.0072 | 0.024 | 0.132 | 0.6 | 0.0196 | 0.072 | - | - | ||||
Cu | 0.324 | 2.52 | 1.24 | 2.36 | 1.56 | 2.8 | 0.164 | 0.48 | - | - | ||||
F | 21.2 | 88 | 28.8 | 44 | 14.4 | 23.2 | 7.2 | 12 | 25.8 | 95.6 | ||||
Fe | 2.08 | 22.4 | 2.8 | 9.2 | 1.56 | 3.6 | 0.376 | 0.48 | 5.74 | 92.8 | ||||
K | - | - | - | - | - | - | - | - | 36.1 | 253 | ||||
Li | 0.044 | 0.116 | 0.0036 | 0.064 | 0.4 | 1.88 | 0.208 | 0.52 | 0.5 | 1.45 | ||||
Mg | 22 | 44 | 19.6 | 52 | 96 | 400 | 48 | 84 | 235 | 1200 | ||||
Mn | 1.48 | 3.12 | 1 | 2.88 | 19.6 | 92 | 7.6 | 13.2 | - | - | ||||
Na | 128 | 184 | 156 | 440 | 400 | 1760 | 264 | 440 | 292 | 1150 | ||||
Pb | 0.104 | 0.96 | 0.014 | 0.044 | 0.0096 | 0.048 | 0.0092 | 0.072 | - | - | ||||
Si | 7.2 | 12.4 | 9.2 | 15.2 | 7.6 | 11.2 | 40 | 56 | - | - | ||||
Sr | 2 | 5.2 | 1.64 | 2.6 | 1.48 | 4.4 | 1.76 | 2.88 | - | - | ||||
U | 0.00108 | 0.0028 | 0.00008 | 0.00048 | 0.0012 | 0.006 | 0 | 0 | - | - | ||||
V | 0.06 | 0.128 | 0.0248 | 0.068 | 0.0364 | 0.08 | 0.0012 | 0.0264 | - | - | ||||
Zn | 0.144 | 0.56 | 5.6 | 18.8 | 2.04 | 8.4 | 2.04 | 26.8 | - | - | ||||
Nitrate | 21.9 | 88.9 | ||||||||||||
Sulfate | 1000 | 1800 | 5190 | 24530 |