Average Compositions and Trace Gases
Sulfur deposits near the summit of Griggs volcano, Alaska.
Photo by Jay Robinson.
Most Common Gases
- Water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2) are the
most common volcanic gases.
- In lesser amounts, volcanoes release carbon monoxide (CO), hydrogen sulfide
(H2S), carbonyl sulfide (COS), carbon disulfide (CS2), hydrogen chloride (HCl),
hydrogen (H2), methane (CH4), hydrogen flouride (HF), boron, hydrogen bromine
(HBr), mercury (Hg) vapor, organic compounds, even gold. From Cadle (1980).
Mercury is released by most volcanoes and has been measured at Kilauea, Mauna
Loa, Hekla, Erebus, at Mount St. Helens (Siegel and Siegel, 1987). Kilauea
produces about 270 tons of mercury each year and has been identified as the source
for mercury on Oahu, 320 km away.
Condensates, sublimates, and incrustations were studied at Merapi volcano by
Symonds and others (1987). The following elements were found:
- Se, Re, Bi, and Cd at concentrations 100,000 that in the magma;
- Au, Br, In, Pb, and W at concentrations 100,000 to 10,000 that in the magma;
- Mo, Cl, Cs, S, Sn, and Ag at concentrations 10,000 to 1,000 that in the magma;
- As, Zn, F, and Rb at concentrations 1,000 to 100 that in the magma;
- Cu, K, Na, Sb, Ni, Ga, V, Fe, Mn, and Li at concentrations 100 to 1 that in the magma.
Gold has condensed from volcanic gases. Meeker and others (1991) reported gold
at Mount Erebus, Antarctica. They found gold in the plume near the crater, in the
air up to 1000 km from the volcano and in near surface samples.
Most of these gases originate in the mantle and are transported to the crust
and surface by complex interactions with magma and rocks encountered along the
way. In general, the gases are dissolved in the magma. At shallow depths, as
pressure on the magma decreases, the gases leave the magma (exsolve). The gases
can interact with surrounding rocks or continue to the surface.