Average Compositions and Trace Gases
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.
Inset Image: Sulfur deposits near the summit of Griggs volcano, Alaska.
Photo by Jay Robinson.