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THE ROLE OF VOLATILES 65

empirical solubility laws for various magma–
volatile combinations. Three examples are given
below.
For H O dissolved in a basalt:
2
n = 0.1078 P 0.7 (5.1)

where n is the amount of dissolved gas given as a
weight percentage (wt%) and P is the pressure in
megapascals (MPa) acting on the magma. For H O
2
dissolved in a rhyolite:
n = 0.4111 P 0.5 (5.2)
Fig. 5.1 Scientists R. Okamura and K. Honma making
measurements at a fumarole on Kilauea volcano, Hawai’I,
For CO dissolved in most magmas:
on August 27, 1973. (Photograph by R.L. Christiansen, 2
Hawaiian Volcano Observatory, courtesy of the U.S.
n = 0.0023 P (5.3)
Geological Survey.)
The equations above apply only if a single volatile
with the fact that volcanoes also release a lot of sul-
species is present in a magma. In fact magmas virtu-
furous gases – the often-noticed smell of “rotten
ally always contain several volatiles, and the solu-
eggs” is caused by the release of hydrogen sulfide
bility functions are then more complex because the
(H S), and frequently deposits of sulfur can be
2 volatiles interact chemically with one another as
found near vents and fumaroles (Fig. 5.1). In fact
well as with the magma. As soon as the least soluble
the most common sulfur compound released by
volatile starts to exsolve and form gas bubbles,
volcanoes is sulfur dioxide (SO ). Sulfurous gases
2 some small amounts of most of the other species
are particularly associated with basaltic magmas –
present will also diffuse into those bubbles.
a basaltic eruption will typically release about 10
However, for simplicity, Fig. 5.2 shows the solubil-
times as much sulfur as a rhyolitic eruption of the
ity of H O alone in rhyolite and basalt as a function
same size. This is an important factor when con- 2
of pressure and depth beneath the surface. This
sidering the effect of volcanic eruptions on climate diagram illustrates several important points about

(see Chapter 12). A wide range of other volatiles the behavior of volatiles in magmas.
can be found in varying amounts in magmas, includ-
ing hydrogen chloride (HCl) and hydrogen fluoride • For both basalt and rhyolite there is a general
(HF). trend in which the amount of water which can be
dissolved in the magma decreases as the pressure
on the magma decreases (i.e., as the magma rises
5.3 The solubility of volatiles in magma towards the surface).
• The solubility of water in rhyolite is considerably
The amount of a given volatile which can be dis- greater than that in basaltic magma. For instance, at
solved in a magma depends on a number of factors a depth of 5 km beneath the surface the maximum
such as the confining pressure, the composition of amount of water which can be dissolved in a rhy-
the magma, and the temperature of the magma. olitic magma is ∼4.8 wt% whereas in a basalt it is
The pressure and the composition (see Table 2.1) only 3.4 wt% (Fig. 5.2).
are generally the most important. To understand • The solubility curves show the maximum
the behavior of volatiles in rising magmas it is nec- amount of water which can be dissolved within the
essary first to know something about the solubility magma at a given pressure. This does not mean that
of volatiles in different magmas. By carrying out the magma will actually contain that amount of
many laboratory experiments scientists have found water dissolved within it. For instance, at a depth of