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5 The role of volatiles

5.1 Introduction bubbles which are distributed throughout the liq-
uid. It is the connecting together of a network of
We saw in the previous chapter how magma moves these bubbles that ultimately causes the continuous
through the mantle and crust. If this magma con- body of liquid to break apart or fragment into a
tains no dissolved volatiles then if it reaches the spray of droplets or clots suspended in the gas
surface it will always be erupted effusively – simply (see Chapter 6). This process is similar to what
pouring out of the vent to form lava flows or domes happens when a bottle of any fizzy drink is opened.
(depending on the chemistry and effusion rate – see When these drinks are bottled they have carbon
Chapter 10). In practice, however, the majority dioxide (CO ) forced into them at high pressures.
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of eruptions which occur subaerially involve some At high pressures the CO dissolves in the liquid.
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degree of explosiveness. As explained in section When the bottle is opened the pressure is reduced
1.2, in volcanology the term “explosive” is used to to atmospheric. The solubility of CO in the liquid is
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denote any eruption in which magma is fragmented lower at lower pressures so when the bottle is
and ejected from a vent within a stream of gas. In opened not all of the CO can remain dissolved and
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some cases volcanic explosions are transient events some of it exsolves and forms gas bubbles that
(these are described in Chapter 7) but often frag- expand and make the drink “fizz”. In magmas, typi-
mentation can occur continuously during a steady cally 95–99% of the “mass” of material erupted is

eruption which might last hours or days (such erup- liquid rock – at most the gas accounts for only a few
tions are discussed in Chapter 6). So, in both a percent of the weight; but that small amount of gas
Hawaiian lava fountain (Fig. 1.1), in which clots of represents a very large “volume” as it expands to
magma up to 1 m or more in diameter are carried atmospheric pressure, and is fundamentally impor-
up to heights of hundreds of meters above the tant in producing explosive eruptions.
vent, and a Plinian eruption, in which mainly tiny This chapter reviews which gases are commonly
ash particles are carried to heights of several tens dissolved in magmas, how the composition of the
of kilometers, an explosive eruption involves frag- magmas influences the amount of dissolved gas,
mented magma being ejected from the vent within and how gases are released from magmas.
a stream of gas.
In some cases, eruptions are explosive because
a volatile substance such as water is mixed with 5.2 Volatiles in magma
the magma as it approaches the surface. However,
in many cases, eruptions are explosive because When magma is in the mantle or the lower crust it
the rising magma has volatiles dissolved within it. contains a range of volatiles dissolved within it. The
As the magma rises towards the surface and the most common dissolved volatiles are H O (water)
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confining pressure decreases, the volatiles gradu- and CO (carbon dioxide). However, anyone who
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ally exsolve from the magma forming the gas has visited an active volcano is probably familiar