Environmental Engineering Reference
In-Depth Information
6
Volcanic-gas monitoring
alessandro aiuppa
6.1 Introduction
The environmental impact of volcanoes is closely related to the rates, style and
chemistry of their gas emissions (Delmelle, 2003 ). Monitoring the composition
and mass
flux of volcanic gases is therefore central to understanding how volcanism
impacts our planet, on both global and local scales.
There are two main modes of volcanic-gas release on Earth ( Chapter 14 ): (i) the
impulsive emission of large quantities of gases during episodic, large-scale vol-
canic eruptions, and (ii) the far more sluggish, but persistent, passive gas release
from quiescent or mildly erupting volcanoes. Characterising the chemical compos-
ition of impulsive emissions has remained a challenge, and direct measurements
have remained limited to satellite-based SO 2 mass estimates (Carn et al ., 2003 ).
These measurements have been combined with indirect estimates of gas budgets
for other gases based on petrological models (e.g. Gerlach et al ., 1996 ). However,
it is the persistent type of emissions that dominates the global volcanic-gas budget
over long-term ( > decadal) timescales ( Chapter 14 ), and this chapter focuses on
the chemical composition of such gas emissions.
It has been known for more than a century that the analysis and interpreta-
tion of compositions and
fluxes of passive volcanic-gas emissions can provide
profound insights into how active volcanoes work. Such insight can contribute
to understanding and possibly even forecasting the transition from quiescence
to an eruption. The basic concept is that magmatic volatiles have
finite solubi-
lities in silicate melts, and hence they inexorably degas to a vapour phase as
magmas are decompressed and cooled as they ascend to the surface (Giggenbach,
1996 ).Sincegasesaremoremobilethanmagmaitself,volcanogeochemists
aim to capture a precursor signal of magma ascent by tracking changes in the
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