Environmental Engineering Reference
In-Depth Information
of straightforward spectroscopic (for plume gases; see also
Chapters 6
and
8
) and
microanalytical (for rock samples) measurement techniques (see also
Chapter 11
).
Where data are available we also discuss volcanic CO
2
because of its climatic
importance. We explore the contrasting roles of volcanoes in maintaining and
perturbing the atmosphere. First (
Section 14.2
), we discuss the
'
background
'
ux
of magma and volcanic gas to Earth
'
s surface and atmosphere, and the signi
cance
of the different scales of activity that contribute to the long-term planetary degas-
sing budget. We then discuss ways in which deviations from long-term average
degassing patterns have been implicated in sustained periods of planetary change
(
Section 14.3
). We draw on evidence both from present-day studies of active
volcanism, and the geological record.
Background
'
activity, short-term perturbations and time-averaging
Like many geological phenomena, volcanic activity spans a vast range of scales
(
Figure 14.1
), both in terms of the typical return-interval for characteristic events (up
to
14.2
'
10
8
years for large
igneous provinces (LIPs)), and in terms of the size of individual eruptions (e.g. up
to 1
10
5
10
6
years for individual
supereruptions
'
andupto10
7
~
-
'
-
10
16
kg for individual silicic
flows from a LIP;
Mason
et al
.,
2004a
;Bryan
et al
.,
2010
). Since the largest events also tend to be the
rarest, individual examples of these events may be signi
-
2
'
supereruptions
'
,orlava
cant short-term perturb-
ations, but contribute relatively little to long-term average
fluxes of magma and gas.
fluxes may be estimated using a
range of different approaches, including direct observations, geological evidence
(reconstruction of past eruptions), proxy records (e.g. volcanic deposition to ice cores),
and experimental and modelling approaches.
Table 14.1
summarises recent
Long-term rates of magmatism and associated gas
best-
guess
'
estimates for current rates of terrestrial magmatism. The present-day long-term-
averaged eruptive
'
3km
3
/
ux associated with subaerial volcanoes is of the order of 0.5
-
yr, or 1,000
8,000 Tg/yr of magma. One major area of uncertainty in these estimates is
the balance between intrusive and extrusive
-
fluxes, which is very poorly known.
Long-term average
fluxes of gas could in principle be estimated from these
magmatic
fluxes, by using solubility and speciation models for volcanic gases in
magmas that are calibrated against experimental data (e.g. Moretti
et al
.,
2003
;
Scaillet and Pichavant,
2003
). However, these models are sensitive to assumptions
about magma compositions and magma storage conditions prior to eruption,
making it hard to generalise to a global volatile budget. Instead, efforts have
focussed on using petrological evidence and experimental approaches to constrain
emissions for individual eruptions (e.g. Chesner and Luhr,
2010
). Rates of volcanic
sulfur emission have also been inferred from analysis of ice-core records, and
present-day remote-sensing measurements (e.g. Pyle
et al
.,
1996
).
