Environmental Engineering Reference
In-Depth Information
was an important global perturbation on a
3-year timescale, on geological
timescales, such eruptions are just a small part of the background outgassing of
the planet. Sulfur emissions from eruptions such as Tambora and the Young Toba
Tuff remain uncertain but have been estimated using petrological methods, or from
proxy measurements of sulfate in ice cores (e.g. Zielinski,
2000
). Tambora is
estimated to have released 50
~
-
58 Tg of SO
2
over 5
-
6 days, while estimates for
Toba range from 35
14 days (Oppenheimer,
2003
; Self
et al
.,
2004
), with petrological estimates suggesting an upper limit of 200
-
3,300 Tg of SO
2
over 9
-
400 Tg
SO
2
(Chesner and Luhr,
2010
). Based on the geological record, a Tambora-scale
eruption occurs about every 1,000 years (Pyle,
1995
) yielding a time-averaged
SO
2
-
flux of
~
0.05
-
0.06 Tg/yr; insigni
cant over geological time compared to the
M9) the
geological record suggests that the recurrence rate drops sharply, to approximately
one every 1 Myr (based on statistics for the last 13.5 Myr; Mason
et al
.,
2004a
).
Thus, SO
2
emissions from Toba-scale eruptions account for less than
flux from
'
background
'
emissions activity. For Toba-scale eruptions (M8
-
10
-
3
Tg/yr. In summary, while large-scale short-lived eruptions are undoubtedly import-
ant global perturbations on timescales of years to decades, over geological time
(e.g. time-averaging over
>
10
5
~
3
10
6
years) sporadic, individual short-lived erup-
-
tions are a relatively insigni
cant part of the background outgassing budget of
the planet (see also
Chapter 11
).
Short-lived explosive eruptions, however, dominate the time-averaged
flux of
volcanic SO
2
emissions to the stratosphere and ash into the environment. The time-
averaged fluxes of ash and gas from explosive eruptions of different magnitudes
are illustrated in
Figure 14.2
. The interplay between magma composition and
plume height (more basaltic magmas are able to carry more sulfur but tend to have
lower plumes) means that medium-sized (M4) eruptions dominate the
flux of
volcanic S to the stratosphere; whilst tephra
flux scales with eruption magnitude
until the drop off in recurrence rate for M8 and 9 eruptions.
The present-day volcanic CO
2
flux is not yet particularly well constrained.
Marty and Tolstikhin (
1998
) suggested a preferred range of 180
440 Tg/yr
(Gerlach,
2011
). More recently Burton
et al
.(
2013
) compiled estimates in the
range 65
-
540 Tg/yr, with the larger estimates including emissions from volcanic
lakes and tectonically and hydrothermally active areas (Burton
et al
.,
2013
).
Some of this
-
flux might be supplied from thermal and metamorphic reactions of
basement rocks (e.g. limestones) with magma (e.g. Parks
et al
.,
2013
), but the
size of this contribution is not known. The submarine volcanic CO
2
flux is likely
to be substantial (
100 Tg/yr), but the current view is that alteration of ocean
crust currently consumes more C (precipitated as carbonates) than is released
at submarine vents (e.g. Burton
et al
.,
2013
). Estimates of CO
2
emissions
from sporadic large eruptions are sparse due to the low solubility of CO
2
in
~
