Page 209 - Fundamentals of Physical Volcanology

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estimates of the mass of sulfuric acid aerosols of the eruption may also have a significant role to
formed during various eruptions compared with play in determining the effects of an eruption.
the volume of the eruptions. The data in the table
can be used to illustrate two important points. First,
12.4.5 The effects of eruption duration on
by comparing two eruptions of similar size, Mount
climate impact
St Helens and El Chichón, we can see the importance
of composition: the Mount St Helens eruption pro- Plinian eruptions which are able to inject ash and
duced only 1/40th of the mass of aerosols produced gas into the stratosphere typically have durations of
in the El Chichón eruption because of the lower less than a day. In such eruptions, then, the injec-
sulfur content of the Mount St Helens magma. tion of ash and gas is short-lived and the atmosphere
The difference in composition meant that the El can begin to “recover” as soon as the eruption ends
Chichón eruption produced a measurable decline (as ash and aerosols begin to be removed by fallout
in global temperature whereas the Mount St Helens and rain out). Basaltic eruptions can have much
eruption did not. Furthermore, eruptions such as El longer durations (it is not unusual for basaltic erup-
Chichón and Agung produced an amount of aerosol tions to last years or even decades) but proceed
loading similar to that of much larger eruptions at much lower eruption rates than Plinian erup-
such as Katmai and Pinatubo because of the higher tions. Whereas the smaller eruption rates mean that
sulfur content of the magma involved in the El plume heights are relatively low in basaltic erup-
Chichón and Agung eruptions. These observations tions, the continuous nature of the eruptions means
show that eruption magnitude alone is not always a that the atmosphere is being constantly loaded with
good guide to the impact of an eruption on climate. acidic aerosols and thus that atmospheric recovery
On the other hand, a very large eruption of sulfur- is delayed and the effects of the eruption can be
poor magma will still cause a massive aerosol load- prolonged well beyond the typical 2–3 year time
ing just because of the sheer size of the eruption. span associated with recovery after Plinian erup-
The Toba eruption is a good example of this tions. The effect of these milder but longer duration
(Table 12.2). So in determining the impact of an eruptions has yet to be adequately investigated.
eruption both composition and eruption size are Furthermore, recent work suggests that larger bas-
important factors. altic eruptions might actually have more potential
The sulfur content of magma varies widely but, in for stratospheric injection of gas than has previ-
general, basaltic magmas have the greatest sulfur ously been appreciated. If this is true it means that

contents and can contain as much as 10 times as basaltic eruptions may be far more important in
much sulfur as more evolved magmas. Thus, on the inducing climate change than Plinian eruptions
basis of composition alone, a basaltic eruption of a because of the higher sulfur contents and the
given volume would be expected to have a con- prolonged nature of the basaltic eruptions.
siderably greater effect on climate than an erup-
tion of similar volume of a more evolved magma.
Basaltic eruptions, however, do not generally 12.5 Volcanoes and mass extinctions
produce high enough eruption plumes to cause
significant stratospheric injection of ash and gas Historical records of volcanic activity and climate
whereas eruptions of more evolved magma com- variation cover only a tiny fraction of Earth history.
monly do. So the eruptions with the greatest sulfur We have seen that eruptions during the histor-
content (and thus the greatest potential to affect ical past have affected climate. Compared with
climate) are, on the face of it, also those with the the scale of eruptions in the geological past these
least capacity to affect climate on a global scale. modern eruptions are of very small volume. Vol-
The situation is not quite this simple, however, canologists are very interested, therefore, in how
for two reasons: (i) the largest basaltic eruptions, the larger eruptions in the geological past may
flood basalt eruptions, may cause stratospheric have affected climate. In particular they are inter-
injection (see section12.5.1) and (ii) the duration ested in possible links between volcanic activity