Page 210 - Fundamentals of Physical Volcanology

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VOLCANOES AND CLIMATE 187

and mass extinction events. Mass extinctions are that the eruptions occur very rapidly (days) at high
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events during which a large number of plant and eruption rates (∼10 kg s ), while others think that
animal species die out in a geologically short period the eruptions occur over longer time periods (years,
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of time. One idea for why these events occur is that perhaps even decades) at slower rates (∼10 kg s ).
they happen when rapid climate change causes Eruption modeling, combined with the evidence
environmental stress which in turn causes a col- that some stratospheric injection occurred during
lapse of the food chain. The most famous of these the Laki eruption, suggests that flood basalt erup-
events is the extinction at the end of the Cretaceous tions are likely to cause some stratospheric
period 65 million years ago in which the dinosaurs injection regardless of which of these two views is
died out. This extinction has been linked by scien- correct. So it now appears that flood basalt erup-
tists to both flood basalt eruptions occurring in tions are likely to be more important in causing
India at the time and to a meteorite impact in the climate change than the largest evolved magma
Yucatan peninsula of Mexico. The meteorite theory eruptions as they would release far more sulfur
is the currently accepted theory for what caused into the atmosphere. Furthermore, as stated above,
the extinction. This section examines ideas about it may be that the longevity of basaltic eruptions
the likely effects of the largest volcanic eruptions proves to be very important in lengthening the
and looks at evidence linking them to mass extinc- duration for which climate is affected. For evolved
tion events. magma eruptions and, indeed, asteroid impact sce-
As seen in Chapter 10, the largest eruptions in narios, the rapid fallout of material means that the
the geological record can be broadly divided into climate impact of the event is only likely to last 2–3
very large Plinian/ignimbrite-forming eruptions years. Is this long enough to trigger an environmen-
and flood basalt eruptions (Table 10.5). The former tal catastrophe sufficient to cause a major collapse
are single eruptions lasting at most a few days that of the food chain and hence a mass extinction?
involve evolved magmas and can have volumes as A longer, but initially less severe, basaltic eruption
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great as ∼2000–3000 km . The latter involve sequ- has the potential to affect climate for decades and
ences of eruptions in which single events have vol- thus seems far more likely to cause such a collapse.
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umes as great as 2000 km and the province formed Furthermore, thus far ideas about mass extinctions
by a sequence can have an erupted volume as great and volcanism have tended to concentrate on cli-
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as 2 × 10 km , being emplaced in a geologically mate change alone. The removal of acidic aerosols
short time period (1–2 Ma). from the atmosphere will cause natural acid rain

Initial interest in the climate impact of the largest and the additional environmental stress caused by
eruptions centered on the large rhyolitic eruptions this in soil, rivers and oceans has yet to be assessed.
because they have the largest volumes and the high- Thus there are many more issues which need to
est eruption rates and hence the greatest plume be investigated before a complete picture can be
heights. As seen in section 12.4.3, models of the developed of the likely environmental impact of
climate effect of these eruptions suggest that they flood basalt eruptions. Figure 12.4, however, gives
could produce the equivalent of a ‘nuclear winter’. us tantalizing evidence that flood basalt eruptions
Basaltic eruptions do not usually cause strato- are a significant factor in triggering mass extinction
spheric injection of ash and gas and so were not ini- events. It shows that when the geological record
tially considered likely to cause significant climate for the past 250 million years is examined there is a
impact. However, recent re-examination of the Laki very strong correlation between the occurrence of
eruption (the largest basaltic eruption to occur in flood basalt eruptions and mass extinction events.
historic time) suggests that it caused some strato-
spheric injection of aerosols. Mass fluxes during the
Laki eruption are estimated to be a maximum of 12.6 Summary
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2 × 10 kg s . Mass fluxes during flood basalt erup-
tions are not known and are currently the subject of • Evidence from the examination of historical
considerable debate by volcanologists. Some think records of volcanic activity and climate variation,