Teaching Earth Sciences - Earth Science Teachers' Association

Explosive volcanoes and the climate
Morgan Jones
Introduction
Volcanic eruptions, an awe inspiring testament
to the power of Nature, have rightly taken their
place alongside such heavyweights as Dinosaurs in
enthusing younger generations to take an active
interest in Earth Science. However, due to the sporadic
and infrequent nature of eruptions, there are still
gaps in scientific understanding of the causes and
effects of such phenomena. An emerging field of
study is the influence that volcanoes exert on the
climate. Effects associated with volcanic eruptions
have been shown to have an adverse effect on the
climate system over a period of days to decades. As
eruptions are infrequent and large eruptions even
more so, we still know little about the nuances of
these climatic effects. While this field of scientific
study has made great strides forward, the lack of
observational evidence available means there are
still gaps in our understanding. In this article, I will
summarise what we currently know about volcanic
eruptions, and then focus on how the atmosphere,
oceans, and terrestrial environments are affected by
these events.
Explosive Eruption Dynamics
When high viscosity magmas move towards the surface,
the volatiles in the magma start to form bubbles that
cannot easily escape. If the rate of ascent is also high, then
the internal pressure of these bubbles eventually exceeds
the viscous relaxation rate of the melt, rupturing the films
between bubbles and causes the magma to disintegrate
in a brittle fashion. The upwards flow of the mixture is
suddenly changed from being dependent on the viscous
properties of the magma (~ 10 7 Pascal seconds) to that
of the inertial forces of the gas phase (10 -5 Pa s). It is this
huge 12 order of magnitude jump in dynamic viscosity
that propels the disintegrating mass out of the vent close
to the speed of sound (Woods & Wohletz, 1991). A good
historical example of such an eruption is the 1991 eruption
of Pinatubo, Philippines.
The rapid injection of the gas-particle mixture into the
atmosphere leads to significant entrainment of the
surrounding air. The air is heated, expands and dilutes
the solid component of the erupted material, which
significantly reduces the eruption column density and
gives the jet buoyancy. This is countered by initial negative
buoyancy and the transference of momentum to the
surrounding air. Sufficient entrainment of air allows part of
the eruption column to become buoyant and rise through
the atmosphere (Sparks et al., 1986). The rising mass of
material forms an umbrella cloud when it reaches neutral
buoyancy, which has been estimated to be between 27
and 38 km above sea level for larger eruptions (Woods &
Wohletz, 1991) (Figure 1). The high intensity of explosive
eruptions and the fine particle size allow tephra to remain
airborne for days, capable of travelling over 3000 km
from the source prior to deposition in extreme cases
(Oppenheimer, 2002).
Eruption size and frequency
Explosive volcanic eruptions can be extremely large. The
standard way of expressing the size of a large eruption
is using the volume of pre-erupted magma that is
subsequently evacuated, termed the Dense Rock Equivalent
(DRE). To give an idea of scale, the 1980 eruption of Mount
St. Helens (USA) erupted about 1 km 3 DRE magma, while
the largest known historical eruption, the 1815 eruption
of Tambora (Indonesia), is estimated to have erupted 30 to
50 km 3 DRE of magma (Self et al., 2004). Explosive supereruptions,
a term made famous by the BBC docudrama on
Yellowstone (USA), are now defined as eruptions with preerupted
volumes in excess of 400 km 3 . This is equivalent
to an erupted mass of 10 15 kg. The largest super-eruption
deposits discovered in the geological record are predicted
to have been over 100 times larger than Tambora (1000’s
of km 3 DRE), dwarfing anything witnessed in historical
times.
These super-eruptions are extremely rare events so it is very
difficult to evaluate their occurrence globally, regionally,
or for an individual volcano (Coles & Sparks, 2006). The
average global repose period for super-eruptions exceeding
1000 km 3 DRE magma, based on geological evidence, is
0.3 to 1 Million years (Mason et al., 2004; Self, 2006).
However, large magnitude volcanism is episodic in nature
24 Teaching Earth Sciences Vol 35 No 1 2010 www.esta-uk.net