Teaching Earth Sciences - Earth Science Teachers' Association

Figure 1 A simple diagram showing the eruption column and cloud for a super-eruption. A) The eruption involves the evacuation of a magma chamber, causing the rocks above
to subside. This forms a Caldera. B) Part of the upwards flow out of the magma chamber remains negatively buoyant in the atmosphere, spreading out radially as pyroclastic
flows. The deposits from these are called ignimbrites. C) Part of the eruption becomes positively buoyant thanks to the entrainment and heating of surrounding air. This rises
until it reaches neutral buoyancy. D) Once at neutral buoyancy, the ash and gas spread out to form an “umbrella cloud” in the high atmosphere.
and each super-volcano has a unique behaviour. It is
plausible to have two super-eruptions from separate centres
in quick succession.
Atmospheric impact
“I had a dream, which was not all a dream.
The bright sun was extinguished, and the stars
Did wander darkling in the eternal space,
Rayless, and pathless, and the icy earth
Swung blind and blackening in the moonless air;
Morn came and went -and came, and brought no day,
And men forgot their passions in the dread
Of this their desolation; and all hearts
Were chilled into a selfish prayer for light.”
This is an excerpt from a poem entitled “The Darkness” by
Lord Byron in 1816. His poetry was inspired by what has
become known as the “year without summer”, where the
far off Tambora volcano in Indonesia had darkened the sky
and lowered global surface temperatures after erupting in
1815. Lord Byron was not alone in his gloomy outlook, his
guest at his summer house that year was Mary Shelley, who
wrote “Frankenstein” in a morbid literary competition with
Lord Byron.
Sulphuric acid aerosol (H 2
SO 4
) is the main cause of this
atmospheric disturbance, which forms from reactions of
H 2
S and SO 2
with water (Figure 2). Atmospheric lifetimes
of H 2
SO 4
are significantly increased in the stratosphere
compared to the troposphere due to the lack of removal
by precipitation. Therefore, stratospheric residence times
are governed by gravitational sedimentation, which takes
between 1 to 3 years for historic eruptions (Robock, 2000).
For explosive eruptions, the formation of an eruption
column means that volcanic emissions can be transported
into the stratosphere, and therefore lengthen the
atmospheric residence time (Figure 2). The 1991 eruption
of Mt. Pinatubo (Philippines) was the first opportunity to
track a sulphate aerosol cloud in the stratosphere using
satellite imagery. The cloud first spread longitudinally,
encircling the globe within a few weeks (Bluth et al.,
1992). Latitudinal mixing appears to take much longer, the
Pinatubo cloud was initially constrained between 20 o S and
30 o N (Long & Stowe, 1994). Thorough mixing throughout
the stratosphere occurred within 6 months (McCormick et
al., 1995), therefore affecting the whole globe.
The solid particles (termed tephra or ash) have a short
atmospheric residence time, with most of the ash deposited
within a few weeks of eruption. Small quantities of very
fine tephra can remain suspended for a few months
(Robock, 2000), but the impact on scattering incoming
radiation is comparatively brief. Sulphur has a large effect
on the Earth’s radiation budget as H 2
SO 4
aerosol particles
have a typical radius of 0.5 µm, comparable to the peak
wavelength in the electromagnetic spectrum from our
Sun. These particles strongly scatter short-wave radiation
and partially absorb in the near infra-red (Andronova et
al., 1999). Incoming light is scattered in all directions, with
back-scattering increasing the net planetary albedo (the
reflectivity of a surface) and forward-scattering increasing
the downwards diffuse radiation (Figure 2). The backscatter
effect is more dominant, causing a net cooling at
the surface during the day as less total radiation reaches
the surface (Harshvardhan, 1979). This phenomenon has
become known as a “Volcanic Winter” (Rampino et al.,
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