Page 194 - Fundamentals of Physical Volcanology

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VOLCANIC HAZARDS AND VOLCANO MONITORING 171

Fig. 11.10 The jökulhlaup from
the 1996 Gjálp eruption, Iceland.
The flood has destroyed part of the
bridge over the river Gygjukvisl on
Skeidararsandur, South Iceland.
(Photograph taken on November 5,
1996, by Magnus Tumi Gudmundsson,
Institute of Earth Sciences, University
of Iceland.)

Unfortunately, every other common volcanic much less. Here it becomes supersaturated, and
volatile is directly dangerous to most living crea- explosively exsolves a dense cloud of carbon diox-
tures. The next commonest volcanic gas is carbon ide. The cloud of gas is denser than air and will hug
dioxide. As a cold gas this is dangerous mainly the ground like a pyroclastic density current and
because we cannot breath it – indeed, carbon diox- travel downhill. Many people died of a mixture of
ide is the waste gas that animals breathe out. After chemical burns and asphyxiation in four villages
it has cooled to the ambient temperature, carbon around Lake Nyos in western Cameroon in 1986
dioxide is ∼50% denser than air and so it collects in when an event of this kind took place. Since then,
topographic hollows, especially if there is no wind a system of pipes has been installed in the lake to
to stir up the atmosphere. Any person or animal syphon water continuously from the bottom to the
walking into a depression filled with carbon diox- surface to allow it to lose gas slowly and steadily,
ide will rapidly become unconscious as no new instead of in catastrophic overturn events.

oxygen goes into their bloodstream and the exist- Other gases can be equally lethal in various ways.
ing supply is used up. An additional problem is that, The volcano Hekla, in Iceland, often erupts magmas
for a small fraction of the human population, car- that are rich in the halogen element fluorine. This
bon dioxide inhibits part of the central nervous gas is even denser than carbon dioxide, and so even
system, so that even if one realizes what is wrong, more likely to collect in hollows in the topography.
one cannot climb out of the depression to safety. On a number of occasions, sheep (and the grass that
Carbon dioxide is a corrosive, as well as poi- they eat) have been poisoned in large numbers by
sonous, acid gas. In certain places, this volatile is accumulations of this gas. Almst equally poisonous
released from shallow magma bodies and seeps are the high concentrations of sulfur dioxide and
upward to collect as a dissolved gas in the water in hydrogen sulfide that basaltic volcanoes sometimes
lakes. The added gas coming from below makes the release. Sulfur dioxide can dissolve in atmospheric
water at the bottom of the lake a little denser than water drops and react with oxygen to form sulfuric
that above, so that it stays at the bottom. If some acid. Tiny droplets of this kind are called aerosols,
event such as a landslide, or even just a very heavy and when present in large amounts can alter the
downpour of rain on one part of the lake, disturbs way the atmosphere reflects and absorbs sunlight,
this density stratification, the water in the lake may thus changing the climate. We discuss the effects of
overturn. Water from the bottom, which is satu- the sulfur dioxide haze from the 1783 eruption of
rated in gas, rises to the top where the pressure is Laki volcano in Iceland in the next chapter.