Page 193 - Fundamentals of Physical Volcanology

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170 CHAPTER 11

Fig. 11.9 A lahar deposit from an
eruption of Mount Pinatubo,
Philippines. The deposit forms the
floor of the channel, and the lahar
material forms a veneer on the channel
walls. (Photograph by Pete Mouginis-
Mark, University of Hawai’I.)

over: if the ash emplaced by the eruption is uncon- accumulation of a large volume of water that can
solidated, then every time that there is a major rain- eventually escape from under the edge of the
storm, it is likely that a new lahar will occur until all glacier in a catastrophic flood (Fig. 11.10). These
of the deposit is eroded away. The many lahars that floods are quite common in parts of Iceland, and
were generated over a several year period following the Icelandic word jökulhlaup is used to describe
the 1991 eruption of Pinatubo in the Philippines them. A recent well-monitored example was the
provide a good example: they led to the evacuation 1996 Gjálp fissure eruption under the Vatnajökull
3
of hundreds of thousands of people long after the ice-cap in southern Iceland. Here ∼3.5 km of water
eruption proper was over. accumulated over a 5 week period and then
drained from under the ice in just 2 days, the flood
destroying roads and bridges in its path (Fig. 11.10).
11.2.6 Jökulhlaups

When an eruption occurs under a glacier, a charac- 11.2.7 Volcanic gases
teristic sequence of events can occur. The weight
of the overlying ice is commonly great enough that We have seen that the commonest volcanic gas is
the magma does not fragment in a volcanic explo- water vapor which, except when it is very hot, is
sion but is extruded along the contact between the harmless. Actually this statement is not quite true,
base of the glacier and the ground, essentially as a because although water has no direct adverse effects
sill. However, melting of the overlying ice produces on plants and animals on the ground, it may have
water, and this interacts with the magma, cooling very adverse effects when high eruption columns
its surface and leading to some of the processes, carry it into the upper atmosphere – the strato-
such as fragmentation of the magma surface into sphere. This is because water molecules become
glassy particles, associated with fuel–coolant inter- involved in a complex chain of chemical reactions
actions (see section 7.3.2). In the subglacial case that involves sunlight. One of the products of these
the process is very much less violent, but it does interactions is the layer of ozone that absorbs many
maximize heat transfer from the magma to the wavelengths of ultraviolet light that, if they reach the
ice. ground, can cause skin cancers. Excessive amounts
Although the magma may melt completely through of water vapor, like other gases present in trace
the ice and produce a hydromagmatic explosive amounts, can modify the reactions that maintain
eruption, the main hazard in these events is the the ozone in ways that are still not fully understood.