Page 195 - Fundamentals of Physical Volcanology

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172 CHAPTER 11

11.3 Hazard assessment eruption taking place within one generation is
extremely small, it could be argued from a prag-
There are several aspects to assessing volcanic matic sociological viewpoint that assessing a vol-
hazards. The first step is to decide what constitutes canic hazard is not the most urgent issue if other
a hazard, and there are both physical and statis- problems, such as flood, earthquake, tsunami, dis-
tical aspects to this. Many centers of population ease and war, are more likely to affect the local
are located close to volcanoes known to have population.
been active in historic time – the city of Naples If a volcano erupts sufficiently frequently, there
near Mount Vesuvius in Italy is a good example. are likely to be well-preserved deposits from at least
For people in areas such as these, being overrun the most recent eruptions. Geologists can identify
by a lava flow or ash flow is an obvious potential and map out these deposits, so that the common
threat; but there is a less obvious hazard: being styles and scales of activity are readily apparent.
killed in the crash of an aircraft with ash-choked The most likely ranges of distances from the vol-
engines, which could happen to those same citi- cano at which people will be at risk from the vari-
zens while on vacation in many other parts of ous kinds of activity can be defined and drawn on a
the world. map of the area. Furthermore, eruption products
This introduces the idea of local, regional, and can be dated in various ways, by radiocarbon or
global volcanic hazards. Global hazards involve tree-ring dating of dead vegetation trapped in the
volcanoes injecting large amounts of gas, aerosols, deposits, or other isotopic dating methods, and the
or small ash particles into the atmosphere and typical repose periods between events of a given
changing the climate, as considered in Chapter 12. scale can be established. Conversely, for volca-
Regional hazards include the effects on aircraft noes that erupt extremely infrequently, the state of
operations of ash falling from eruption clouds, preservation of even the most recent deposit may
and such phenomena as the collapse of volcanoes be very poor, to the point where they are unrecog-
erupting in the ocean, which can generate tsunami nizable. In the cases of many of the mountains of
waves traveling great distances. Local hazards the Andes and central America, for example, it was
include all of the products of eruptions that are not realized until the advent of modern geology that
emplaced close to the volcanic source – lava flows, they “were” volcanoes. For these types of volca-
pyroclastic density currents, and fall deposits. noes we have to fall back on our knowledge of how
Historically, it is local hazards that have received other similar volcanic centers in similar tectonic

most attention because they are generally the easi- settings have behaved to assess the likely extent of
est to identify. Even so, prior to the 20th century the hazards. Once the range of likely future behav-
the best indicators of volcanic activity were quite ior patterns of a particular volcano has been deter-
likely to be the memories of local inhabitants, and mined, the next step is to estimate when the next
this underlines one of the potential problems in eruption is likely to occur. On short time scales this
dealing with hazards. If a volcanic eruption (or any is best done by geophysical monitoring, which is
other kind of catastrophe for that matter) has not considered in the next section. On longer time
happened within the memory of one’s grandpar- scales, it is done mainly on the basis of the statis-
ents, it is simply not perceived as a likely threat. Yet tics of previous eruptions. The best that can be
many kinds of natural disaster, including eruptions, produced is the probability of an eruption of a
have return periods (the most likely intervals given magnitude occurring within a given period
between occurrences) of at least tens of thousands of time.
of years. It is only since the emergence of system- One way of making a statistical appraisal of pos-
atic geological mapping, and the development of sible future activity is to classify previous eruptions
a basic theoretical understanding of what kind of by the Volcanic Explosivity Index (VEI) as defined
eruption produces a given type of deposit, that the in Table 10.3 and to compile a list of how many
true potential hazard of many volcanic systems eruptions of a given VEI rating have occurred in a
has been perceived. And where the likelihood of an given interval of time. Table 11.1 shows an example