Page 177 - Fundamentals of Physical Volcanology

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154 CHAPTER 10

The VEI produces a single number between 0 and 8
which gives a combined measure of magnitude and
intensity. It therefore assumes a link between these
two properties of an eruption. Table 10.3 illustrates
the VEI classification system. Note that in this sys-
tem the intensity of the eruption is denoted by the
height of the eruption column (because column
height is directly related to mass eruption rate –
eqn 6.7). Table 10.4 shows the classification of a
number of historical eruptions using this system.
Although the VEI system is widely used it has a
number of drawbacks. One is that it is concerned
with explosive eruptions, so that any eruption
which is primarily lava-producing would be classified
Fig. 10.5 View of the ruins of the Roman city of as having a low VEI value even if the eruption had
Herculaneum, destroyed in the AD 79 eruption of Vesuvius.
a high eruption rate, produced a large volume of
The present-day city of Ercolano, built on top of the
lava, and was very destructive. Furthermore, the VEI
deposits from the eruption, and the volcano Vesuvius itself
assumes a connection between the magnitude and
are visible in the background. (Photograph by Lucia Gurioli,
University of Hawai’I.) intensity of an eruption which is not borne out by
observations. A more reliable system is to use inde-
pendent measures of magnitude and intensity. One
3200 km away!) or the AD 79 eruption of Ves- such system, for example, defines the magnitude of
uvius which destroyed Pompeii and Herculaneum an eruption as
(Fig. 10.5).
magnitude = log (M ) − 7 (10.1)
10 e
10.6.1 The magnitude of historic volcanic
where M is the total erupted mass (in kg), and the
eruptions e
intensity of an eruption as
Volcanologists keep detailed records of the Earth’s
volcanic activity. Such records are compiled using a intensity = log (M ) + 3 (10.2)

10
f
range of information including eyewitness accounts,
−1
geophysical data collected by volcano observator- where M is the mass flux (in kg s ). The integers in
f
ies, geological mapping and remote sensing stud- these equations are chosen simply for conve-
ies. Details of past activity have been painstak- nience: few eruptions involve masses less than
−1
7
3
ingly reconstructed using information such as his- 10 kg or mass fluxes less than 10 kg s . This sys-
torical records and geological mapping. One such tem has the advantage that it allows direct compar-
record is that of the Global Volcanism Program ison of explosive and lava-producing eruptions.
of the Smithsonian Institution in Washington, DC Table 10.4 illustrates the use of this system for
(http://www.volcano.si.edu/gvp/index.htm). This classifying a number of historical eruptions. For pri-
lists known eruptive activity which has occurred marily explosive eruptions the VEI value is similar
during the past 10,000 years. The scale of eruptions to the magnitude value, but notice the difference in
in this catalog is classified using an index called these values for lava-producing eruptions such as
the Volcanic Explosivity Index or VEI. This is a the 1950 Mauna Loa eruption and the 1991–93 Etna
widely used method of classifying the magnitude eruption.
and intensity of volcanic eruptions. Table 10.4 includes examples of both the typical
The magnitude of an eruption is defined as the total small magnitude and low intensity activity which is
volume or mass of material erupted. The intensity is going on all the time on the Earth and of the largest
a measure of the volumetric or mass eruption rate. eruptions to have occurred in the recent past. In