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158 CHAPTER 10

fraction of the chamber volume. For example, the
10.6.3 The frequency of volcanic eruptions
magma chamber beneath Hekla in Iceland is esti-
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Each individual volcano tends to have its own mated to have a volume of up to 145 km . The 1991
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pattern of activity, with some volcanoes erupting eruption at Hekla produced 0.15 km of lava or
much more frequently than others. Intervals between ∼0.1% of the magma chamber volume. The cham-
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eruptions are commonly tens of minutes at Strom- ber volume at Kilauea is ∼50 km . Typical erupted
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boli but thousands to hundreds of thousands of years volumes at Kilauea are between 10 −3 and 0.1 km ,
for large volcanoes erupting rhyolites. However, or 0.002 and 0.2% of the chamber volume. Large
there is a tendency, for individual volcanoes and for pyroclastic density current eruptions (Table 10.5)
volcanoes as a whole, for eruptions of small magni- producing thousands of cubic kilometers of mater-
tude to occur frequently whereas larger eruptions ial require storage in magma chambers which are
are rarer. This is why the magnitude of eruptions much larger than the small basaltic chambers even
experienced during human history is considerably if all the magma could be evacuated from them.
smaller than the scale of eruptions which are found If, as in the basaltic case, only ∼0.1% of the total
in the geological record. Eruptions of very large chamber volume could be erupted, then an erup-
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magnitude are (fortunately) very rare and so on the tion producing 1000 km of material would require
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small time frame of human history we have gener- storage in a chamber with a volume of 10 km 3
ally experienced small magnitude eruptions. Figure (equivalent to a sphere of radius ∼60 km).
4.19 showed the number of eruptions of a given Further evidence for a general link between
magnitude which are expected to occur per thou- magma chamber size and eruption magnitude comes
sand years. About 100 eruptions of the scale of the from comparing caldera size with erupted volumes.
1980 Mount St Helens eruption could be expected Calderas form by collapse of the magma chamber
per 1000 years, or one per decade. On the scale roof as magma is withdrawn during eruptions
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of the 1883 Krakatau eruption about 10 eruptions which generate more than ∼10–50 km of material.
could be expected per 1000 years or one per cen- It is thought that the caldera diameter is a reflection
tury. Events on the scale of the 1815 eruption of of the diameter of the underlying magma chamber.
Tambora occur about once in a millennium. The When the area of a caldera produced in a given
figure shows a distinct change in slope at VEI values eruption is compared with the volume of material
greater than 7. This is probably a reflection of the erupted a good correlation is observed (Fig. 10.8).
incomplete nature of our records of large eruptions For example, the caldera formed by the 2.2 Ma

rather than a real feature of volcanic behavior. Cerro Galan eruption which produced ∼2000 km 3
of material (Table 10.5) was 25 by 35 km in
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diameter, an area of ∼690 km , and the caldera
10.6.4 Magma chambers and eruption
formed in the 5000 km 3 La Garita eruption
magnitude and frequency
(Table 10.5) measured 35 by 75 km, an area of
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As seen in Chapter 4, magma is generally stored ∼2000 km . The ratios of the erupted volume to
at some level within the crust prior to eruption. It caldera area for these two eruptions are ∼2.9 km and
is plausible to imagine then that a link would exist 2.4 km, respectively, suggesting that these were
between the magnitude of an eruption and the size approximately the vertical extents of the parts of
of the magma chamber which feeds it. For instance, the chambers evacuated.
small but frequent basaltic eruptions from volca-
noes in Hawai’I and Iceland occur from magma
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chambers with typical volumes of only 35–150 km , 10.7 Elastic and inelastic eruptions
determined by the methods described in sec-
tion 4.2.3. Such chambers could, therefore, only Simple models such as the one described in section
produce this volume of magma during an eruption 4.4 can explain the general behavior of volcanic sys-
assuming the chamber could be fully evacuated. tems and predict the pattern of behavior expected
Typical eruption volumes are, in fact, only a small at a single volcano. For a magmatic system with a