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MAGMA STORAGE 59

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and Icelandic systems seems primarily to be the Tephra volume (m )
way magma is supplied to the shallow magma cham-
10 6 10 8 10 10 10 12
ber. In Hawai’I the supply is either continuous 5
10
or sufficiently frequent that the magma in the
shallow chamber never cools enough to allow 4 Ruiz 1985
10
significant evolution of the magma composition
and so all eruptions are basaltic. In Iceland, which 10 3 Mt St Helens 1980
is located astride the Mid-Atlantic spreading ridge,
the supply of magma to shallow chambers is influ- 10 2 Krakatau 1883
enced by the spreading process, which is intermit- Number of eruptions
tent rather than smooth. As a result, resupply can 10 Tambora 1815
be infrequent enough to allow cooling and crystal-
lization of the magma between resupply events, so 1
that eruption of evolved magmas can occur. Yellowstone 2 Ma
The consequences of these different patterns of 0.1 Eruptions per
cooling and fractionation are profound because thousand years
they fundamentally influence the style of volcanic 0.01
activity which will occur. Considering the contrast
between Hawai’I and Iceland again, the lack of sig- Fig. 4.19 The relationship between the magnitude of
eruptions, expressed as volume of tephra, and their
nificant fractionation in Hawaiian magma chambers
frequency, expressed as number of eruptions per thousand
means that eruptions from them are associated
years, illustrated by some well-studied volcanoes. The
with the typical range of mild basaltic eruption
pattern that small eruptions occur much more frequently
styles (see Chapters 9 and 10). By contrast, an than large eruptions is clear. (Modified from fig. 6 in Simkin,
Icelandic chamber which has had an opportunity to T. and Siebert, L. (2000) Earth’s volcanoes and eruptions:
cool may erupt highly evolved magmas in major an overview. Encyclopedia of Volcanoes. Academic Press,
explosive eruptions. The 1875 eruption of Askja in 249–261. Copyright Elsevier (2002).)
Iceland, for example, involved the eruption of both
basaltic and rhyolitic magmas (a fresh influx of intervals of ∼600,000 years but these can have
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basaltic magma apparently triggering the eruption). volumes in excess of 2000 km , making them 10 12
The main eruption occurred from the Askja caldera (a trillion!) times larger than a typical Stromboli

and involved the predominantly rhyolitic magma. eruption! Detailed study of volcanic systems
This eruption was Plinian in style (see Chapters 6 shows that:
and 8) and generated a plume 26 km high, making
• there is a link between the magnitude and fre-
it one of the largest explosive eruptions to have
quency of eruptions such that small eruptions
occurred in Iceland in historical times.
occur frequently and larger eruptions are less
frequent (Fig. 4.19);
4.4.2 Regulation of eruption frequency • there is a broad link between the magnitude of
and magnitude an eruption and the size of the magma chamber
feeding it (Table 4.1).
Volcanic eruptions vary tremendously in magni-
tude and frequency. Some volcanoes erupt almost These two points taken together suggest that the
continuously, with individual eruptions producing presence of a magma chamber within a magmatic
very small volumes of material. For example, mildly system plays a crucial role in controlling eruption
explosive eruptions at Stromboli volcano typically frequency and magnitude. To understand why this
occur every few minutes to tens of minutes, with should be the case we can examine the findings of
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eruptions producing 1–2 m of ejected material. a simple mathematical model of magma chamber
By contrast, Yellowstone has major eruptions at behavior.