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58 CHAPTER 4

Mount Etna, in Sicily, currently has no permanent of magma in shallow magma chambers play an
shallow magma storage system of any significant important role in generating it. Storage of magma
size. This volcano is particularly interesting in that within a magma chamber will inevitably lead to
there have been major structural events during cooling and fractional crystallization of the
its history where parts of the eastern flank of the magma. Fractional crystallization is the process in
mountain have collapsed into the sea forming which a cooling magma crystallizes out successive
giant landslides. These processes have exposed minerals and in doing so progressively evolves in
ancient lava flows whose compositions can only be composition. The study of fractional crystallization
explained if the magmas that erupted to form them falls within the realm of igneous petrology rather
were stored for a time at the kind of low pressure than physical volcanology and we are concerned here
found in a shallow magma chamber. Furthermore, only with the ways in which crystallization affects
there are signs of caldera collapse events having the physical processes of the magmatic system.
occurred, another indicator of shallow magma stor- Basaltic magmas will only undergo fractional crys-
age. Why should the internal plumbing of the vol- tallization if their residence time within a magma
cano have changed? Any major collapse event must chamber is long enough for cooling to become
cause a very great alteration to the distribution of significant. Two critical (and linked) factors con-
stress within the body of the volcano, and if this trolling the cooling and crystallization history of
interrupts the internal pathways supplying magma a particular magma chamber are the nature of
to shallow levels for long enough, any existing magma resupply to the chamber from depth, and
magma chamber may solidify. Furthermore, if the the frequency with which magma is erupted from
same stress changes happen to make it easier for the magma chamber. If magma is supplied to the
magma to rise directly to the surface, there is no magma chamber continuously then cooling will be
reason for a new shallow chamber to form. minimized and the magma chemistry is unlikely
to alter greatly. In this situation eruptions are also
likely to be frequent and so the residence time of
4.4 Magma chambers and their impact on magma in the chamber is minimized, also limiting
volcanic systems the cooling experienced by a particular batch of
magma. If, however, magma supply is intermittent,
We have seen that magma chambers represent then the magma in the chamber is likely to cool
a staging post for magma on its journey from the between resupply events and so to evolve in

mantle to the surface and are fed by semi-permanent, composition.
deep plumbing systems which represent the evo- These differences can be illustrated by compar-
lution of magma supply from simple transport in ing activity in Hawai’I with that in Iceland. In both
dikes to the ability of pulses of magma to travel up- Hawai’I and Iceland, basaltic magma is supplied
ward in a mush column (Fig. 1.21). This section to shallow magma chambers from the mantle. In
examines some of the profound effects that the Hawai’I the vast majority of eruptions produce only
presence of shallow magma chambers has on the basaltic magmas. There is little evidence for the
nature and behavior of volcanic systems. formation of significantly more evolved magmas
by fractional crystallization (except during the very
final stages of activity of a given volcano when the
4.4.1 Fractionation in magma chambers
rate of supply of mantle magma to it becomes very
Magmas generated in the mantle are typically of small). In Iceland many eruptions are also purely
basaltic compositions. Erupted magmas have a far basaltic in character but it is not uncommon for
wider range of compositions ranging from these there to be eruptions of more evolved magmas or
same basaltic compositions up to highly evolved mixed-magma eruptions (eruptions in which both
rhyolitic lavas. While there are many reasons for basaltic and more evolved magmas are released
this compositional diversity, storage and evolution together). The difference between the Hawaiian