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62 CHAPTER 4

low viscosity magmas, such as basalts, bubbles can • Chamber shapes appear to vary widely from
move with relative ease and thus have the great- simple sheet-like and laccolithic structures to
est opportunity to rise to the chamber roof and more equant shapes. The shape of a chamber is
escape through cracks in the overlying rocks. In likely to evolve through time by a combination of
high-viscosity magmas, movement of bubbles is processes, including deformation and fracturing
inhibited and the gas is less liable to escape. Thus of the surrounding rocks, cooling and stoping.
the importance of gas exsolution as a method of • The existence of a magma chamber has a pro-
pressurization of a magma chamber is likely to be found effect on the chemistry of erupting mag-
more significant in chambers containing chemically mas and hence also on the physical properties of
evolved, viscous magmas. Indeed, observations the magmas. The storage of magma in a shallow
show that eruptions of evolved magmas frequently magma chamber allows the magma to cool and
start with a highly explosive phase which appears crystallize and so lets its chemistry progressively
to be caused by the exsolution of water from the evolve.
magma while it is still in the magma chamber. The • The presence of a magma chamber within a vol-
exsolution is thought to both trigger the chamber canic system has a profound effect on the scale
failure which leads to eruption and explain the and frequency of volcanic eruptions and thus
highly explosive nature of the opening stage of magma chambers are crucial regulators of vol-
such an eruption. Once this bubble-rich magma has canic activity. Small chambers give rise to small
been erupted, less gas-rich magma from deeper in but frequent eruptions while larger chambers
the chamber may be erupted with a corresponding erupt less frequently but generate larger indi-
decline in the explosivity of the eruption. vidual eruptions.
The magma replenishment and gas exsolution • Magma chamber failure, which is a prerequisite
models described here represent end-member for eruption, can be triggered by both the influx
cases for chamber failure. They are not, however, of fresh magma or by the exsolution of gas within
mutually exclusive, and in reality both magma the magma chamber as a result of magma crys-
replenishment and gas exsolution may contribute tallization. Either process can raise the internal
to the pressure rise within, and failure of, a particu- chamber pressure to the point where the tensile
lar magma chamber. strength of the chamber walls is exceeded.

4.6 Further reading
4.5 Summary
Blake, S. (1981) Volcanism and the dynamics of open
• Evidence from petrology, volcano morphology,
magma chambers. Nature 289, 783–5.
geophysics and geology suggests that storage
Blake, S. (1984) Volatile oversaturation during the
of magma in a crustal magma chamber prior to
evolution of silicic magma chambers as an eruption
eruption is extremely common. The sizes and
trigger. J. Geophys. Res. 89, 8237–44.
depths of these storage zones vary widely
Marsh, B.D. (2000) Magma chambers. In Encyclopedia
(Table 4.2).
of Volcanoes (Ed. H. Sigurdsson), pp. 191–206.
• The development of magma chambers is not
Academic Press, San Diego, CA.
well understood, but study of eroded chambers
Norton, D., Taylor, H.P. & Bird, D.K. (1984) The geo-
suggests that they often develop from an initial
metry and high-temperature brittle deformation of
sill-like body. As any intrusive body is susceptible the Skaergaard intrusion. J. Geophys. Res. 89, 10,
to cooling and solidification, a chamber can 178–92.
develop only if it is frequently resupplied with Tait, S., Jaupart, C. & Vergniolle, S. (1989) Pressure,
fresh magma. For this reason, establishment of gas content and eruption periodicity of a shallow,
a deep plumbing system to feed the magma crystallising magma chamber. Earth Planet. Sci.
chamber is essential to its survival. Lett. 92, 107–23.