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MAGMA STORAGE 61

volume (s > 0) then the chamber pressure in- added). Just as in the replenishment model, failure
creases less for a given volume of magma added and occurs because the pressure in the chamber gra-
so more magma must be added for the chamber dually increases to the point where the tensile
failure criterion to be met. This simple model has strength of the chamber walls is exceeded and
two important implications for the patterns of fracturing occurs. However, in this case the pres-
activity at a given volcanic center. sure increases because of the exsolution of volatiles
First, as long as the chamber walls behave elasti- from the magma. Volatile species such as carbon
cally, the volume added prior to failure is equal dioxide (CO ) and water (H O) are contained dis-
2 2
to the maximum volume that can be erupted from solved in magmas as they form at great depths in
the chamber during a single eruption without the the mantle. Chapter 5 describes in detail how the
danger of caldera collapse, and this volume is ∼0.1 solubility of these gases decreases with decreasing
to 1% of the chamber volume (see Fig. 4.20). Thus pressure so that as magma rises it will eventually
this model predicts that the larger the size of the reach a pressure level at which the magma be-
magma chamber feeding an eruption, the larger the comes saturated in one or more volatile species
volume of the eruptions it can produce. Second, and gas will exsolve from the magma forming gas
the frequency of eruptions can be thought of in bubbles. The low solubility of CO in magmas
2
terms of the repose time, R , between eruptions means that it is often present as a separate gas phase
T
and expressed as: within shallow magma chambers. In addition, as
magma cools and crystallizes within a magma cham-
R =∆V/M (4.4) ber, the volatile species become increasingly
T m
concentrated in the residual melt and, by this
where, as above, ∆V is the volume of magma added mechanism, the melt may also eventually become
to the magma chamber prior to failure and M is saturated in more soluble volatiles, particularly
m
the volume rate at which magma is supplied to the water. The formation of gas bubbles within the
chamber. If M does not vary too greatly between magma requires space to be created. This space can
m
different magma chambers, it follows that for big- be created by the compression of the magma, the
ger chambers the repose time between eruptions deformation of the chamber walls and by the crys-
will be longer. Or, put another way, the bigger the tallization process itself. Just as with the addition
magma chamber, the less frequent its eruptions. of magma to the chamber, the exsolution of gas
Taken together, these two implications of the and formation of gas bubbles increases the pressure

model match the observed behavior of real vol- in the chamber. Modeling suggests that only a
canic systems, i.e., that larger magma chambers few percent crystallization is necessary to cause
feed larger but less frequent volcanic eruptions. So sufficient exsolution to raise the pressure to failure
magma chamber size plays a crucial role in regula- levels.
ting the scale and frequency of volcanic activity at This mechanism of chamber failure can occur
any given volcanic center. only if the gas stays trapped within the chamber.
Observations in the summit regions of many volca-
noes show that gas is constantly escaping upwards
4.4.3 Volatiles and chamber failure
from the magma chamber. (Visitors to volcanoes
The model just discussed represents one extreme such as Kilauea in Hawai’I will be immediately
scenario for magma chamber failure: failure occurs aware of this gas leakage in the summit region
because of the replenishment of the magma cham- because of the sulfurous fumes, and caution is
ber with fresh magma from deeper levels. At the advised in spending too much time in certain local-
opposite extreme, models have been developed in ities because the gas release is so great that it is
which the failure of the magma chamber occurs potentially lethal to someone with breathing prob-
solely as the result of exsolution of volatiles as lems!) The ability of gas to escape will depend
magma cools and crystallizes within a closed cham- primarily on whether the gas bubbles can move
ber (i.e., one in which no fresh magma is being upwards through the magma in the chamber. In