Page 422 - Caldera Volcanism Analysis, Modelling and Response

Hydrothermal Fluid Circulation and its Effect on Caldera Unrest 397

Bishoff et al., 1986; Pitzer and Pabalan, 1986; Fournier, 1987; Bishoff, 1991). At
shallower depths density differences may drive separation of hot rising gases from
the denser liquid phase. As a result hot acid-sulfate springs and fumaroles, fed by
CO 2 - and H 2 S-rich gases, tend to dominate at higher elevation, whereas neutral-
chloride waters will dominate at greater depths within the volcanic edifice and feed
springs at lower elevation and at the periphery of the main upflow region (White
et al., 1971; Ingebritsen and Sorey, 1988). Such zonation of discharge features may
not exist in wide calderas, which are commonly characterised by a rather flat
ground surface (Ingebritsen and Sanford, 1998; Goff and Janik, 2000).
Rock properties also greatly affect the onset and evolution of fluid circulation.
Rock permeability expresses the resistance offered by a porous medium to fluid
flow. It depends on lithology, varying by several orders of magnitude among
different rock types. It also varies with the degree of rock fracturing or alteration.
Within the same rock formation permeability may change at different locations
and can be anisotropic. Stratigraphic discontinuities, faults or fractures may act as
channelways for fluid propagation, focusing the flow along preferential directions
(Ingebritsen and Sanford, 1998). Where fractures are present fluid flow can be
relatively fast, and thermal disequilibrium between rock and fluid may exist. If more
than one phase is present fractures can also affect phase distribution, as the liquid
phase is preferentially held into the smaller pores while gas occupies the fractures
(Helmig, 1997). Rock permeability tends to decrease with depth as higher
confining pressure, diagenetic and metamorphic processes reduce pore size
(Ingebritsen and Sanford, 1998).
Rock permeability can be modified via several processes and at different spatial
scales, and this affects the fluid flow pattern through time. Some of these processes
are intimately associated with chemical and physical interactions between fluids
and host rocks: dissolution or precipitation of mineral phases changes pore size and
connectivity, whilst modifying fluid composition and properties (Norton and
Knight, 1977; Fournier, 1987; Verma and Pruess, 1988; Ingebritsen and Sanford,
1998). Permeability may also increase in response to hydro-fracturing induced by
high pore pressure (Norton and Knight, 1977; Burnham, 1985; Gudmundsson
et al., 2002). As rock permeability within the hydrothermal system changes through
time, surface features associated with fluid circulation change accordingly as the
system adjusts to new flow rates and directions.

3. Modelling of Hydrothermal Fluid Circulation

Physical modelling of fluid flow problems is based on the solution of the
fundamental equations that govern fluid motion. However, in the case of porous
media flow the classical approaches of fluid mechanics, such as those based on
the solution of the Navier–Stokes equations, are not immediately applicable as the
details of the microscopic pore geometry are not known and flow variables and
parameters cannot be defined everywhere. In this case, it is necessary to work on a
macroscopic scale, where each quantity is defined as an average over an appropriate
volume of material (representative elementary volume). Within this volume it is