Page 434 - Caldera Volcanism Analysis, Modelling and Response

Hydrothermal Fluid Circulation and its Effect on Caldera Unrest 409

6. Discussion and Conclusions
Hydrothermal fluid circulation is a very special feature of active volcanic
systems. By controlling the transport of heat and fluids from the magma reservoir
to the surface, hydrothermal fluids play a significant role in the evolution of
volcanic centres. Circulating fluids affect rock properties, generate various types of
geochemical and geophysical signals and can trigger shallow seismicity and ground
deformation. Pore pressure build-up, followed by intense degassing, may explain
uplift and subsidence cycles observed during non-eruptive unrest at many calderas
in the world. Repeated crises, accompanied by ground deformation and by
widespread hydrothermal alteration, can progressively weaken rocks strength and,
on the long-term, this can be one of the many features favouring a renewal of the
eruptive activity. Hydrothermal systems therefore represent an important key to
understanding volcanic unrest. Physical modelling of hydrothermal fluid circulation
is a powerful tool to study the evolution of hydrothermal systems, and to quantify
their effects on selected geochemical and geophysical parameters. Volcanic
surveillance collects geochemical and geophysical data, whose evolution depends
on the complex interactions between the magmatic source, hot circulating fluids
and the host rocks through which they circulate. It is impossible to fully assess all
these interactions, but the interpretation of monitoring data should account for this
complexity. Results from numerical modelling of hydrothermal circulation are
promising, but the complexity of the natural systems demand further improvement
to achieve satisfactory results, beyond the theoretical study. A fully multi-
disciplinary approach is required to improve and further connect conceptual
models, numerical models and monitoring data. We need robust conceptual models
capable of describing caldera evolution in all its geochemical and geophysical
aspects, consistently incorporating all available information.
Several aspects need to be further investigated to fully understand the role of
fluids in unrest crises. Mechanisms controlling magma degassing and the transport of
heat and fluid from the magma chamber to the hydrothermal system are still poorly
constrained. Pulsating degassing, alternating phases of higher and lower gas flow
rates, seems to be a common behaviour, but we lack a coherent explanation for it.
A better characterisation of magmatic degassing could greatly improve our descrip-
tion of hydrothermal circulation, and provide further insights on the functioning
and ultimate meaning of non-eruptive unrest crises. Subsurface rock properties
are usually poorly defined — few measurements may be available on thermal and
acoustic properties of subsurface rocks, but information on hydraulic properties
is usually missing. Data used to set up numerical models are commonly taken from
the literature and are hardly representative of the natural system. Changes of rock
properties through time, or with changing system conditions, are also poorly
constrained and generally are not accounted for in models applied to volcanological
problems. Nevertheless, these changes could play an important role during unrest
crises and their effects on fluid-flow pattern, and on pressure and temperature
distribution, should be assessed. More data are available on hydrothermal alteration
and, in general, on chemical reactions taking place during fluid circulation.
However, modelling of volcanic unrest has not yet included their description, and