Page 420 - Caldera Volcanism Analysis, Modelling and Response

Hydrothermal Fluid Circulation and its Effect on Caldera Unrest 395

on the region involved. More research is needed to completely understand volcanic
unrest, and to provide tools for the early recognition of eruptive crises.
A common characteristic of active calderas is the presence of hydrothermal
systems, which can play an important role during unrest crises. The presence of
a hydrothermal system may affect the eruptive style of a volcano, favouring the
occurrence of phreatic or phreatomagmatic events, but it also controls heat and
fluid transport within the volcanic edifice during both quiescent and unrest periods.
A hydrothermal system represents the complex interface between the magma
chamber and the surface. Geochemical or geophysical signals originating at the
magma chamber level can be distorted, magnified or reduced by the presence of
hydrothermal fluids. Hydrothermal systems may themselves generate additional
measurable signals in response to changing conditions.
Physical modelling of hydrothermal systems is an important tool for studying
the role of hydrothermal fluids during caldera unrest. Once an appropriate
conceptual model is identified, physical modelling of heat and fluid circulation in
the caldera then allows the quantification of relevant geochemical and geophysical
signals generated by the hydrothermal system. This is a first step toward identifying
the relationship between magmatic and hydrothermal systems, and hence toward
the interpretation of volcanic unrest at calderas.
This paper focuses on illustrating the role of hydrothermal systems during
caldera unrest. After describing the main features of hydrothermal circulation
(see Section 2), Section 3 focuses on the governing equations and physical models
which describe fluid circulation through porous media. The role of hydrothermal
fluids during volcanic unrest is reviewed in Section 4, while the final section
describes results from numerical modelling of hydrothermal fluid circulation at
Solfatara (Phlegrean Fields caldera, Italy).

2. The Hydrothermal Fluid Circulation

Hydrothermal fluid circulation develops where a thermal anomaly heats
pore fluids (i.e.: groundwater, meteoric water or seawater). In volcanic areas,
the heat source is commonly a shallow magma reservoir which releases not
only heat but also exsolved magmatic gases, such as water, carbon dioxide and
sulphur compounds, with minor HCl and HF. Hydrothermal features detect-
able at the surface depend on several parameters which include the size and
geometry of the magmatic source, the properties of fluid components and the
characteristics of subsurface rocks. Calderas are commonly characterised by wide
heat anomalies and host larger and longer-lived hydrothermal system than
stratovolcanoes.
Although magmatic volatiles are commonly identified in hydrothermal systems,
mechanisms of fluid transfer from the magma chamber to the surroundings are not
completely understood. It is generally assumed that a transition zone exists between
the molten magma and the brittle host rock, where partially molten rock acts as a
ductile medium deforming plastically, and heat is mainly transferred by conduction.
Across this transition zone, porosity tends to vanish, because of high temperatures