Publication Date:
2019-03-29
Description:
The Solfatara area and its fumaroles are the main
surface expression of the vigorous hydrothermal activity within
the active Campi Flegrei caldera system. At depth, a range of
volcanic and structural processes dictate the actual state of the
hydrothermal system below the crater. The presence of a large
variety of volcanic products at shallow depth (including pyroclastic
fallout ash beds, pyroclastic density current deposits, breccias,
and lavas), and the existence of a maar-related fault system
appears to exert major controls on the degassing and alteration
behavior. Adding further to the complexity of this environment,
variations in permeability and porosity, due to subsoil lithology
and alteration effects, may further influence fluid flow towards
the surface. Here, we report results from a field campaign conducted
in July 2015 that was designed to characterize the in situ
physical (temperature, humidity) and mechanical (permeability,
strength, stiffness) properties of the Solfatara crater subsoil. The
survey also included a mapping of the surficial hydrothermal
features and their distributions. Finally, laboratory measurements
(porosity, granulometry) of selected samples were performed.
Our results enable the discrimination of four main subsoils
around the crater: (1) the Fangaia domain located in a topographic
low in the southwestern sector, (2) the silica flat domain on the
western altered side, (3) the new crust domain in the central area,
and (4) the crusted hummocks domain that dominates the north,
east, and south parts. These domains are surrounded by encrusted
areas, reworked material, and vegetated soil. The distribution of
these heterogeneous subsoils suggests that their formation is
mostly related to (i) the presence of the Fangaia domain within
the crater and (ii) a system of ring faults bordering it. The subsoils
show an alternation between very high and very low permeabilities,
a fact which seems to affect both the temperature distribution
and surficial degassing. A large range of surface temperatures
(from 25 up to 95 °C) has been measured across these
surfaces, with the hottest spot corresponding to the mud pools,
the area of new crust formation, and the crusted hummocks. In
the subsoil, the distribution of temperature is more complex and
controlled by the presence of coarser, and more permeable,
sandy/pebbly levels. These act as preferential pathways for hot
hydrothermal fluid circulation. In contrast, low permeability,
fine-grained levels act as thermal insulators that remain relatively
cold and hinder fluid escape to the surface. Hot gases reach the
surface predominantly along (vertical) fractures. When this occurs,
mound-like structures can be formed by a cracking and healing process associated with significant degassing. It is anticipated
that the results presented here may contribute to an improved
understanding of the hazard potential associated with the
ongoing hydrothermal activity within the Solfatara crater. At this
site the permeability of the near-surface environment and its
changes in space and time can affect the spatial and temporal
distribution of gas and heat emission. Particularly, in areas where
reduction in permeability occurs, it can produce pore pressure
augmentation that may result in explosive events.
Description:
Published
Description:
83
Description:
3V. Proprietà dei magmi e dei prodotti vulcanici
Description:
JCR Journal
Keywords:
Solfatara crater
;
Subsoil
;
Hydrothermal alteration
;
Degassing
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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