Publication Date:
2017-04-04
Description:
The physical integrity of a sub-volcanic basement is crucial in controlling the stability of a volcanic edifice. For
many volcanoes, this basement can comprise thick sequences of carbonates that are prone to significant
thermally-induced alteration. These debilitating thermal reactions, facilitated by heat from proximal magma
storage volumes, promote the weakening of the rock mass and likely therefore encourage edifice instability.
Such instability can result in slow, gravitational spreading and episodic to continuous slippage of unstable
flanks, and may also facilitate catastrophic flank collapse. Understanding the propensity of a particular
sub-volcanic basement to such instability requires a detailed understanding of the influence of high temperatures
on the chemical, physical, and mechanical properties of the rocks involved. The juxtaposition of a thick
carbonate substratum and magmatic heat sources makes Mt. Etna volcano an ideal candidate for our study.
We investigated experimentally the effect of temperature on two carbonate rocks that have been chosen to represent
the deep, heterogeneous sedimentary substratum under Mt. Etna volcano. This study has demonstrated
that thermal-stressing resulted in a progressive and significant change in the physical properties of the two
rocks. Porosity, wet (i.e., water-saturated) dynamic Poisson's ratio and wet Vp/Vs ratio all increased, whilst
P- and S-wave velocities, bulk sample density, dynamic and static Young's modulus, dry Vp/Vs ratio, and dry
dynamic Poisson's ratio all decreased. At temperatures of 800 °C, the carbonate in these rocks completely
dissociated, resulting in a total mass loss of about 45% and the release of about 44 wt.% of CO2. Uniaxial deformation
experiments showed that high in-situ temperatures (〉500 °C) significantly reduced the strength of the
carbonates and altered their deformation behaviour. Above 500 °C the rocks deformed in a ductile manner and
the output of acoustic emissions was greatly reduced. We speculate that thermally-induced weakening and the
ductile behaviour of the carbonate substratum could be a key factor in explaining the large-scale deformation
observed at Mt. Etna volcano. Our findings are consistent with several field observations at Mt. Etna volcano
and can quantitatively support the interpretation of (1) the irregularly low seismic velocity zones present within
the sub-volcanic sedimentary basement, (2) the anomalously high CO2 degassing observed, (3) the anomalously
high Vp/Vs ratios and the rapid migration of fluids, and (4) the increasing instability of volcanic edifices in
the lifespan of a magmatic system. We speculate that carbonate sub-volcanic basement may emerge as one of
the decisive fundamentals in controlling volcanic stability.
Description:
Published
Description:
42-60
Description:
2R. Laboratori sperimentali e analitici
Description:
JCR Journal
Description:
restricted
Keywords:
Decarbonation
;
04. Solid Earth::04.08. Volcanology::04.08.08. Volcanic risk
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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