Thermal weakening of the carbonate basement under Mt. Etna volcano (Italy): Implications for volcano instability

Abstract

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 thermalstressing 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 CO 2 . 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 A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPTthe 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 CO 2 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.

Increasing the Permeability of Hydrothermally Altered Andesite by Transitory Heating

“…Additionally, calcite crystals in the moderately altered andesite have reaction textures after having been treated to 739 °C (Figure c). The relatively large second reaction observed in the TG analyses of the moderately altered andesite (Figure a) is consistent with decarbonation reactions of calcite in this temperature range, as shown in equation (e.g., Mollo et al, ; Heap, Mollo, et al, ). …”

Matrix permeability of reservoir rocks, Ngatamariki geothermal field, Taupo Volcanic Zone, New Zealand

“…The sonic wave results show that the saturated samples have a noticeable P-wave velocity increase (average 4%) in wave velocity compared to the dry samples, while the saturated and dry S-wave velocities are generally smaller (average − 2%). This phenomenon has been observed in other studies (e.g., Heap et al 2013Heap et al , 2014. The microfracture-dominated samples tended to have a higher dry P-and S-wave velocities.…”

What Controls Sill Formation: An Overview From Analogue Models

“…Sill emplacement above previous sill may also occur (Rocchi et al, ; Westerman et al, ), as probably due to low rigidity and/or high‐viscosity contrasts due to advanced but not complete solidification of the older sill, or to the lateral feeding of the younger sills, without any interaction with the older magma. Rigidity inversions may be due to thermal weakening (i.e., decrease of rock rigidity; Heap et al, ), so that portions of the crust become more compliant at high depths, as geophysically detected in Afar (Desissa et al, ), Iceland (Mitchell et al, ; Schuler et al, ), and Canary Islands (Martí et al, ).…”