Professeur invité - Géosciences : Sergio Vinciguerra (INGV Rome)
Vendredi 1er juillet, 11h, Salle E314, 24 rue Lhomond 75005 Paris
Mt. Etna (Sicily, Italy) is the largest volcano in Europe, and one of the most active on Earth. Its volcanic edifice is 1200-km2-wide and 3300-m-high, made up of a pile of lava flows above a thick sedimentary sequence. In particular cycles of hazardous flank eruptions occurred in the recent years, preceded by intense seismic activity and high rates of ground deformation, eventually leading to abrupt opening of eruptive fracture systems in the eastern flank. Consequently, basaltic rocks of Mt. Etna volcano have been the subject of a number of experimental studies in recent years, with the aim to determine the mechanical parameters needed for ground deformation and seismicity modelling, as well as for the modelling of the weakening mechanisms destabilising the volcano eastern sector.
However, reconstructions of the sub-volcanic morphology show that only 373 km3 of the bulk total volume of 1400 km3 of the edifice at Mt. Etna and its substratum is comprised of volcanics. The rest comprise a laterally-extensive culmination of sedimentary rocks that reach a vertical thickness of about 2 km in the central portion of the edifice.
An increase in instability, promoted by the thermal weakening of the basement, can result in devastating and deadly flank collapse. A detailed understanding of the influence of high temperatures, as a result of magma intrusion for example, on the physical and chemical properties of representative rock that forms the sub-volcanic basement of active volcanoes is thus essential.
Here therefore we report results from an experimental study of the influence of temperature on the mechanical, physical and chemical properties of representative volcanic and sedimentary rocks that comprise the edifice of Mt. Etna, such marly limestone, pure and porous limestones (one of which contains dolomite) representative of the substratum sedimentary rocks at Mt. Etna heated at 760 and 800 °C and loaded to failure in a uniaxial apparatus ; seismic velocity of these rock samples were also recorded in-situ.
Back scattered SEM images, XRPD data and TG analyses reveal that this observed thermal weakening is the result of two thermo-chemical transformations : i) at temperatures below 600 °C, clay dehydroxylation causes the collapse of kaolinite crystal structure ; ii) at temperatures above 600 °C, calcite initiates its dissolution, releasing CO2 and forming new CaO-bearing minerals.
These debilitating chemical changes have a dramatic influence on the physical properties of the two investigated limestones. Porosity, dynamic Poisson’s ratio and the Vp/Vs ratio all show a substantial increase, whilst P- and S-wave velocities and dynamic Young’s modulus all show a substantial decrease. These changes in physical properties can explain (1) the increased edifice instability seen to follow magmatic events, (2) the anomalously low seismic velocity zone present in the sub-volcanic basement at Mt Etna, (3) the anomalously high CO2 degassing seen at large faults at Mt Etna volcano and the conflict between the calculated magma volume at depth (using CO2 emissions) and the volume of erupted magma, and (4) the anomalously high Vp/Vs ratios and the rapid migration of fluids.
