Petrophysical characterisation of volcanic ejecta to constrain subsurface lithological heterogeneities: implications for edifice stability at basaltic volcanoes

Abstract

Piton de la Fournaise (PdF) is an active and dominantly effusive basaltic volcano. The geologic record preserves evidence of rare explosive eruptions, associated with the seaward sliding of the steep east flank and the collapse of large calderas emplacing lithic-rich breccias. Breccias of PdF offer the opportunity to sample a wide range of crustal lithologies. Unaltered blocks are representative of plutonic (gabbros), sub-volcanic (dolerites emplaced in sills and dykes), and volcanic (lavas) units. We document a large variability in density, porosity, P-wave velocity (dry and wet), and uniaxial compressive strength (UCS). The large variation in P-wave velocity and UCS results from the wide ranges in texture and lithology. Notably, some of the dense gabbroic units having experienced several cycles of natural reheating are comparatively weak. We infer that volcano instability should not be interpreted solely in terms of altered rock units. Rather, the large petrophysical heterogeneity of crustal rocks at PdF, and likely at many other basaltic volcanoes, must be considered when interpreting monitoring data and assessing volcano stability. RésuméLe Piton de la Fournaise (PdF) est un volcan basaltique actif et à dominante effusive. Les archives géologiques conservent des preuves d'éruptions explosives rares, associées au glissement vers la mer du flanc est escarpé et à l'effondrement de grandes calderas produisant des brèches riches en lithiques. Les brèches de PdF offrent l'opportunité d'échantillonner une large gamme de lithologies crustales. Les blocs non modifiés sont représentatifs des unités plutoniques (gabbros), subvolcaniques (dolérites placées dans sills et les dykes) et volcaniques (laves). Nous documentons une grande variabilité de la densité, de la porosité, de la vitesse de l'onde P (sèche et humide) et de la résistance à la compression uniaxiale (UCS). La grande variation de la vitesse de l'onde P et du UCS résulte des larges gammes de texture et de lithologie. Notamment, certaines des unités gabbroïques denses ayant subi plusieurs cycles de réchauffement naturel sont relativement faibles. Nous en déduisons que l'instabilité du volcan ne doit pas être interprétée uniquement en termes d'unités de roches altérées. Au contraire, la grande hétérogénéité pétrophysique des roches crustales à PdF, et probablement dans de nombreux autres volcans basaltiques, doit être prise en compte lors de l'interprétation des données de surveillance et de l'évaluation de la stabilité du volcan.

The 1965 Mahavel Landslide (Réunion Island, Indian Ocean): Morphology, Volumes, Flow Dynamics, and Causes of a Rock Avalanche in Tropical Setting

“…As mentioned above, the collapsed material was made of 'a’ā lava flows and scoria deposits. The volcanic rock vesicularity significantly varies with the type of formations from 45% to 60% for scoria to around 25% for the lava flow massive core (Bato et al., 2016; Di Muro et al., 2014, 2021; Harris et al., 2017). Since the geological formations involved in the scarp collapse are dominantly scoria layers, an average vesicularity of around 35%–45% is a reasonable estimate for the total volume of rock.…”

Geochemistry and Petrology of the Bellecombe Lava Sequence, Enclos Fouqué Caldera, Piton de la Fournaise Volcano (Réunion, France)

“…This approach allowed for a quantitative evaluation of the bubble abundance and morphology in three dimensions. Vesicles are present in all the analyzed samples in amounts from 2.04 to 38.34 vol.%, a common range for effusive products of Piton de la Fournaise [58], and with a number density (number of vesicles per unit volume) from 21 to 179 #/mm 3 (Table 3) that matches that of recent lavas emitted at the volcano [59,60]. The results of the 3D analysis of bubble amounts are in agreement with those obtained by thin section analysis (cfr.…”

Geothermal potential of small sub-volcanic intrusions in a typical Icelandic caldera setting

“…Each of these rock types is known to have a wide range of physical and mechanical properties [e.g. Schaefer et al 2015;Eggertsson et al 2020;Heap et al 2020a;Weaver et al 2020;Di Muro et al 2021;Heap and Violay 2021], due to composition, eruption type and style, and alteration, amongst other factors [as discussed by Heap and Violay 2021]. It is also of note that the response of volcanic rocks to thermal stimulation is complex and temperature-dependent [Mordensky et al 2019;Kennedy et al 2022].…”