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

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.