Abstract
Piton de la Fournaise (PdF) is a basaltic volcano whose activity is characterized by effusive to mildly explosive eruptions. The geologic record preserves evidence of explosive eruptions associated with the seaward sliding of the steep east flank. Such eruptions formed calderas that are several km in diameter and their products have been emplaced as breccias. The breccias of PdF offer the opportunity to sample a wide range of different lithologies covering most of the stratigraphy of the edifice. Petrophysical measurements revealed a corresponding variability in density, porosity, P-wave velocity, and uniaxial compressive strength, confirming the petrophysical consequences of the lithological diversity. Different lithologies cannot simply be distinguished on the basis of their physical properties. We infer that volcano instability should not be interpreted solely in terms of altered rock units. The large heterogeneity of crustal rocks must be considered when interpreting monitoring data and assessing hazards related to volcano stability.
Highlights
Caldera collapses and flank failures punctuate the history of volcanoes worldwide and represent major highly hazardous events in their evolution [Poland et al 2017]
All vesicular lavas we studied are characterised by a uniaxial compressive strength (UCS) of 30–120 MPa, a stress range that overlaps with the measured entrapment pressure of most melt and fluid inclusions hosted in olivine crystals of Piton de la Fournaise (PdF) magmas
Our new petrophysical data show that, in spite of the little changing bulk composition, such volcanoes can be characterised by rocks with highly variable rock physical and mechanical properties, a factor that could play a major role in their destabilisation
Summary
Caldera collapses and flank failures punctuate the history of volcanoes worldwide and represent major highly hazardous events in their evolution [Poland et al 2017]. Caldera and pit collapse events are usually related to magma withdrawal from the shallow part of the plumbing system during flank eruptions or lateral intrusion. The nature of magma storage beneath most calderas is subject to significant speculation as storage zones can be complex and span a large range of depths [Kennedy et al 2018, and references therein]. Volcano stability and shape evolution are strongly dependent on the rheology of the deforming crust. Volcanoes can undergo slow deformation and spreading over extended time periods, punctuated by events of rapid slumping and flank failure [De Vries and Francis 1997]. Instability factors include slope steepening and overloading, peripheral erosion, hydrothermal alteration, and rainfall infiltration and, next to magma emplacement, they can be triggered by hydrothermal pressurisation and large earthquakes (for reviews of processes and trigger mechanisms, see McGuire [1996] and Borgia et al [2000])
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