Weakening of the volcanic edifice before an eruption: consequence for the magma pressure and flow

What is it about?

Eruptions in basaltic volcanoes are often preceded by increasing seismicity and surface deformation. In this article we study the processes at work in the volcanic edifice rock mass. A rock mass, deformed and fractured, weakens: this process is called damage. Rock elastic characteristics decrease with damage. The way damage evolves under the effect of magma pressure contributes to determine the time evolution of the pre‐eruptive deformation. We show that continuity, a measure of damage, is a decreasing exponential of the cumulative number of earthquakes; this relationship is comparable to the definition of Boltzmann entropy. A model of initially elastic edifice, damaged under the pressure of a viscous, incompressible, magma in a reservoir has been developed. It is shown that, when fractures interact, continuity and magma pressure strongly drops, magma flux increases and the power of the magma‐edifice interaction reaches a maximum before the eruption. A relation between the time of this maximum and the eruption time has been established and checked using 24 summit eruptions at Piton de la Fournaise; it provided an estimation within 10% of the eruption time in 60%–75% of the cases. Checking relations of this type with very numerous eruptions on basaltic volcanoes will allow knowing its reliability.

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Photo by Askara Svarga on Unsplash

Photo by Askara Svarga on Unsplash

Why is it important?

Understanding pre-eruptive deformation and seismicity on basaltic volcanoes requires understanding the coupling between magma and volcanic edifices. In this article, we are specifically interested in studying the volcanic edifice rock mass and its evolution with deformation. The volcanic edifice shows extensive cracks on its surface; the deformations it undergoes cannot therefore be considered as small, and the volcanic edifice cannot simply be regarded as linearly elastic. When deformation is slow (quasi-static), the edifice and the magma are close to equilibrium. In this case, the pressure in the magma reservoir evolves as the strength of the edifice. If we wish to understand the deformations and the temporal evolution of the magma pressure in the reservoir, it is therefore important to correctly model the evolution of the strength of the edifice as a function of its deformation. One important point is that when the deformation becomes too large, the edifice is no longer elastic, its strength passes its peak and decreases with deformation. In this case, the magma pressure may decrease, but it is controlled by the flow rate of the supply system. The rate of deformation is therefore controlled by the weakening of the edifice and the magma flow rate. When the weakening becomes too strong, the deformation of the edifice accelerates and a dyke forms.

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