Pressure conditions in coupled magma bodies and their evolution during eruptions and caldera collapse: Piton de la Fournaise 2007

Understanding the interplay between pressure evolution in magma bodies in volcano roots and caldera collapses is important to for the general understanding of volcanoes and how calderas form. We use lessons-learned regarding caldera collapse dynamics and inferred 2014-2015 pressure evolution in a magma body under the Bardarbunga caldera, Iceland, to revisit the dynamics of the 2007 caldera collapse at Piton de La Fournaise volcano, La Reunion, in a project supported by EUROVOLC trans-national access. At Piton de la Fournaise, (rising to 2632 m above sea leve) a shallow and small magma body (close to sea-level; volume less than one cubic kilometer) played a central role. The overpressure compared to lithostatic prior to collapse is inferred to have been small (< 5 MPa), based on models of driving pressure for minor eruptions that occurred on 18-19 February and 30 March prior to the caldera forming lateral flank eruption that occurred 2 April – 1 May, 2007. The site of the lateral flank eruption occurred at an elevation of 500 m, much lower than the summit.  This elevation difference is inferred to a key factor for creating high driving pressure for magma flow. We infer that rapid flow of magma led to fast drop in pressure in a shallow magma body under the caldera, triggering inflow of magma from a deeper magma body under Piton de la Fournaise, that was in important element of the 2007 eruptive activity.  This deep inflow did, however, not sustain enough the pressure in the shallow magma body during the eruption, causing development of significant under-pressure leading to the collapse.