Basaltic calderas: Collapse dynamics, edifice deformation, and variations of magma withdrawal

Abstract

[1] The incremental caldera collapses of Fernandina (1968), Miyakejima (2000), and Piton de la Fournaise (2007) are analyzed in order to understand the collapse dynamics in basaltic setting and the associated edifice deformation. For each caldera, the collapse dynamics is assessed through the evolution of the (1) time interval T between two successive collapse increments, (2) amount of vertical displacement during each collapse increment, and (3) magma outflow rate during the whole collapse caldera process. We show from the evolution of T that Piton de la Fournaise and Fernandina were characterized by a similar collapse dynamics, despite large differences in the caldera geometry and the duration of the whole collapse caldera process. This evolution significantly differs from that of Miyakejima where T strongly fluctuated throughout the whole collapse process. Quantification of the piston vertical displacements enables us to determine the magma outflow rates between each collapse increment. Displacement data (tiltmeter and/or GPS) for Piton de la Fournaise and Miyakejima are used to constrain the edifice overall deformation and the edifice deformation rates. These data reveal that both volcanoes experienced edifice inflation once the piston collapsed into the magma chamber. Such a deformation, which lasts during the first collapse increments only, is interpreted as the result of larger volume of piston intruded in the magma chamber than magma withdrawn before each collapse increment. Once the effect of the collapsing rock column vanishes, edifice deflates. We also determine for each caldera the critical amount of magma evacuated before collapse initiation and compare it to analog models. The significant differences between models and nature are explained by the occurrence of preexisting weak zones in nature, i.e., the ring faults, that are not taken into account in analog models. Finally, we show that T at Piton de la Fournaise and Fernandina was equally controlled by the frictional resistance along the ring faults and the magma outflow rate. In addition to these two parameters, the collapse dynamics of Miyakejima was also influenced by variations of the magma bulk modulus, which changed after the influx of deep gas-rich magma into the collapse-related magma chamber. Altogether, our results show that the dynamics of caldera collapse in basaltic volcanoes proceeds in two phases: Phase 1, starting with the first collapse, is characterized by the largest collapse amplitude, an incremental edifice inflation, and a step-by-step increase of the rate of magma outflow. Phase 2 shows a rapid decrease of the magma discharge rate to a low level concomitant with the continuous edifice deflation. If deep magma is injected into the magma chamber, as at Miyakejima, an additional phase occurs (phase 3).Citation: Michon, L., F. Massin, V. Famin, V. Ferrazzini, and G. Roult (2011), Basaltic calderas: Collapse dynamics, edifice deformation, and variations of magma withdrawal,

Stress evolution during caldera collapse

“…7a). In combination with other factors, such as low magma compressibility or pre-existing ring faults, this effect of host rock modulus may in part explain why some critical depletion levels estimated in nature (Michon et al, 2011) are much less than those predicted by past analogue models (Geyer et al, 2006) or analytical studies (Roche and Druitt, 2001).…”

“…Answers to these questions inform the interpretation of geodetic, seismic and other geophysical data collected before, during and after a collapse event (Ekstrom, 1994;Fichtner and Tkalcic, 2010;Massin et al, 2011;Michon et al, 2011;Shuler et al, 2013). They are hence important for deformation monitoring and hazard assessment at not only volcanoes, but also other natural or man-made instances of depletion-induced subsidence (Cesca et al, 2011;Dahm et al, 2011;Lenhardt and Pascher, 1996;Segall, 1989).…”

Laboratory Modelling of Volcano Plumbing Systems: A Review

“…Crumpler et al 1996;Geyer and Martí 2008;Hansen and Olive 2010). Both ancient geological evidence and recent observations from active volcanoes indicate that calderas form through subsidence of the roof of a sub-surface magma body, as a consequence of magma withdrawal from that body (Fouqué 1879;Verbeek 1884;Clough et al 1909;Williams 1941;Smith and Bailey 1968;Druitt and Sparks 1984;Lipman 1984Lipman , 1997McBirney 1990;Geshi et al 2002;Cole et al 2005;Michon et al 2011). Subsidence usually occurs along a ring fault; magma may intrude this fault to form a ring dyke and may ultimately erupt from it.…”

“…Gas exsolution during magma intrusion and eruption may strongly affect collapse dynamics, especially with more evolved magma compositions (Stix and Kobayashi 2008;Michon et al 2011). Such effects remain to be examined experimentally.…”

Great challenges in volcanology: how does the volcano factory work?

“…Caldera collapse occurs under specific eruptive conditions and, in the last decades, it has occurred a few times only (Geyer and Martì, 2008;Stix and Kobayashi, 2008;Michon et al, 2011). While caldera collapse is relatively infrequent, it is still difficult to forecast it, as there is uncertainty on its triggering causes and the proper detection of these causes at the surface: in fact, caldera collapse may be triggered by both overpressure or underpressure conditions within a magma chamber, including lateral intrusion of magma, giving the system a wide spectrum of dynamic variability (e.g., Gudmundsson, 1998;Martì et al, 2009).…”