How caldera collapse shapes the shallow emplacement and transfer of magma in active volcanoes

Abstract

Calderas are topographic depressions formed by the collapse of a partly drained magma reservoir. At volcanic edifices with calderas, eruptive fissures can circumscribe the outer caldera rim, be oriented radially and/or align with the regional tectonic stress field. Constraining the mechanisms that govern this spatial arrangement is fundamental to understand the dynamics of shallow magma storage and transport and evaluate volcanic hazard. Here we show with numerical models that the previously unappreciated unloading effect of caldera formation may contribute significantly to the stress budget of a volcano. We first test this hypothesis against the ideal case of Fernandina, Gal ‡pagos, where previous models only partly explained the peculiar pattern of circumferential and radial eruptive fissures and the geometry of the intrusions determined by inverting the deformation data. We show that by taking into account the decompression due to the caldera formation, the modeled edifice stress field is consistent with all the observations. We then develop a general model for the stress state at volcanic edifices with ! 1 calderas based on the competition of caldera decompression, magma buoyancy forces and tectonic stresses. These factors control: 1) the shallow accumulation of magma in stacked sills, consistently with observations; 2) the conditions for the development of circumferential and/or radial eruptive fissures, as observed on active volcanoes. This top-down control exerted by changes in the distribution of mass at the surface allows better understanding of how shallow magma is transferred at active calderas, contributing to forecasting the location and type of opening fissures.

Historical Volcanism and the State of Stress in the East African Rift System

“…Multiple factors combining to generate such composite fields have been advocated and analyzed: loading due to the edifice (e.g., Dahm, 2000;Pinel and Jaupart, 2000;Maccaferri et al, 2011) and unloading (e.g., Maccaferri et al, 2014), the effects of volcano morphology (e.g., Tibaldi et al, 2014;Corbi et al, 2015), the generation of magma reservoirs and calderas (e.g., Tibaldi, 2015) and the anisotropy of host rocks (Gudmundsson, 2011a). Many dykes do not propagate all the way to the surface, but may be arrested by layers with variable associated stress (Gudmundsson and Philipp, 2006).…”

Orientation of the Eruption Fissures Controlled by a Shallow Magma Chamber in Miyakejima

“…Petrological evidences of the lavas support the tapping of magma from the shallow magma chamber (Chadwick et al, 2011). The path of the feeder dike followed the maximum compressive stress generated by the overpressurized sill-like chamber (Chadwick and Dieterich, 1995) and also the existence of caldera (Corbi et al, 2015) to form a circumferential fissure. Development of the circumferential inclined sheets in Fernandina is controlled by the shape of the shallow magma chamber (horizontal sill), whereas the vertical dike-like shape of the shallow chamber in Miyakejima forms preferred oriented vertical feeder dikes.…”

“…The propagation direction of dikes in a volcanic edifice can be also affected by the topographic contrast, especially in an asymmetric edifice (e.g., Tibaldi et al, 2014;Corbi et al, 2015;Rivalta et al, 2015 and references therein).The volcanic edifice of Miyakejima, including its submarine part, has a conical shape without any remarkable bulge along the rift zone ( Figure 1B). The development of the NE-SW rift zone became significant within the last 2800 years.…”

“…However, the distribution of dikes and eruption fissures in volcanoes display radial, circumferential, and curvature patterns reflecting the local stress in the volcanic edifice (e.g., Chadwick and Howard, 1991). Local disturbances in the stress field of a volcano are caused by the prominent topography of a volcanic edifice (e.g., Tibaldi et al, 2014), flank instability (e.g., Walter et al, 2005), unloading by collapse (Corbi et al, 2015), active faults near the volcanic system (e.g., Seebeck and Nicol, 2009), mechanical heterogeneity in the volcanic edifice and basement (e.g., Letourneur et al, 2008), and the magmatic activity within the volcano (e.g., Chadwick and Dieterich, 1995;Takada, 1997). Recent observations of the dike intrusion events reveal that the stress field in the host rock is affected by the emplacement of intrusions, and consequently feedbacks to the growth of the next dikes (e.g., Bagnardi et al, 2013;Falsaperla and Neri, 2015).…”

From magma ascent to ash generation: investigating volcanic conduit processes by integrating experiments, numerical modeling, and observations

“…Moreover, dike pathways through the crust may not be vertical and may not coincide with pre-existing weaknesses in the host rock as is often assumed . Dike pathways are often tortuous as evidenced by recent studies [Bagnardi et al 2013] and strongly influenced by the distribution of surface loads [Maccaferri et al 2014, Corbi et al 2015, layering [Rivalta et al 2005, Maccaferri et al 2010 and faulting [Passarelli et al 2015. In addition to this, there are problems with the terminology used to define magma chambers: there is confusion when we refer to what different techniques are detecting at different time scales.…”