Abstract
Many calderas show remarkable unrests, which often do not culminate in eruptions (non-eruptive unrest). In this context the interpretation of the geophysical data collected by the monitoring networks is difficult. When the unrest is eruptive, a vent opening process occurs, which leads to an eruption. In volcanic calderas, vent locations typically are scattered over a large area and monogenic cones form. The resulting pattern is characterized by a wide dispersion of eruptive vents, therefore, the location of the future vent in calderas is not easily predictable. We propose an interpretation of the deformation associated to unrest and vent pattern commonly observed at volcanic calderas, based on a physical model that simulates the intrusion and the expansion of a sill. The model can explain both the uplift and the subsequent subsidence, through a single geological process. In particular, we simulate the vertical displacement that occurred at the central area of Campi Flegrei caldera during the last decades, and we obtain good agreement with the data of a leveling benchmark near the center of the caldera. Considering that the stress mainly controls the vent opening process, we try to gain insight on the vent opening in calderas through the study of the stress field produced by the intrusion of an expanding sill. We find that the tensile stress in the rock above the sill is concentrated in a ring-shaped area with radius depending on the physical properties of magma and rock, the feeding rate and the magma cooling rate. This stress field is consistent with widely dispersed eruptive vents and monogenic cone formation, which are often observed in the calderas. However, considering the mechanical properties of the elastic plate and the rheology of magma, we show that remarkable deformations may be associated with low values of stress in the rock at the top of the intrusion, thereby resulting in non-eruptive unrests. Moreover, we have found that, under the assumption of isothermal conditions, the stress values decrease over time during the intrusion process. This result may explain why the long-term unrests, in general, do not culminate in an eruption.
Highlights
Calderas typically show strong ground deformation, with uplift episodes followed by subsidence, remarkable geothermal activity and seismicity mainly characterized by earthquake swarms
Examples are the non-eruptive unrest at Campi Flegrei caldera (Italy) during 1968–1970 and 1982–1984 (Del Gaudio et al, 2010; D’Auria et al, 2011), Long Valley (California) during 1978–2000 (Hill, 2006), Rabaul (Papua New Guinea) in 1984 (McKee et al, 1984), Sierra Negra (Galápagos Islands, Ecuador) in 1996 (Chadwick et al, 2006), Yellowstone (USA) between 2004 and 2010 (Chang et al, 2010), Santorini (Greece) in 2011–2012 (Newman et al, 2012; Lagios et al, 2013), which were accompanied by significant variations in the monitoring parameters but were not followed by an eruption (Segall, 2013; Acocella et al, 2015)
The model is basically governed by the feeding rate, the rheology of magma and the mechanical properties of the rock above the intrusion (Bunger and Cruden, 2011; Michaut, 2011; Macedonio et al, 2014; Giudicepietro et al, 2016), we do not exclude that magma degassing can contribute to the deformation processes, especially in felsic calderas
Summary
Calderas typically show strong ground deformation, with uplift episodes followed by subsidence, remarkable geothermal activity and seismicity mainly characterized by earthquake swarms. This behavior stimulated a long debate if the deformation is due to hydrothermal or magma migration (see e.g., Lundgren et al, 2001; Gottsmann et al, 2006; Trasatti et al, 2008). New technology allowed an improvement in volcano monitoring and produced valuable data set of observations and geophysical measurements This improvement of volcano monitoring allowed us to increase our knowledge upon the calderas behavior (Newhall and Dzurisin, 1988; Geyer and Martí, 2008; Sobradelo et al, 2010; Acocella et al, 2015). The model is basically governed by the feeding rate, the rheology of magma and the mechanical properties of the rock above the intrusion (Bunger and Cruden, 2011; Michaut, 2011; Macedonio et al, 2014; Giudicepietro et al, 2016), we do not exclude that magma degassing can contribute to the deformation processes, especially in felsic calderas
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