Magma accumulating in a shallow subsurface reservoir tends to have a lower density than the crustal rocks within which the reservoir is embedded. Intuitively, then, buoyant stresses derived from this rock-magma density difference should contribute to how a reservoir evolves, and many volcanologists argue that magma buoyancy is a critical driver of magma reservoir rupture that leads to eruption. Quite recently, two increasingly cited 2014 papers published in Nature Geoscience continued this trend by contending that rock-magma density differences in fact produce sufficient buoyancy to destabilize a massive magma reservoir, leading directly to rupture that can yield a supereruption.
Magma buoyancy, of course, is just one of many stresses that will interact to dictate the stability of a magma reservoir, and it is important to examine these stress interactions carefully when characterizing reservoir rupture and the possibility that an eruption will occur. In a 2007 paper published in the Journal of Volcanology and Geothermal Research, for example, I used elastic finite element models to show that rock-magma density differences, relative to the other factors involved, play almost no role in the rupture of a small magma reservoir, i.e. one typical of what is found beneath a shield or composite volcano (Figure 1).
![Fig 1: For host rock of density 2600 kg/m^3, demonstration that rupture location (small arrows) is invariant for magma densities ranging from 1000-3400 kg/m^3 [for full details, see Figure 6 in Grosfils (2007)]](https://research.pomona.edu/eric-grosfils/files/2015/11/2007Fig-321x600.jpg)
Fig 1: For host rock of density 2600 kg/m^3, rupture location (at small arrow locations in (a), at depth h=0 for all cases in (b)) is invariant for magma densities ranging from 1000-3400 kg/m^3 [for full details, see Figure 6 in Grosfils (2007)].
![Fig 2: when buoyancy is treated without correctly integrating it into the complex suite of stresses in the host-reservoir system, significant deformation and faulting is predicted (a), but modeling buoyant stresses as an integral part of the system reveals that it has minimal impact (b). [For full details, see Figure 5 of Gregg et al. (2015)]](https://research.pomona.edu/eric-grosfils/files/2015/11/2015Fig.jpg)
Fig 2: When buoyancy is treated without correctly integrating it into the complex suite of stresses in the host-reservoir system, significant deformation and faulting is predicted (a), but modeling buoyant stresses as an integral part of the system reveals that it has minimal impact (b) [for full details, see Figure 5 of Gregg et al. (2015)].
![Fig 3: Viscoelastic model for incremental reservoir pressurization and fault evolution of a supereruption-sized magma reservoir system. [See Figure 10 of Gregg et al. (2012) for full details]](https://research.pomona.edu/eric-grosfils/files/2015/11/2012Fig10-600x344.jpg)
Fig 3: Cartoon of viscoelastic model results depicting incremental reservoir pressurization and fault evolution for a supereruption-sized magma reservoir system [for more details see Figure 10 of Gregg et al. (2012)].
For more information see:
- Grosfils, E.B., Magma reservoir failure on the terrestrial planets: Assessing the importance of gravitational loading in simple elastic models, Journal of Volcanology and Geothermal Research, 166, 47-75, 2007.
- Gregg, P.M., S.L. de Silva, E.B. Grosfils, and J.P. Parmigiani, Catastrophic caldera-forming eruptions: Thermomechanics and implications for eruption triggering and maximum caldera dimensions on Earth, Journal of Volcanology and Geothermal Research, 241-242, 1-12, 2012.
- Gregg, P.M., E.B. Grosfils, and S.L. de Silva, Catastrophic caldera-forming eruptions II: The subordinate role of magma buoyancy as an eruption trigger, Journal of Volcanology and Geothermal Research, 305, 100-113, 2015.