Summary

The dynamics of magma fragmentation is a controlling factor in the behavior of explosive volcanic eruptions. Fragmentation changes the eruption dynamics from a system of bubbly flow to one of gas-particle flow. To date, the influence of the fragmentation process itself on the eruption dynamics has been largely neglected in eruption models. This is understandable, as the explosive expansion of mixtures of pressurized gases and pyroclasts in volcanic eruptions is a complex process that cannot be studied directly. The dynamics of the gas-particle mixture resulting after magma fragmentation in volcanic eruptions was experimentally investigated in a shock-tube apparatus. We performed fragmentation experiments with natural volcanic samples with diverse porosities (10 – 67 vol. %), different applied pressures (4-20 MPa) and distinct temperatures (room temperature and 850°C). Two different types of experiments were performed. In the first type, we measured the ejection velocity of a plate placed loosely on top of a volcanic sample in order to account for the ejection of the caprock in Vulcanian eruptions. In the second type we simultaneously measured the fragmentation speed and the ejection velocity of the gas-particle mixture in the absence of a plate. In both cases the results are in good agreement with a general model for Vulcanian eruptions based on 1-D shock-tube theory, including magma fragmentation, and considering the specific conditions of each experiment.

Our results show that the fragmentation process plays an important role in the dynamics of the gas-particle mixture. The reasons include the following: 1) the energy consumed by fragmentation reduces the energy available to accelerate the gas-particle mixture; 2) the fragmentation speed controls the pressure available for the ejection of the gas-particle mixture which in turn determines the velocity, density and mass discharge rate; 3) the grain-size distribution produced during fragmentation controls the mechanical and thermal coupling between the gas phase and the particles; 4) the fragmentation process may produce heterogeneities in the concentration of particles. In volcanic eruptions all these factors can affect the eruption dynamics significantly.

The model presented herein is consistent with the experimental results and is capable of describing the dynamics of brittle fragmentation in Vulcanian eruptions and yielding more realistic initial pressures at the onset of fragmentation than previous models. We applied this model to recent Vulcanian eruptions of Popocatépetl and Colima volcanoes (Mexico) and estimated the initial gas pressure required to disrupt the caprock, fragment the underlying magma and eject ballistic projectiles to the observed distances. Further, the model is used in concert with a ballistic model to relate initial pressure and gas content with ballistic range. This coupled model was calibrated and validated with field and video observations of ballistics ejected during different Vulcanian eruptions at Popocatépetl Volcano. The model relates the zones which could be affected by the impact of ballistic projectiles to the initial pressure that can be estimated from seismic and geophysical monitoring, providing valuable information for more refined short-term hazard assessment at active explosive volcanoes.

Finally, a general methodology to delimit the zones that can be affected by ballistic projectiles is presented and applied to Popocatépetl Volcano. Three explosive scenarios with different intensities have been defined according to the past activity of the volcano and parameterized considering the maximum kinetic energy associated with ballistic projectiles ejected during previous eruptions. For each explosive scenario, the ballistic model is used to calculate the maximum range of the projectiles considering the optimum launch conditions. Our results are presented in a ballistic hazard map with topographic profiles that depict the likely