Visiting Soufriere Hills Volcanic Conduit Before An Explosion (Montserrat, Lesser Indies)

At arc volcanoes, forecasting the brutal shift from effusive to explosive activity is challenging. Pre-explosive conduit conditions were retrieved from pumices and, together with numerical models, helped to constrain magma flow dynamics before the explosion.

According to a 2015 UN global assessment report on disaster risk reduction,1 “800 million people are currently living within 100 km of a volcano that could erupt.” Among volcanic hazards, the switch from an effusive, quiescent behaving eruption to a sudden explosive and devastating event can be extremely deadly. For example, in 1997, after a long period of effusive activity, the Soufrière Hills Volcano (Montserrat island, British Lesser Indies) produced 88 vulcanian explosions that forced the authorities to enlarge the evacuation perimeter and caused destruction to Plymouth.2 Another more recent example is the 2010 eruption of Merapi (Java, Indonesia), during which an explosive event caused hundreds of casualties.3 Improved understanding of the processes that drive such events is mandatory for enabling better monitoring and forecasting of volcanic explosions.

In their search for understanding, scientists are confronted by two main issues: (1) there is no direct access to the conduit conditions (e.g., porosity and pressure), which must be guessed from lava samples a posteriori analysis and eruption monitoring observations;  and (2) magma dynamics in the conduit, which control the eruptive style, are complex (Figure 1) and ruled by parameters, such as magma viscosity and porosity, that can vary by several orders of magnitude on a short length scale.

Figure 1 (diagram): Interactions between conduit conditions parameters and consequences for eruptive style at silicic volcanoes. The effect of some external factors that change boundary conditions were also indicated. Only the main interactions between porosity and other conduit parameters were represented here. The effects of viscosity (although important in the model), crystals and temperature (constant in the model), and other parameters such as more volatile species (e.g. CO2, Sulfides) were not represented in this diagram for readability and simplicity reasons. A →+ B: an increase of A causes an increase of B. A → B: an increase of A causes a decrease of B. Figure courtesy Laure Chevalier.

Among them, magma gas content was identified as a critical parameter for understanding volcanic erupting style variations. Indeed, when magma rises in the conduit, the gas, dissolved at depth, exsolves and expands due to lower pressures. If it remains within the magma, the resulting high gas content causes the magma to fragment in an explosive eruptive style. If, however, the magma loses gas through permeable outflow, it loses explosiveness and may erupt effusively, forming lava flows and domes. External factors, such as magma chamber recharge and dome collapse, add complexity to this scenario by driving transient conditions.

Numerical modeling of magma flow in the volcanic conduit is a powerful tool for scientists to understand the controls on eruptive activity and, from constraining field observations and lava analysis, interpret possible scenarios of conduit conditions’ evolution during an eruption. The 2010 eruption of Soufrière Hills led to a noticeable explosion on the 11th of February. This explosion was triggered by a sudden decompression due to dome collapse at the top of the conduit, possibly preserving information on the preceding effusive phase conduit conditions. This gave the scientists a unique opportunity to better characterize conduit conditions during an effusive regime.

Researchers4 first estimated some conduit parameters from the analysis of pumices, i.e. samples from the pre-explosive magma column that were expelled from the conduit during the explosive event. On each sample, glass (i.e. not crystalized magma) water content, a good indicator of magma pressure, was measured using an elementary analyzer. The porosity was also measured with Scanning Electron Microscopy and 3D imaging of the sampled pumices. From these constraining parameters, conduit pressure5 and depth6 retrieval models were used to estimate pre-explosive porosity, pressure, depth, and water solubility conditions in the conduit. Results show that prior to the explosion, the magmatic column was extensively degassed (few bubbles = low porosity) and that the explosion ejected samples from the magma column down to at least 3km in the conduit (Figure 2).

Figure 2 – Top: results of the two retrievering models giving pre-explosive pressures, depths, and porosities. According to magma fragmentation studies8, the explosive threshold is reached when porosity is over 80%. Our results show a very low porosity prior to the explosion, thus underlying the fact that the magma was strongly outgassed at that moment. On the bottom: a sketch of the Montserrat plumbing system modified after Wadge et al. (2014)7. The black rectangle represents the part of the magmatic system affected by our data. The magmatic chambers are magma storage places while dykes or conduit stands for fractures in the basement through which the magma may move to the surface. The difference between dykes and the conduit is the geometry of the fractures that are linear for the first one and star-shaped for the second (on a horizontal cross-section). Figures courtesy Tonin Bechon.

A 1D numerical conduit flow model was then used to constrain magma dynamics in the conduit. In this model, magma flow conditions in the conduit, as well as eruptive style, depend on a few input parameters, including magma incoming rate at the conduit base, conduit radius, and magma permeability dependence on porosity. These parameters were varied and fitted to retrieve conduit conditions (porosity, pressure, extrusion rate) close to the observations.

Two possible scenarios resulted from this study (Figure 3): in the first one, magma influx was steady and high just before the explosion, in agreement with magma extrusion rate observations,7 and magma permeability was high even at low porosities. The weakness of this scenario resides in the difficulty to get high permeabilities at low porosities deep in the conduit. The second scenario rather supports the presence of unsteady conditions and the alternation of high magma velocities periods, with little degassing, and magma stalling during which sustained degassing occurred. This second scenario is supported by the observation of rapidly changing eruptive activity and cyclicity behaviors at Soufrière Hills Volcano.7

Figure 3 (scenarios): Sketches of two possible scenarios for the pre-explosive conduit dynamics. Red and blue arrows represent magma flux and gas flux to the country rock respectively. Bigger arrows mean higher flux; the scale is arbitrary. Figure courtesy Laure Chevalier.

In a nutshell, Burgisser et al (2019)4 retrieved pre-explosive natural conditions (Pressure, depth, porosity) for the explosion of the 2/11/2010 at Soufriere Hills (Montserrat, Lesser Indies). Confronting these values with conduit flow numerical models allowed to explain observed outcomes by two different hypotheses.

Overall, this study supports the importance of understanding how the ascending magma loses gas in order to improve transient activity modeling. Researchers’ future work aims to develop along two different axes: (1) exploring new technological development to better constrain pre-explosive natural conditions; and (2) improving magma gas loss modeling by taking into account greater complexity like magma flow shearing effects on bubble deformation and permeability evolution.

These findings are described in the article entitled Conduit processes during the February 11, 2010 Vulcanian eruption of Soufrière Hills, Montserrat, recently published in the Journal of Volcanology and Geothermal Research.


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