The explosivity of volcanic eruptions: Quantifying the critical role of permeability in magmas

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Volcanic eruptions are one of the most powerful and spectacular natural phenomena and even relatively small eruptions can have major social, environmental and economic impacts, on a global scale. Around 9% of the world's population are directly affected by volcanic eruptions and so it is a primary goal of volcanology to be able to forecast the style of volcanic eruptions so that vulnerable communities can be safely evacuated. Yet, in times of volcanic unrest, we still cannot say with any certainty what type of eruption will occur and therefore how hazardous it might be.As magma rises inside the volcano, fluids that are dissolved in the magma begin to form bubbles. Eruption explosivity is principally controlled by the ability of these fluids to escape from magma. We know that a primary control of the movement of fluids in magma is its permeability. Permeability is defined as the ease at which fluids can move through a medium and, is controlled by the physical properties of the magma (chemical composition, crystal content and porosity) and, the environmental conditions and processes occurring in the conduit that modify these properties. Two models of fluid movement are generally considered, one related to the fracture of magma and a second related to the formation of bubble chains. However, these models are not quantified or verified.During times of volcanic unrest, internal conduit processes produce geophysical signals, such as seismicity. Understanding the relationship between geophysical signals and the processes that generate them can help us to produce more robust eruption forecasts. A certain type of seismicity that we frequently register at volcanoes are proposed to be generated by the movement of fluids. These are called low frequency (LF) seismic events. However, there are multiple models for the source processes of LF seismicity, hindering our capacity to interpret them.As we cannot visit a volcanic conduit, we use laboratory experiments with natural rock samples to simulate what is happening inside a volcano and to produce analogue geophysical data (acoustic emission, AE, are the laboratory analogue of natural seismicity) to better interpret these processes. However, the environmental conditions in volcanic conduits are extreme and unfortunately the majority of published experimental studies do not fully replicate these conditions (temperature, pressure, fluids and stress) simultaneously. This is due to the difficulties of high-temperature, high-pressure experiments and the limitations of current equipment designs. Therefore, we still do not understand how fluids move through magma or what specific processes generate types of LF seismicity.Using unique, high-temperature, triaxial deformation experiments; the first comprehensive study of the permeability of natural magma samples, under simulated volcanic conditions of temperature, pressure and stress will be performed. Within these experiments, deformation processes common to volcanic conduits and hypothesised to have significant effect on permeability will be replicated, using an applied axial load. AE will be monitored throughout all experiments using a newly designed sensor array that will allow high spatial-resolution AE measurements to be made at magmatic temperatures. Understanding how fluids move in magma and, the generation of laboratory analogue geophysical data will open a window into the volcanic system. Key processes that control the movement of fluids in magmas and the geophysical signals produced will be identified. These unique experimental data will link the processes dictating permeability evolution to the explosive potential of a volcanic system and more accurately define the source processes that generate LF seismicity, underpinning a new generation of models to forecast the state of the volcanic systems and potential future eruptive activity.