Mobilising magma in the largest eruptions: Quantifying critical processes using in situ real time x-ray tomography

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Volcanic eruptions are one the most powerful and impressive natural phenomena, and even relatively small eruptions can have major global impacts. The magma stored beneath volcanoes is an evolving mixture of molten rock (liquid), crystals (solid) and bubbles (gas). As magma cools the number of crystals increases and in principle, when magma reaches ~45% crystals the crystals jam together, 'locking up' and making it too stiff to move: the magma becomes 'uneruptible'. However, some of the most devastating explosive eruptions (including the largest super-eruption ever known) erupt large volumes (100-5000 km3) of this 'uneruptible' crystal-rich (45-60%) magma. So how do these crystal-rich eruptions happen? What lets the magma move?As we cannot visit a magma chamber, laboratory experiments with natural rock samples and synthetic approximations (analogues) are used to simulate what is happening beneath the volcano. From these experiments, we have developed models that describe how crystal-poor magma will flow when a force is applied (its rheology). However, these rheological models fail for more crystal-rich magma (concentrated suspensions). It is thought that in crystal-rich systems the magmas ability to move is critically controlled by the crystal-crystal, crystal-bubble and bubble-bubble interactions, and the variable spatial distribution of the crystals, bubbles and melt within the sample. In one hypothesis a build-up of pressure drives bubbles through the crystal network, and causes the network to break into pieces. Despite still having the same high crystal content, deformation can then occur in the crystal-poor regions between the pieces, and the magma becomes mobile. Crystal-rich magmas and their analogues are opaque, and conventional experimental methods do not allow us to observe the internal micro-scale processes. Therefore we have only been able to quantify the average behaviour of a volume of magma. While many possible microstructural interaction processes have been hypothesised, they remain untested. In this project the equipment used for conventional rheological experiments will be modified to allow the collection of 3D images in real time using X-ray computed Micro-Tomography (XMT). At the Diamond Light Source synchrotron facility this revolutionising imaging technology can capture the 3D internal structure of a sample (i.e. the distribution of crystals, bubbles and melt in a magma) in as little as a few seconds: producing a 3D 'movie' of what happens when the magma is deformed. By applying standard image analysis techniques to the 3D images captured over the course of an experiment, the distribution of bubbles, crystals, and melt can be quantified; every crystal and bubble can be tracked through time; and the nature of every interaction can be identified. For the first time we will be able to see what is happening inside the magma in 4D (3D + time).By working with analogue materials, and systematically testing the microstructural behaviour as we change the crystal content, crystal shape, bubble volume and a range of other parameters known to vary in magma chambers (e.g. temperature, pressure) the high speed 4D data will be used to map out the nature and importance of the different interactions, and define the role of micro-scale variability (phase distributions and interactions) on flow. These data will be used to build a new generation of rheological models that describe the mobility of complex two- and three-phase concentrated magmatic suspensions based on an accurate understanding of the microstructural physics and micro-scale variability. By running 4D experiments on natural samples and testing the model against the results, the project will identify the conditions under which crystal-rich magmas can erupt, and begin to identify the magmatic processes that lead to the most devastating eruptions.