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E315: Mixing and eruption of crystal laden bubbly magmas in the shallow crust (Lead Supervisor: Andy Woods, BP Institute and Department of Earth Sciences)

Supervisors: Andy Woods (BP Institute and Earth Sciences) and Marie Edmonds (Earth Sciences)

Importance of the area of research:

It has become clear that in many volcanic systems, there may be a number of interconnected sills at different depths in the crust, and as magmas rise through the shallow crust, they can migrate from one sill to the next. On entering a sill, new magma will interact with the more evolved magma already present, exchanging heat, and leading to crystallisation and the release of volatiles. This can in turn cause the density of the magmas to evolve, leading to convective mixing and formation of hybrid magma, or the formation of inclusions of one magma in the other. As the hybrid magma rises through multiple sills, continued mixing and intermingling of the magmas, will lead to progressively more evolved magmas. It has also become clear that some of these magmas are highly crystalline, and the way in which this affects the mixing and ascent of the magma is an emerging area of research. The fluid dynamics of the ascent of highly crystalline bubbly magma through multiple sills will be explored in this PhD project, drawing from the extensive data base of Soufriere Hills Volcano, Montserrat. Here evidence from the evolving gas chemistry and the increasing presence of mafic inclusions in the erupting magmas, coupled with geophysical evidence of the successive inflation-deflation cycles of the volcano, provides constraints on this progressive evolution of the magma.

Project summary:

This PhD project will develop a series of new models to help interpret the evolution and mixing of magmas as they rise through the shallow crust, passing through multiple sills in which they may interact with more evolved magmas. A major and emerging question about these processes concerns the role of crystals in controlling the rheology and associated fluid dynamical behaviour of the magmas, and in particular the ability of the magmas to mix as magma density evolves through the release of bubbles. Although magmas with low crystal contents may behave as Newtonian liquids, as the crystal content increases, the behaviour becomes progressively more non-Newtonian, and the efficiency of overturn and mixing, or of the formation of inclusions of one magma in a second as volatiles form in the lower magma will be strongly influenced by this rheology. The project will explore the fluid dynamics of magma mixing in a crystalline magma, through novel laboratory experiments and supporting quantitative models. These models will then be tested against some of the detailed data sets associated with a range of well-studied intermediate arc volcanoes (e.g. Soufriere Hills, Mount St Helens, Santiguito volcanoes). These data will relate to: the petrology of magma suites (including mafic enclaves) and the geochemistry of bulk rocks, melt inclusions and volcanic gases; as well as the geophysical manifestations of unrest and eruption such as the inflation-deflation cycles associated with the eruption of Soufriere Hills volcano Montserrat (1995-present).

What the student will do:

The student will develop new laboratory experiments in which bubbles are injected into crystalline layers of viscous particle laden liquid. Initially the project will involve exploring the dynamics of the inflation of the mixture, and the mechanisms of  bubble-separation in liquids with different crystal contents. Then the student will build on these experiments, examining the potential for overturn and mixing into an overlying, originally less dense liquid, as a result of the supply of bubbles and associated reduction in density of the lower layer. In addition, the student will run experiments in which the liquids are driven upwards from the containing vessel into a second reservoir, to explore the role of crystal content and volatile content on the ordering of the magma  which erupts, in reference to the classical picture of selective withdrawal of two magmas from a stratified reservoir. As the project evolves, the student will then apply the findings from these experiments to model magma mixing, degassing and eruption to the interpretation of some key observations that are generic to many arc volcanoes. These include: an inability to image geophysically a magma storage area beneath the volcano; decoupling of the gas phase from the magma; extensive magma mixing recorded in bulk rocks; and changes in proportions of mafic magmas in the erupted magmas over time.

Please contact the lead supervisor directly for further information relating to what the successful applicant will be expected to do, training to be provided, and any specific educational background requirements.

References:

Phillips, JC; Woods, AW, 2002, Suppression of large-scale magma mixing by melt-volatile separation, EARTH AND PLANETARY SCIENCE LETTERS, 204, 1-2, 47-60, Citations: 22.

Huppert, HE; Woods, AW, 2002, The role of volatiles in magma chamber dynamics, NATURE, 420, 6915, 493-495, Citations: 62.

Tarasewicz, Jon; White, Robert S.; Woods, Andrew W.; Brandsdottir, Bryndis; Gudmundsson, Magnus T., 2012, Magma mobilization by downward-propagating decompression of the Eyjafjallajokull volcanic plumbing system, GEOPHYSICAL RESEARCH LETTERS, 39, , Citations: 11

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