Browning, John; (2016) Thermo-mechanical effects of magma chambers and caldera faults. Doctoral thesis (Ph.D), Royal Holloway, University of London.

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Abstract

Shallow magma chambers influence a range of crustal processes at active volcanoes.For example many and perhaps most dyke fed eruptions originate from shallow magma storage regions. Magma chamber failure, resulting in the initiation of caldera faults or magma filled fractures (dyke) is likely governed by a complex interplay between regional and local mechanical stresses and thermal effects. This study utilises a multitude of techniques to decipher salient thermo-mechanical processes occurring as a result of magma stored at shallow (<10 km) depth. A new model to forecast magma chamber rupture and dyke initiation is proposed. The analytical solutions presented are applied to field data from Santorini, Greece, and combine poro-elastic material constraints with geodetic data to estimate both magma volumes stored beneath the caldera and internal excess pressure generated during periods of magmatic recharge. Predicting the path or propagation of magma once it has left a shallow magma chamber is an important but so far unachievable goal in volcanology. Caldera faults offer pathways for magma and often develop ring-dykes. A previously unreported mechanism for the formation of ring-dykes is the capture of inclined sheets at caldera fault boundaries. Geological field observations of an exceptionally well-exposed ring-fault at Hafnarfjall in Western Iceland were used as input to the finite element method numerical modelling software COMSOL multiphysics to infer a mechanism of principal stress rotation within a fault damage zone. The same modelling technique was then used to estimate the far-field crustal displacements resulting from the failure and collapse of a shallow magma chamber roof. This study is framed within the context of the 2014-15 Bardarbunga-Holuhraun (Iceland) dyke injection and eruptive episode, and hypothesises that significant ice subsidence was not solely associated with crustal subsidence but instead related to ice flow within Bardarbunga caldera generated by the dyke emplacement. Thermal stresses resulting from hot magma emplacement and gradual cooling likely combine to weaken volcanic edifices. For example, field evidence suggests many normal faults nucleate from cooling joints. A suite of thermal stressing experiments finds that cooling and contraction produces larger and more abundant micro-cracks when compared with heating expansion. This is an important result when considering that almost all previous studies concerned with thermal stressing focused on the heating cycle.

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