Densification of permeable liquids and magmas

Wadsworth, Fabian B. (2016): Densification of permeable liquids and magmas. Dissertation, LMU München: Fakultät für Geowissenschaften

Abstract

A central question pertinent to our understanding of volcanic eruptions is: how long can magma remain permeable during shallow ascent? The answer to this complex question has implications for whether or not volcanic plugs can form at the top of silicic conduits and for the longevity of overpressure in magma, which is key to understanding the likelihood that a magma will fragment explosively in eruption. In this thesis a conceptual, and then mathematical framework for addressing this problem is established before experimental data is presented. The mathematical treatment of the problem progresses from processes that affect single droplets and that can be explicitly constrained, such as heat and mass transfer and shape changes in volcanic droplets, before applying these concepts to arrays of many droplets and porous liquids in general. In testing experimental data, a step-by-step approach is taken in which (1) a controlled analogue dataset is used to differentiate the model that best describes the data; (2) the chosen model is extended to more volcanically relevant conditions by testing it against experiments performed using natural materials; and (3) the consequences of the densification process for the time dependence of permeability are assessed. A universal scaling is found between the porosity and the permeability of densifying systems and this is used to calibrate a numerical model for the kinetics of permeability decay in volcanic plugs. Finally, a densification map is provided on which the dominant timescales and lengthscales are compared such that specific volcanic conditions or observations can be plotted to assess whether or not they are consistent with the densification process. In conclusion, it is noted that permeable magmas and viscous liquids in general will densify until an equilibrium volume is reached. This densification is driven by either the surface tension stresses internal to the permeable pore network or by additional external stresses and is limited by the liquid viscosity and the lengthscale of the pores. All volcanic eruptions are driven by exsolved gas and the buoyancy and pressure they contribute to the system and must be outgassed in either explosive or passive events. While the explosive contribution of magma outgassing has received much attention, the physical process by which passive outgassing, and the resultant densification, occur remains poorly understood. Future work could constrain the regime in which explosive and passive degassing are coincident and compete to release the gas-pressure built up during the shallowest portions of magma ascent to the Earth's surface.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 550 Geowissenschaften
• Fakultäten: Fakultät für Geowissenschaften
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 14. Juni 2016
• 1. Bericht­erstatter:in: Dingwell, Donald Bruce