The degassing of basaltic magma chambers

  • Fred Witham

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

I present an integrated study of volcanic degassing processes coupled with the impact of degassing on magma chamber dynamics. A one-dimensional model of heat flow inside an incipient magma reservoir is derived by considering the thermal effect of repeated basaltic sill intrusions into the crust. The minimum magma flux required to maintain melt indefinitely, i.e.\ to form a ‘magma chamber’, is found as a function of intrusion depth. Despite the time-dependent nature of the modelled solutions, fluxes can be approximated by a steady state solution when cooling is dominated by heat loss through the earth's surface. Degassing enhances crystallisation rates, so greater magma fluxes are required to produce a long-lived magma chamber. A physical model of a generic lava lake system is developed and its stability tested by investigating the response of the system to perturbation. The stability of the system controlled by the lake--conduit geometry and the magma's volatile content. Such a mechanism is consistent with lava lake behaviour during the 1983-4 Pu`u `O`o eruption of Kilauea. The Boiling Lake of Dominica exhibits stability punctuated by occasional crises involving rapid draining and filling of the lake and changes in water temperature. The lava lake stability model explains the behaviour well.Different volatile species exsolve to differing extents during the near-surface degassing. Under conditions of equilibrium, closed-system degassing, emitted volatile ratios remain constant despite persistent degassing. This prediction is consistent with the remarkably constant sulphur to chlorine ratios observed at Stromboli, Masaya and Nyiragongo volcanoes. This result has implications for volcanic hazard assessments based on gas monitoring -- injection of fresh, volatile-rich magma will not necessarily manifest itself as a change in emitted gas ratios.The turbulent motion of negatively-buoyant plumes of degassed magma released into turbulently convecting magma chambers is studied. By assuming that the turbulent environment removes fluid from the plume at a rate proportional to a characteristic environmental velocity scale, I derive a model describing the fluid behaviour. I extend the study to include the evolution of a finite, confined environment, the end-member regimes of which are a well-mixed environment at all times (high convective velocities), and a `filling box' model similar to that of \citet{Baines69FW} (low convective velocities). I find that the convecting filling box is not stable indefinitely, but that the density stratification will eventually be overcome by thermal convection.
Date of Award2008
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorJeremy C Phillips (Supervisor)

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