This thesis describes the investigation of fluid dynamic processes involved in maintained explosive volcanic eruptions. The thesis is divided into chapters relating to dynamical processes in a volcanic system: evolution and evacuation of a reservoir of molten rock, flow in a narrow conduit to the Earth's surface, and subsequent transport in the atmosphere. In chapter 2, we calculate the mass erupted, prior to caldera collapse, from a chamber as the pressure changes from a certain overpressure to a specified underpressure at which wall collapse occurs. The compressibility of the magma increases significantly as the pressure falls and the magma becomes saturated in volatiles. Magma saturation exerts a dominant control on the amount of magma erupted. We also examine the effects on mass erupted of the chamber shape, size and depth beneath the Earth's surface, the magma composition and the strength of country rock. Finally, we demonstrate applications of our results to various historical eruptions, including the eruption at Vesuvius in 79A.D. and the eruption at Mt St Helens in 1980. During maintained explosive volcanic eruptions, fragmented silicic magma and volatiles exit the vent with pressures typically in the range 10-100 atm and at the speed of sound of the mixture. In chapter 3, we review previous models of magma ascent up a conduit and identify some new scalings for the exit velocity as a function of the speed of sound of the mixture. In chapter 4, we combine models of evolution of the magma chamber with models of ascent of magma up the conduit to make estimates of the duration of the eruption and examine the rate of change in eruption rate with time under conditions of decreasing chamber pressure, changing magma volatile content and conduit widening due to erosion. Finally, we demonstrate an application of our results to the historical eruptions at Vesuvius in 79A.D. and at Mt St Helens in 1980. After decompression, the bulk of the material may ascend as a larger convecting eruption column or collapse to form a dense fountain which sheds ash flows around the vent. In chapter 5, we model the decompression of jets beyond the vent. We describe a jet freely decompressing into the atmosphere or into a crater, coupling our results with models of eruption column formation. We show that decompression through a crater may cause collapse at relatively small eruption rates, while it may promote formation of buoyant eruption columns at higher eruption rates. If a crater grows through erosion during an eruption, then typically a transition in eruption style may occur from an eruption column to column collapse.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:596823 |
| Date | January 1996 |
| Creators | Bower, S. M. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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