Computational Modeling of Lava Domes using Particle Dynamics to investigate the effect of conduit flow mechanics on flow patterns

Open Access
Author:
Husain, Taha Murtuza
Graduate Program:
Energy and Mineral Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 22, 2014
Committee Members:
  • Derek Elsworth, Dissertation Advisor
  • Derek Elsworth, Committee Chair
  • Barry Voight, Committee Member
  • Tong Qiu, Committee Member
  • Zuleima T Karpyn, Committee Member
  • Jamal Rostami, Committee Member
  • Glen Mattioli, Special Member
Keywords:
  • computational modeling
  • lava domes
Abstract:
Large (1 – 4 x 106 m3) to major (> 4 x 106 m3) dome collapses for andesitic lava domes such as Soufriere Hills Volcano, Montserrat are observed for elevated magma discharge rates (6 – 13 m3/s). The gas rich magma pulses lead to pressure build up in the lava dome that result in structural failure of the over steepened canyon-like walls which may lead to rockfall or pyroclastic flow. This indicates that dome collapse at Soufriere Hills Volcano, Montserrat is intimately related to magma extrusion rate. Variation in magma extrusion rate for open-system magma chambers is observed to follow alternating periods of high and low activity. Periodic behavior of magma exhibits a rich diversity in the nature of its eruptive history due to variation in magma chamber size, total crystal content, linear crystal growth rate and magma replenishment rate. Distinguished patterns of growth were observed at different magma flow rates ranging from endogenous to exogenous dome growth for magma with varying strengths. Determining the key parameters that control the transition in flow pattern of the magma during its lava dome building eruption is the main focus of this work. This dissertation examines the mechanical effects on the morphology of the evolving lava dome on the extrusion of magma from a central vent using a 2D particle dynamics model. The particle dynamics model is coupled with a conduit flow model that incorporates the kinetics of crystallization and rheological stiffening to investigate important mechanisms during lava dome building eruptions. Chapter I of this dissertation explores lava dome growth and failure mechanics using a two-dimensional particle-dynamics model. The model follows the evolution of fractured lava, with solidification driven by degassing induced crystallization of magma. The particle-dynamics model emulates the natural development of dome growth and rearrangement of the lava dome which is difficult in mesh-based analyses due to mesh entanglement effects. The deformable talus evolves naturally as a frictional carapace that caps a ductile magma core. Extrusion rate and magma rheology together with crystallization temperature and volatile content govern the distribution of strength in the composite structure. This new model is calibrated against existing observational models of lava dome growth. Results show that the shape and extent of the ductile core and the overall structure of the lava dome are strongly controlled by the infusion rate. The effects of extrusion rate on magma rheology are sensitive to material stiffness, which in turn is a function of volatile content and crystallinity. Material stiffness and material strength are key model parameters which govern magma rheology and subsequently the morphological character of the lava dome and in turn stability. Degassing induced crystallization causes material stiffening and enhances material strength reflected in non-Newtonian magma behavior. The increase in stiffness and strength of the injected magma causes a transition in the style of dome growth, from endogenous expansion of a ductile core, to stiffer and stronger intruding material capable of punching through the overlying material and resulting in the development of a spine or possibly inducing dome collapse. Simulation results mimic development of a megaspine upon the influx of fresh magma which leads to the re-direction of magma flow, creating a new shear zone and the switching of dome growth from one side to the other. Our model shows similar dome growth dynamics as observed at Soufriere Hills Volcano, Montserrat, indicating a strong correlation between extrusion rate and its subsequent effect on mechanical properties and variations in magma rheology. Chapter II of this dissertation explores the effects of a spectrum of different rheological regimes, on eruptive style and morphologic evolution of lava domes, using a two-dimensional (2D) particle-dynamics model for a spreading viscoplastic (Bingham) fluid. We assume that the ductile magma core of a 2-D synthetic lava dome develops finite yield strength, and that deformable frictional talus evolves from a carapace that caps the magma core. Our new model is calibrated against an existing analytical model for a spreading viscoplastic lava dome and is further compared against observational data of lava dome growth. Results indicate that a degassing-induced increase in strength of the injected magma causes a transition in the lava dome morphology from a low-relief lava dome evolving endogenously (with bulk yield strength - 104 < τo < 106 Pa), to a Pelean lava dome with spines (τo > 105 - 106 Pa) extruded through the dome carapace. The virtual lava dome with τo = 0.6 MPa shows good agreement with the observed dome heights observed at the Soufriere Hills Volcano, Montserrat during a period of endogenous growth. The calculated apparent flow viscosity (1.36 x 1011 Pa.s for τo = 0.6 MPa) is in the range of bulk viscosity (109 to 1012 Pa.s) measured for crystal-rich lavas. Our model results indicate a strong correlation between apparent yield strength and dome morphology, as controlled by degassing-induced crystallization and extrusion rate. Chapter III of this dissertation explores different lava-dome styles by developing a two-dimensional particle-dynamics model. These growth patterns range from endogenous lava dome growth comprising expansion of a ductile dome core to the exogenous extrusion of a degassed lava plug resulting in generation of a lava spine. We couple conduit flow dynamics with surface growth of the evolving lava dome, fueled by an open-system magma chamber undergoing continuous replenishment. The conduit flow model accounts for the variation in rheology of ascending magma that results from degassing-induced crystallization. A period of reduced effusive flow rates promote enhanced degassing-induced crystallization. A degassed lava plug extrudes exogenously for magmas with crystal contents (ϕ) of 78%, yield strength >1.62 MPa, and at flow rates of <0.5 m3/s, while endogenous dome growth is predicted at higher flow rates (Qout > 3 m3/s) for magma with lower relative yield strengths (<1 MPa). At moderately high flow rates (Qout = 4 m3/s), the extrusion of magma with lower crystal content (62%) and low interparticulate yield strength (0.6 MPa) results in the development of endogenous shear lobes. Our simulations model the periodic extrusion history at Mount St. Helens (1980 – 1983). Endogenous growth initiates in the simulated lava dome with the extrusion of low yield strength magma (ϕ = 0.63 and τ = 0.76 MPa) after the crystallized viscous plug (ϕ = 0.87 and τ = 3 MPa) at the conduit exit is forced out by the high discharge rate pulse (2 < Qout < 12 m3/s). The size of the endogenous viscous plug and the occurrence of exogenous growth depend on magma yield strength and the magma chamber volume, which control the periodicity of the effusion. Chapter IV of this dissertation explores the Variation in the extruding lava flow patterns range from endogenous dome growth with a ductile core to the exogenous extrusion of a degassed lava plug that results in the generation of a spine. The variations are a manifestation of the changes in the magma rheology which is governed by magma composition and rate of decompression of the ascending magma. We simulate using a two-dimensional particle-dynamics model, the cyclic behavior of lava dome growth with endogenous growth at high discharge rates followed by exogenous extrusion of rheologically stiffened lava due to degassing induced crystallization at low discharge rates. We couple conduit flow dynamics with surface growth of the evolving lava dome which is fueled by an overpressured reservoir undergoing constant replenishment. The periodic behavior between magma chamber pressure and discharge rate is reproduced as a result of the temporal and spatial change in magma viscosity controlled by crystallization kinetics. Dimensionless numbers are used to map the flow behaviors with the changing extrusion regime. A dimensionless plot identifying the flow transition region during the growth cycle of an evolving lava dome in its lava dome eruptive period is presented. The plot provides a the threshold value of a dimensionless strength parameter (π2 < 3.31 x 10-4) below which the transition in flow pattern occurs from endogenously evolving lava dome with a ductile core to the development of a shear lobe for short or long lived periodic episode of the extrusion of magma.