p6 alina

Modelling Conduit and Dome Flow Dynamics of Volcanoes: 

Modelling Conduit and Dome Flow Dynamics of Volcanoes Alina Hale

Outline of talk: 

Outline of talk Overview of volcanoes Different styles of eruption Magma rheology Lava domes and Soufrière Hills Volcano Ph.D. research Future research at ESSCC

Volcano Overview: 

Volcano Overview Volcano origin: Divergent boundary (plate separation) Convergent boundary (mantle, remelted crust, subducted slab material) 3. Hotspots Oceanic Continental Mantle and plate interaction 1511 known eruptions in last 10000 years and 30 a year 238000 deaths in 400 yrs Poorly understood Produced most of the metallic minerals: copper, gold, silver, lead and zinc

Geological classes of volcanoes: 

Geological classes of volcanoes Cinder cone Steep, conical hill of volcanic fragments that accumulate around and downwind from a vent. Rock fragments are often called cinders or scoria. Shield volcano Broad, gentle slopes built by the eruption of fluid basalt lava. Tends to build enormous, low-angle cones because of the low viscosity upon eruption. Volcanic (lava) dome Rounded, steep-sided mounds of very viscous magma, usually dacite or rhyolite. May consist of several lava flows. Composite volcanoes Alternating layers of pyroclastic and rock solidified from lava flows. The most common form for volcanoes of magmatic arcs. Rheology ultimately governs eruption style

Soufrière Hills Volcano, Montserrat: 

Soufrière Hills Volcano, Montserrat Montserrat situated in Lesser Antilles, a volcanic island arc British over-seas territory Current eruption began 1995 and still on-going Comprised almost entirely of volcanic deposits Enhanced seismicity in 1897-98, 1933-37, and in 1966-67

Motivation for Research: 

Motivation for Research Growth of numerous lava domes Collapse events can result in generation of pyroclastic flows, tsunamis and explosions Statistics used for a general trend for collapse events Subtle modelling techniques required to fully understand the science: little prior research Analogue models

Dome Growth Styles: 

Dome Growth Styles Axisymmetrical dome Platy dome Ross Griffiths andamp; Jonathan Fink Lobate dome Spiny dome Endogenous vs. Exogenous

Ph.D. Research: 

Ph.D. Research MODELLING TECHNIQUES: Conduit flow models Non-Newtonian flow model for rheology and flow profile Finite Element Method (FASTFLO) Fluid and solid combined motion Complex geometries WHAT’S TO BE MODELLED: Growth of the lava dome Change in lava dome growth style Stability of the lava dome

Quasi 1D Conduit Flow Model: 

Quasi 1D Conduit Flow Model Perfect cylindrical conduit, actual plumbing unknown Extrusion rate a combination of: 'Fluid' flow (Bingham) Plug slip Boundary condition governed by rheology: Yield strength Shear strength of magma Velocity gradient Rheology constrained by: Observations Monte Carlo simulations Initial condition: Over pressure (Pa) Magma Chamber Shear zones within dome originate at conduit/dome base interface Shear zones form within lava dome Slip boundary condition No-slip boundary condition

Total Extrusion Rate: 

Total Extrusion Rate (m3s-1) (m3s-1) (Pa) Shear plane length can give information about the plumbing of the upper conduit system and dome stability (m) Depth of shear zone in conduit (m) Extrusion a combination of flow and slip Well defined boundary between the two growth regimes

Oct – Nov 1996 Dome: 

Oct – Nov 1996 Dome Oct-Nov 1996 lava dome extruded on Soufrière Hills Volcano Observed to initially grow endogenously before becoming exogenous after reaching a height of 115m Dome growth creates a feedback mechanism into conduit flow dynamics Run 7 reproduces observations Rich variety of extrusion rates and growth styles for slight variations in state variable values

FEM Exogenous Model: 

Shear zones within lava dome Shear zones along conduit wall Exogenous behaviour considered with FEM modelling techniques: Magma channelled to free surface Shear zones originate at conduit walls Model interior discontinuities (shear zones) Understanding processes associated with exogenous dome growth will help to predict future growth and internal discontinuities capable of promoting collapse What is required for shear boundaries to occur and evolve? FEM Exogenous Model

General Elasto-Viscoplasticity Model: 

General Elasto-Viscoplasticity Model Strains split into elastic and viscoplastic components Substitute into equilibrium equation Viscoplastic strain calculated by: Where the plastic potential is defined by: To achieve convergence time stepping is given by:

Exogenous Model Results: 

Exogenous Model Results Growth of shear zone in initially endogenous dome for 1 sec extrusion Peak shear zone growth i.e. maximum unstable dome growth region Peak in shear zone production due to viscosity and extrusion rate (time-dependent rheology) Real dome more complex Initial shear zone depth (metres)

Conclusion: 

Conclusion Initial motivation was to model lava domes for a greater understanding of growth and stability. Ph.D. work shows that the problem is more complex than previously thought and requires further modelling work. Shear planes in conduit Conduit transition from a dyke into a cylinder Link to seismicity Cross conduit gradients in rheology Next: 2D FEM conduit models to: Better understand volcano plumbing Link dome growth style to stability Consider cross conduit gradients in rheology and velocity Many Unsolved Problems

ESSCC Research: 

Utilise existing ACcESS models: Discharge of material from a silo. Model tracks free surface Has parallels to magma flow in a volcanic system (shear zones) Colouring indicates strain rate as the material flows out through the bottom. Red = rapid deformation rates Yellow = low deformation rates The flow is driven by gravity here but will be modified to be driven by pressure with appropriate rheology ESSCC Research