TY - JOUR

T1 - Effects of conduit geometry on magma ascent dynamics in dome-forming eruptions

AU - de' Michieli Vitturi, M.

AU - Clarke, Amanda

AU - Neri, A.

AU - Voight, B.

N1 - Funding Information:
This research was supported by project AIRPLANE funded by MUR (Italy, PI Neri) and NSF grant EAR-0310329 (PI Clarke). Valuable comments, which significantly improved the manuscript, were provided by Helene Massol and three anonymous reviewers.
Copyright:
Copyright 2011 Elsevier B.V., All rights reserved.

PY - 2008/8/15

Y1 - 2008/8/15

N2 - We develop a steady-state, two-phase flow model of magma ascent through an axisymmetric conduit of variable radius R and length L in order to quantify relationships between conduit geometry and magma ascent dynamics. Holding boundary conditions and chamber magma properties constant, we vary conduit geometry systematically and independently, such that the upper conduit radius increases or decreases by a factor of Rt/Rb (radius ratio; 0.4 ≤ Rt/Rb ≤ 2.5), above a change initiation height H (0.1 ≤ H/L ≤ 0.7), and over length Le (Le/L = 0.2), where Rt and Rb are conduit radius above (t) and below (b) the radius change and H is the height above the top of the magma chamber. Conduit widening causes a drop in overpressure and corresponding increase in gas volume fraction and magma acceleration over the whole length of the conduit, with all changes increasing in magnitude with increasing radius ratio. Magma ascent rate increases roughly as R2 and volumetric flow rate subsequently increases as R4 when Rt = Rb = R. Both increasing Rt for a fixed Rb (increasing radius ratio) and increasing Rb for a fixed Rt (decreasing radius ratio), increase volume flow and magma ascent rates. Compared to changes in geometry, small changes in chamber pressure (< 5%) have a weak effect on flow rate. Many model runs produce a magma plug at the top of the conduit, largely due to permeable gas loss through conduit walls. In general, large radii and low radius ratios (i.e., nearly cylindrical conduits) favor thin, low-density plugs, which may facilitate sudden destruction of a plug, and thus enhance the likelihood of explosive over extrusive eruptions. These findings suggest that changes in conduit geometry, such as those caused by conduit erosion during explosive eruptions or by accretion of magma along conduit walls, are strongly coupled to magma ascent dynamics and should not be ignored when interpreting changes in eruptive behavior.

AB - We develop a steady-state, two-phase flow model of magma ascent through an axisymmetric conduit of variable radius R and length L in order to quantify relationships between conduit geometry and magma ascent dynamics. Holding boundary conditions and chamber magma properties constant, we vary conduit geometry systematically and independently, such that the upper conduit radius increases or decreases by a factor of Rt/Rb (radius ratio; 0.4 ≤ Rt/Rb ≤ 2.5), above a change initiation height H (0.1 ≤ H/L ≤ 0.7), and over length Le (Le/L = 0.2), where Rt and Rb are conduit radius above (t) and below (b) the radius change and H is the height above the top of the magma chamber. Conduit widening causes a drop in overpressure and corresponding increase in gas volume fraction and magma acceleration over the whole length of the conduit, with all changes increasing in magnitude with increasing radius ratio. Magma ascent rate increases roughly as R2 and volumetric flow rate subsequently increases as R4 when Rt = Rb = R. Both increasing Rt for a fixed Rb (increasing radius ratio) and increasing Rb for a fixed Rt (decreasing radius ratio), increase volume flow and magma ascent rates. Compared to changes in geometry, small changes in chamber pressure (< 5%) have a weak effect on flow rate. Many model runs produce a magma plug at the top of the conduit, largely due to permeable gas loss through conduit walls. In general, large radii and low radius ratios (i.e., nearly cylindrical conduits) favor thin, low-density plugs, which may facilitate sudden destruction of a plug, and thus enhance the likelihood of explosive over extrusive eruptions. These findings suggest that changes in conduit geometry, such as those caused by conduit erosion during explosive eruptions or by accretion of magma along conduit walls, are strongly coupled to magma ascent dynamics and should not be ignored when interpreting changes in eruptive behavior.

KW - computational model

KW - conduit dynamics

KW - conduit geometry

KW - effusion rate

KW - magma ascent

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U2 - 10.1016/j.epsl.2008.05.025

DO - 10.1016/j.epsl.2008.05.025

M3 - Article

AN - SCOPUS:48149091407

VL - 272

SP - 567

EP - 578

JO - Earth and Planetary Sciences Letters

JF - Earth and Planetary Sciences Letters

SN - 0012-821X

IS - 3-4

ER -