The role of magma composition and water content in explosive eruptions: 2. Pyroclastic dispersion dynamics

Abstract The net effect of magma composition and water content on the behavior of explosive volcanic eruptions was investigated by the use of thermofluid-dynamic models and numerical simulation. The two-phase flow models employed allowed the description of the eruptive process from the conduit entrance at the top of the magma chamber up to the pyroclastic dispersion process in the atmosphere. In this paper, vent conditions computed in the companion paper by Papale et al. [Papale, P., Neri, A., Macedonio, G., this issue. The role of magma composition and water content in explosive eruptions: I. Conduit ascent dynamics. J. Volcanol. Geotherm. Res.], as a function of magma composition and water content, are employed as boundary conditions at the crater base of the pyroclastic dispersion model. The eruptive mixture was described as a two-phase gas–particle fluid. Pyroclasts are represented as solid particles of one size whereas the gas phase consists of superheated water vapor leaving the vent and atmospheric air. Simulations were performed on physical domains extending several kilometers in both radial and vertical directions and for several minutes of real time. The results clearly show the net effects of the anhydrous magma composition, crystals, and water content on the spatial and temporal distribution of pyroclastic material during the first minutes of eruption. In detail, the results indicate a strong influence of magma composition on eruption intensity, whereas the eruptive style, i.e., the generation of Plinian, transitional, or collapsing columns, appears to be mainly controlled by the total water content of the magma, i.e., the weight fraction of water with respect to the entire magmatic mixture. The main effect of the crystals is to reduce the total water content of the mixture and lead to a less buoyant behavior of the column. As a summary, for the investigated ranges of variations, simulation results indicate a progressive change to a more collapsing behavior of the column whenever water-poorer or crystal-richer magma is erupted from the vent with the anhydrous composition of magma affecting just the intensity of the eruption. For most conditions, such a conclusion does not appear to be strongly dependent on a relatively large variation of vent diameter and, therefore, of the scale of the event. Simulation results appear to be consistent with field studies even though more complete data on specific eruptions are recommended for a more thorough comparison.

[1]  Mark Settle,et al.  Volcanic eruption clouds and the thermal power output of explosive eruptions , 1978 .

[2]  A. A. Amsden,et al.  Numerical calculation of multiphase fluid flow , 1975 .

[3]  P. Papale,et al.  Magma flow along the volcanic conduit during the Plinian and pyroclastic flow phases of the May 18, 1980, Mount St. Helens eruption , 1994 .

[4]  Paolo Papale,et al.  The role of magma composition and water content in explosive eruptions: 1. Conduit ascent dynamics , 1998 .

[5]  Lionel Wilson,et al.  The Control of Volcanic Column Heights by Eruption Energetics and Dynamics , 1978 .

[6]  R. Cioni,et al.  Dynamics of the A.D. 79 eruption: stratigraphic, sedimentologic and geochemical data on the successions of the Somma-Vesuvius southern sector , 1992 .

[7]  Michael Herzog,et al.  Volcanic plume simulation on large scales , 1998 .

[8]  L. Wilson.  Relationships between pressure, volatile content and ejecta velocity in three types of volcanic explosion , 1980 .

[9]  H. Sigurdsson,et al.  The intensity of plinian eruptions , 1989 .

[10]  Roberto Scarpa,et al.  Monitoring and Mitigation of Volcano Hazards , 1996 .

[11]  L. Wilson,et al.  Explosive volcanic eruptions - VI. Ejecta dispersal in plinian eruptions: the control of eruption conditions and atmospheric properties , 1987 .

[12]  R. Cioni,et al.  Compositional Layering and Syn-eruptive Mixing of a Periodically Refilled Shallow Magma Chamber: the AD 79 Plinian Eruption of Vesuvius , 1995 .

[13]  Greg A. Valentine,et al.  Numerical models of Plinian eruption columns and pyroclastic flows , 1989 .

[14]  M. A. Sommer.  Volatiles H2O, CO2, and CO in Silicate Melt Inclusions in Quartz Phenocrysts from the Rhyolitic Bandelier Air-Fall and Ash-Flow Tuff, New Mexico , 1977, The Journal of Geology.

[15]  M. Rosi,et al.  The 1631 Vesuvius eruption. A reconstruction based on historical and stratigraphical data , 1993 .

[16]  Lionel Wilson,et al.  Explosive volcanic eruptions — IV. The control of magma properties and conduit geometry on eruption column behaviour , 1980 .

[17]  H. Sigurdsson,et al.  Eruption dynamics and magma withdrawal during the Plinian Phase of the Bishop Tuff Eruption, Long Valley Caldera , 1991 .

[18]  N. Wakao,et al.  Effect of fluid dispersion coefficients on particle-to-fluid heat transfer coefficients in packed beds , 1978 .

[19]  S. Baloga,et al.  Sensitivity of buoyant plume heights to ambient atmospheric conditions: Implications for volcanic eruption columns , 1996 .

[20]  H. Sigurdsson,et al.  Temporal variations in column height and magma discharge rate during the 79 A.D. eruption of Vesuvius , 1987 .

[21]  Augusto Neri,et al.  Numerical simulation of collapsing volcanic columns with particles of two sizes , 1996 .

[22]  G. Valentine.  Eruption column physics , 1997 .

[23]  H. Sigurdsson,et al.  Variations in column height and magma discharge during the May 18, 1980 eruption of Mount St. Helens , 1990 .

[24]  L. Lirer,et al.  Two Plinian Pumice-Fall Deposits from Somma-Vesuvius, Italy , 1973 .

[25]  Andrew W. Woods,et al.  The decompression of volcanic jets in a crater during explosive volcanic eruptions , 1995 .

[26]  John R. Holloway,et al.  Volatiles in magmas , 1994 .

[27]  D. Gidaspow.  Multiphase Flow and Fluidization: Continuum and Kinetic Theory Descriptions , 1994 .

[28]  Andrew W. Woods,et al.  The fluid dynamics and thermodynamics of eruption columns , 1988 .

[29]  Augusto Neri,et al.  Influence of eruption parameters on the thermofluid dynamics of collapsing volcanic columns , 1994 .

[30]  Michael Herzog,et al.  The effect of phase changes of water on the development of volcanic plumes , 1998 .

[31]  A. T. Anderson,et al.  H2O, CO2, CI, and gas in Plinian and ash-flow Bishop rhyolite , 1989 .

[32]  Lionel Wilson,et al.  A model for the formation of ignimbrite by gravitational column collapse , 1976, Journal of the Geological Society.

[33]  S. Kieffer,et al.  Sources of unsteady column dynamics in pyroclastic flow eruptions , 1991 .

[34]  Micol Todesco,et al.  Assessing the pyroclastic flow hazard at Vesuvius , 1994, Nature.

[35]  R. Sparks,et al.  Quantitative models of the fallout and dispersal of tephra from volcanic eruption columns , 1986 .

[36]  C. W. Criswell,et al.  Chronology and pyroclastic stratigraphy of the May 18, 1980, eruption of Mount St. Helens, Washington , 1987 .

[37]  A. Neri,et al.  Physical Modeling of Collapsing Volcanic Columns and Pyroclastic Flows , 1996 .

[38]  A. Woods,et al.  A laboratory study of explosive volcanic eruptions , 1992 .

[39]  C. Newhall,et al.  Fire and mud: eruptions and lahars of Mount Pinatubo, Philippines , 1998 .

[40]  Augusto Neri,et al.  NUMERICAL SIMULATION OF COLLAPSING VOLCANIC COLUMNS , 1993 .

[41]  N. Wakao,et al.  EFFECT OF FLUID DISPERSION COEFFICIENTS ON PARTICLE-TO-FLUID MASS TRANSFER COEFFICIENTS IN PACKED BEDS. CORRELATION OF SHERWOOD NUMBERS , 1978 .

[42]  Lionel Wilson,et al.  Explosive Volcanic Eruptions–III. Plinian Eruption Columns , 1958 .