Bulletin of Volcanology

, Volume 54, Issue 4, pp 329–344 | Cite as

Sedimentation of tephra by volcanic plumes: I. Theory and its comparison with a study of the Fogo A plinian deposit, Sao Miguel (Azores)

  • M I Bursik
  • R S J Sparks
  • J S Gilbert
  • S N Carey
Article

Abstract

Sedimentation of ejecta from volcanic plumes has been studied as a function of distance from the source in the Fogo A plinian deposit, Sao Miguel, Azores. The Fogo A trachytic pumice deposit is reversely graded and can be divided into two parts on the basis of pumice colour, abundance of syenite accessory lithic clasts and distribution. The lower syenite-poor part was dispersed to the south and was clearly influenced by wind. The upper syenite-rich part is coarsegrained and has a nearly symmetrical distribution around the vent. Elongation of isopachs to the east indicate a weak wind influence. The grain-size variations of lithic and crystal components in the upper coarse part were studied. Total accumulation and accumulation per unit area (expressed in kg/m2) show good fits to a gaussian function at distances greater than ∼7 km for grain diameters less than 2 cm. These results agree with a theoretical model for a radially spreading turbulent current moving over a quiescent fluid. The gaussian coefficient is shown to be a function of grain size and the flow rate of material into the umbrella region of the eruption column. The coefficient is therefore also a function of column height. The column height deduced from these data is 21 km, which is in broad agrrement with the column height of 27 km deduced from maximum clast dispersal using the method of Carey and Sparks (1986). The accumulation of clasts larger than 2 cm agrees with a theory for the fallout of clasts from the margins of the ascending eruption column, which treats the plume as a succession of large eddies that decrease their mass of particles as an exponential function of time. Calculations are also presented for the influence of the radial inflow of surrounding air into the column on the deposition of clasts. These calculations constrain the wind speed during the later part of the Fogo A eruption to be at most a few metres per second. The study has allowed four different dynamic categories of clast behaviour to be recognised in eruption columns.

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References

  1. Allen JRL (1982) Particle motions at low concentrations: grading in pyroclastic fall deposits. Chapter 3 in Sedimentary Structures: their character and physical basis (Elsevier) Vol 1:111–135Google Scholar
  2. Armienti P, Macedonio G, Pareschi MT (1988) A numerical model for simulation of tephra transport and deposition: applications to May 18 1980 Mount St Helens eruption. J Geophys Res 93:6463–6476Google Scholar
  3. Bursik MI, Woods AW (1991) Buoyant, superbuoyant and collapsing eruption columns. J Volcanol Geotherm Res 45:347–350Google Scholar
  4. Carey S, Sparks RSJ (1986) Quantitative models of the fallout and dispersal of tephra from volcanic eruption columns. Bull Volcanol 48:109–125Google Scholar
  5. Hazen A (1904) On sedimentation. Trans Amer Soc Civ Eng 53:45–88Google Scholar
  6. Knox JB, Short NM (1964) A diagnostic model using ashfall data to determine eruption characteristic and atmospheric conditions during a major volcanic event. Bull Volcanol 27:5–24Google Scholar
  7. Macedonio G, Pareschi MT, Santacroce R (1988) A numerical simulation of the plinian phase of 79 AD eruption of Vesuvius. J Geophys Res 93:14817–14827Google Scholar
  8. Martin D, Nokes R (1988) Crystal settling in a vigorously convecting magma chamber. Nature 332:534–536Google Scholar
  9. Morton BR, Taylor GI, Turner JS (1956) Turbulent gravitational convection from maintained and instantaneous sources. Proc R Soc London (A) 234:1–23Google Scholar
  10. Papantoniou D, List EJ (1989) Large-scale structures in the farfield of buoyant jets. J Fluid Mech 29:151–190Google Scholar
  11. Pescatore T, Sparks RSJ, Brazier S (1987) Reverse grading in the Avellino plinian deposit of Vesuvius. Boll Soc Geol Italy 106:667–672Google Scholar
  12. Pyle DM (1989) The thickness, volume and grain size of tephra fall deposits. Bull Volcanol 51:1–15Google Scholar
  13. Sparks RSJ (1986) The dimensions and dynamics of volcanic eruption columns. Bull Volcanol 48:3–15Google Scholar
  14. Sparks RSJ, Carey SN, Sigurdsson H (1991) Sedimentation from gravity currents generated by turbulent plumes. Sedimentology 38:839–856Google Scholar
  15. Susuki T (1983) A theoretical model for dispersion of tephra. In: Shimozura D, Yokoyama I. (eds) Arc volcanism: physics and tectonics. TERRAPUB, Tokyo, pp 95–113Google Scholar
  16. Walker GPL (1973) Explosive volcanic eruptions — a new classification scheme. Geol Rundsch 62:431–446Google Scholar
  17. Walker GPL, Croasdale R (1971) Two plinian type eruptions in the Azores. J Geol Soc London 127:17–55Google Scholar
  18. Wilson L (1972) Explosive volcanic eruptions — II. The atmospheric trajectories of pyroclasts. Geophys JR Astron Soc 30:381–392Google Scholar
  19. Wilson L, Walker GPL (1987) Explosive volcanic eruptions — VI. Ejecta dispersal in plinian eruptions: the control of eruption conditions and atmospheric properties. Geophys JR Astron Soc 89:657–679Google Scholar
  20. Wilson L, Sparks RSJ, Huang TC, Watkins ND (1978) The control of eruption column heights by eruption energetics and dynamics. J Geophys Res 83:1829–1836Google Scholar
  21. Woods AW (1988) The fluid dynamics and thermodynamics of plinian eruption columns. Bull Volcanol 50:169–193Google Scholar
  22. Woods AW, Bursik MI (1991) Particle fallout, thermal disequilibrium and volcanic plumes. Bull Volcanol 53:559–570Google Scholar

Copyright information

© Springer-Verlag 1992

Authors and Affiliations

  • M I Bursik
    • 1
  • R S J Sparks
    • 2
  • J S Gilbert
    • 2
  • S N Carey
    • 3
  1. 1.Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadenaUSA
  2. 2.Department of GeologyUniversity of BristolBristolUK
  3. 3.Graduate School of OceanographyUniversity of Rhode Island KingstonUSA

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