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Bulletin of Volcanology

, Volume 74, Issue 1, pp 187–205 | Cite as

Causes and consequences of bimodal grain-size distribution of tephra fall deposited during the August 2006 Tungurahua eruption (Ecuador)

  • Julia Eychenne
  • Jean-Luc Le Pennec
  • Liliana Troncoso
  • Mathieu Gouhier
  • Jean-Marie Nedelec
Research Article

Abstract

The violent August 16–17, 2006 Tungurahua eruption in Ecuador witnessed the emplacement of numerous scoria flows and the deposition of a widespread tephra layer west of the volcano. We assess the size of the eruption by determining a bulk tephra volume in the range 42–57 × 106 m3, which supports a Volcanic Explosivity Index 3 event, consistent with calculated column height of 16–18 km above the vent and making it the strongest eruptive phase since the volcano’s magmatic reactivation in 1999. Isopachs west of the volcano are sub-bilobate in shape, while sieve and laser diffraction grain-size analyses of tephra samples reveal strongly bimodal distributions. Based on a new grain-size deconvolution algorithm and extended sampling area, we propose here a mechanism to account for the bimodal grain-size distribution. The deconvolution procedure allows us to identify two particle subpopulations in the deposit with distinct characteristics that indicate dissimilar transport-depositional processes. The log-normal coarse-grained subpopulation is typical of particles transported downwind by the main volcanic plume. The positively skewed, fine-grained subpopulation in the tephra fall layer shares close similarities with the elutriated co-pyroclastic flow ash cloud layers preserved on top of the scoria flow deposits. The area with the higher fine particle content in the tephra layer coincides with the downwind prolongation of the pyroclastic flow deposits. These results indicate that the bimodal distribution of grain size in the Tungurahua fall deposit results from synchronous deposition of lapilli from the main plume and fine ash elutriated from scoria flows emplaced on the western flank of the volcano. Our study also reveals that inappropriate grain-size data processing may produce misleading determination of eruptive type.

Keywords

Tephra fall Bimodal grain size Volcanic Explosivity Index Eruption type Tungurahua volcano 

Notes

Acknowledgements

This work is part of a PhD project by JE and has been completed in the context of a French-Ecuadorian cooperation program. Discussions with P Ramon, P Samaniego, H Yepes, C Robin, K Kelfoun, P Hall, P Mothes and many other individuals in the Tungurahua region improved our understanding of the August 2006 event. Reviews of the manuscript by RJ Carey and D Andronico and editorial handling by R Cioni and J White are warmly acknowledged.

Supplementary material

445_2011_517_MOESM1_ESM.docx (37 kb)
Table S1 Thickness measurements and samples collected in the PFall layer. Projected coordinates system: PSAD_1956_UTM_Zone 17S (DOC 36.7 KB)

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Copyright information

© Springer-Verlag 2011

Authors and Affiliations

  • Julia Eychenne
    • 1
    • 2
    • 3
  • Jean-Luc Le Pennec
    • 1
    • 2
    • 3
  • Liliana Troncoso
    • 4
  • Mathieu Gouhier
    • 1
    • 2
    • 3
  • Jean-Marie Nedelec
    • 5
    • 6
  1. 1.Laboratoire Magmas et VolcansClermont Université, Université Blaise PascalClermont-FerrandFrance
  2. 2.CNRS, UMR 6524Laboratoire Magmas et VolcansClermont-Ferrand cedexFrance
  3. 3.IRD, R 163Laboratoire Magmas et VolcansClermont-Ferrand cedexFrance
  4. 4.Instituto GeofísicoEscuela Politécnica NacionalQuitoEcuador
  5. 5.ENSCCF, LMIClermont UniversitéAubièreFrance
  6. 6.CNRS, UMR 6002AubièreFrance

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