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Bulletin of Volcanology

, Volume 73, Issue 10, pp 1583–1609 | Cite as

The flow dynamics of an extremely large volume pyroclastic flow, the 2.08-Ma Cerro Galán Ignimbrite, NW Argentina, and comparison with other flow types

  • Ray A. F. Cas
  • Heather M. N. Wright
  • Christopher B. Folkes
  • Chiara Lesti
  • Massimiliano Porreca
  • Guido Giordano
  • Jose G. Viramonte
Research Article

Abstract

The 2.08-Ma Cerro Galán Ignimbrite (CGI) represents a >630-km3 dense rock equivalent (VEI 8) eruption from the long-lived Cerro Galán magma system (∼6 Ma). It is a crystal-rich (35–60%), pumice (<10% generally) and lithic-poor (<5% generally) rhyodacitic ignimbrite, lacking a preceding plinian fallout deposit. The CGI is preserved up to 80 km from the structural margins of the caldera, but almost certainly was deposited up to 100 km from the caldera in some places. Only one emplacement unit is preserved in proximal to medial settings and in most distal settings, suggesting constant flow conditions, but where the pyroclastic flow moved into a palaeotopography of substantial valleys and ridges, it interacted with valley walls, resulting in flow instabilities that generated multiple depositional units, often separated by pyroclastic surge deposits. The CGI preserves a widespread sub-horizontal fabric, defined by aligned elongate pumice and lithic clasts, and minerals (e.g. biotite). A sub-horizontal anisotropy of magnetic susceptibility fabric is defined by minute magnetic minerals in all localities where it has been analysed. The CGI is poor in both vent-derived (‘accessory’) lithics and locally derived lithics from the ground surface (‘accidental’) lithics. Locally derived lithics are small (<20 cm) and were not transported far from source points. All data suggest that the pyroclastic flow system producing the CGI was characterised throughout by high sedimentation rates, resulting from high particle concentration and suppressed turbulence at the depositional boundary layer, despite being a low aspect ratio ignimbrite. Based on these features, we question whether high velocity and momentum are necessary to account for extensive flow mobility. It is proposed that the CGI was deposited by a pyroclastic flow system that developed a substantial, high particle concentration granular under-flow, which flowed with suppressed turbulence. High particle concentration and fine-ash content hindered gas loss and maintained flow mobility. In order to explain the contemporaneous maintenance of high particle concentration, high sedimentation rate at the depositional boundary layer and a high level of mobility, it is also proposed that the flow(s) was continuously supplied at a high mass feeding rate. It is also proposed that internal gas pressure within the flow, directed downwards onto the substrate over which the flow was passing, reduced the friction between the flow and the substrate and also enhanced its mobility. The pervasive sub-horizontal fabric of aligned pumice, lithic and even biotite crystals indicates a consistent horizontal shear force existed during transport and deposition in the basal granular flow, consistent with the existence of a laminar, shearing, granular flow regime during the final stages of transport and deposition.

Keywords

Cerro Galán caldera/ignimbrite VEI 8 magnitude eruption Crystal-rich ignimbrite Laminar-like Shearing Granular flow 

Notes

Acknowledgements

We thank the Australian Research Council Discovery Grant Scheme for funding most of the costs of this research through grant DP0663560 to undertake research on the volcanology of the Cerro Galán ignimbrite. Careful reviews by Greg Valentine and Tim Druitt and very helpful editorial suggestions by Kathy Cashman and James White have greatly improved the paper.

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Copyright information

© Springer-Verlag 2011

Authors and Affiliations

  • Ray A. F. Cas
    • 1
  • Heather M. N. Wright
    • 1
    • 4
  • Christopher B. Folkes
    • 1
  • Chiara Lesti
    • 2
  • Massimiliano Porreca
    • 2
    • 5
  • Guido Giordano
    • 2
  • Jose G. Viramonte
    • 3
  1. 1.School of GeosciencesMonash UniversityClaytonAustralia
  2. 2.Dipartimento di Scienze GeologicheUniversità di Roma TreRomeItaly
  3. 3.Instituto GEONORTE and CONICETUniversidad Nacional de SaltaSaltaArgentina
  4. 4.US Geological SurveyMenlo ParkUSA
  5. 5.Centro de Vulcanologia e Avaliação de Riscos Geológicos (CVARG)Universidade dos AçoresPonta DelgadaPortugal

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