[3] In the past ten years, evidence from volcanic deposits. [e.g., Fierstein and ... deposits of fallout and pyroclastic currents emplaced during the same eruptive ...
GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L10607, doi:10.1029/2004GL019709, 2004
Contemporaneous convective and collapsing eruptive dynamics: The transitional regime of explosive eruptions A. Di Muro,1,2 A. Neri,3 and M. Rosi1,4 Received 12 February 2004; revised 24 March 2004; accepted 7 April 2004; published 21 May 2004.
[1] Two contrasting eruptive regimes have been classically postulated to describe the behavior of explosive eruptions: convective and collapsing. Early studies have recognized that many eruptions evolve from the first to the second regime and have assumed the existence of a sharp boundary between them. Consequently, the dynamics of such transition has never been investigated in detail. Here, we present results of integrated deposit analyses and numerical simulations which demonstrate univocally that both dynamics can, and often do, coexist for hours in a new intermediate transitional regime of explosive eruptions. The study elucidates the features of the new regime in terms of mass partitioning, pyroclastic current dynamics and INDEX interplay among convective and collapsing styles. T ERMS: 3210 Mathematical Geophysics: Modeling; 3220 Mathematical Geophysics: Nonlinear dynamics; 8414 Volcanology: Eruption mechanisms; 8404 Volcanology: Ash deposits; 8450 Volcanology: Planetary volcanism (5480). Citation: Di Muro, A., A. Neri, and M. Rosi (2004), Contemporaneous convective and collapsing eruptive dynamics: The transitional regime of explosive eruptions, Geophys. Res. Lett., 31, L10607, doi:10.1029/2004GL019709.
1. Introduction [2] Depending on the degree of dilution attained by the pyroclastic mixture in the atmosphere, explosive eruptions can undergo two end-member behaviors: convective or collapsing [e.g., Sparks et al., 1978; Woods, 1988; Valentine and Wohletz, 1989]. In the fully convective regime, the mixture dilutes enough to buoyantly rise into the atmosphere, forming a convective column and umbrella cloud system. The hours-long fallout of pyroclasts from the convective system forms a well-sorted regionally-dispersed sheet which drapes the topography (fall deposit). In the fully collapsing regime, the erupted mass collapses back to the ground under the action of gravity, spreading dense pyroclastic currents (pyroclastic flows). Gravitational segregation of the solid load within pyroclastic flows accumulates a poorly-sorted, massive
1 Dipartimento di Scienze della Terra, Universita` degli Studi di Pisa, Pisa, Italy. 2 Now at Laboratoire de Physique et Chimie des Syste`mes Volcaniques, IPGP-Paris VI, Paris, France. 3 Istituto Nazionale di Geofisica e Vulcanologia, Centro per la Modellistica Fisica e Pericolosita` dei Processi Vulcanici, Pisa, Italy. 4 Also at Istituto Nazionale di Geofisica e Vulcanologia, Centro per la Modellistica Fisica e Pericolosita` dei Processi Vulcanici, Pisa, Italy.
Copyright 2004 by the American Geophysical Union. 0094-8276/04/2004GL019709
deposit, which ponds within topographic lows (pyroclastic flow deposit). [3] In the past ten years, evidence from volcanic deposits [e.g., Fierstein and Hildreth, 1992; Self et al., 1996; Wilson and Hildreth, 1997; Rosi et al., 1999, 2001], remote sensing [Holasek et al., 1996], physical modeling [Kaminski and Jaupart, 2001; Neri and Dobran, 1994; Neri et al., 2002; Veitch and Woods, 2002] and laboratory experiments [e.g., Carey et al., 1988; Woods and Caulfield, 1992] have pointed to a highly complex dynamics at the convectivecollapsing transition. However, previous the assumption of the existence of a sharp boundary between the two styles implicates that the entire erupted mass feeds either the convective column or the pyroclastic currents. As a consequence, the common occurrence of alternating or hybrid deposits of fallout and pyroclastic currents emplaced during the same eruptive phase has been often interpreted as the result of multiple shifts from one regime to the other. Such shifts have been variably ascribed to a set of causes such as modification of conduit/crater geometry [e.g., Woods and Bower, 1995], pyroclast and magma properties [Carey et al., 1990], magma discharge rate [Wilson et al., 1980], and magma fragmentation style [e.g., Valentine and Giannetti, 1995]. [4] In this paper, we present new results of integrated sedimentological and modeling studies specifically designed to investigate the dynamics of the transition between the two end-member eruptive regimes. Eruptive dynamics were reconstructed through the analysis of exceptionally well-exposed deposits of two recent Plinian events: the June 15, 1991, Pinatubo (Philippines) and the 1280 AD Quilotoa (Ecuador) eruptions [Di Muro, 2002]. Both eruptions were fed by a crystal-rich dacitic magma, had similar intensity and magnitude, and underwent a gradual passage from fully convective to fully collapsing behavior. In addition, numerical simulations using a 2D time-dependent multiparticle fluid-dynamic model were performed. Input parameters were consistent with these natural events. This approach allowed to quantify the partitioning of the erupted mass among pyroclastic currents and buoyant columns and the temporal change of their physical properties.
2. Dynamics and Pyroclastic Deposits of Transitional Eruptions [5] The well-monitored June 15, 1991, Mt. Pinatubo dacite Plinian eruption provided a first opportunity to clearly observe and to document the gradual evolution of the eruptive regime [Scott et al., 1996; Rosi et al., 2001; Torres, 2001]. Stratigraphic analysis combined with remote sensing data indicate that during the initial 3 hrs a
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gradual shift occurred from an early fully convective column, to a phase of partial collapse. This intermediate phase produced lapilli fallout and emplacement of variably diluted pyroclastic currents (syn-Plinian surges) which propagated up to 10 km from the crater. Dense pyroclastic flows with a maximum runout of 16 km associated with huge ash-carrying columns (co-ignimbrite columns) were produced during the following
