Deposits and Physical Properties of Pyroclastic Density Currents During Complex Subplinian Eruptions: the AD 472 (Pollena) Eruption of Somma-Vesuvius, Italy
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Sedimentology (2007) 54, 607–635 doi: 10.1111/j.1365-3091.2006.00852.x Deposits and physical properties of pyroclastic density currents during complex Subplinian eruptions: the AD 472 (Pollena) eruption of Somma-Vesuvius, Italy ROBERTO SULPIZIO, DANIELA MELE, PIERFRANCESCO DELLINO and LUIGI LA VOLPE Dipartimento Geomineralogico, Universita` di Bari, and Centro Interdipartimentale di ricerca per la valutazione e mitigazione del RIschio SIsmico e VUlcanico (CIRISIVU), via Orabona 4, 70125 Bari, Italy (E-mail: [email protected]) ABSTRACT Small-scale pyroclastic density currents (PDCs) associated with the AD 472 (Pollena) eruption of Somma-Vesuvius, Italy, were generated by both magmatic and phreatomagmatic explosive fragmentation. The resulting deposits were emplaced under flow boundary conditions dominated by varying combinations of grain interaction, fluid escape and traction processes. Stratigraphic and lithofacies analysis of these PDCs offers a new perspective on the en masse versus progressive aggradation debate for PDC deposition. In particular, the analyses indicate that PDCs were density stratified with a basal underflow dominated by grain interactions. The underflows comprised trains of self-organized granular pulses of variable thickness and magnitude, depending on the overall particle concentration and fluid turbulence. A change in gradient between the upper and lower slopes of the volcano promoted deposition and the different pulses aggraded sequentially (stepwise). In this model each pulse stops en masse and the whole deposit aggrades progressively. Particle concentration, density, mean velocity, and flow height were assessed for the studied PDCs using different methods for massive and stratified deposits. The calculated mobility of the flows was 0Æ2to 0Æ3, in the expected range for small-scale PDCs. Keywords Flow parameters, pyroclastic density currents, sedimentology, Somma-Vesuvius, Subplinian eruptions. INTRODUCTION & Dingwell, 1996; Sparks et al., 1997; Dingwell, 1998). The particle concentration and eruption Pyroclastic density currents (PDCs) are among the style are crucial in determining the physical most complex and dangerous volcanic pheno- parameters (i.e. velocity, density, clast-support mena. They have a diverse origin forming during mechanism) and mobility of the flows (distance both explosive eruptions and as a consequence travelled versus difference in height between of gravity-driven collapse of lava domes. They source and deposit: Sparks, 1976; Middleton & may be short-lived (highly unsteady) or relatively Neal, 1989; Bonnecaze et al., 1993; Hallworth & long-lived (sustained unsteady to quasi-steady) Huppert, 1998; Branney & Kokelaar, 2002; Cao phenomena associated with either magmatic or et al., 2003). Regardless of whether they are phreatomagmatic fragmentation (e.g. Cas & Wright, concentrate or dilute suspensions of gas and 1987; Carey, 1991; Branney & Kokelaar, 2002). In particles, PDCs consist of two essential and inter- currents linked to explosive eruptions, the clast gradational parts: an underflow and a phoenix concentration reflects the eruption style (collapse plume (e.g. Fisher, 1966; Dade & Huppert, 1996; of a pyroclastic fountain or radial expansion of an Baer et al., 1997; Branney & Kokelaar, 2002). The over-pressurized jet), the volatile content of the underflow is denser than the atmosphere and erupting mixture, the degree of fragmentation and flows in direct contact with the ground, whereas the abundance of accidental fragments (Alidibirov the phoenix plume (or co-ignimbrite plume) is Ó 2007 The Authors. Journal compilation Ó 2007 International Association of Sedimentologists 607 608 R. Sulpizio et al. less dense than the atmosphere and lofts convec- OUTLINE OF THE POLLENA ERUPTION tively (Dobran et al., 1993; Sparks et al., 1997). Mass partitioning between the underflow and the This study builds on the work of Sulpizio et al. phoenix plume changes continuously during (2005), which identified three main phases in the motion and is particularly pronounced when a Pollena eruption: a first Subplinian phase with slope change induces sedimentation (concave-up oscillating sustained columns dominated by local curvature; Macias et al., 1998; Denlinger & pyroclastic fallout deposition (I); a second Sub- Iverson, 2001; Saucedo et al., 2004) or enhances plinian phase with generation of pyroclastic fall mixing with air (i.e. accelerating the moving and PDCs (II); and a final phreatomagmatic phase mixture, convex-up local curvature; Branney & dominated by generation of PDCs (III; Fig. 2A). Kokelaar, 2002), or where a change in the substrate The oscillating Subplinian phase produced (e.g. topographic jumps, surface roughness, stand- fall deposits of alternating lapilli and ash beds. ing water) affects the current (Fisher, 1990; Carey Massive, lithic-rich PDC deposits occur only at the et al., 1996; Gurioli et al., 2002). The rate and top of the unit attributed to this phase (LRPF; behaviour of mass partitioning between the under- Fig. 2A). Massive and dune-bedded PDC deposits flow and the phoenix plume influence the runout (S1,S2, NA and Fg1–2 deposits; Fig. 2A) makeup the distance of a current as well as the mass flux phase II succession, alternating with a pair of fall (e.g. Bursik & Woods, 1996; Branney & Kokelaar, deposits (L8 and L9) that record the climax of the 2002), density and grain size of pyroclasts (e.g. eruption. PDC deposits dominate the final phase III Taddeucci & Palladino, 2002) and rate of air phreatomagmatic stage, with alternation of dune- entrainment (e.g. Huppert et al., 1986; Woods & bedded deposits with internal cross-stratification Bursik, 1994; Nield & Woods, 2003). Therefore, (Sy) and massive, ash-rich deposits (Fy; Fig. 2A). data on physical properties of both solid particles The massive PDC deposits crop out mainly in the and the fluid phase are crucial for a better under- northern sector and in part of the south-eastern standing of the physics of PDCs and for improved sector of the volcano (Fig. 2C, D and F), although numerical simulation of their behaviour. scattered outcrops are reported in the western This paper presents the results of a study of sector (Di Vito et al., 1998). Dune-bedded PDC PDC deposits that originated during the AD 472 deposits (S units; Fig. 2A) crop out almost con- Pollena Subplinian eruption of Somma-Vesuvius, tinuously from the northern to the south-eastern Italy. The eruptive mechanisms and deposits of sector (Fig. 2B and E). The estimated total volume this eruption have been recently described by of the eruption (on land volume) was 1Æ53 km3, Rolandi et al. (2004) and Sulpizio et al. (2005). with 1Æ38 km3 consisting of fallout deposits (Sul- The Pollena eruption provides an ideal oppor- pizio et al., 2005) and only 0Æ15 km3 consisting of tunity to study PDC deposits, as the succession PDC deposits. The peak mass discharge rate (MDR) contains stacked deposits of flows that range from during the phases with sustained columns ranges 6 )1 very dilute to highly concentrated related to both between 7 · 10 kg sec (bed L4), and 3Æ4 · 7 )1 magmatic and phreatomagmatic activity (Sulpizio 10 kg sec (bed L8), equating to column heights et al., 2005). The dispersal and sedimentological between 14 and 20 km (Sulpizio et al., 2005). characteristics of the different PDC deposits are The composition of the erupted material ranges described drawing on a new geological map of from phonolite (first erupted) to tephritic phono- Somma-Vesuvius (Santacroce & Sbrana, 2003) lite (Fig. 3A). Variations of selected major and and detailed field study of more than 35 strati- trace elements with stratigraphic height are graphic sections (Fig. 1). Sampling and grain-size shown in Figure 3B. The SiO2 content shows a analysis of the concentrated PDC deposits allows regular decrease from the L4 bed, accompanied by the bulk density of the flow boundary at the time a significant fall in correspondence of NA unit and of deposition to be estimated. For the dilute PDC a slight increase in Fy unit (Fig. 3B). The contents deposits, flow parameters were constrained using of CaO and Ba show similar trends, with an the method of Dellino et al. (2004), which calcu- increasing abundance versus stratigraphic height lates the mean flow velocity and flow height by and almost constant concentration from NA unit assuming the flow is a turbulent boundary layer (CaO trend) and L6 top bed (Ba trend) up to Fy unit shear flow. The field and laboratory data presen- (Fig. 3B). Juvenile material (pumice and scoria) is ted here support a new model for transport and porphyritic, with a constant mineralogy (leucite, deposition of small-scale PDCs, which conjoins clinopyroxene, and minor sanidine, biotite and some principles of the en masse deposition and opaque grains) over the entire eruptive sequence. progressive aggradation theories. However, variations in texture and petrographic Ó 2007 The Authors. Journal compilation Ó 2007 International Association of Sedimentologists, Sedimentology, 54, 607–635 Sedimentological variability of pyroclastic density currents 609 Fig. 1. Location map of the study area. The pale brown area indicates the superficial exposures of the pyroclastic density currents (PDCs) of the Pollena eruption (modified from Santacroce & Sbrana, 2003). The main depositional fans on the northern and eastern slopes of Mt Somma are labelled from a to e. Blue lines and the blue arrow indicate the main valleys that channeled the PDCs. The red line defines the upslope depositional limit of PDCs of the Pollena eruption.