Bull Volcanol (2003) 65:418–432 DOI 10.1007/s00445-002-0268-4 RESEARCH ARTICLE Margherita Polacci · Laura Pioli · Mauro Rosi The Plinian phase of the Campanian Ignimbrite eruption (Phlegrean Fields, Italy): evidence from density measurements and textural characterization of pumice Received: 30 January 2002 / Accepted: 22 November 2002 / Published online: 8 February 2003  Springer-Verlag 2003 Abstract Textural characterization of pumice clasts from explosive volcanic eruptions provides constraints on magmatic processes through the quantification of crystal and vesicle content, size, shape, vesicle wall thickness and the degree of interconnectivity. The Plinian fallout deposit directly underlying the Campanian Ignimbrite (CI) eruption represents a suitable case to investigate pumice products with different textural characteristics and to link the findings to processes accompanying conduit magma ascent to the crater. The deposit consists of a lower (LFU) and upper (UFU) pumice lapilli bed generated by the sub-steady eruption of trachytic magma with <5 vol%. crystals and a peak discharge rate of 3.210 8 kg/s. Density measurements were performed on samples collected from different stratigraphic intervals at the Voscone-type outcrop, and their textural characteris- tics were investigated at different magnifications through image analysis techniques. According to clast densities, morphologies and vesicle textures pumice clasts were classified into microvesicular (heterogeneous vesicles), tube (elongated/deformed vesicles) and expanded (coa- lesced/inflated vesicles).The combination of density data and textural investigations allowed us to characterize both representative areas and textural extremes of pumice products. Bulk vesicularity spans a broad interval varying from 0.46 to >0.90, with vesicle number density ranging from 10 7 –10 8 cm 3 . The degree of vesicle coalescence is high for all pumice types, with interconnected vesicles generally representing more than 90% of the bulk vesicle population. The results show a high degree of heteroge- neous textures among pumice clasts from both phases of the eruption and within each eruption phase, the different pumice types and also within each single pumice type fragment. The origin of pumice clasts with different textural characteristics is ascribed to the development of conduit regions marked by different rheological behavior. The conclusions of this study are that vesicle deformation, degree of coalescence and intense shear at the conduit walls play a major role on the degassing process, hence affecting the entire conduit dynamics. Keywords Coalescence · Conduit · Phlegrean Fields · Plinian eruption · Reticulite · Tube pumice · Vesiculation Introduction Motivation for the study Pumice is commonly found in volcanic deposits and, along with ash and loose crystals, represents one of the three main types of juvenile products discharged by explosive volcanic eruptions (Cas and Wright 1987). Quantification of pumice clast morphologies and textures has proved to be an important complementary approach in studying conditions related to magma vesiculation, frag- mentation and conduit ascent to the earth’s surface (Houghton and Wilson 1989; Klug and Cashman 1994, 1996; Gardner et al. 1996, 1998; Hammer et al. 1999; Polacci et al. 2001; Klug et al. 2002). Pumices from pyroclastic deposits differ in their macroscopic properties, such as color and shape, but they also show distinct microscopic features generated by different vesicularities, crystallinities, vesicle and crystal sizes, shapes and distributions, vesicle wall thickness and extent of vesicle interconnectivity (magma permeability). Previous studies found that this wide morphological and textural variability characterizes pyroclasts discharged during different phases or even within the same eruption phase and related the findings to either variations in eruption dynamics (Gardner et al. 1996; Klug et al. 2002) Editorial responsibility: J. Stix M. Polacci ( ) ) Istituto Nazionale di Geofisica e Vulcanologia, via della Faggiola 32, 56126 Pisa, Italy e-mail: polacci@pi.ingv.it Tel.: +39-050-8311932 Fax: +39-050-8311942 L. Pioli · M. Rosi Dipartimento di Scienze della Terra, v. S. Maria 53, 56126 Pisa, Italy