Bull Volcanol (2003) 65:55–72 DOI 10.1007/s00445-002-0241-2 RESEARCH ARTICLE Michael H. Ort · Giovanni Orsi · Lucia Pappalardo · Richard V. Fisher Anisotropy of magnetic susceptibility studies of depositional processes in the Campanian Ignimbrite, Italy Received: 24 August 2000 / Accepted: 15 August 2002 / Published online: 1 November 2002  Springer-Verlag 2002 Abstract The late Pleistocene trachytic Campanian Ign- imbrite underlies much of the Campanian Plain near Naples, Italy, and occurs in valleys in the mountainous area surrounding the plain out to about 80 km from its source, the Campi Flegrei caldera. At sites within 15 km of the Campi Flegrei, anisotropy of magnetic suscepti- bility (AMS) principal directions indicate that, in the absence of significant topography, deposition came from a flow moving in a roughly radial direction. AMS studies of the more distal ignimbrite reveal downhill and/or downvalley flow directions prior to deposition, even where these directions are at high angles to a generally radial transport direction from the vent. On the flanks of Roccamonfina Volcano, flow was directly downhill, as if the source of the ignimbrite was the summit of the volcano. In most localities, the ignimbrite consists of a single massive deposit. In a few localities in the Apennine Mountains, however, the confluence of multiple drainage systems off mountains resulted in multiple local flow units that cannot be correlated between valleys. A detailed study of the ignimbrite in the flat Titerno River valley near Massa shows that the AMS fabrics are not due to late-stage creeping during deposition or compaction. Well-defined, but non-parallel AMS fabrics from vertical and lateral sections in the Massa area are best explained by the merging of gravity currents flowing down the valley and steep valley sides to form a single aggrada- tional deposit. Clast compositions and AMS axes at Mondragone indicate that the pyroclastic flow encoun- tered the Monte Massico massif and was partially blocked, so that flow during deposition was toward the Campi Flegrei. Similar AMS data from sites along the edge of the Campanian Plain indicate back-flow off the first ridge of the Apennine Mountains reached at least 5 km from their base. The Campanian Ignimbrite was deposited from a density-stratified pyroclastic flow. The depositional system consisted of the lower, denser portion of the current, and was controlled by topography. The grouping of the AMS axes is interpreted to indicate that deposition occurred under laminar flow conditions. Keywords Anisotropy of magnetic susceptibility · Campanian Ignimbrite · Depositional system · Pyroclastic flows Introduction Fisher et al. (1993) presented field and laboratory data for the Campanian Ignimbrite and concluded that it was deposited from a density-stratified pyroclastic flow of considerable thickness (hundreds of meters). They also suggested that the pyroclastic flow was separated into transport and depositional systems, with deposition occurring only from the dense base of the current. The deposits were interpreted to reflect the conditions at the base of the flow, but other characteristics indicated that the upper parts were more dilute. In this paper, we present more information, primarily from anisotropy of magnetic susceptibility (AMS) studies, on the depositional processes involved in the emplace- ment of the Campanian Ignimbrite. We use AMS axis directions to interpret the flow directions during deposi- tion at each locality and test different emplacement models. We concur with Fisher et al. (1993) that the Campanian Ignimbrite was deposited from a depositional system that was decoupled from the dilute transport system of the pyroclastic flow. At all localities sampled Editorial responsibility: J. McPhie M.H. Ort ( ) ) Center for Environmental Sciences and Education and Department of Geology, P.O. Box 4099, Northern Arizona University, Flagstaff, AZ 86011-4099, USA e-mail: michael.ort@nau.edu Tel.: +1-928-5239363 Fax: +1-928-5239220 G. Orsi · L. Pappalardo Osservatorio Vesuviano, Via Diocleziano 328, 80124, Napoli, Italy R.V. Fisher Department of Geological Sciences, University of California, Santa Barbara, CA 93106, USA (Deceased)