MOBILITY OF A LARGE-VOLUME PYROCLASTIC FLOW - EMPLACEMENT OF THE CAMPANIAN IGNIMBRITE, ITALY

The trachytic Campanian Ignimbrite, originally exposed over a 30,000-km2 area around Naples, Italy, is the product of a highly energetic, gas-rich eruption. The deposit lies in valleys and isolated watersheds, and in its medial and distal extent to the south, north and east of Naples, the Campanian pyroclastic current encountered mountains exceeding 1 000 meters. Anisotropy of magnetic susceptibility (AMS) measurements indicate that the Campanian pyroclastic current (the transport system) traveled radially outward from the Phlegrean Fields area, but the ignimbrite-forming flow (the deposition system that developed from the base of the transport system) moved downslope from mountainsides to valleys, including slopes facing the eruption source, and flowed down drainage systems from intermontane basins. The Campanian pyroclastic current flowed approximately 35 km over the water of the Bay of Naples to deposit > 43 m of ignimbrite on the south shore and also overtopped a 685-1000-m-high ridge of the Sorrento Peninsula to deposit more on the other side. The distribution of the ignimbrite and the measured flow directions suggest that the Campanian pyroclastic current moved across the landscape as an expanded (and therefore turbulent) decompressing flow rather than as a high-density, nonturbulent sheet-like current moving over mountains by momentum acquired by eruption column collapse. Strong expansion is corroborated by shard morphology that indicates derivation from highly inflated pumice and suggests vesicles must have been at least 80% by volume of the original magma.