THE EMPLACEMENT OF INTERMEDIATE VOLUME IGNIMBRITES - A CASE-STUDY FROM ROCCAMONFINA VOLCANO, SOUTHERN ITALY

A model is presented for the emplacement of intermediate volume ignimb rites based on a study of two approximately 6 km3 volume ignimbrites o n Roccamonfina Volcano, Italy. The model considers that the flows were slow moving, and quickly deflated from turbulent to non-turbulent con ditions. Yield strength and density increased whereas fluidisation dec reased with time and runout of the pyroclastic flows. In proximal loca tions, on the caldera rim, heterogeneous exposures including discontin uous lithic breccias, stratified and cross-stratified units interbedde d with massive ignimbrite suggest deposition from turbulent flows. In medial locations thick, massive ignimbrite occurs associated with thre e types of co-ignimbrite lithic breccia which we interpret as being em placed by non-turbulent flows. Multiple grading of different breccia/l ithic concentration types within single flow units indicates that inte rnal shear occurred producing overriding or overlapping of the rear of the flow onto the slower-moving front part. This overriding of differ ent parts of non-turbulent pyroclastic flows could be caused by at lea st two different mechanisms: (1) changes in flow regime, such as hydra ulic jumps that may occur at breaks in slope; and (2) periods of incre ased discharge rate, possibly associated with caldera collapse, produc ing fresh pulses of lithic-rich material that sheared onto the slower- moving part of the flow in front. We propose that ground surge deposit s enriched in pumice compared with their associated ignimbrite probabl y formed by a flow separation mechanism from the top and front of the pyroclastic flow. These turbulent clouds moved ahead of the non-turbul ent lower part of the flow to form stratified pumice-rich deposits. In distal regions well-developed coarse, often clast-supported, pumice c oncentration zones and coarse intra-flow-unit lithic concentrations oc cur within the massive ignimbrite. We suggest that the flows were non- turbulent, possessed a relatively high yield strength and may have mov ed by plug flow prior to emplacement.