The volcaniclastic deposits of the main caldera and the evolution of the Galluccio Tuff of Roccamonfina volcano, Southern Italy
The south-west portion of the main caldera was mapped and a stratigraphy for the caldera-fill was constructed. The exact timing of formation of the main caldera is unclear; However, caldera collapse either predates or was synchronous with the eruption of the Campagnola Tuff. The proximal facies of the Campagnola Tuff exists as a complex relation of ignimbrite, lithic breccia and pyroclastic surge deposits. Overlying this the Galluccio Tuff a compound ignimbrite, ~6 km3 D.R.E, forms the base of the exposed caldera fill. Caldera lakes then became well established and following activity was predominantly phreatomagmatic. Pyroclastic surge deposits possess sand wave structures of several types and their migration direction was apparently controlled by the velocity/flow regime of the surge rather than the moisutre content. The morphology of juvenile clasts from phreatomagmatic deposits indicates that the eruptions were driven by a combination of vesiculation and magma/water interaction. The uppermost pyroclastic deposits are thought to represent the early phase of dome building where water still had access to the vent. The construction of the lava domes brought activity to a close within the main caldera. The Galluccio Tuff on the flanks of the volcano may be divided into three compositionally distinct eruptive units. The Lower Galluccio Tuff, correlated with the bulk of the Galluccio Tuff filling the main caldera. The Middle Galluccio Tuff commenced with the eruption of pumice-rich pyroclastic flows followed by flows enriched in both the size and amount of lithic fragments forming lithic-rich ignimbrite and co-ignimbrite lithic breccias of which several types exist. The Upper Galluccio Tuff is composed of lithic-rich ignimbrite which possess dense pumice fragments and are thought to be the product of a combination of both vesiculation and magma water interaction. Field relations indicate that pyroclastic flows were sometimes generated in quick succession and may have overrun earlier slower moving flows. Occasionally internal shear may have caused the overriding of portions of the same flow, these often coincide with lithic breccias and represent the climax of the eruptive phases. The grading of lithic fragments indicates that the expansion and fluidization decreased and yield strength increased with time in a pyroclastic flow.