R Cioni

Magmatic and phreatomagmatic phases in explosive eruptions of Vesuvius as deduced by grain-size and component analysis of the pyroclastic deposits

Abstract Grain-size and component analyses have been carried out on pyroclastic deposits of three well-known explosive eruptions of Vesuvius: those of 79 A.D., 1631 and 1906. These eruptions cover a wide energy spectrum, from Plinian to Strombolian, and each includes a transition from a magmatic to a well-documented and clearly distinguishable phreatomagmatic phase of activity. The chronicles of the eruptions are revisited in light of the granulometric and component data obtained from the study of the deposits and interpretation of the dynamics of the main eruptive episodes is provided for each eruption. The phreatomagmatic products (originated from explosive interaction of magma with groundwater) have characteristic grain-size and component distribution patterns distinct from associated purely magmatic pyroclastic products: in any eruption phreatomagmatic products exhibit: (a) a marked increase in both the lithic/juvenile and the crystal/total juvenile ratios, and (b) a preferential fragmentation of the juvenile fraction. Moreover, eruption energy, degree of evolution of erupted magma, lithic content and depth of provenance are all clearly correlated. The abundance of lithics and crystals in phreatomagmatic deposits can be ascribed to (a) preferential fragmentation of the aquifer-hosting rocks due to explosive vaporization of ground water; (b) vent flaring and craterization possibly related to overpressure conditions following the entrance of large quantities of steam in the conduit; (c) indirect enrichment in both crystal and lithic fractions by removal of juvenile fines from the eruptive cloud. In each eruption some of the deposits with “phreatomagmatic” component distribution patterns also exhibit evidence of steam condensation (accretionary lapilli, vesiculated tuff, mud coating, soft deformations) and some do not: these are therefore to be considered sufficient but not necessary diagnostic features for a hydromagmatic origin of the deposit (“wet” in the sense of Sheridan and Wohletz, 1983). “Dry” phreatomagmatic products correspond to higher energy conditions allowing production and maintenance of overheated steam. The magma/water ratio in terms of heat exchange surface (i.e. the degree of primary fragmentation of the magma) is certainly the main factor controlling the efficiency and the energy balance of the interaction. A series of observations strongly suggest that a primarily fragmented magma is a basic requisite for the explosive magma/water interaction: (a) occurrence of short phreatomagmatic episodes interrupting lava fountains activity (1906 eruption) in connection with drawdown of the magma column in the conduit; (b) parallel increase in depth of magma fragmentation level and magma/water interaction level (as deduced by the nature of lithics), and (c) preferential occurrence of phreatomagmatic activity at the end of explosive eruptions (when juvenile gas pressure declines and water from the confining aquifer more easily enters the conduit).