ERUPTIVE MECHANISMS OF THE NEAPOLITAN-YELLOW-TUFF INTERPRETED FROM STRATIGRAPHIC, CHEMICAL, AND GRANULOMETRIC DATA

The Neapolitan Yellow Tuff (12 ka) is the second largest pyroclastic deposit of the Campanian Volcanic Area covering at least 1000 km(2) with conservative estimates of volume placed at 40 km(3). Previous studies showed that this mainly trachytic deposit, composed of two members, was erupted by (1) a central-vent, mostly phreatoplinian phase (Lower Member) that generated pyroclastic surges and fallout reaching 34 km from the vent, followed by (2) a multiple-vent, phreatomagmatic and magmatic phase (Upper Member) associated with onset of caldera collapse that produced surges extending 14 km from the vent. The Lower Member is well bedded and comprises 13 subunits that alternate between phreatoplinian surges/flows and Plinian pumice-and-ash fallout. The Upper Member is relatively lithic-rich and more massive in character. For both members, magma compositions vary from alkali trachyte through trachyte to latite, which does not fit a simple inversion of magma chamber gradients. Calculations based on magma chemistry show an increase in magma density, a decrease followed by an increase in viscosity, and a general decrease of gas fraction during the course of both eruptive phases. After migrating to a depth of about 400 m, calculated fragmentation depths gradually rise during each phase. Application of sequential fragmentation/transport analysis to granulometric data shows for the lower member an average ratio of phreatomagmatic to magmatic components of 70:30 while the upper member shows an average ratio of 80:20. However, considering the tephra volume represented by samples, computed water/magma interaction ratios (R) are shown to fluctuate, but generally decrease from about 0.65 to 0.05 during eruption of the Lower Member while the Upper Member shows R fairly constant at about 0.1. Furthermore, surge/flow runout distances and estimates of eruptive velocities from R values show that column collapse heights were extremely high (6 to 7 km) during the first phase and were substantially lower during the second phase (2 to 3 km). Vent radii required for calculated eruption velocities of 180 to 370 m/s are between 70 and 300 m, suggesting a cumulative eruption duration of over 10 hours, perhaps spanning of one to several days.