WET SURGE DEPOSITS AT LA-FOSSA-DI-VULCANO - DEPOSITIONAL AND ERUPTIVE MECHANISMS

Wet surge deposits of different volcanic cycles of the recent Fossa activity at Vulcano have been measured on a bed-by-bed basis, with data recorded to millimeter detail. The wet surge layers are varicoloured with variable thickness, with the most recurrent thickness being about 1 cm. The beds consist of fine ash without internal structures. Textural features include: (a) accretionary lapilli, of maximum size of 0.5 cm, dispersed thoroughout the layer or forming continuous layers of submillimeter size; (b) vesiculated layers which represent 10% to 65% of the total deposit; vesicles have different shapes and smooth walls, varying in volume from 1% to 15-20%; (c) soft-sediment types of bedding deformation, such as gravity flowage ripples, load cast and slumps. The slope angle has not influenced either the concentration and size of the accretionary lapilli or the shape, size, and distribution of vesicles. Only the thickness of the layers decreases with distance from the vent. SEM investigations show features indicating the hydromagmatic origin of the deposits and stressing the role of the fluid phase. Noteworthy is the presence of vesiculated grains, produced by magmatic exsolution, which show chilling effects on the internal walls of the broken bubbles. Grain size analyses reveal that the layers are not graded and most of the samples have a median size finer than 50 μm. The grain size distributions are frequently polymodal, suggesting several closely timed explosions. As all the beds exhibit the same textures, grain size, and particle morphology a single mechanism can explain their deposition. The depositional unit formed at the base of the cloud through the lateral expansion on the ground of a sticky muddy medium consisting of ash and fluid. In general each layer lost its plasticity before the deposition of the next layer. The deposition occurred in a nearly continuous sequence without periods of rest long enough to permit erosional discontinuity. The eruptions are hydromagmatic and occur where magma, at least partially fragmented, comes into effective contact with subsurface water. The process follows a model suggested by Wohletz (1983b, 1986), developing in more than one fragmentation event. In the turbulent surge cloud both severe hydration and alteration of glassy grains and the formation of accretionary lapilli occur. © 1990.