Complexities of plinian fall deposition at vent: an example from the 1912 Novarupta eruption (Alaska)

An extremely proximal ejecta ring, with exposures to within 100 m of vent, was deposited during later-stage plinian fall activity during the 1912 Novarupta eruption in Alaska. One bed in the ejecta ring (bed S) contains predominantly andesitic clasts which serve to delineate the striking contrast in thinning rates along dispersal axis of the ejecta ring [Pyle bt values of 70 m (bed S alone) or 190 m (whole ejecta ring)] and the coeval dacitic plinian fall deposits [Pyle bt values of 4 km (proximal) and 37 km (medial-distal)]. The locally deposited andesitic and dacitic clasts of the ejecta ring are interpreted as products of an irregular ‘collar’ of low-fountaining ejecta partially sheathing the core of higher-velocity dacitic ejecta that fed the stable, converting 23-km-high column. The presence of such an extremely proximal accumulation of ejecta appears to be a feature common to several other historic eruptions that generated widespread fall deposits. This feature in part accounts for conflicts between measured and calculated values for thickness maxima in plinian fall deposits and suggests that modifications may be required of existing models for plinian eruption columns.