Complex Proximal Geometry of Fall Deposits From a Plinian Eruption: Implications for Eruption Dynamics

The 1886 eruption of Tarawera, New Zealand, is unusual for a Plinian eruption because (1) it involved entirely basaltic magma and (2) it produced abundant proximal deposits with a complex geometry not predicted by standard models of Plinian eruption columns. The eruption occurred along a 17 km fissure that extended from Mt Tarawera to Rotomahana. The Plinian activity is interpreted to be restricted to the 8 km segment on Mt Tarawera. During the 5 hour eruption, over 50 point source vents were active along this segment with a variety of styles and dispersals. These vents primarily produced localized, cone-building tephra fall, while at the same time some vents contributed to the Plinian plume in a random and spasmodic fashion. The proximal deposits that resulted comprise a series of lensoid packages that can be mapped along continuous exposures on both sides of the 1886 fissure. We map package thicknesses on cross sections using a combination of field observations, photographs, and detailed stratigraphic logs. The cross sections allow us to determine which vents contributed to the proximal deposits at any instant in time, and thus to contrast style and intensity among vents. There is clear evidence for rapid and localized accumulation of bombs and lapilli coeval with the Plinian plume. Clasts from both low fountains and high plume were deposited simultaneously, thus a range of explosive styles is represented by the particles in each sample. The 1886 dispersal data require much more complex models for the velocity distribution in the lower portion of the plume than are afforded by existing numerical models.