Variations in juvenile pyroclast density-size distributions among layers, pits, and eruptions of silicic magmas

Abstract:

Characterizing variations in pyroclast density with size is imperative for creating accurate tephra dispersion models. However, current models use single values of density or simplified linear trends from ash to lapilli for a given magma composition. Recent studies report up to three-fold increases in density over a small decrease in pyroclast size from the millimeter to micron scale, a trend that holds for both mafic and silicic pyroclasts. However, the reason for the overall sigmoidal shape of a density-size trend, the size at which density begins to drastically increase, and the rapidity of the change is not fully understood. Increases in density can correlate with shifts in the internal texture of clasts and lateral location within the conduit during an eruption, as recently evidenced by rhyolitic pumices from the 1060 CE eruption of Medicine Lake Volcano. The density-size trend for basaltic scoria from the 2006 Tungurahua eruption varied with sampling location, though the reason for which has yet to be explored.

Here we compare density-size trends for pyroclasts 0.125 mm < d < 64 mm from multiple layers and 3 pits from the 1300 BP rhyolitic eruption of Newberry Volcano, OR. First, we examine whether these density-size trends differ significantly from layer to layer in a given pit and whether these increases correspond to changes in the overall size distribution of the pyroclasts and/or in internal texture (e.g. a shift in predominance of circular-bubble-bearing pumices to elongate-bubble-bearing pumices with increasing density). Second, we assess the consistency in these density-size trends for a single layer across pits, and whether these trends can be used to stratigraphically correlate layers. Third, we contextualize our results with those of pyroclasts from the 1060 CE eruption Medicine Lake Volcano to quantify differences in density-size trends amongst similarly-silicic plinian eruptions. In doing so, we will create a ground-truthed density-size model for rhyolitic pyroclasts that can be used for tephra dispersion modeling. Preliminary results show that density-size trends for silicic eruptions tend to be sigmoidal in shape; however, the range over which density rapidly increases with decreasing clast size varies by eruption. This change in curvature may mirror variations in internal texture as a result of differing conduit dynamics.