What Grain Size Distribution And Sedimentology Can Tell Us About The Propagation Of Volcanic Mass Wasting Flows

Abstract:

Debris avalanches and lahars with high mobility are among the most destructive and hazardous mass flows in volcanic environments; making them important to understand from a hazard assessment perspective. Models of their propagation and emplacement need to account for this mobility, while being consistent with deposit sedimentology and geomorphology. Thus, sedimentological characteristics of mass-wasting flow deposits are important for assessing their propagation and emplacement mechanisms and differentiating between flow types. Here, we compare the sedimentology of volcanic debris avalanches and lahars, using data published in the literature, in order to identify differences and understand their propagation mechanisms. We compare their sedimentology and grain size distribution as expressed by the descriptive statistics: median grain size, sand, gravel and finer particle proportion, skewness, and sorting.

Results suggest that lahars and debris avalanches diverge in their sedimentology and grain size distribution evolution during propagation, even when sourced from the same material. Increasing bimodality, evolution to negative skewness, with decreasing sediment size, accompanied by very poor sorting suggest comminution of particles due to particle-particle interactions in debris avalanches. Instead, preferential deposition of the coarsest particles and improved sorting suggest that the decrease in grain size of lahars is the result of debulking. The divergence results from the high water content of lahars, in contrast to debris avalanches, introducing different processes during propagation. Therefore, the findings support previous studies suggesting that debris avalanches can be considered as dense granular flows where the effect of inertial collisions of solid fragments are more important than fluid effects.