Grain size distribution and sedimentology in volcanic mass-wasting flows: implications for propagation and mobility

The sedimentological characteristics of mass-wasting flow deposits are important for assessing the differences between phenomena and their propagation and emplacement mechanisms. In the present study, nine volcanic debris avalanche deposits and eight lahar deposits are considered, from the literature. Their sedimentology is expressed in the descriptive statistics: median grain size, sand, gravel and finer particle fractions, skewness and sorting. Analysis of the data confirms that lahars and debris avalanches diverge in their grain size distribution and in their evolution during propagation. Water saturation in lahars is the main factor enabling debulking, a mechanism that is not recorded in the data derived from debris avalanches deposits. On the contrary, evidence of comminution of particles due to particle-particle interactions is observed in debris avalanches, and not in lahars. These findings support previous studies suggesting that although water content in debris avalanches plays a role in propagation, the effects of inertial collision of solid fragments are more important than fluid effects, confirming that particle-particle interactions are the main factor influencing the mobility of non-saturated mass wasting flows.