Grain-size features of two large eruptions from Cotopaxi volcano (Ecuador) and implications for the calculation of the total grain-size distribution

Studies of grain-size distributions of explosive volcanic eruptions provide important insights into fragmentation mechanisms and eruptive conditions and are crucial to the modeling of tephra dispersal. As a result of sedimentation processes and plume dynamics, grain-size features vary significantly both in the downwind and crosswind directions and are difficult to characterize. We have analyzed grain-size features in the downwind and crosswind directions of the two largest eruptions of the last 2000 years of Cotopaxi volcano activity (Ecuador). Crosswind grain-size variations are similar for both eruptions (i.e., layers 3 and 5), while at any given downwind distance from vent, the layer 3 deposit is coarser than the layer 5 one. This suggests that layers 3 and 5 were characterized by similar plume height but that layer 3 was advected by a stronger wind. In addition, both deposits are coarsest along the dispersal axis and become richer in ash in the crosswind direction showing a Gaussian decreasing rate. Deposit thickness also shows a Gaussian crosswind decay, but layer 3 is significantly thicker at all points than is layer 5 due to the former's larger erupted mass. Based on both quantitative analysis of field data and on numerical simulations, we show that tephra deposits associated with large explosive eruptions (i.e., plume height of 30 km) should be sampled out to at least 200 km from the vent (depending on wind speed and tropopause height) in order to derive complete grain-size distributions that are not depleted in fines. Eruptions occurring in a strong wind field at high latitudes (e.g., Iceland) require lesser representative-sampling distances because of the lower tropopause heights.