Vegetation damage as a proxy for physical characteristics of PDCs

As one of Ecuador’s most active volcanoes, Tungurahua poses a constant threat to the surrounding area. Pyroclastic density currents (PDCs) are the most significant hazard owing to their mobility and the difficulty of measuring their physical properties during emplacement. We present a study of the deposits from the 2006 eruptions at Tungurahua, which produced PDCs that descended multiple valleys, destroying over a dozen towns and claiming at least six lives. There were two main periods of activity during the summer of 2006: July 14-16, which produced 20 PDC events, and August 16-17, when 32 PDCs descended the flanks in only eight hours. The deposits consist of 3 main facies: scoria-bomb accumulations, ash-rich deposits, and ash cloud surges surrounding the dense flows. The currents were generated by boiling-over activity, and the deposits are characterized by concentrations of juvenile bread crust bombs up to 2 m in diameter predominately in levees and flow snouts. Two of the largest deposits are in Juive Grande and Cusua valleys, which experienced 12 and 26 events, respectively. Despite the petrologic and granulometric similarities of the two valley deposits, they have markedly different emplacement characteristics as recorded by damage to vegetation. In Juive Grande, wood (up to 8 cm diameter) was charred to its center at multiple locations. At Cusua, vegetation is charred only within the outermost few millimeters and has no interior damage; no completely charred wood was found in Cusua. To address this paradox, damaged vegetation was collected from both deposits to examine emplacement temperature, velocity, and particle concentration of the flows using laboratory experiments. Methods include charring wood in lab using materials from PDCs at Tungurahua, abrasion of wood with ash and lapilli, examination of cellular damage to wood using the SEM, and vitrinite reflectance measurements. Charring experiments conducted at varying temperatures with different wood diameters indicate that the deposits at Juive Grande remained hotter for a longer time than those at Cusua. An SEM examination of the wood samples collected from Juive Grande shows that cell walls in wood from Juive Grande have been homogenized, indicating emplacement temperatures above 300-325°C. Abrasion simulations involve firing pyroclastic material collected from the 2006 eruptions at green trees, which will provide constraints on the density, concentrations, and forces within the PDC as they descended the valleys. Also, applying the modeling of Clarke and Voight (2000) to trees snapped in half in zones inundated by dilute ash-cloud currents, it is possible to estimate the dynamic pressure, density, and velocity of the PDCs. Using observed damage to vegetation in and near the PDC deposits, laboratory simulations, and modeling, it is possible to quantify physical properties of the PDCs that descended Tungurahua in 2006, including emplacement temperature, density, concentration, and dynamic pressure, and how they change in both time and space. Furthermore, these methods are not dependent on specific events at Tungurahua, and are therefore applicable at other PDC localities.