Vegetation dieback as a proxy for temperature within a wet pyroclastic density current: A novel experiment and observations from the 6th of August 2012 Tongariro eruption

Abstract

The 6th of August 2012 eruption of Te Maari (Mt Tongariro, New Zealand) generated wet pyroclastic density currents (PDCs) which caused widespread dieback of vegetation (singed, brown foliage) in their path. An absence of significant charcoal formation suggests that PDC temperatures were mostly below 250°C. Textural evidence for liquid water being present in the matrices during emplacement (vesicles) suggests that temperatures were b 100°C. We determined a probable minimum PDC temperature using an experiment replicating the critical temperatures required to induce foliar browning in seven species affected by the eruption. In locations where all species exhibited browned foliage (or were defoliated), temperatures were probably ≥ 64°C assuming a PDC duration of 60 s. In the more distal areas, where only the most susceptible species were browned while others remained healthy and unaffected, temperatures were probably around 51-58°C. These results have relevance to volcanic hazard mitigation and risk assessment, especially on the popular Tongariro Alpine Crossing.

The Fluid Mechanics of Pyroclastic Density Currents

“…The blue dashed line depicts an equivalent Rouse profile adapting the model of Valentine (1987). (Efford et al 2014). These proxies have indicated considerable variability in the thermal conditions in small-volume pyroclastic density currents (Rader et al 2015) but with a greater degree of homogenization and higher temperature in the bed load (Benage et al 2014).…”

“…Observed and inferred velocities of these currents range from 10 m/s to >300 m/s; they can transport volcanic ash from micrometers in size to clasts larger than 1 m; and the largest currents can travel 100 km and are remarkably mobile when encountering rough topography (Figure 1) (Branney & Kokelaar 2002, Bursik & Woods 1996, Fisher et al 1993, Miller & Smith 1977. Their temperatures range from eruptive (∼600-1,000 • C) to much cooler if the currents entrain a substantial amount of atmosphere or interact with water (Efford et al 2014, Heap et al 2014, Sheridan & Wang 2005, Wallace et al 2003). Pyroclastic density current deposits can be loose granular material or can be hot enough at deposition to weld together, forming a dense volcanic tuff (Figure 1).…”

Reconstruction of the 2014 eruption sequence of Ontake Volcano from recorded images and interviews

“…The melting temperature of typical chocolate is approximately 30 °C and that of the discolored part of hair is >100 °C in hot air (e.g., Yamashita et al 2012). The estimated temperature of the PDCs accompanying the eruption was higher than or equal to that of the 2012 eruption of Te Maari Volcano (100-64 °C in proximal, 51-58 °C in distal: Efford et al 2014), and lower than that of the 1900 eruption of Adatara Volcano (over 100 °C, under 400 °C: Fujinawa et al 2006).…”

New Zealand forest dynamics: a review of past and present vegetation responses to disturbance, and development of conceptual forest models

“…Minor eruptions from active volcanoes and geothermal areas continue to cause significant localised vegetation damage (e.g. active volcanoes in the central North Island such as Tarawera, Ngāuruhoe, and Ruapehu; Efford et al 2014).…”