Ash clouds temperature estimation. Implication on dilute and concentrated PDCs coupling and topography confinement

Abstract

Pyroclastic density currents (PDCs) are among the most hazardous of all volcanic processes in terms of high speeds and unpredictable extent. While concentrated PDCs are usually topographically confined, the dilute counterpart (ash cloud) is able to overrun topographic barriers, with unexpected trajectories posing a high risk for human settlements around the volcano. Here, for the first time, the temperature of an ash could, for a PDC originated during the 11 July, 2015 Volcán de Colima eruption, is determined, without pre-installed instruments, based on the degree of charcoaling of trees affected by the ash cloud. Temperature estimations were performed using Reflectance analysis and microtomography images processing of pine wood charred fragments. The combination of these two independent and well-established methods to organic matter charred in a volcanic environment constitutes a pioneering attempt for the indirect temperature estimation of dilute pyroclastic density currents (PDCs). Charcoal fragments were sampled at different heights along tree trunks outstanding from the PDC deposit. Both the temperatures obtained from charcoal analyses (reflectance and microtomography) and observation of damages to the tree trunks allowed to distinguish: (i) a lower Zone A, which extends 150–180 cm above the top of the PDC deposit, where trunks show peeled bark and multiple lithic impacts; temperature values are equal or slightly higher than the underlying deposit for the entire length of the valley; (ii) an upper Zone B, developed above 150–180 cm from the top of the PDC deposit, where trees are only burned without any block impact marks; temperature estimations for Zone B are comparable with the PDC deposit temperature range from proximal to distal areas. The temperature data indicate that the 11 July, 2015 Colima PDC event, the ash cloud was always thermally coupled with the under-running concentrated flow for the entire length of the ravine, explaining the observed strong vertical uplift of the ash cloud and the substantial absence of ash cloud detachments along flow. A corollary of our study is that, should a detachment have occurred, the ash cloud surge would have had initial temperatures as high as the one carried by the high concentration part of the PDC. A major outcome of our study is that the temperature estimation of ash clouds bears important implication in terms of hazard assessment for pyroclastic density currents along narrow valleys that usually cut the steep slopes of stratovolcanoes.

Preservation of neurons in an AD 79 vitrified human brain

“…These remains are unique for the excellent quality of tissue preservation, giving us the opportunity to examine in detail the ultrastructure of a 2000-year-old human brain: due to a natural process of vitrification, at Herculaneum the CNS was “frozen” in its native condition, preserving intact remnant cell structures in the neuronal tissue [ 9 ]. The conversion of human tissue to glass (vitrification) occurred as a result of the rapid cooling of the volcanic ash deposit after exposure to the hot ash cloud at a temperature of about 500°C [ 10 – 12 ]. Previous heating bone experiments showed analogous temperatures [ 13 ] that were also confirmed by our recent reflectance analysis on carbonized wood from Herculaneum [ 9 ].…”

Energy facies: A global view of pyroclastic currents from vent to deposit

“…This will locally further result in a forced pyroclastic current and might result in deposit bypassing and emplacement at the break in slope, in strongly positive gradients of the topological aspect ratio and competence ratio for juvenile and lithic clasts, and in an abrupt decrease of the emplacement temperature with distance (Table 2). This facies has been recognized for Colima (Pensa et al, 2019; see Table 1).…”

“…An example is in a channelization or 2). This facies has been recognized for Colima, whereas Energy facies 1 and 2 have been recognized for Bolsena WOB and the Deseado Caldera (Pensa et al, 2019;Palladino & Pettini, 2020; see Table 1).…”

The hazards of unconfined pyroclastic density currents: a new synthesis and classification according to their deposits, dynamics, and thermal and impact characteristics

“…A single PDC is commonly composed of two distinct layers -the denser, gravity driven basal "flow" layer and a more dilute, buoyant upper "surge" layer (Fisher 1995). While these layers can flow in unison under certain conditions (e.g., coupled PDCs in the 2015 eruption at Colima, Mexico; Pensa et al 2019), it is common that they behave independently of each other, with the upper and lower layers moving at different speeds and having different characteristics (Breard and Lube 2017). The dense basal layer of a PDC (the flow) is topographically constrained so that their path typically remains confined to within a pre-existing channel, while the dilute, upper layer of PDCs (the surge) is less topographically constrained.…”