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.
The emplacement temperatures of three ignimbrites belonging to the 4.6-ka Fogo A plinian eruption sequence in São Miguel Island (Azores, Portugal) were determined using partial thermal remanent magnetization (pTRM) of lithic clasts and reflectance (Ro%) of charcoal fragments embedded within the deposits and collected at the same localities close to each other. The Fogo A sequence is characterised by a complex stratigraphy consisting of a thick plinian deposit interbedded with two intraplinian ignimbrites (here named “pink” and “black” intraplinian ignimbrite, respectively) and capped by a final ignimbrite (here named “dark brown” ignimbrite). A total of 140 oriented lithic clasts from the three ignimbrites were collected from 15 localities distributed along the northern and southern flanks of the volcano. The pTRM analyses show different paleomagnetic behaviours, which correspond to different emplacement temperatures of the ignimbrites. The emplacement temperatures of the pink and black intraplinian ignimbrites inferred from pTRM analysis were respectively ≥400 and ≥600 °C; the temperatures of the dark brown ignimbrite are lower, estimated between 300 and 350 °C. Thermal estimations of three key sites were compared with the results of the analysis of reflectance (Ro%) measured on eight specimens derived from charcoal fragments collected from the pink intraplinian ignimbrite and the dark brown ignimbrite. Results indicate Ro% values between 1.61 and 1.37 for the pink intraplinian ignimbrite, whereas fragments collected from the dark brown ignimbrite show Ro% values between 0.85 and 0.50. No charred wood was found in the black intraplinian ignimbrite. Ro% values indicate that charcoal fragments in the pink intraplinian ignimbrite reached temperatures of 380–460 °C, whereas the Ro% values of the dark brown ignimbrite indicate slightly lower temperatures of 330–350 °C. TRM and Ro% results are comparable and validate the use of both methods. Greatest accuracy in the determination of emplacement temperatures of ignimbrites is achieved when both methods can be applied at the same locations
The 4.6 ka Fogo A Plinian eruption was a caldera-forming volcanic event on São Miguel Island, Azores. The deposit succession is very complex, composed of a thick trachytic Plinian fallout deposit interstratified with two intra-Plinian ignimbrites (named “pink ignimbrite” and “black ignimbrite” sequentially). The succession ends with a main ignimbrite (named “dark brown ignimbrite”), which represents the deposit of complete collapse of the eruption column and the end of the eruption. In this work, emplacement temperatures of the three ignimbrites are estimated by study of partial thermal remanent magnetization (pTRM) of lithic clasts. A total of 140 oriented lithic clasts were collected from 15 localities distributed along the northern and southern flanks of Fogo volcano. The paleomagnetic data reveal different emplacement temperatures and thermal histories that were experienced by each ignimbrite. The results indicate the presence of five different paleomagnetic behaviours that suggest emplacement temperatures of 350–400 °C for the first (pink) intra-Plinian ignimbrite, temperatures higher than 580–600 °C for the second (black) intra-Plinian ignimbrite and 250–370 °C for the last (dark brown) climactic ignimbrite. The thermal history experienced by each pyroclastic flow and its ignimbrite deposit was also assessed by the use of the magnetite-ilmenite geothermometer to determine the pre-eruptive magma temperature (estimated to be around 900 °C). We interpret the different emplacement temperatures of the Fogo A ignimbrites as being due to a combination of factors. These include (i) collapse from different heights of the eruption column and the resultant different amounts of air entrainment into the gas-particle mixture, (ii) variable content of lithic clasts and (iii) different types of juvenile clasts in the ignimbrites
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