Emplacement Temperature of the Overbank and Dilute-Detached Pyroclastic Density Currents of Merapi 5 November 2010 Events using Reflectance Analysis of Associated Charcoal

Abstract

Merapi eruption in 2010 produced 17 km high column of ash and southward pyroclastic density current (PDC). Based on the deposits characteristics and distributions, the PDC is divided into channel and overbank facies (pyroclastic flow), and associated diluted PDC (pyroclastic surge). The hot overbank PDCs and the associated dilute-detached PDCs are the main cause of high casualty (367 fatalities) in medial-distal area (5–16 km), especially near main valley of Kali Gendol. We reported the emplacement temperature of these two deposits using reflectance analysis of charcoal. We used both entombed charcoals in the overbank PDC and charcoals in singed house nearby. Samples were collected on 6–13 km distance southward from summit. Charcoalification temperatures of the entombed charcoals represent deposition temperature of the overbank PDCs, whereas those of charcoals in the singed house resembles temperature of the associated dilute-detached PDCs. Results show mean random reflectance (Ro%) values of entombed charcoal mainly range 1.1–1.9 correspond to temperature range 328–444 °C, whereas charcoal in singed house range 0.61–1.12 with estimated temperature range 304–358 °C. The new temperature data of the dilute-detached PDCs in the medial-distal area is crucial for assessing impact scenarios for exposed populations as it affects them lethally and destructively

Generation of Overspill Pyroclastic Density Currents in Sinuous Channels

“…Pensa et al (2018) estimated the temperatures of the unconfined ash cloud from the 2015 Volcán de Colima eruptions and found temperatures of ∼200°C which was approximately ∼170°C lower than that of the valley-confined area using charcoal reflectance methods. At Merapi using the 2010 deposits, Wibowo et al (2018) estimated temperatures of detached PDC of the overbank to be ∼300-440°C which is close in temperature to estimates of the channelized flow. The significant cooling due to spreading on an unconfined surface is mirrored in our simulations.…”

“…At Merapi using the 2010 deposits, Wibowo et al. (2018) estimated temperatures of detached PDC of the overbank to be ∼300–440°C which is close in temperature to estimates of the channelized flow. The significant cooling due to spreading on an unconfined surface is mirrored in our simulations.…”

“…In the Merapi 2010 eruption, lethal PDCs overspilled at Bronggang, approximately 13.5 km from the summit, causing fatalities both outside and inside homes due to burns (Baxter et al, 2017;Jenkins et al, 2013). The temperatures in the overspilled current and the channelized flow are estimated at 200-300°C for Merapi 2010 eruption (Trolese et al, 2018;Wibowo et al, 2018) and 300-350°C for the Volcán de Colima 2015 eruption (Pensa et al, 2018(Pensa et al, , 2019. These high temperatures suggest that the decoupled current was fed by the insulated dense underflow and not fully detached.…”

“…High temperatures are a particular hazard of channelized PDCs which can transport material nearly isothermally from initiation to deposition (Trolese et al, 2018;Wibowo et al, 2018). The overspilled portions show similar or slightly lower temperatures to the valley confined deposits (Baxter et al, 2017;Pensa et al, 2018Pensa et al, , 2019Trolese et al, 2018).…”

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

“…Overspill PDCs maintain both their high temperature and dynamic pressure, and thus cause damage through both heat and force. Parent channel-confined flows from hot lava dome collapse can be over 600 °C, and fast overspills have been determined in several occurrences to have temperatures at or near their parent flows (e.g., Trolese et al 2018, Wibowo et al 2018, which can have dynamic pressures over 100 kPa near source and over 15 kPa in distal areas (Macorps et al 2018). As with channel-confined PDCs, overspill flows typically destroy everything in their path, and cause the death and burial of people in inundated areas.…”