Volcanic deposits accumulated over time along the flanks of volcanoes can be an important risk factor as they can cause pyroclastic flows in the months and years following eruptions.
Through new and in-depth investigations into the eruption of the Guatemalan volcano Fuego which took place on June 3, 2018, it was discovered that the devastating pyroclastic flow that destroyed the village of San Miguel de Los Lotes and which caused hundreds of victims, was actually caused by the collapse of lava and pyroclastic material, i.e. from the set of products emitted during the activity of the volcano which had accumulated in the upper part of the volcano itself in the weeks and months preceding the eruption.
This is the result presented in the study " recently published in the AGU's 'Journal of Geophysical Research- Solid Earth'.
The research, carried out by an international team of scientists from the National Institute of Geophysics and Volcanology (INGV), the Departments of Earth Sciences of the Universities of Florence and Pisa and the School of Geography, Geology and the Environment of the of Leicester (UK), has allowed to deepen the dynamics of the most destructive phase of the 2018 eruption of Volcán de Fuego in Guatemala, which took place about an hour and a half after the most violent explosive activity, when the volcanic risk for the inhabitants of the area seemed to have decreased.
“Pyroclastic flows are some of the most dangerous phenomena related to the activity of volcanoes”, explains Gilda Risica, Research Fellow at the University of Florence and author of the article. “These are real currents composed of gases and volcanic fragments of various sizes, 'rivers' which flow at very high temperatures along the sides of the volcanoes and which, depending on factors such as the slope of the volcanic edifice and the violence of the eruption, they can also reach very high speeds, destroying everything in their path in a short time, including entire villages, as happened in 2018 in the case of San Miguel de Los Lotes".
The study of the deposits of the pyroclastic flow and the paleomagnetic analyzes of the materials arriving downstream, about 12 kilometers from the crater, have allowed the researchers to reconstruct the most dramatic phases of the eruption and the nature of the pyroclastic flow itself.
“The results we obtained show how the pyroclastic current of 2018 at Volcán de Fuego was generated by the unexpected collapse of pyroclastic deposits alternating with lava materials which, over the previous two or three years, following small explosive and effusive eruptions, they had accumulated on the upper part of the volcano”, continues Fabio Speranza, Director of the Rome 2 Section of INGV and co-author of the article. “The paleomagnetic analyzes carried out on the materials, in fact, showed that only 6% of the pyroclastic flow deposit was composed of material attributable to the ongoing eruption with temperatures above 590°C, while 39% of the material dragged downstream had a temperature between 200 and 500°C and, finally, more than half of the products were 'cold' and characterized by temperatures below 200°C. The detection of the different temperatures showed that a large part of the volcanic material dragged downstream had, in reality, accumulated on the sides of the volcano in previous eruptions and slowly partially cooled".
"During the land surveys in the valleys filled by the devastating flows of 2018, however, the presence of other older deposits emerged and very similar to those studied, suggesting that Volcán de Fuego may have already had a similar behavior in the past" highlight Mauro Rosi and Marco Pistolesi, professors of the Department of Earth Sciences of the University of Pisa, co-authors and among the creators of the study
“The implications of our study concern in particular the possibility that events like this can occur in volcanoes that have characteristics similar to those of Fuego. Implementing new and appropriate monitoring and mitigation procedures for this type of risk therefore becomes crucial to help safeguard the natural heritage and save human lives", concludes Gilda Risica.
Risica G., Rosi M., Pistolesi M, Speranza F, Branney MJ (2022) - “Deposit-Derived Block-and-Ash Flows: The Hazard Posed by Perched Temporary Tephra Accumulations on Volcanoes; 2018 Fuego Disaster, Guatemala”. JGR Solid Earth, Volume 127, Issue 6.
Fig. 1 - Creation and destruction of pyroclastic deposits accumulated over time. Sequential images from Google Earth® showing the gradual filling of the upper part of the Las Lajas canyon around the Fuego summit area. The fumarolic activity indicates the emplacement of new hot materials in September 2016. The highest part of the canyon appears to be filled again in May 2018, before the eruption of June 3, 2018, during which it suddenly emptied, exhuming the buried canyon walls (March 2019).
Fig 2 - Satellite view (a) before and (b) after the pyroclastic currents of the June 3 eruption in the Las Lajas valley, showing the destroyed bridge on the RN14 road, and the overflow zones near the golf resort (modified by Google Earth®). The larger overfill to the south caused the destruction of San Miguel de Los Lotes. (c) Ridges and erosion furrows in areas where the pyroclastic current overflowed southward on 3 June, removing forest cover and substrate. (d) Small trees at the edge of the overflow zones, bent and leafless by the more diluted parts of the pyroclastic current.
Fig. 3 - (a): Trees in life position upstream of San Miguel de Los Lotes, broken 1-2m above ground level, suggest June 3 were initially buried by deposit up to 1-2m high, and subsequently broken as the flow velocity increases over time. Tree stump shows a deeply embedded stone block and slight charring. The current goes from left to right. (b): Standing trees in flow margin deposits near San Miguel Los Lotes show bark stripped in the upper parts by erosion of the diluted parts of the pyroclastic current, while the lower parts were protected by the more concentrated deposit. (c): Reinforced concrete parts of the houses severely damaged above the upper part of the pyroclastic flow deposit (golf resort).