Search

In Ecuador, a country with numerous potentially active volcanoes, recurrent large earthquakes, and regular climate-related events, it is surmised that phenomena such as debris flows have affected pre-Hispanic populations since their settlement in ~5000 cal BC. Here, using a multidisciplinary approach, we studied the most recent debris flow events that affected the Cayambe city area, located 15 km west of the active glacier-clad Cayambe volcano. Based on detailed characterization of the deposits, including sedimentological, archaeological, and paleontological analyses, as well as radiocarbon dating. We found that two debris flow (i.e., Río Blanco I and II) destroyed Caranqui settlements in 665–775 cal AD and 774–892 cal AD, respectively, while another event impacted a Spanish colonial farm in 1590 –1620 cal AD (Río Blanco III). The grain size distribution of these deposits indicates a gravel-rich flow for Río Blanco I and clay-rich flow for Río Blanco II and III, whilst componentry suggests low juvenile volcanic content for all three deposits. Juvenile components include pumice and lustrous dense dacites, while accidental clasts are dull dense dacites, oxidized and hydrothermally-altered material, as well as archaeological artifacts. These results, in addition to radiocarbon ages, suggest that the debris flows could either be post-eruptive or not related to volcanic eruptions. Potential non-volcanic trigger mechanisms for these events include rainfall and/or earthquakes, which implies that they can occur at any time and without forecast. Currently, the city of Cayambe is rapidly expanding and, consequently, our findings are relevant for creating impact scenarios for future debris flows forming in the Rio Blanco headwaters and descending to the city.

Periglacial melting processes can provide the water source for secondary lahars triggered by volcanic and/or meteorological phenomena on volcanoes. Between December 2015 and April 2016, four major lahars were reported southeast of Chimborazo volcano (Ecuador). Fieldwork allowed determining the area (1.670.37 km2), volume (3E+05 to 7E+05 m3), peak discharge (100 - 150 m3/s) and mean speed (2 - 4 m/s) of these flows, which affected the local infrastructure and threatened several towns downstream (>1000 inhabitants). This case study suggests that anomalous periglacial melting could have been induced by: i) an increase in temperatures at periglacial altitudes partly ascribed to El Niño phenomenon, ii) albedo reduction of the glacier due to ash fallout from Tungurahua volcano (40 km east of Chimborazo) which erupted from 1999 to 2016 and, iii) a slight increase in internal activity at Chimborazo prior and during the lahars occurrence, as evidenced by more seismic events and thermal anomalies. These simultaneous factors could have led to the formation, outburst and/or overflow of superficial and intra-glacier ponds providing the water source to generate lahars on a dormant volcano.

International audience; Sangay is one of the most active volcanoes in Ecuador, as it has been almost continuously erupting at least since the seventeenth century. However, because of its remote location and low associated risk to human population, little is known about its eruptions. Here we summarize Sangay's volcanic activity from January 2001 to May 2020, based on ground-based data, satellite-derived observations and chemical analysis of its erupted products. During the analyzed period, Sangay's activity changed from continuous to episodic, as revealed by seismic, thermal and ash emission data. We identified three main eruptive periods: the first, from 2001 to 2013, extruded a cumulative volume of 100 ± 50 million m3 of lava through long-lived activity; the second emitted 54 ± 27 million m3 in four short-lived episodes, which occurred once every year from 2015 to 2018; and finally, a third period since 2019, which has continuous but fluctuating intensity, and shows a significant increase in the extruded lava volume (172 ± 86 million m3 until May 31, 2020). Our results show a marked change in the eruption frequency and a significant increase in average discharge rate over time, although surface activity remained similar, with lava flows, small explosions and ash venting. We propose that three magmatic processes acted to explain the observed changes: between 2001 and 2013 the long-living low-intensity eruptions were promoted by buoyancy, while since 2019 similar but more intense activity was triggered by mass injections. In contrast, the episodic activity in between probably resulted from volatile exsolution due to crystallization (second boiling). Transitions between these three regimes are presumably the result of varying mass inflow rates. Our results provide insights into eruptive style transitions commonly observed at volcanoes of intermediate composition, such as Sangay, over a timescale of several years.

Pululahua is an active volcano located 15 km north of Quito, Ecuador, that comprises sixteen dacitic-andesitic lava domes and a 13 kmThe online version contains supplementary material available at 10.1007/s00445-022-01590-4.El Pululahua es un volcán activo situado 15 km al norte de Quito, Ecuador, que comprende dieciséis domos de lava dacítica-andesítica y una depresión sub-rectangular de 13 km² formada entre ~2,6 y ~2,3 mil años atrás. En este artículo realizamos un estudio detallado de 70 depósitos de flujos y caídas que componen la secuencia piroclástica para mostrar que la depresión, previamente clasificada como caldera, se formó en realidad por numerosas erupciones Vulcanianas a (sub-) Plinianas que destruyeron domos de lava pre-existentes y co-eruptivos. Fundamentamos esta interpretación con trabajo de campo, el análisis de densidad, componentes y distribución del tamaño de grano de 24 depósitos clave, además del análisis textural y petrológico de 16 piroclastos juveniles muestreados a lo largo de toda la secuencia. Los datos documentan una alternancia entre erupciones (sub-) Plinianas y Vulcanianas. La predominancia de material juvenil denso que conserva variaciones microtexturales dentro de los depósitos Vulcanianos indica cambios en el almacenamiento de magma a poca profundidad antes de estas erupciones. Un análisis exploratorio de las texturas de los fenocristales y de las composiciones de los bordes de plagioclasa y anfíbol sugiere que gran parte de la actividad eruptiva fue desencadenada por repetidas inyecciones de magma menos diferenciado al sistema magmático del Pululahua. La secuencia inferida de eventos eruptivos entre ~2,6 y ~2,3 mil años proporciona una nueva hipótesis para la formación de la actual morfología del Pululahua, incluyendo múltiples episodios de erupciones efusivas y explosivas acompañadas de la migración de los ventos eruptivos. Nuestros resultados aportan una nueva interpretación de los posibles futuros escenarios de peligro asociados al volcán Pululahua, los cuales podrían afectar a millones de personas.