Secondary Lahars Triggered by Periglacial Melting at Chimborazo Volcano, Ecuador

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.

Open volcanic systems: evidence for deep gas loss

<p>Previous studies of Vulcanian eruptive products have shown that the respective volcanic conduits were filled for the<br>most part with low-porosity magma prior to eruption. Comparison with the theoretical porosity distribution<br>expected from closed-system degassing suggests that gas loss must have taken place at great depth within the<br>magmatic column. At such high pressures, however, porosities are low enough to rule out traditional gas loss<br>mechanisms. We tested if channelling, an outgassing mechanism based on bubble connection due to high crystal<br>content proposed to occur in mushy magma reservoirs, could also happen in volcanic conduits. We reanalysed<br>phenocryst, microlite, and porosity data from recent eruptions of Merapi volcano, Indonesia, Soufrière Hills<br>volcano, Montserrat, and Tungurahua volcano, Ecuador. Overall, these magmas had crystal contents high enough<br>for outgassing to occur by channelling. Gases could be channelled out of the magma columns at various levels<br>during ascent to yield mostly gas-depleted magma columns prior to explosive behaviour. Such outgassing by<br>channelling has thus the capacity to influence eruptive style. Depending on the phenocryst content, microlite<br>growth during ascent can either foster, or impede gas escape by channelling. Considering the pervasive occurrence<br>of microlites and ensuing high crystal contents in volcanic conduits, the high likelihood of channelling implies that<br>other outgassing mechanisms might not be as dominant as previously envisioned.</p>

Impact of metal content in agricultural soils near the Tungurahua volcano on the cultivation of Allium fistulosum L

The Tungurahua volcano, located in the eastern mountain range of Ecuador, since its reactivation in 1999 has had several phases of volcanic activity, which have produced gas, ash and lava emissions. These emissions release a large amount of metals to nearby soils that are currently used for agricultural purposes. Metal pollution can cause serious problems for human health; while other metals are necessary as nutrients in most agricultural crops. In this investigation, the metal content in agricultural soils of the Quero canton was evaluated, as well as its bioavailability and content in the culture of Allium fistulosum L., in order to obtain information on the impact of potentially polluting metals (cadmium, lead, nickel, strontium, cobalt, copper and zinc) and nutrients (potassium, magnesium, iron and manganese) on crops. For the estimation of total metals in soil an acid digestion was performed; for bioavailable metals an extractant mixture (EDTA-Triethanolamine-CaCl2, pH 7) was used and for the branch onion a calcination followed by acid digestion was carried out. The quantification of the metals was carried out by flame atomic absorption spectroscopy or graphite furnace. The results showed that the metal content, both in the soil samples and in the branch onion, was below the maximum values allowed in the local regulations for all the metals studied. In addition,the intake of the metal by the branch onion was independent of the bioavailable fraction.

Isomass and Probability Maps of Ash Fallout Due to Vulcanian Eruptions at Tungurahua Volcano (Ecuador) Deduced from Historical Forecasting

Since April of 2015, the ash dispersion and ash fallout due to Vulcanian eruptions at Tungurahua, one of the most active volcanoes in Ecuador, have been forecasted daily. For this purpose, our forecasting system uses the meteorological Weather Research and Forecasting (WRF) and the FALL3D models. Previously, and based on field data, laboratory, and numerical studies, corresponding eruption source parameters (ESP) have been defined. We analyzed the historically forecasted results of the ash fallout quantities over four years (April 2015 to March 2019), in order to obtain the average isomass and probability maps for three-month periods: February–March–April (FMA), May–June–July (MJJ), August–September–October (ASO), and November–December–January (NDJ). Our results indicate similar ash fallout shapes during MJJ and ASO, with a clear and defined tendency toward the west of the volcano; this tendency is less defined during NDJ and FMA. The proximal region west of the volcano (about 100 km to the west) has the highest probability (>70%) of being affected by ash fallout. The distant region to the west (more than 100 km west) presented low to medium probabilities (10%–70%) of ash fallout. The cities of Guaranda (W, 60% to 90%), Riobamba (SW, 70%), and Ambato (NW, 50% to 60%) have the highest probabilities of being affected by ash fallout. Among the large Ecuadorian cities, Guayaquil (SW, 10% to 30%) has low probability, and Quito (N, ≤5%) and Cuenca (SSE, <5%) have very low probabilities of being affected by ash fallout. High ash clouds can move in different directions, compared to wind transport near the surface. Therefore, it is possible to detect ash clouds by remote sensing which, in Ecuador, is limited to the layers over the meteorological clouds, which move in a different direction than low wind; the latter produces ash fallout over regions in different directions compared to the detected ash clouds. In addition to the isomass/probability maps and detected ash clouds, forecasting is permanently required in Ecuador.

Pb-Sr isotope temporal variations on juvenile ash samples from the last eruptive period of Tungurahua volcano (1999-2016)

&lt;p&gt;Andean volcanic activity consists in long quiescence periods interrupted by violent explosive eruptions of diverse intensity, magnitude and duration. Anticipating these transitions between low energy eruptions and violent major explosions is a challenge for modern volcanology. Many parameters could play a role in explaining these fast transitions, such as the conduit process (degassing and microlite crystallizations) but also deeper processes (crustal assimilation, nature of the mantle-derived primitive liquids).&lt;/p&gt;&lt;p&gt;Geophysical methods (seismic, acoustic, heat flow, ground deformation, gas emissions) are currently used by volcano observatories in order to identify unrest phases at medium to short time-scale. Furthermore, the evolution of pre-eruptive magmatic reservoirs is often studied via micro-petrological techniques, such as the detailed study of growth-zoned crystals emitted during paroxysmal phases. These zonation patterns are related to changes in composition and/or P-T-XH&lt;sub&gt;2&lt;/sub&gt;O conditions in the magma reservoir caused by its complex evolution through time (injection/recharge, crystallization, degassing processes). Thanks to these methods, a tight relation between mafic magma recharge and explosive volcanic reactivation has been constrained with delay time estimated from years to months. Yet, such petrologic methods are not efficient as predictive tools.&lt;/p&gt;&lt;p&gt;In this study, we performed micro-geochemical analysis (trace elements, Pb-Sr isotopes) on time-series of juvenile ash samples (80) emitted by Tungurahua volcano over its most recent period of activity (1999-2016). Geochemical time series display an oscillating pattern with good correlations between Pb-Sr isotope compositions evolution and most trace elements as well as between whole rocks and ashes compositions. Pb-Sr isotope cyclic signal seems to reveal that paroxysmal phases first emissions are the most radiogenic of each eruption phase. As the eruption goes on, we observe a rapid decrease in Pb-Sr isotope composition. Finally, it appears that both &lt;sup&gt;206&lt;/sup&gt;Pb/&lt;sup&gt;204&lt;/sup&gt;Pb and &lt;sup&gt;87&lt;/sup&gt;Sr/&lt;sup&gt;86&lt;/sup&gt;Sr ratios of juvenile ash samples decrease from 2013 to the last year of activity in 2016, leaving the final emissions with the less radiogenic Pb-Sr isotope compositions.&lt;/p&gt;&lt;p&gt;The oscillating geochemical pattern of juvenile ash samples has been meticulously compared to the well-known eruptive dynamics of the volcano through time, providing clues on the processes triggering the violent reactivation phases. Indeed, the rapid geochemical evolution of the magma reservoir with a high temporal resolution has been interpreted as the effect of deep magma recharges in the shallow reservoir of Tungurahua volcano.&lt;/p&gt;&lt;p&gt;Coupling continuous geochemical analysis to geophysical monitoring puts in evidence a geochemical precursor for eruptive phases, delivering a better understanding on the global stratovolcano system and assuring an improvement in volcanic monitoring.&lt;/p&gt;

From examination of natural events to a proposal for risk mitigation of lahars by a cellular-automata methodology: a case study for Vascún valley, Ecuador

Lahars are erosive floods, mixtures of water and pyroclastic detritus, known for being the biggest environmental disaster and causing a large number of fatalities in volcanic areas. Safety measures have been recently adopted in the threatened territories by constructing retaining dams and embankments in key positions. More disastrous events could be generated by the difficulty of maintaining these works in efficiency and for the changed risk conditions originating from their presence and the effects of their functioning. LLUNPIY/3r, a version of the cellular-automaton model LLUNPIY for lahar simulations, is presented. The growing frequency of lahars in the Vascún valley of Tungurahua volcano (Ecuador), probably due to the effects of the climatic change, has recently produced smaller and less dangerous events, sometimes favored by the collapse of ponds generated by small landslides. An investigation is performed here in order to reproduce such situations in a controlled way by the use of LLUNPIY/3r simulations. Using precise field data, points are individuated where dams by backfill, which are easy to collapse, can produce the formation of ponds; LLUNPIY/3r simulations permit projecting the triggering of small lahars by minor rainfall events or projecting, in the case of more rainfall, the anticipation of lahar detachment, avoiding simultaneous and dangerous confluence with other lahars.