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Abstract: Since the eruption which affected Quito in AD 1660, Guagua Pichincha has been considered a hazardous volcano. Based on field studies and twenty 14C dates, this paper discusses the eruptive activity of this volcano, especially that of the last 2000 years. Three major Plinian eruptions with substantial pumice discharge occurred in the 1st century, the 10th century, and in AD 1660. The ages of organic paleosols and charcoal from block-and-ash flow and fallout deposits indicate that these eruptions occurred near the end of 100 to 200 year-long cycles of discontinuous activity which was comprised of dome growth episodes and minor pumice fallouts. The first cycle took place from ~AD 1 to 140. The second one developed during the 9th and 10th centuries, lasted 150–180 yr, and included the largest Plinian event, with a VEI of 5. The third, historic cycle, about 200 yr in duration, includes pyroclastic episodes around AD 1450 and AD 1500, explosive activity between AD 1566 and AD 1582, possible precursors of the 1660 eruption in the early decades of the 17th century, and finally the 1660 eruption (VEI 4). A fourth event probably occurred around AD 500, but its authenticity requires confirmation. The Plinian events occurred at the end of these cycles which were separated by repose periods of at least 300 yr. Older volcanic activity of similar type occurred between ~4000 and ~3000 yr BP. Because ash fallout and related mudflows represent a serious hazard for Quito''s metropolitan area, the significance of the increasing phreatic activity observed from 1981 to 1998, and the 1999–2001 magmatic episode of dome growth and collapse are discussed. These probably represent a short step in a longer evolution which may result in a major Plinian event in the future decades or in the next century, comparable to that which occurred during the 1st, 10th, and 17th centuries. [Copyright &y& Elsevier]

Abstract: Ice-capped Chimborazo is one of the large composite Ecuadorian volcanoes whose recent eruptive activity is poorly known. This paper presents the characteristics and the ages of a newly discovered Holocene sequence of pyroclastic deposits on the east and north sides of the cone. Lying upon a moraine of the Late-Glacial period, the most complete section of ~4.5 m in thickness is located 5 km from the present summit crater. It consists of seven massive or diffusely stratified ash flow layers and four fallout layers interbedded with seven paleosoils. Based on field study, most flow deposits were assessed as surge layers, and six radiocarbon analyses obtained from charcoal fragments and paleosoils indicate that eruptions occurred at quite regular intervals between about 8000 and 1000 years ago. The first two and most potent events generated thick lahars over the north and west flanks of the cone. Surface textures of volcanic clasts were analysed by scanning electron microscopy. Blocky and blocky/fluidal vitric clasts indicate fragmentation during vulcanian explosions of a quite solidified shallow magma body. In addition, aggregates either cemented at a cooling stage (with surface fluidal textures), or consisting of fine particles (moss-looking aggregates), form a large part of the surge deposits. These characteristics indicate powerful explosions and intense fragmentation due to phreatic water reaching the conduit, probably from the ice-cap. Since the last eruption occurred between the early part of the 5th century (~AD 420) and the end of the 7th century, these results highlight that Chimborazo is a potentially active volcano. Given its dominating presence over the densely populated Ambato and Riobamba basins, and owing its large ice-cap, Chimborazo should be considered a dangerous volcano. [Copyright &y& Elsevier]

The geology and long-term structural development of Imbabura volcano (Ecuador) have been discerned from field surveys, as well as petrographical and geochemical studies. The volcano is composed of an ancient edifice (Imbabura I), which has experienced one major volcanic landslide directed N 330–340°. After the emplacement of that debris-avalanche deposit, the petrographically and geochemically different edifices Imbabura II-1 and Imbabura II-2 developed on the N and NE flank of Imbabura I, as well as in its summit zone. Finally the smaller Imbabura II-3 edifice, a dome-complex locally known as Huarmi Imbabura, was emplaced in the SSW flank of Imbabura I. The relatively complex structure of the edifice (i.e. flank collapse direction and spatial distribution of the major stratigraphical units) cannot be explained solely by volcano-tectonic interactions with the regional left-lateral Río Ambi strike-slip fault, which traverses the edifice from NNE to SSW. Instead, the fact that the whole Imbabura edifice stands on a substratum dipping to the NW (i.e. the NW flank of the neighbor Cubilche volcano) is proposed to be the main factor influencing the departure of Imbabura structure from previously known models of volcanic edifices interacting with strike-slip faults. This hypothesis was tested with scaled experiments, during which the dip-direction of the substratum was systematically changed with respect to the strike of the fault. The structures developed by the experimental prototypes are highly coherent with those observed at Imbabura volcano. [ABSTRACT FROM AUTHOR]

Abstract: New fieldwork, radiometric and whole-rock chemical data permit the reconstruction of the main eruptive stages of the Chimborazo compound volcano, the highest summit of the Northern Andes. Chimborazo is composed of three successive edifices. The Basal Edifice (CH-I) was active from ~120 to 60ka and resulted in a large, mostly effusive edifice which was built up during two stages of cone-building, terminating with the formation of a dome complex. This edifice was affected by a huge sector collapse around 65–60ka which produced a major debris avalanche that spread out into the Riobamba basin, covering about 280km2 with an average thickness of 40m and a total volume of ~10–12km3. After the emplacement of the Riobamba debris avalanche, eruptive activity resumed at the eastern outlet of the avalanche scar and was responsible for the construction of a less voluminous, Intermediary Edifice (CH-II), whose current remnants are the Politécnica and Martínez peaks. This edifice developed from 60 to 35ka. Lastly, eruptive activity shifted to the west, leading to the construction of the morphologically well-preserved Young Cone (CH-III) which currently forms the highest summit (Whymper). The average eruptive rate of Chimborazo volcano is 0.5–0.7km3/ka. However, looking at the three successive edifices individually, we estimate that there has been a progressive decrease in magma output rate from the Basal Edifice (0.7–1.0km3/ka), through the Intermediary Edifice (0.4–0.7km3/ka) to the Young Cone (~0.1km3/ka). However, during the main cone-building stages, the peak eruption rates are markedly higher, indicating significant variations in the magma output rate during the lifespan of this arc volcano. During the Holocene, the Chimborazo eruptive activity consisted of small-volume explosive events that occurred at quite regular intervals, between about 8000 and 1000yr ago. Since the last eruption occurred between the early part of the 5th century and the end of the 7th century, and the average time interval between the events is about 1000yr, Chimborazo must be considered as a potentially active volcano. The presence of a thick ice cap covering the summit, its steep flanks and its position above the populated lowland area of Riobamba and Ambato, are factors that result in a high potential risk. [Copyright &y& Elsevier]

Abstract: Understanding the processes at the origin of explosive events is crucial for volcanic hazard mitigation, especially during long-lasting eruptions at andesitic volcanoes. This work exposes the case of Tungurahua volcano, whose unrest occurred in 1999. Since this date, the eruptive activity was characterized by low-to moderate explosiveness, including phases with stronger canon-like explosions and regional ash fallout. However, in 2006, a sudden increase of the explosiveness led to pyroclastic flow-forming eruptions on July 14th (VEI 2) and August 16–17th (VEI 3). All magmas emitted from 1999 to 2005, as well as the samples from the 2006 eruptions, have homogeneous bulk-rock andesitic compositions (58–59wt.% SiO2), and contain the same mineral assemblage consisting of pl+cpx+opx+mag±ol. However, during the August 16–17th event, the erupted tephra comprise two types of magmas: a dominant, brown andesitic scoria; and scarce, light-grey pumice representing a subordinate, silica-rich juvenile component. For the andesitic magma, thermobarometric data point to magmatic temperatures ranging from 950 to 1015°C and pressures in the range of 200 to 250MPa, which corresponds to 7.5–9.5km below the summit. Disequilibrium textures in plagioclase and pyroxene phenocrysts, particularly thin overgrowth rims, indicate the recharge of this magma body by mafic magma. Between 1999 and 2005, repeated injections from depth fed the intermittent eruptive activity observed while silica-rich melts were produced by in-situ crystallization in the peripheral parts of the reservoir. In April 2006, the recharge of a primitive magma produced strong convection and homogenisation in the reservoir, as well as pressure increase and higher magma ascent rate after seven years of only moderately explosive activity. This work emphasizes the importance of petrological studies in constraining the pre-eruptive magmatic conditions and processes, as a tool for understanding the fundamental causes of the changes in the eruptive dynamism, particularly the occurrence of paroxysmal phases in andesitic systems with open-vent behaviour. [Copyright &y& Elsevier]

Gaua Island (also called Santa Maria), from the central part of the Vanuatu arc, consists of a large volcano marked by a caldera that hosts the active Mount Garet summit cone. In this paper, a geochemical study including Sr, Nd, Pb and Hf isotopic compositions of 25 lavas emitted since 1.8 Ma is presented, with a focus on the volcanic products that preceded (old volcanics, main cone and pyroclastic series) and followed (Mount Garet) the caldera forming event. All lavas show an island arc signature with enrichment in LILE and depletion in HFSE. Post-caldera lavas define a medium-K calc-alkaline trend, whereas lavas from the former main cone have high-K calc-alkaline compositions. Compared to the pre-caldera volcanic suite, the Mount Garet lavas have similar Th/Nb (~ 1.5), 143 Nd/ 144 Nd (~ 0.51295) and 176 Hf/ 177 Hf (~ 0.28316) ratios, but higher Ba/La (~ 42 vs. ~ 27) and 87 Sr/ 86 Sr (0.70417 vs. 0.70405) ratios and lower Ce/Pb (~ 2.7 vs. ~ 4.6), La/Sm (~ 2.5 vs. ~ 4.0) and 206 Pb/ 204 Pb (18.105 vs. 18.176) ratios. High Th/Nb and low Nd and Hf isotopic ratios compared to N-MORB suggest the contribution of ~ 2% of subducted sediment melt to the mantle source of Gaua magmas. Most of the observed differences between pre- and post-caldera lavas can be accounted for by the involvement of at least two portions of the mantle wedge, metasomatized by different slab-derived aqueous fluids. In addition, the lower La/Sm (at a given 143 Nd/ 144 Nd) ratios of Mount Garet lavas suggest a higher degree of partial melting (~ 10–15%) compared to the pre-caldera lavas (~ 5%). The Santa Maria Pyroclastic Series (SMPS) eruption probably triggered the caldera collapse, in response to emptying of the magmatic chamber. This event may have allowed new access to the surface for a geochemically distinct batch of magma issued from a separate magma chamber, resulting in the birth and construction of Mount Garet within the caldera. As both magmatic suites were emitted over a very short time, the storage of their parental magmas beneath the volcano is still possible. [ABSTRACT FROM AUTHOR]

Abstract: The Licán Ignimbrite represents a large explosive basaltic andesite event in the eruptive history of the Villarrica Volcano, an active stratovolcano of the Southern Andean Volcanic Zone. A mineralogical study of this ignimbrite indicates important disequilibrium features. Resoption textures were observed around Ca-rich (An74–92; Group 1) and more sodic (An35–44; Group 2) plagioclase phenocryst cores. Rims on these phenocrysts as well as microlites grew with an intermediate composition (An47–63; Group 3). Low- and high-Mg clinopyroxene and orthopyroxene phenocrysts were also identified; most of them showing reverse Mg zoning. Based on these mineralogical data and thermodynamic modeling, an increase of about 200°C in temperature (from ~900–1000° to ~1100–1150°C), a decompression of ~400bar (equivalent to ~1.5km) and a decrease in H2O content in the melt of ~1.6wt.% are identified during crystallization. Addition of hot magma from below would have caused heating of the reservoir with the consecutive ascent of magma pulses, H2O exsolution and convective mixing (compositional and/or thermal mixing). A numerical model using the Finite Element Method was developed to assess the response of the Licán Ignimbrite reservoir upon conductive heating by 200°C by mafic underplating; this model indicates a rapid (tens of years) temperature homogenization of this system, with ascending magma plumes with velocities as high as 6m/h. Because the Licán Ignimbrite erupted at the end of the Pleistocene glaciation, glacial unloading could have also contributed to depressurization of the reservoir. [Copyright &y& Elsevier]

Abstract: Chimborazo is a Late Pleistocene to Holocene stratovolcano located at the southwest end of the main Ecuadorian volcanic arc. It experienced a large sector collapse and debris avalanche (DA) of the initial edifice (CH-I). This left a 4 km wide scar, removing 8.0±0.5 km3 of the edifice. The debris avalanche deposit (DAD) is abundantly exposed throughout the Riobamba Basin to the Río Chambo, more than 35 km southeast of the volcano. The DAD averages a thickness of 40 m, covers about 280 km2, and has a volume of >11 km3. Two main DAD facies are recognized: block and mixed facies. The block facies is derived predominantly from edifice lava and forms >80 vol.% of the DAD, with a probable volume increase of 15–25 vol.%. The mixed facies was essentially created by mixing brecciated edifice rock with substratum and is found mainly in distal and marginal areas. The DAD has clear surface ridges and hummocks, and internal structures such as jigsaw cracks, injections, and shear-zone features are widespread. Structures such as stretched blocks along the base contact indicate high basal shear. Substratum incorporation is directly observed at the base and is inferred from the presence of substratum-derived material in the DAD body. Based on the facies and structural interpretation, we propose an emplacement model of a lava-rich avalanche strongly cataclased before and/or during failure initiation. The flow mobilises and incorporates significant substrata (10–14 vol.%) while developing a fine lubricating basal layer. The substrata-dominated mixed facies is transported to the DAD interior and top in dykes invading previously-formed fractures. [Copyright &y& Elsevier]

Abstract: A petrological study of the eruptive products of El Reventador allowed us to infer the magmatic processes related to the 2002 and 2004–05 eruptions of this andesitic stratovolcano. On November 3, 2002, El Reventador experienced a highly explosive event, which was followed by emplacement of two lava flows in November–December 2002. Silica contents range from 62 to 58 wt.% SiO2 for the November 3 pyroclastic deposits to 58–56 and 54–53 wt.% SiO2 for the successive lava flows. In November 2004 eruptive activity resumed supplying four new lava flows (56–54 wt.% SiO2) between November 2004 and August 2005. Volatile contents in matrix glasses and glass inclusions from the November 3 pyroclastic deposits allow us to estimate the total amount of SO2 and HCl released into the atmosphere during the paroxysmal phase (i.e. 80 kT of SO2 and 280 kT of HCl). Pre-eruptive pressure-temperature conditions of the magmas range from 300 to 150 MPa and ~1000 °C with high water contents (~5 wt.%). We propose the existence of an andesitic magma body located at ~7–12 km depth that is frequently intruded by more primitive, hydrous magmas from a deeper source. The initial crystallization of amphibole from the hydrous primitive magma seems typical of El Reventador, as well as the historically recurrent and regular periods of eruptive activity lasting several years. This eruptive behaviour coupled with the fractionation and mixing processes inferred from the 2002 and 2004–05 petrologic data suggest that deep magmatic recharge at El Reventador is frequent, and is probably responsible for the high frequency of eruptions. [Copyright &y& Elsevier]