Ubinas: the evolution of the historically most active volcano in southern Peru

Abstract

Ubinas volcano has had 23 degassing and ashfall episodes since A.D. 1550, making it the historically most active volcano in southern Peru. Based on fieldwork, on interpretation of aerial photographs and satellite images, and on radiometric ages, the eruptive history of Ubinas is divided into two major periods. Ubinas I (Middle Pleistocene >376 ka) is characterized by lava flow activity that formed the lower part of the edifice. This edifice collapsed and resulted in a debris-avalanche deposit distributed as far as 12 km downstream the Rio Ubinas. Non-welded ignimbrites were erupted subsequently and ponded to a thickness of 150 m as far as 7 km south of the summit. These eruptions probably left a small collapse caldera on the summit of Ubinas I. A 100-m-thick sequence of ash-and-pumice flow deposits followed, filling paleo-valleys 6 km from the summit. Ubinas II, 376 ky to present comprises several stages. The summit cone was built by andesite and dacite flows between 376 and 142 ky. A series of domes grew on the southern flank and the largest one was dated at 250 ky; block-and-ash flow deposits from these domes filled the upper Rio Ubinas valley 10 km to the south. The summit caldera was formed between 25 and 9.7 ky. Ash-flow deposits and two Plinian deposits reflect explosive eruptions of more differentiated magmas. A debris-avalanche deposit (about 1.2 km 3 ) formed hummocks at the base of the 1,000-m-high, fractured and unstable south flank before 3.6 ka. Countless explosive events took place inside the summit caldera during the last 9.7 ky. The last Plinian eruption, dated A.D.1000-1160, produced an andesitic pumice-fall deposit, which achieved a thickness of 25 cm 40 km SE of the summit. Minor eruptions since then show phreatomagmatic characteristics and a wide range in composition (mafic to rhyolitic): the events reported since A.D. 1550 include many degassing episodes, four moderate (VEI 2-3) eruptions, and one VEI 3 eruption in A.D. 1667. Ubinas erupted high-K, calc-alkaline magmas (SiO 2 =56 to 71%). Magmatic processes include fractional crystallization and mixing of deeply derived mafic andesites in a shallow magma chamber. Parent magmas have been relatively homogeneous through time but reflect variable conditions of deep-crustal assimilation, as shown in the large variations in Sr/Y and LREE/HREE. Depleted HREE and Y values in some lavas, mostly late mafic rocks, suggest contamination of magmas near the base of the >60-km-thick continental crust. The most recently erupted products (mostly scoria) show a wide range in composition and a trend towards more mafic magmas. Editorial responsibility: D Dingwell J.-C. Thouret ( ) ) Laboratoire Magmas et Volcans,Recent eruptions indicate that Ubinas poses a severe threat to at least 5,000 people living in the valley of the Rio Ubinas, and within a 15-km radius of the summit. The threat includes thick tephra falls, phreatomagmatic ejecta, failure of the unstable south flank with subsequent debris avalanches, rain-triggered lahars, and pyroclastic flows. Should Pl...

Geomorphology of the Ampato volcanic complex (Southern Peru)

“…HualcaHualca is considered an extinct volcano (Thouret et al, 2005). This edifice is characterized by a horseshoe-shaped caldera resulting from the collapse of the northern flank.…”

“…Ampato is classified as a dormant volcano (Thouret et al, 2005) and presents a fresh volcanic morphology only slightly modelled by glaciers. However, Sabancaya is the youngest volcanic structure of the Ampato volcanic complex.…”

“…Most previous work in the Central Andes focused on glacial landform schemes to date glacier advances or deglaciation (Bromley et al, 2009(Bromley et al, , 2011Glasser, Clemmens, Schnabel, Fenton, & Mchargue, 2009;Hall et al, 2009;Kelly et al, 2012;May et al, 2011;Smith, Lowell, & Caffee, 2009), geology (Thouret et al, 2001(Thouret et al, , 2005 and hazards (Cobeñas, Thouret, Bonadonna, & Boivin, 2012;Sandri, Thouret, Constantinescu, Blass, & Tonini, 2014). Nevado Coropuna presents the most complete glacial and periglacial mapping of the Central Andes (Forget, Thouret, Kuentz, & Fontugne, 2008;Úbeda, 2013), but its volcanic landforms have not been represented accurately enough.…”

Volcanic facies of the debris avalanche deposit of Tata Sabaya Volcano, Central Andes.

“…1). Se han desarrollado nuevos trabajos in situ en algunos de estos depósitos (e.g., van Wyk de Vries et al, 2001;Clavero et al, 2002;Clavero et al, 2004), así como también se han identificado nuevos depósitos de este tipo en la zona (e.g., Thouret et al, 2005;. Se ha propuesto que estos depósitos de avalancha de detritos han sido generados por deslizamiento gravitacional (e.g., van Wyk de Vries et al, 2001), alteración hidrotermal (e.g., , erupciones magmáticas (e.g., O'Callaghan y Francis, 1986;Thouret et al, 2005;Godoy y Clavero, 2008) o procesos mixtos (Clavero et al, 2004), influyendo en gran parte de estos colapsos el estrés tectónico local (e.g., Francis y Wells, 1988;Vezzoli et al, 2008;Arancibia et al, 2009).…”

“…La inestabilidad y el colapso de edificios volcánicos han sido objeto de gran cantidad de estudios en el mundo, en particular con posterioridad a la erupción del Monte Santa Helena en 1980 (e.g., Voight et al, 1981;Ui, 1983;Siebert, 1984;Francis y Wells, 1988;Ponomareva et al, 2006). El estudio de estos eventos es de gran utilidad para reconocer la estabilidad de edificios volcánicos propensos a colapsar y que se encuentren cercanos a áreas pobladas, de manera de mitigar los posibles efectos que un flujo de estas características pudiese generar (e.g., Capra et al, 2002;Thouret et al, 2005;Catane et al, 2008).…”