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Bulletin of Volcanology

, Volume 67, Issue 6, pp 557–589 | Cite as

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

  • Jean-Claude Thouret
  • Marco Rivera
  • Gerhard Wörner
  • Marie-Christine Gerbe
  • Anthony Finizola
  • Michel Fornari
  • Katherine Gonzales
Research Article

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 km3) 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 (SiO2=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.

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 Plinian eruptions of the size of the Holocene events recur at Ubinas, tephra fall would affect about one million people living in the Arequipa area 60 km west of the summit.

Keywords

Andes Ubinas Eruption history Radiometric dating Geochemistry Fractional crystallization Mafic magma Hazards 

Notes

Acknowledgments

This work has been carried out through a cooperation agreement between IRD Institut Français de Recherche pour le Développement and IGP Instituto Geofísico del Perú. We thank O. Macedo, J. Dávila, D. Ramos, R. Pinto and J. C. Gómez, N. Fournier, V. Glock and F. Sortino for their help in the field. The research program has been supported by IRD and the Laboratoire Magmas et Volcans, Université Blaise-Pascal and CNRS in Clermont-Ferrand (France) where M. Rivera carried out a Master Sc. project (1999–2000). G. Hartman and K. Simon (both GZG) are thanked for isotope and ICPMS trace element analyses, respectively. This cooperation was supported financially by a DAAD-PROCOPE programme to JCT and GW as well as by DFG Leibniz award. Géosciences Azur contribution no. 567. Valuable comments provided by Dr. T. Spell, Dr. C. Siebe, and Dr. E. Smith, are greatly acknowledged.

Supplementary material

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Copyright information

© Springer-Verlag 2005

Authors and Affiliations

  • Jean-Claude Thouret
    • 1
  • Marco Rivera
    • 2
  • Gerhard Wörner
    • 3
  • Marie-Christine Gerbe
    • 4
  • Anthony Finizola
    • 5
  • Michel Fornari
    • 6
  • Katherine Gonzales
    • 7
  1. 1.Laboratoire Magmas et VolcansUniversité Blaise-Pascal et CNRS, OPGCClermont-Fd CedexFrance
  2. 2.INGEMMETDirección de Geología AmbientalLa VictoriaPerú
  3. 3.GZG, Abt. GeochemieUniversität GöttingenGöttingenGermany
  4. 4.Département de Géologie-Pétrologie-GéochimieUniversité Jean Monnet et Laboratoire Magmas et VolcansSaint Etienne CedexFrance
  5. 5.Istituto Nazionale di Geofisica e Vulcanologia (INVG-Palermo)PalermoItaly
  6. 6.IRD, Géosciences AzurUniversité de Nice-Sophia AntipolisNice Cedex 2France
  7. 7.IGP, Instituto Geofísico del PerúRegional ArequipaCaymaPeru

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