Browsing by Author "Fornari, Michel"
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Item Restricted Geology of El Misti volcano near the city of Arequipa, Peru(Geological Society of America, 2001-12) Thouret, Jean-Claude; Finizola, Anthony; Fornari, Michel; Legeley-Padovani, Annick; Suni, Jaime; Frechen, ManfredApproximately 750 000 people live at risk in the city of Arequipa, whose center lies 17 km from the summit (5820 masl [meters above sea level]) of the active El Misti volcano. The composite edifice comprises a stratovolcano designated Misti 1 (ca. 833– 112 ka), partially overlapped by two stratocones designated Misti 2 and Misti 3 (112 ka and younger), and a summit cone Misti 4 (11 ka and younger). Eight groups of lava flows and pyroclastic deposits indicate the following volcanic history. (1) Three cones have been built up since ca. 112 ka at an average eruptive rate of 0.63 km3/k.y. (2) Several episodes of growth and destruction of andesitic and dacitic domes triggered dome-collapse avalanches and block-and-ash-flows. Deposition of these flows alternated with explosive events, which produced pyroclastic-flow deposits and tephra-fall and surge deposits. (3) Nonwelded, dacitic ignimbrites may reflect the formation of a 6 × 5 km incremental caldera collapse on Misti 2 (ca. 50 000 and 40 000 yr B.P.) and a 2 × 1.5 km summit caldera on Misti 3 (ca. 13 700 to 11 300 yr B.P.). (4) Tens of pyroclastic flows and at least 20 tephra falls were produced by Vulcanian and sub-Plinian eruptions since ca. 50 ka. On average, ash falls have occurred every 500 to 1500 yr, and pumice falls, every 2000 to 4000 yr. (5) Misti erupted relatively homogeneous andesites and dacites with a few rhyolites, but Misti 4 reveals a distinct mineral suite. Less evolved andesites prevail in scoriaceous products of group 4–1 including historical ash falls. Scoriae of Misti 4 and the ca. 2300–2050 yr B.P. banded pumice commonly show heterogeneous textures of andesite and rhyolite composition. This heterogeneity may reflect changes in physical conditions and magma mixing in the reservoir. (6) Deposits emplaced during the Vulcanian A.D. 1440– 1470 event and the sub-Plinian eruption(s) at ca. 2050 yr B.P. are portrayed on one map. The extent and volume of these deposits indicate that future eruptions of El Misti, even if moderate in magnitude, will entail considerable hazards to the densely populated area of Arequipa.Item Restricted Ubinas: the evolution of the historically most active volcano in southern Peru(Springer, 2005) Thouret, Jean-Claude; Rivera, Marco; Wörner, Gerhard; Gerbe, Marie-Christine; Finizola, Anthony; Fornari, Michel; Gonzales Zuñiga, KatherineUbinas 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.