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Largest Explosive Eruption in Historical Times in the Andes at A.D

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Largest Explosive Eruption in Historical Times in the Andes at A.D b- Largest explosive eruption in historical times in the Andes at A.D. Huaynaputina volcano, 1600, southern Peru Jean-Claude ThOuret* IRD and Instituto Geofisicodel PerÚ, Calle Calatrava 21 6. Urbanización Camino Real, La Molina, Lima 12, Perú Jasmine Davila Instituto Geofísico del Perú, Lima, Perú Jean-Philippe Eissen IRD Centre de Brest BP 70, 29280 Plouzane Cedex, France ABSTRACT lent [DREI volume. km': Table The The largest esplosive eruption (volcanic explosivity index of 6) in historical times in the 4.4-5.6 I). tephra is composed of -SO% pumice with rhyolitic Andes took place in 1600 at Huaynaputina volcano in southern Peru. According to chroni- A.D. glass, crystals (mostly amphibole and bio- cles, the eruption began on February 19 with a Plinian phase and lasted until March 6. Repeated 15% tite), and 5% xenoliths including hydrothermally tephra falls, pyroclastic flows, and surges devastated an area 70 x 40 kni? west of the vent and altered fragments from the volcanic and sedimen- affected all of southern Peru, and earthquakes shook the city of Arequipa 75 km away. Eight tnry bedrock, and a small amount of accessory lwa deposits, totaling 10.2-13.1 km3 in bulk volume, are attributed to this eruption: (1) a widespread, fragments. The erupted pumice is a white, , -S.1 kn3 pumice-fall deposit; (2) channeled ignimbrites (1.6-2 km3) with (3) ground-surge and medium-potassicdacite (average SiO, and ash-cloud-surge deposits; (4) widespread co-ignimbrite ash layers; (5) base-surge deposits; (6) 63.6% 1.85% hlgO) of the potassic calc-alkalic suite. The unconfined ash-flow deposits; (7) crystal-rich deposits; and (S) late ash-fall and surge deposits. phenocryst assemblage comprises plagioclase, bio- Disruption of a hydrothermal system and hydromagmatic interactions are thought to have tite, amphibole, magnetite, quartz, ilmenite, and fueled the large-volume explosive eruption. Although the event triggired no caldera collapse, apatite. The white color, phenocryst assemblage, ring fractures that cut the vent area point to the onset of a funnel-type caldera collapse. and stratigraphic position distingirish the A.D. 1600 tephra from ash from adjacent volcanoes. INTRODUCTIOS LARGE VOLUME AND WIDE 2. Three types of nonwelded ignimbrites, The largest explosive eruption in historical SPECTRUM OF TEPHRA 1.6-2 km' in volume, are preserved in radial tinies in the Andes took place from 19 February Eight types of enipted deposits encompass an valleys and on the plateau. Lowest in strati- to 6 March 1600 at Huaynaputina. a small vol- estimated bulk volume of 10.2-13.1 km? (Figs. 2 graphic order, an ash-flow deposit is interbedded canic center 75 kin east of Arequipa in southern and 3. Table I). in the pumice-fall deposit in proximal sections Peni (Fig. 1: Thouret et al., 1997). Huaynaputina I. The Plinian pu,niice-fall deposit was dis- (Fig. 3A). Second, pyroclastic-flow deposits is not a typical stratovolcano; instead, four vents persed in a widespread, I-cni-thick lobe 11 15000 5-15 m thick above the Plinian fallout are chan- are nested on the floor (4200 m) of a horseshoe- kni? to the west and west-northwest toward the neled in all catchments and as far as 45 kni away shaped 2.5 x 1.5 km' caldera. open on the east Pacific coast (Fig. 2A). According to a log-nomial from the vent in the Rio Tambo canyon: they are edge of a high volcanic plateau toward the plot of thickness versus the square root of area composed of inversely graded pumice lapilli and canyon of the Rio Tambo. This caldera breached (Fig. 2B) and to the 360000 hn2 area of reported normally graded vitreous lava clasts in ash. ivith the summit (4SOO ni) of the volcano before the ash fall (Fig. 2C), the computed bulk volume of a few breadcrust bombs (Fig. 3A). Third, thick A.D. 1600 eruption. The plateau has been built up the Plinian fallout is -8.1 km'(dense-rock equiva- ash- and pumice-rich pyroclastic-flow deposits, of no more than 500-m-thick lava flows and ignimbrites of late hliocene to Quatemary age. tvhich overlie sedimentary and intrusive rocks of hlesozoic age. Repeated tephra falls. pyroclastic flows, and surges buried seven villages, killed -1500 people (Navarro. 1994). and devastated the landscape ivithin an elliptical area about 70 x 40 km west of the volcano. Pyroclastic flows choked the Rio Tambo canyon and reportedly dammed two teni- porary lakes (Barriga. 195 1). The catastrophic breaching of the lakes released large-scale debris Figure 1. Schematic map flows that swept down the 120-km-longvalley to of area of Huaynaputina the Pacific Ocean. volcano and Rio Tambo Understanding the voltrminous A.D. 1600 area. Inset: Location of Plinian eruption at Huaynaputina is important Huaynaputina and central because of the correlation of a wide spectrum of Andean volcanic zone (CAVZ) in Peru. erupted deposits with reported events, the severe afterniuth on the once-populated area around a poorly known volcano, and the probable global climatic impact. This paper shows the large mag- nittide of the eruption. the variety of the erupted deposits across an area of high relief, and the sig- nificance of the event. v. 1 Geology: Mny 1999: 27: no. 5: p. 4354&; 4 fi~ures:I table. 435 I I Fonds Documentaire ORSTOM Figure 2. A: Map showing ex- tent and isopachs (thickness A.D. in centimeters) of 1600 Plinian tephra fall from Huay- naputina (HP). 8: Plot of thickness vs. square root of area (after Pyle, 1989) for computing bulk volume of Plinian tephra-fall deposit: 6.91 km3 within 1 cm isopach and 8.12 km3 within limit of reported ash fall, shown in sketch map C. preserved on the plateau, are underlain by cres- (Fig. 3A). The sandwave facies and cross-bedded ash and lapilli. Both thickness and grain size of cent-shaped dunes as far as 1.5 km from the cal- beds as well as the pinch-and-swell and dunelike the ash layers increase toward the valley-con- dera rim (Fig. l).The dunes consist'of breccia, a features point to ground-surge deposits. Other fined ignimbrites. mixture of lava clasts, pumiceous and porphyritic thin, fine-ash layers from ash-cloud surges over- 5. Cross-bedded ash and pumice deposits, dacite lumps, and xenolithic blocks. lie the pyroclastic-flow units (deposit 3, Fig. 3A). 1-3 m thick and showing sandwave facies, accre- 3. One-decimeter- to several-decimeters-thick 4. Several massive ash layers totaling 5-20 tionary lapilli, and soft deformation, are interbed- ash and pumice deposits, fine grained and with cm in thickness mantle the pumice-fall deposit ded in the middle part of the ignimbrite sequence few'lithic fragments, underlie most of the pyro- as far as 75 km away from the vent (Fig. 3B). as far as 15 km from the vent in the Rio Tambo clastic-flow units in proximal valley sections They consist of fine ash alternating with coarse valley and tributaries (Fig. 3A). The base-surge Figure 3. Composite strati- graphic sections of tephra from A.D. olluviurn; ash-fall deposit 1600 eruption. Deposits fine vitric numbered 1 to 8 are described sh-cloudsurge, ash in text. rystal-rich surge deposit sh-cloud surge. fine vitic ash rystal-rich lapilli and ash Pre-A.D.1600soil Massive with increasing Unconfined medial to distal sections I3 %15km from the vent area L c Pre-A.D.1600soil in ash 436 f 1999 GEOLOGY, May e k-.-#f LI F deposits of phreatomagmatic origin represent an TABLE 1. VOLCANOLOGICAL DATA FOR A.D. 1600 PLINIAN ERUPTION estimated volume of 0.05 km'. Extent of the Plinianfallout (within 1cm isopach) 5 115 O00 km2 6. Unconfined ash-flow deposits2-3 m thick in Extent of reported ash and dust fallout (Fig.' 2C). -360 O00 kmz proximal areas (Fig. 3A) correlate with a massive Volume of Plinian ejecta (within 1cm isopach and -6.9 and 8.1 km3 vitric-ash layer 3-4 cm thick as far as 70 km west- from Fig. 2C) northwest of the vent. The flows that emplaced Minimum volume percent beyond 1cm isopach 17.5% (80% 4.4-5.6 the ash-flow deposits must have surmounted the Minimum volume DRE pumice, average km3 density 1.5 g/cm3) 2400-m-high left wall of the Rio Tambo canyon Minimum volume of lithic fragments 1.O5 km3 15 km away from source toward the southeast and Minimum volume of magma 3.754.25 km3 a series of 1000- to 1400-m-high topographic Duration of the Plinian phase 13-19 h 9 barriers at least as far as 60 km to the west. Mass eruption rate (1 h average, DRE magma density 1.3-1.5.105 kgls 2.4 g/cm3) 7. A gray, crystal-rich deposit, 15-35 cm thick io4 0.5-0.6 Volumetric eruption rate (same characteristicsas above) 7.~7.7 m3/s in proximal to medial sections and km3 Peak mass eruption rate* 1-2 10' kg/s in volume, overlaps a large part of the area man- Column height' 33-37 km tled by the Plinian fallout as far as 75 km west- Peak wind velocity' 10-20 mls northwest of the vent. The crystal-rich deposit, Weight percent ejecta <lmm (at 0.1 T thickness and 0.01 73.5%,98.3% thicker on slopes that face the vent, is interbedded T maximum) Weight percent ejecta <63 pm (at 0.1 T and 0.01 T 57.6%,93.4% between ash-flow deposits 6 and 8 in confined maximum) sections (Fig.
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