Jean-Claude Thouret

Geology of El Misti volcano near the city of Arequipa, Peru

Approximately 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.

Kinematic characteristics of pyroclastic density currents at Merapi and controls on their avulsion from natural and engineered channels

We herein report an example of pyroclastic density current avulsion on 14 June 2006 at Merapi, Indonesia. Four discrete series of multiple dome collapses led to the generation of four individual block-and-ash flows into Kali Gendol valley. All four pyroclastic density currents locally overflowed the channel margins to devastate cultivated terraces along each side of the box-shaped canyon while propagating as much as 2.2 km within adjacent tributaries. The largest destruction was caused by the second and third pyroclastic density currents. Both of these flows partially spilled out of the Gendol valley at a travel distance of 4.9 km, bypassing a sabo dam just upstream of the village of Kaliadem and leaving the village almost completely destroyed and buried by several meters of overbank deposits. The main mechanism of flow avulsion on June 14 was the overflow of up to 20 vol% of the dense, basal part of the pyroclastic density currents onto interfluves. The relative proportions of valley-escaped material increased dramatically in the succession of each of the four pyroclastic density currents. The main geometric parameters controlling flow avulsion and their critical values were quantified through high-resolution real-time kinetic–global positioning system (RTK-GPS) data of the pre-event topography for each of the 14 June flows. This case highlights the way in which a sabo dam can significantly increase the potential of flow avulsion. Key lessons are derived for the future hazard mitigation in valleys subject to volcanic mass flows. Flow-observational and geometric data are combined into a model to derive the kinematic characteristics of the basal, valley-ponding avalanche and the valley-escaping veneer flows, which are otherwise hidden by overriding clouds of elutriated ash.

Reconstruction of the AD 1600 Huaynaputina eruption based on the correlation of geologic evidence with early Spanish chronicles

The largest historical eruption (VEI 6) in the Andes began on February 19 and continued until March 6 or 15, AD 1600 at Huaynaputina, a dacitic stratovolcano located on a high volcanic plateau in south Peru. Tephra falls, pyroclastic flows and surges disrupted life in an area of ∼4900 km2 around the volcano, and ash-fall was reported 200–500 km away in south Peru, west Bolivia, and north Chile. The aftermath of the large-scale eruption was severe and protracted for the people and colonial economy of south Peru. By linking up the series of events inferred from Spanish chronicles with the lithofacies and composition of the tephra (bulk volume 11.4–12.1 km3, dense rock equivalent (DRE) 4.6–4.95 km3), we distinguish five eruptive phases. (1) During the plinian phase, a sustained plinian column 27–35 km high on February 19–20 delivered a dacitic pumice fall of ∼3.1 km3 DRE volume. The plinian pumice formed a widespread lobe of ∼95 000 km2 within the 1-cm isopach; strong winds carried fine ash >500 km to the west, and west-northwest into the Pacific Ocean. The computed volumetric eruption rate was in the range of 5.4–6.6×104 to 1×105 m3/s and the mass eruption rate 1.3–1.6×108 kg/s. The onset and high discharge of the sustained plinian eruption was fueled by the disruption of an active hydrothermal system enclosed in the pre-AD 1600 amphitheater. The plinian column shut off as the vent was choked when the fragmentation focus deepened to beneath the weathered bedrock, >1600 m below the vent area. (2) During the second phase, a dwindling column sent ash-falls on proximal to medial areas and possibly pyroclastic surges on proximal slopes. (3) During the third ignimbrite-forming phase with interspersed hydromagmatic events, pyroclastic flows 1.5–2 km3 in volume were channeled into the Rio Tambo canyon and tributaries. The flows with interbedded base-surge deposits in proximal tributaries probably produced vigorous columns over high, rugged relief around the Huaynaputina plateau. Winds winnowing the columns dispersed a widespread co-ignimbrite ash, probably mixed with co-plinian ash, over an area of ∼265 000 km2. (4) During the fourth phase, an unusual crystal ash-fall was deposited when the residual magma with a crystal content as high as 80% was tapped near the end of the eruption. (5) During the fifth phase, ash-flows produced surge deposits and lag-fall breccias near vent, small-volume ash-flow deposits in proximal catchments, and a thin ash-fall layer in medial to distal areas. The proximal deposits were also produced by diluted flows able to surmount ridges 1000 m high as far as 15 km east from the vent. The ignimbritic and hydromagmatic phases greatly modified the ≤400-m-diameter plinian vent. Tapping of the crystal-rich magma and ash flows towards the end of the eruption led to the formation of two youthful vents in domes. Geochemistry and mineralogy of the plinian and post-plinian units point to an unusual zoned magma sequence. The ignimbrite-forming phase tapped a magma batch richer in silica than the less differentiated plinian magma. The crystal-rich magma of unit 4 was fed by ‘crystal mush’ in a layered magma reservoir and (or) from two magma reservoirs at distinct depths. The geochemical and mineralogical trend throughout the eruption, and preliminary measurements of geobarometers suggest a complex model linking a shallow (6–7 km) magma reservoir to a deeper (∼15 km) reservoir. The total DRE volume (4.6–4.95 km3) of erupted tephra did not lead to caldera collapse. Ring fractures cutting multiple vents are associated with a dyke swarm intruding the weathered volcanic bedrock. This suggests the onset of a funnel-type or piecemeal collapse.

Largest explosive eruption in historical times in the Andes at Huaynaputina volcano, a.d. 1600, southern Peru

The largest explosive eruption (volcanic explosivity index of 6) in historical times in the Andes took place in a.d. 1600 at Huaynaputina volcano in southern Peru. According to chronicles, the eruption began on February 19 with a Plinian phase and lasted until March 6. Repeated tephra falls, pyroclastic flows, and surges devastated an area 70 × 40 km2 west of the vent and affected all of southern Peru, and earthquakes shook the city of Arequipa 75 km away. Eight deposits, totaling 10.2–13.1 km3 in bulk volume, are attributed to this eruption: (1) a widespread, ∼8.1 km3 pumice-fall deposit; (2) channeled ignimbrites (1.6–2 km3) with (3) ground-surge and ash-cloud-surge deposits; (4) widespread co-ignimbrite ash layers; (5) base-surge deposits; (6) unconfined ash-flow deposits; (7) crystal-rich deposits; and (8) late ash-fall and surge deposits. Disruption of a hydrothermal system and hydromagmatic interactions are thought to have fueled the large-volume explosive eruption. Although the event triggered no caldera collapse, ring fractures that cut the vent area point to the onset of a funnel-type caldera collapse.

Quaternary eruptive history and hazard-zone model at Nevado del Tolima and Cerro Machin Volcanoes, Colombia

The ice-clad and fumarolic Nevado del Tourna volcano (4 ° 39′N, 75 ° 20′W) south of Nevado del Ruiz, is offset toward the southeast from the axis of the volcanic Ruiz-Tolima massif with respect to the major NE-trending strike-slip Palestina fault. It is composed of four units: (1) a pre-Tolima plateau-like basement of basaltic andesite lava flows of early Quaternary age; (2) a dissected, ancestral Tourna stratovolcano, cut by a presumed collapse caldera of middle Pleistocene age; (3) an older Tolima stratovolcano of late Pleistocene age, partly destroyed by a summit caldera; and (4) composite domes of the cone-shaped young and present Tolima. Young Tolima volcano is an andesitic and dacitic composite cone formed over the past 40,000 years within a 3-km-wide caldera that opened around 0.14 Ma. Deposits of welded and nonwelded pumice- and scoria-flows were emplaced toward the southeast (Rio Combeima) and northeast (Rio Totare). Repeated growth of lava domes over the past 16,000 years is witnessed by thick block-lava flows on the southern and eastern flanks and by block-and-ash or scoria-rich pyroclastic-flow deposits. This activity occurred during at least six eruptives stages as follows: El Placer, ca. 16,200-14,000 yr B.P.; Romerales, ca. 13,000–12,300 yr B.P.; Canalones, ca. 11,500–9750 yr B.P.; Mesetas, ca. 7200 – 4600 yr. B.P.; Encanto, ca. 3600 – 1700 yr B.P., and Nieves, historical. Interactions with the ice cap probably triggered debris flows that partly filled the Combeima and Totare valleys and formed the Holocene terraces on the upper Pleistocene volcaniclastic fans of Ibagueand Venadillo as much as 60 km from the source. The latest major activity was a plinian eruption, which deposited a pumice-fall layer ca. 3600 yr B.P. (0.5 km3 actual volume) mainly toward the west and northwest. Minor tephra-falls and debris flows occurred during the historical period before the reported 1918 and 1943 small (phreatic ?) events. A general hazard-zone map shows areas potentially affected by future eruptions both at Nevado del Tolima and at active Cerro Machin 12 km southward. The extent of areas likely to be affected by tephra-falls, debris flows, pyroclastic flows or surges, debris avalanches and lava flows is shown. Subplinian and plinian eruptions of Nevado del Tolima were used to represent the moderate and large events to be expected. 300,000 people live within a 35-km distance from those volcanoes, which have exhibited a behaviour more explosive than Nevado del Ruiz. Despite the small-sized ice cap, debris flows are the most probable hazard for even a minor eruption, because of the very steep slope gradient, and because of probable interactions of hot eruptive products with ice and snow. Additionally, scoria flows and debris avalanches can be directed toward the southeast and could be transformed into debris flows that would devastate the Combeima valley and suburbs of Ibaguecity, where about 50,000 people live

Defining conditions for bulking and debulking in lahars

Through measurements at Semeru Volcano, East Java, we define the conditions under which bulking (entrainment of sediment and pore water) and debulking (dilution and sedimentation) occur in rain-triggered volcanic floods (lahars). Two observation sites were installed 510 m apart, along the Curah Lengkong River, 11.5 km southeast of Semerus summit. This 30-m-wide box valley, with a gravel and lava base, represents a real-world flume analogy. Pore-pressure sensors provided stage measurements, a broad-band seismograph gave insight into sediment content and frictional-collisional behavior, video cameras were used to measure surface velocities, and direct bucket samples were taken. Eight rainfall-induced lahars were recorded, lasting 1–3 h with heights of 0.5–2 m, peak velocities of 3–7 m/s, and discharges of 25–250 m3/s. Flows ranged from typical (<40 wt% sediment) to coarse and dense hyperconcentrated flows (50–60 wt% sediment). Multiple distinct flow “packets” occurred within the complex lahars, and were used to determine internal changes between sites. From the multiparameter data set at each site, volumetric bulking and wave shortening, due to portions of the lahar accelerating toward the flow front, are identified. Initial debulking of lahars between sites may reflect drainage into the dry substrate. Estimates of discharge and volume at each site lead to the quantification of bulking and debulking by these actively flowing lahars along the channel reach. From this, we observe that bulking can be localized to certain parts of lahars, resulting in intraevent increases in peak discharge that are greater than what would occur if bulking was evenly distributed throughout the flow. Such data are essential for the development of numerical descriptions and hazard models for mass flows.

Environmental changes in the highlands of the western Andean Cordillera, southern Peru, during the Holocene

In the tropical Andes, impacts of both natural and anthropogenic disturbances have been detected over a period exceeding 4000 years. However, the history of the environment remains unknown in most Andean regions. To infer possible interactions between climate and humans, we analysed the pollen content of an 8.5 m deep peat core extracted from a peat bog located near the Nevado Coropuna volcano on the slope of the Western Cordillera in southern Peru (15°30S, 72°40W, 6380 m). Results showed that taxa of the upper Puna expanded when cooler and moister climatic conditions prevailed during a time period that includes the early and the mid Holocene (9700–5200 cal. yr BP). An increase in shrub pollen frequencies and a decrease in the Poaceae/Asteraceae ratio are attributed to a drier climate during the late Holocene (5200–3000 cal. yr BP). After 3000 cal. yr BP, the vegetation cover resembled that of today. Both archaeological and pollen data attest to the beginning of agriculture from 2200 cal. yr BP. Around 900 cal. yr BP, the vegetation cover suddenly changed, probably because of a colder and drier climate. Past societies continued their agricultural activities despite this abrupt change. Our results emphasize the specific geographical location of Nevado Coropuna astride the Western Cordillera and the western edge of the Altiplano, which is consequently subject to both Pacific and Atlantic influences.

Low-temperature thermochronology in the Peruvian Central Andes: implications for long-term continental denudation, timing of plateau uplift, canyon incision and lithosphere dynamics

Abstract: In Peru, the western edge of the 4.5 km high Western Cordillera is cut by a >3 km deep canyon. To understand incision by the Cotahuasi–Ocoña River and the regional uplift history of this orogenic plateau capped by volcanic rocks, 26 crystalline rock samples were collected for low-temperature thermochronology from vertical profiles parallel and perpendicular to the canyon. Rock cooling histories confirm that most plateau denudation had occurred prior to 24 Ma but plateau incision peaked after c. 14–9 Ma in response to rapid surface uplift. The abrupt occurrence of a rock heating event is also detected during middle Miocene time. This was either a response to the emplacement of low-conductivity, regionally extensive ignimbritic caprock or a response to crustal-scale fluid circulation caused by wet melting of the overriding plate when magmatism resumed c. 24 Ma. The potential for thermochronology to provide information on past geothermal gradients is discussed, showing how it can be used as a proxy for understanding change in subducting slab dynamics, with oscillations in subduction angle having perhaps been the main on–off switch for magmatism in this Cordilleran setting. Supplementary material: Fission-track and apatite (U–Th)/He results and apatite chemistry indicators are available at http://www/geolsoc.org.uk/SUP18405.

SPOT satellite monitoring of the eruption of Nevado Sabancaya volcano (Southern Peru)

Abstract The Nevado Sabancaya volcano (Southern Peru) began a phreatomagmatic eruptive process on 28 May 1990 after 4 years of weak seismic activity and fumarolic emission. SPOT images were acquired on 21 July 1986 and 1 July 1989. Differences between the two dates in snow and ice cover and morphology of the crater allowed us to foresee an increase in activity and possible change in the eruptive process. After the beginning of the eruptive stage, five SPOT images were provided by the CNES (France), the last one dated 12 December 1990. They show variations of ash cover, the opening of fractures, an increase of the crater size, and new mudflows. The 10-m ground resolution and stereoscopic capabilities of Panchromatic SPOT imagery permitted us to follow the major changes of geomorphic features during a volcanic eruptive stage.

Quantitative scanning-electron microscope analysis of volcanic ash surfaces: Application to the 1982–1983 Galunggung eruption (Indonesia)

Qualitative analyses of volcanic ash are time-consuming and subjective, whereas quantitative analyses are methodical and automated. Not only volcanic ash particles, but also many natural particles have been widely described and quantified by their outlines. The qualitative data of volcanic ash surfaces need to be expressed quantitatively, supported by supplementary methods such as statistical analysis and artificial intelligence. Well-defined surface descriptors can be applied to volcanic ash particles. In this study, roughness and texture descriptors of pyroclastic material from the 1982–1983 eruption of Galunggung (Java, Indonesia) were used to describe the vesicle surfaces of the particles, alteration intensity, and/or fine particle abundance. These parameters are important for distinguishing the products of magmatic eruptions from those of phre-atomagmatic eruptions. Further application of this method may allow these descriptors to be easily converted to alteration grade, vesicularity index, intensity of the fragmentation mechanism, and relative proportions of the pyroclast types. Hence, discrimination between products of different fragmentation mechanisms may permit forecasting of volcanic hazards.