Misti volcano in southern Peru has a record of explosive eruptions and a nearby population of over 810,000, making it a hazardous volcano. The city center of Arequipa, Peru's second most populous city, is 15 km from the summit of Misti, and many neighborhoods are closer. As the population increases yearly, the urban boundary continues to move up the south side of the volcano. Many parts of the city are built upon the deposits from Misti's most recent Plinian eruption at ca. 2 ka. The 2 ka Plinian eruption (Volcanic Explosivity Index [VEI] 5) produced a 1.4 km 3 tephra-fall deposit and 0.01 km 3 of pyroclastic-flow deposits in ~2–5 h. Column height varied during the eruption but ascended up to 29 km. Pyroclastic flows descended only the south side of the volcano. The tephra fall spread southwest, resulting in ~20 cm of tephra accumulation in the area now occupied by the city center. The flowage deposits were previously identified as pyroclastic-flow deposits, but new sedimentologic and textural evidence suggests that ~80% (by volume) of the deposits were emplaced wet and relatively cold. As such, they are lahar deposits. A Neoglacial advance concurrent with the eruption supports evidence for voluminous snow and ice on the edifice. Pyroclastic flows melted between 0.01 km 3 and 0.04 km 3 of ice and snow on the volcano, triggering lahars that descended the volcano and inundated channels and some interfluves on the south flank. The lahars evolved downstream from proximal debris flows to distal hyperconcentrated flows, emplacing ~0.04 km 3 of deposits. Four facies of lahar deposits are present in the channels and another facies occurs on the interfluves. Such a comprehensive understanding of the 2 ka eruption will help to inform future volcanic hazards assessments. Pyroclastic-flow and tephra-fall deposits of the same magnitude could occur again and are useful in hazards assessment. The 2 ka lahars required voluminous water, which is no longer available on the volcano, and, within modern climate conditions, these deposits are not representative of possible future events. Estimations of water available from modern rain and snow suggest that lahars with volumes between 1 × 10 5 m 3 and 3 × 10 6 m 3 are possible. Lahars are more likely if an eruption occurs during a period of high snow accumulation or during subsequent heavy rainfall. Lahars up to 1 × 10 7 m 3 are possible if the Río Chili is dammed during an eruption. Lahar hazard zones generated using these volumes suggest the largest lahars could enter Arequipa.

In this work, unconformity-bounded stratigraphic (UBS) units are applied to a complex volcanic terrain. UBS units allow us to summarize the spatial-temporal relationships between the cartographic units of a volcanic district or complex, and identify individual successions of volcanic deposits and their probable connection with morphogenetic and/or tectonic phases. The UBS unit approach for the interpretation of significant preserved unconformity surfaces is supported by data from geomorphology, sedimentology, petrology, tectonics, and volcano-tectonics. The result is a stratigraphic scheme that combines the recognized lithostratigraphic units into a series of synthems, in which several phases of geological evolution of Ustica Island are recognized. The emplacement in the early Pleistocene of the basaltic Monte Guardia dei Turchi lithosome, which represents the emerged part of the volcanic seamount, was followed by hydromagmatic activity and construction of a tuff cone that is part of the Monte Costa del Fallo synthem. Following a period of quiescence, volcanism on the island resumed with a Plinian eruption that deposited a thick sequence of trachytic pumice breccias and was accompanied by a caldera collapse. Subsequently, marine terracing occurred in the southwest, while extrusive volcanics filled the caldera in the northern part of the island. Lava flows associated with this volcanism are present on the western flank of the volcano, whereas along the sublittoral zones, dike swarms provide evidence of phreatomagmatic activity. This volcanic activity was followed by repeated alluvial deposition and marine erosion. Finally, recent subaerial monogenetic centers produced shallow subvolcanic bodies, scoria and tuff cones, and lava flows.

Campi Flegrei is a densely populated active volcanic field. Two major explosive volcanic events have led to the formation of nested calderas. Detailed stratigraphy of the volcanic rocks outcropping in part of this area contributes toward a better understanding and definition of the volcanic hazard. Our research activity focuses on the southwestern sector of Campi Flegrei including Procida Island. This area is particularly suitable for stratigraphic reconstruction due to the thick pyroclastic sequences exposed on the coastal cliffs. These sequences include several paleosol horizons and substantially represent the products of all the volcanic activity of Campi Flegrei. The onset of volcanic activity in this area is represented by products related to the activity of scattered vents of unconstrained age. From 74 to 55 ka, they were mantled by products erupted in the nearby island of Ischia. Circa 39 ka, the Campanian Ignim-brite eruption occurred in the Campi Flegrei area, producing a large caldera. A thick succession of welded pyroclasts, lithic breccias, and associated ash- and pumice-flow deposits was emplaced in the proximal area. The local activity resumed at 19–17 ka with the formation of monogenetic volcanoes. A phreatoplinian eruption (Neapolitan Yellow Tuff) occurred at 15 ka, which produced a second, nested caldera. Stratified yellow tuff volcanoes, ranging in age between 9 and 5 ka, developed along the Neapolitan Yellow Tuff caldera boundary. Minor tephra layers testify to the final explosive activity vented in this area.

Abstract Here we describe a two-day field trip to examine Quaternary volcanism in the Canadian Cascade arc, named the Garibaldi volcanic belt. Day 1 of the trip proceeds along the Whistler corridor from Squamish to Pemberton and focuses on Quaternary glaciovolcanic deposits. Interactions between volcanoes and ice in the Garibaldi volcanic belt have been common during the past two million years and this has resulted in a diverse array of landforms, including subglacial domes, tuyas, impounded lava masses, and sinuous lavas that exploited within-ice drainage systems. On Day 2, the trip heads northwest of Pemberton, British Columbia, along logging roads to see deposits from the 2360 yr B.P. eruption of the Mount Meager volcanic complex. This eruption began Plinian-style, generating pyroclastic fall and flow deposits and ended with the production of block and ash pyroclastic flows by explosive (Vulcanian) collapse of lava domes (e.g., Soufriére Hills). Many of the traits of the deposits seen on this two day trip are a reflection of, both, the style of eruption and the nature of the surrounding landscape. In this regard, the trip provides a spectacular window into the nature and hazards of effusive and explosive volcanism occurring in mountainous terrains and the role of water and ice.

The youngest highly explosive basaltic eruptions from Masaya Caldera in central western Nicaragua produced five main pyroclastic deposits: the San Antonio Tephra, La Concepción Tephra, the Masaya Triple Layer, and the Masaya Tuff with the Ti cuan-te pe Lapilli. This tephra sequence was deposited over the past ∼6000 yr. The distribution and physical characteristics of these deposits suggest they originated from the Masaya Caldera. They have volumes ranging from 0.2 km 3 for La Concepción Tephra to 3.9 km 3 for the Masaya Tuff and cover minimum areas of 600–1600 km 2 . All deposits formed by violent eruptions discharging 10 11 to 10 12 kg of magma, thus reaching eruption magnitudes between 4.3 and 5.9 and volcanic explosivity indices of 3–4. An analysis of hazards for the main population centers around the Masaya Caldera shows that, if there were a similar eruption today, the most vulnerable communities would be Ticuantepe, Nindirí, and Masaya. In addition, La Concepción, southwest of the caldera, and the capital Managua, more than 15 km to the northwest, could be affected.

Fontana Tephra was erupted from the Masaya area in west-central Nicaragua in the late Pleistocene. This basaltic-andesitic Plinian eruption evolved through (1) an initial sequence of short, highly explosive pulses emplacing thinly stratified fallout lapilli, (2) emplacement of a surge to the southwest while fallout took place in the northwesterly dispersal sectors, (3) a series of quasi-steady Plinian episodes depositing massive fallout beds of highly vesicular scoria lapilli, and (4) a terminal phase of the eruption comprising numerous subplinian eruption pulses in which varying amounts of external water were involved, forming a well-stratified sequence of lapilli beds. The Plinian episodes were repeatedly interrupted by phreatomagmatically affected pulses, evidenced by layers of higher lithic contents and scoria clasts with quenched rims, as well as by proximal cross-bedded fine to medium lapilli pyroclastic surge deposits, which left pale ash partings at distal locations. Erupted tephra volumes, column heights, and wind velocities have been estimated for three different vent scenarios because no firm source location could be identified. The minimum total erupted tephra volume is between 1.4 and 1.8 km 3 , much lower than previous estimates for this eruption. Eruption column heights ranging from 24 to 30 km for the Plinian eruptive phases were obtained by comparing lithic and scoria distribution data with modeling results. Consistent results from different approaches suggest that these models, which were developed for dacitic to rhyolitic Plinian eruptions, also provide good approximations for basaltic Plinian eruptions considering all sources of uncertainty.

This study focuses on two issues that are still a matter of debate in subduction zones, particularly in western México: (1) the close association within the same volcanic complex of typical amphibole-free andesites to rhyolites and amphibole-bearing andesites to rhyolites, characteristic of the hydrated front of the Mexican arc; and (2) the occurrence of bimodal magmatism without evidence for interaction between mafic and intermediate to silicic magmas, which are in addition characterized by different petrogenetic affinities. Our case study is the San Pedro–Cerro Grande volcanic complex, a Quaternary silicic to intermediate dome complex located in western Mexico. Volcanic activity has been divided into two periods. In the middle Pleistocene, andesitic to dacitic magmas were emplaced along WNW-trending faults in the southern portion of the complex. The Las Cuevas pyroclastic sequence (older than ca. 500 ka) was emplaced during this episode, most likely from a local source. This first period of activity ended before ca. 280 ka with the emplacement of the Cuastecomate Plinian deposit, which is related to the formation of the San Pedro caldera, an ∼4-km-wide subcircular depression that is today partially buried by younger volcanic products. During the second period of activity (ca. 280–30 ka), rhyolitic and dacitic domes were mostly emplaced along the caldera rim and inside the caldera. In addition, hawaiites and mugearites built the Amado Nervo shield volcano on the caldera rim. Intermediate- to high-silica lava and pyroclastic rocks are subalkaline, whereas the Amado Nervo mafic lavas are transitional toward the alkaline series (Na-alkaline). No genetic relationships have been found between subalkaline and transitional Na-alkaline rocks, which are thought to represent different batches of magma from different mantle sources. Petrographic, geochemical, and isotopic variations observed in the transitional Na-alkaline Amado Nervo lavas point to a parental magma from a mantle melt that underwent limited olivine separation during its ascent to the surface. Among subalkaline rocks, two groups showing contrasting petrographical and geochemical features are recognized based on the presence of amphibole. Amphibole-bearing intermediate to silicic rocks are characterized by lower Ce and other incompatible trace element contents and lower 87 Sr/ 86 Sr (0.70382–0.70401) compared to amphibole-free rocks (0.70411–0.70424). On the basis of petrological characteristics, the two groups of magmas are interpreted to have evolved in two different magmatic reservoirs under different pressures and water contents in the mid-upper crust. Both groups of magmas were differentiated by open-system processes. We propose that assimilation and equilibrium crystallization (AEC) processes account for the amphibole-bearing rocks. Hotter and less evolved magmas interacted to a higher degree with the crust than the more evolved and colder magmas. This produced the observed higher 87 Sr/ 86 Sr in the less differentiated rocks of the amphibole-bearing group. On the other hand, amphibole-free rocks have chemical and isotopic characteristics that can be modeled by assimilation and fractional crystallization (AFC) processes. All data suggest that the two groups of subalkaline rocks have been generated by a common parental hydrous magma, but evolved in two different reservoirs. Amphibole-bearing magmas underwent amphibole fractionation in a mid-upper crustal reservoir and show assimilation of two types of basement: one akin to Oaxaquia and another akin to the Guerrero terrane. Amphibole-free magma only shows assimilation of an Oaxaquia-type basement.

The evolution of Popocatépetl volcano was determined through the definition of rock units following morphostratigraphic criteria and detailed geological sections. The primitive volcano, named Nexpayantla, probably contemporaneous with Pies volcano, a part of the Iztaccíhuatl volcanic complex to the north, grew beneath the site of today's cone. This volcano produced mainly andesitic to dacitic lava flows, presented flank activity in the form of several large dacitic lava domes, and was intruded by dacitic to rhyolitic dikes. The evolution of Nexpayantla volcano finished with a large collapse to the south that produced the Lower Tlayecac avalanche deposit. A new cone, Ventorrillo volcano, was built on the remains of Nexpayantla and was formed mostly by andesitic lava flows, but did not present any recognizable flank activity. Ventorrillo volcano collapsed in a large Bezymianny-type eruption toward the southwest, producing the Upper Tlayecac avalanche deposit and the Tochimilco pumice. The Calpan fan was derived from collapse and eruptions of Pies volcano. The present-day cone grew through the emission of many andesitic to dacitic lava flows, which were grouped into eroded or covered lava slopes (Malpaís, Las Mesas, Metepec, and San Pedro Benito Juarez lava flows), and glaciated (Fraile lava flows) and nonglaciated (Las Cruces, Buenavista, Quimichule, Atlimiyaya, Chiquipixle, and Nealtican lava flows) lava slopes with marked features, both from the central vent and from flank eruptions, mainly to the northeast and southwest of the cone. The Ecatzingo and Ombligo-Xalipilcáyatl flank vents formed two well-defined lineaments. The relative ages of the lava flows were determined through morphology, stratigraphic relations, and tephra cover. Two stages of growth were separated by a large Plinian eruption, which emplaced the Black and White (B&W) and Pumice with Andesite (PWA) fall deposits, which were used as stratigraphic markers. Another twelve Plinian pumice deposits are interstratified with the lava flows. Four large volcaniclastic fans and five valley fill deposits form the volcano's piedmont, and have resulted from the successive emplacement of pyroclastic flows, lahars, and fluvial deposits along several gullies that mark the lower slopes of the volcano. Glacier melting coincident with several of the Plinian eruptions could have been responsible for some of the extensive lahar deposits.

Approximately 21,700 yr B.P., after a period of quiescence of ∼4800 yr, Nevado de Toluca volcano erupted, producing the Lower Toluca Pumice deposit. The activity generated a 24-km-high Plinian column that lasted ∼11–13 h and dispersed 2.3 km 3 (0.8 km 3 dense rock equivalent) of tephra toward the NE, blanketing the Lerma basin, an area occupied today by the city of Toluca, with up to 5 cm of ash. Subsequent eruptive pulses were sub-Plinian in style, accompanied by phreatomagmatic explosions that emplaced surge deposits. Finally, the column collapsed toward the NE with the emplacement of a pumice flow deposit. The high vesicularity of the pumice from the basal Plinian layer, up to 83% by volume, indicates that exsolution was dominantly magmatic, and that pressurization of the magma chamber was probably due to a magma mixing process. Evidence for this includes the compositional range of juvenile products (61–65 wt% SiO 2 ), as well as the presence of two types of plagioclase, one in equilibrium and the other one with disequilibrium textures and reverse zoning. This suggests input of an andesitic liquid into the dacitic magma chamber. Based on the eruptive record, the most likely future eruptive activity at Nevado de Toluca volcano will be Plinian. Although quiet for more than 3250 yr, Plinian activity could occur after a long period of quiescence, and it could represent a hazard for the entire Toluca basin, where more than one million people live today.

Abstract Tephrochronological studies carried out over the past decade in the area surrounding Mexico City have yielded a wealth of new radiocarbon ages from eruptions at Popocatépetl, Nevado de Toluca, and Jocotitlán stratovolcanoes and monogenetic scoria cones in the Sierra Chichinautzin Volcanic Field. These dates allow us to constrain the frequency and types of eruptions that have affected this area during the course of the past 25,000 yr. They have important implications for archaeology as well as future hazard evaluations. Late Pleistocene and Holocene volcanic activities at the stratovolcanoes are characterized by recurrent cataclysmic Plinian eruptions of considerable magnitude. They have affected vast areas, including zones that today are occupied by large population centers at Puebla, Toluca, and Mexico City. During Holocene time, Nevado de Toluca and Jocotitlán have each experienced only one Plinian eruption, ca. 10,500 yr B.P. and 9700 yr B.P. respectively. During the same period of time, Popocatépetl had at least four such eruptions, ca. 8000, 5000, 2100, and 1100 yr B.P. Therefore, the recurrence interval for Plinian eruptions is less than 2000 yr in this region. The last two Plinian eruptions at Popocatépetl are of particular interest because they destroyed several human settlements in the Basin of Puebla. Evidence for these disasters stems from pottery shards and other artifacts covered by Plinian pumice falls, ash-flow deposits, and lahars on the plains to the east and northeast of the volcanic edifice. Several monogenetic scoria cones located within the Sierra Chichinautzin Volcanic Field at the southern margin of Mexico City were also dated by the radiocarbon method in recent years. Most previous research in this area was concentrated on Xitle scoria cone, whose lavas destroyed and buried the pre-Hispanic town of Cuicuilco ca. 1665 ± 35 yr B.P. The new dates indicate that the recurrence interval for monogenetic eruptions in the close vicinity of Mexico City is also <2000 yr. The longest lava flow associated with a scoria cone was erupted by Guespalapa and reached 24 km from its source; total areas covered by lava flows from each monogenetic eruption typically range between 30 and 80 km2, and total erupted volumes range between 0.5 and 2 km3/cone. An average eruption rate for the entire Chichinautzin was estimated at ~0.5 km3/1000 yr. These findings are of great importance for archaeological as well as volcanic hazard studies in this heavily populated region.