Abstract Subaerial volcaniclastic deposits are produced principally by volcanic debris avalanches, pyroclastic density currents, lahars, and tephra falls. Those deposits have widely ranging geomorphic and sedimentologic characteristics; they can mantle, modify, or create new topography, and their emplacement and subsequent reworking can have an outsized impact on the geomorphic and sedimentologic responses of watersheds surrounding, and channels draining, volcanoes. Volcaniclastic deposits provide a wealth of information about eruptive histories, volcanic processes, and landscape responses to eruptions. The volcanic processes that produce these deposits, and consequently the character and sedimentary structures of the deposits themselves, are influenced by initiation mechanism. Deposit preservation is affected by deposit magnitude, texture, and composition, depositional environment, and climate regime. Innovative analyses of deposits from several modern eruptions and advancements in physical and numerical modelling have vastly improved our understanding of volcanic processes, interpretations of eruptive histories, and recognition of the hazards posed by volcanic eruptions. This contribution highlights and summarizes major advances that have occurred in the past few decades in understanding of volcaniclastic deposits and linkages with volcanic processes.

Abstract The 1980 eruption of Mount St. Helens caused instantaneous landscape disturbance on a grand scale. On 18 May 1980, an ensemble of volcanic processes, including a debris avalanche, a directed pyroclastic density current, voluminous lahars, and widespread tephra fall, abruptly altered landscape hydrology and geomorphology, and created distinctive disturbance zones having varying impacts on regional biota. Response to the geological and ecological disturbances has been varied and complex. In general, eruption-induced alterations in landscape hydrology and geomorphology led to enhanced stormflow discharge and sediment transport. Although the hydrolog-ical response to landscape perturbation has diminished, enhanced sediment transport persists in some basins. In the nearly 30 years since the eruption, 350 million (metric) tons of suspended sediment has been delivered from the Toutle River watershed to the Cowlitz River (roughly 40 times the average annual preeruption suspended-sediment discharge of the Columbia River). Such prodigious sediment loading has wreaked considerable socioeconomic havoc, causing significant channel aggradation and loss of flood conveyance capacity. significant and ongoing engineering efforts have been required to mitigate these problems. The overall biological evolution of the eruption-impacted landscape can be viewed in terms of a framework of survivor legacies. Despite appearances to the contrary, a surprising number of species survived the eruption, even in the most heavily devastated areas. With time, survivor “hotspots” have coalesced into larger patches, and have served as stepping stones for immigrant colonization. The importance of biological legacies will diminish with time, but the intertwined trajectories of geophysical and biological successions will influence the geological and biological responses to the 1980 eruption for decades to come.

This volume brings together papers on current research into natural hazards in El Salvador and efforts to mitigate their impact on the society. It recognizes the need and potential for such work around the world and especially in many developing countries. In El Salvador, researchers on geological hazards obtain very frequent experience with real hazards and, through their work, aim to help develop strategies to mitigate the terrible suffering and monetary cost that is associated with their impact.

Collapse of Santa Ana volcano during the late Pleistocene produced the voluminous and extremely mobile Acajutla debris avalanche, which traveled ∼50 km south into the Pacific Ocean, forming the broad Acajutla Peninsula. The subaerial deposit covers ∼390 km 2 ; inclusion of a possible additional ∼150 km 2 submarine component gives an estimated volume of 16 ± 5 km 3 . Hummocks are present to beyond the coast-line but are most prominent in four clusters corresponding to the location of buried bedrock ridges. Bulking in distal portions incorporated accessory Tertiary-to-Quaternary volcaniclastic rocks and ignimbrites. Modern Santa Ana volcano was constructed within the collapse scarp, visible only on its northwest side, following an apparent transition in eruptive style. More than 286,000 people, the country's main port, and important agricultural land now overlie the Acajutla debris-avalanche deposit, which is one of only a few in Central America to exceed 10 km 3 in size. Because major edifice failures are high-impact, low-frequency events, the probability of a future Acajutla-scale collapse is very low. However, a collapse even an order of magnitude smaller in volume from modern Santa Ana volcano would impact heavily populated areas. The Acajutla failure was perpendicular to a NW-trending fissure system cutting across Santa Ana volcano, which may also influence future failure orientations. The current structure of Santa Ana volcano suggests that future collapses are most likely to the southwest, but the possibility of northward failures cannot be excluded.

The Las Colinas landslide was one of thousands of landslides triggered by the January 13th El Salvador earthquake (M W 7.6) in early 2001. The landslide was highly destructive. It led to the death of ∼585 people when it swept into a residential area of Santa Tecla, a suburb of San Salvador. The landslide originated from the top of a steep escarpment and involved pyroclastic deposits (silty sands and sandy silts) interbedded with paleosol horizons. The initial volume of the landslide was only ∼130,000 m 3 . The runout distance of the landslide, which developed into a rapid flowslide, was 735 m over a vertical distance of 166 m giving a H/L ratio of 0.23. The flowslide ran its final 460 m over a slope of only 3°. The flowslide debris was mainly dry but may have been partially saturated. It is postulated that strong seismic shaking amplified by topographic effects led to tensile stripping of the initial failure mass, which then lost strength very rapidly as it moved downslope and disintegrated into cohesionless debris. Urban topography consisting of buildings and streets may have inhibited debris spreading and channelized debris resulting in a long runout. The Las Colinas flowslide illustrates that runout behavior determines the landslide hazard at the base of the source slope and raises the question of landslide risk at the base of the Balsamo Escarpment, where existing residential developments are located within the runout distance of similar flowslide events that could occur in the future.

Two devastating earthquakes struck El Salvador within a month. The first quake of January 13, 2001, which was centered off El Salvador's southern coast, damaged or destroyed nearly 108,000 houses and killed at least 944 people. A considerable amount of soil (∼200,000 m 3 ) was fluidized on a mountain ridge rising south behind the Las Colinas area of Nueva San Salvador (Santa Tecla). The average slope was at most ∼13°, and yet the fluidized soil flowed ∼400 m across the residential area, destroying many houses and killing more than 700 people. This report outlines the findings obtained through reconnaissance by a mission dispatched by the Japan Society of Civil Engineers and the laboratory tests that followed it.

The pyroclastic deposits, known as Tierra Blanca Joven, underlie most of metropolitan San Salvador and other areas surrounding Lake Ilopango. The Tierra Blanca Joven deposits are products of a complex sequence of pyroclastic flows and falls that occurred during the A.D. 430 eruption of Ilopango Caldera. Very fine, compact white ash-lapilli predominates in both flow and fall units. Laboratory tests carried out on high-quality, undisturbed Tierra Blanca Joven samples show negative pore-water pressures and weak cementation. They also reveal how the strength and compressibility of these sediments can change significantly when the suction and bonding are lost upon soaking or remolding. Thick Tierra Blanca Joven deposits contribute to landslide risk during heavy rainfalls and strong earthquakes in the region.

During a one-month period in early 2001, El Salvador experienced two devastating earthquakes. On 13 January, a M-7.7 earthquake centered ∼40 km off the southern coast in the Pacific Ocean caused widespread damage and fatalities throughout much of the country. The earthquake triggered thousands of landslides that were broadly scattered across the southern half of the country. The most damaging landslide, a rapidly moving mass of ∼130,000 m 3 , occurred in the Las Colinas neighborhood of Santa Tecla, where ∼585 people were killed. Another large landslide (∼750,000 m 3 ) near the city of San Vicente blocked the Pan-American Highway for several weeks. One month later, on 13 February, a M-6.6 earthquake occurred ∼40 km east-southeast of San Salvador and triggered additional thousands of landslides in the area east of Lake Ilopango. The landslides were concentrated in a 2500 km 2 area and were particularly abundant in areas underlain by thick deposits of poorly consolidated, late Pleistocene and Holocene Tierra Blanca rhyolitic tephras erupted from Ilopango caldera. Most of the triggered landslides were relatively small, shallow failures, but two large landslides occurred that blocked the El Desagüe River and the Jiboa River. The two earthquakes triggered similar types of landslides, but the distribution of triggered landslides differed because of different earthquake source parameters. The large-magnitude, deep, offshore earthquake triggered broadly scattered landslides over a large region, whereas the shallow, moderate-magnitude earthquake centered within the country triggered a much smaller, denser concentration of landslides. These results are significant in the context of seismic-hazard mitigation for various earthquake scenarios.

Volcanic debris flows (lahars) in El Salvador pose a significant risk to tens of thousands of people as well as to property and important infrastructure. Major cities and nearly a third of the country's population are located near San Salvador, San Vicente, and San Miguel volcanoes. Debris flows traveling as little as 4 km from source at these volcanoes put hundreds to thousands of lives, property, and infrastructure at risk. We used a statistically based model that relates debris-flow volume to cross-sectional and planimetric inundation areas to evaluate spatial patterns of inundation from a suite of debris flows ranging in volume from 100,000 m 3 to as large as 100 million m 3 and examined prehistoric deposits and a limited number of historical events at these volcanoes to estimate probable frequencies of recurrence. Our analyses show that zones of greatest debris-flow hazard generally are focused within 10 km of the summits of the volcanoes. For typical debris-flow velocities (3–10 m/s), these hazard areas can be inundated within a few minutes to a few tens of minutes after the onset of a debris flow. Our analyses of debris-flow recurrence at these volcanoes suggest that debris flows with volumes of 100,000 m 3 to as large as 500,000 m 3 have probable return periods broadly in the range of ∼10 to 100 yr. Debris flows having volumes less than 100,000 m 3 probably recur more frequently, especially at San Miguel volcano. Despite the limited extents of the hazard zones portrayed in our analyses, even the smallest debris flows could be devastating. Urban and agricultural expansions have encroached onto the flanks of the volcanoes, and debris-flow–hazard zones extend well into areas that are settled densely or used for agriculture. Therefore, people living, working, or recreating along channels that drain the volcanoes must learn to recognize potentially hazardous conditions, be aware of the extents of debris-flow–hazard zones, and be prepared to evacuate to safer ground when hazardous conditions develop.

Preindustrial human societies living in ancient Central America apparently varied considerably in their vulnerabilities to the sudden massive stresses caused by explosive volcanic eruptions. The focus is on ancient complex societies and volcanic eruptions in the Zapotitán Valley during the past two millennia in what is now El Salvador, within a contextual and comparative framework of other eruptions and societies from other areas of Central America. Comparing eruptions and their societal effects indicates that there was a threshold of magnitude of these eruptions beyond which the society affected did not recover, and reoccupation was by a different ethnic group. An example is the TBJ (Tierra Blanca Joven) eruption of Ilopango where the original inhabitants never recovered to reoccupy the area. In contrast, the smaller Boquerón eruption devastated the eastern half of the Zapotitán Valley, but recovery was achieved by the indigenous society. The concept of “scalar vulnerabilities” is introduced, where physical factors such as the magnitude of eruptions are compared along with cultural factors such as variation in organizational complexity and institutionalized hostility. Simpler egalitarian societies in this sample were more resilient to sudden massive stresses than were the more complex societies. Simpler societies relied less on intensive agriculture, staple crops, fixed facilities, redistributive economies, and hierarchical institutions. Cultural factors such as chronic political hostilities can greatly increase the vulnerability of societies at any point on the simple-to-complex range. It is argued here that assessing a society's vulnerability must include the magnitude of stress, the complexity of the society (including the related factors of adaptation, demography, and the “built environment”), and the political landscape.

Physical and chemical parameters were obtained from the crater lake of Santa Ana during a two-year monitoring program between 2000 and 2002. The lake contains cool (20 °C) acid-sulfate-chloride waters with a pH ∼1, SO 4 = 11,000 mg/kg, Cl = 7000 mg/kg, and total dissolved solids concentration = 23,000 mg/kg. A bathymetric survey revealed a shallow lake with a maximum depth of 27 m and a volume of 0.47 million m 3 . Chemical data obtained from the lake show that the major cations are derived essentially from the congruent dissolution of the basaltic andesite host rock. Thermodynamic modeling shows that the acid waters last equilibrated with the host andesite at low temperature, ∼100 °C. Stable isotopic data of the lake waters indicate that D/H and 18 O/ 16 O isotopic ratios reflect the combination of evaporation effects at the lake surface and the contribution of deep magmatic fluids. δ 34 S HSO 4 = 16.3‰ suggests that the main source of dissolved bisulfate ions is magmatic SO 2 . No δ 18 O equilibrium is observed between water and bisulfate ion, suggesting slow kinetics of the isotopic exchange at the low-temperature environment of the lake. Gas emissions from the fumarolic field increased in May 2000; lake temperature increased to 30 °C, and dissolved chloride and sulfate increased as well. Following this change in activity, deuterium and oxygen isotopic ratios shifted toward heavier compositions due to enhanced evaporation at the lake surface.

We report the first detailed study of spatial and temporal variations on the diffuse emission of carbon dioxide from the Santa Ana–Izalco–Coatepeque volcanic complex. Soil CO 2 efflux measurements were performed at 447 sampling sites and reached values up to 293 g m −2 d −1 for the March 2001 survey. Most of the diffuse CO 2 degassing is occurring at the center of this volcanic complex, near the Cerro Pacho dome where the intersection of one of the regional NW-SE fault/fracture systems and the south Coatepeque caldera rim occurs. Soil CO 2 efflux measurements were not performed inside Santa Ana's summit crater due to accessibility problems at the time of this survey. The total diffuse CO 2 emission for this volcanic complex was estimated at ∼600 t/d. Low CO 2 efflux values were identified on the flanks and summit regions of the Izalco and Santa Ana volcanoes. Isotopic data of gases collected from low-temperature fumaroles in the study area showed 3 He/ 4 He ratios close to the atmospheric composition, except for the fumarolic discharges inside Santa Ana summit crater (7.54). Carbon isotopic signature of these gases suggested that 74% of CO 2 is limestone-derived from thermodecarbonation processes, while the fumarole sample from Santa Ana crater showed ∼48% of limestone-derived CO 2 . Temporal variations of the diffuse CO 2 degassing observed at the Cerro Pacho dome ranged from 4.3 to 327 g m −2 d −1 , with a median value of 98 g m −2 d −1 . Time domain analysis of the soil CO 2 efflux showed a strong autoregressive behavior, whose covariance slowly dies through time. Spectral analysis showed the existence of soil CO 2 efflux variations at diurnal and semidiurnal frequencies. This finding suggests that soil CO 2 efflux variations are coupled with those of meteorological variables (i.e., barometric pressure, wind speed, etc.), accounting for much of the behavior of the soil CO 2 efflux time series at the short-term scale.

Volcán San Salvador (110 km 3 ) looms over the capital of El Salvador, with the same given name. The volcano has been dormant since its last eruption in 1917. Meanwhile, a metropolis with more than two million people has developed around and encroached upon the volcano flanks. This paper details the volcano's eruptive history and discusses the particular hazards associated with Volcán San Salvador. Additional thickness measurements (new data) are used to recalculate and improve aerial extent and volume estimates of widespread tephra. Five eruption scenarios are outlined that describe the major types of eruptions observed in the geologic record. They include monogenetic (magmatic and hydromagmatic) flank eruptions and three increasingly explosive eruption scenarios originating from a central vent. A map delineates hazard zones based on the types of volcanic hazards from the central vent. The relative risk of a monogenetic flank vent eruption is identified. This paper summarizes the most accurate data available to provide better understanding of the volcanic hazards so that the risks associated with the next volcanic eruption can be minimized.

The Ilopango Caldera, located 10 km east of San Salvador, has erupted voluminous silicic pyroclastics four times in the last 57,000 years. The present caldera has a quasi-rectangular shape and is filled by Lake Ilopango. This paper provides a detailed description of a segment of the intracaldera stratigraphy at Ilopango caldera, with emphasis on the San Agustín Block Unit. Physical volcanology, petrology, and geochemistry establish the depositional environment and eruptive conditions of the intracaldera sequence and help to model the emplacement of the San Agustín Block Unit. The intracaldera stratigraphy comprises a sequence of pyroclastic density currents, unconformably overlain by lacustrine sediments and conformably overlain by the San Agustín Block Unit. A new radiocarbon age on wood near the top of the Lacustrine Unit indicates that a lake was present ≥43,670 years ago. The intracaldera sequence displays abundant evidence of emplacement in a subaqueous environment. The San Agustín Block Unit comprises a basal Fine Ash facies and an overlying Pumice Breccia facies. The basal Fine Ash facies is a hydromagmatic layer containing pumiceous and blocky angular glass shards, aggregates of fine ash and phenocryst fragments, and phenocrysts with a fine ash coating. The overlying Pumice Breccia facies is composed of pumice clasts up to three meters in length. The pumice clasts display a series of jointing textures indicative of hot emplacement and rapid cooling. These two facies suggest an initial subaqueous explosive eruption in which a vesiculated silicic melt fragmented upon contact with the water. When the magma had degassed sufficiently, the eruption style evolved to subaqueous dome growth that spalled quenched pumice clasts from a moderately vesiculated carapace. La Caldera de Ilopango se encuentra a 10 km al este de San Salvador. Ilopango ha tenido cuatro grandes erupciones de piroclastos ricos en sílice durante los últimos 57,000 años. La caldera actualmente tiene forma rectangular y el Lago de Ilopango se encuentra dentro de ella. Este artículo proporciona una descripción detallada de un segmento de la estratigrafía en el interior de la caldera de Ilopango con énfasis en una unidad que se llama “Unidad de Bloque San Agustín.” La volcanología física, petrología y geoquímica describen el medio ambiente deposicional y las condiciones eruptivas de la secuencia en el interior de la caldera y ayudan a modelar el emplazamiento de la “Unidad de Bloque San Agustín.” La secuencia en el interior de la caldera es constituida de flujos piroclásticos discordantes sobre ella yacen sedimentos lacustres y sobre estos últimos yace de manera concordante la “Unidad de Bloque San Agustín.” Una datación reciente con el método de radiocarbón de madera encontrada cerca del techo de la secuencia lacustre indica que un lago ya existia hace ≥43,670 años. La secuencia en el interior de la caldera muestra evidencia de un emplazamiento en un medio ambiente subacuático. La “Unidad de Bloque San Agustín” constituye una facie basal de ceniza fina y una facie superpuesta de pómez brecha. La facie basal de ceniza fina es un estrato hidromagmático conteniendo pedazos de vidrio, algunos en forma de bloque y otros vesiculares con textura de pómez. Contiene además un agregado de ceniza fina con fragmentos fenocristales y fenocristales con una recubierta de ceniza fina. La pómez brecha está compuesta de pómez con una elongación a trece metros. Los clastos pómez muestran diaclasas radiales, diaclasas concéntricas y diaclasas perpendiculares a la superficie que indican una deposición caliente y un enfriamiento rápido. Las dos facies sugieren una erupcíon inicialemente explosiva donde un magma sílice vesiculado es fragmentado cuando se pone en contacto con el agua. Cuando el magma está lo suficientemente desgasificado, la erupcíon desarrolla un domo subacuático des-boronando clastos templados de pómez proveniendo de una caparazón vesiculada.

The most recent activity at Ilopango Caldera occurred in 1880 when a dacitic lava dome (68.4 wt% SiO 2 ) emerged from the center of the caldera lake. Dark inclusions of basaltic andesite (53.9 wt% SiO 2 ) represent 2 to 3 vol% of the dacite dome. Petrographic textures and geochemistry indicate that the mafic inclusions were solidified after injection into partially crystallized dacite magma. During and before solidification, plagioclase crystals observed in the dacite actively transferred into the mafic magma, entraining dacitic melt and producing a reaction rim on crystals. Plagioclase crystals with sodic cores (An 40–50 ) and more calcic overgrowth rims (∼An 70 ) derived from the dacitic melt are common inside mafic inclusions. Intimately mingled dacitic glass exhibits small but significant differences in chemistry relative to the host dacite, most notably a potassium enrichment, which we attribute to diffusion processes between the two magmas. We propose that an injection of mafic magma at the base of partially crystallized dacite magma triggered the 1880 eruption. The overpressure necessary for eruption was generated by the addition of mafic magma, which triggered vesiculation of the rising dacite. The presence of mafic magma in both the A.D. 429 and 1880 eruptions may indicate that this process is of general importance in the eruptive history of Ilopango. La última erupción de la caldera Ilopango fue en 1880, cuando emergió un domo dacítico (68.4% SiO 2 ) del centro del lago. Inclusiones oscuras de andesitas basálticas (53.9% SiO 2 ) constituyen de 2 a 3% del volumen del domo dacítico. Las texturas petrográficas y los análisis geoquímicos sugieren que las inclusiones máficas se solidificaron al ser inyectadas en un magma dacítico parcialmente cristalizado. Antes y durante la solidificación, cristales de la dacita fueron transferidos al magma máfico, entrando liquido dacítico y produciendo coronas de reacción sobre los cristales. Los cristales de plagioclasa originalmente derivados de la dacita presentan núcleos sódicos (An 40–50 ) y contornos cálcicos (∼An 70 ), encontrándose comunmente dentro de las inclusiones máficas. También, se registró la presencia de vidrio dacítico dentro de las inclusiones, este vidrio presenta pequeñas, pero importantes diferencias en su composición química con respecto a la dacita. Particularmente, se observó un enriquecimiento en potasio, el cual atribuimos a procesos de difusión entre ambos magmas. Por lo tanto, proponemos que una inyección de magma máfico en la base de un magma dacítico parcialmente cristalizado originó la erupción de 1880. La presión necesaria para la erupción fue producida por la adición de un volumen de magma máfico, por el recalentamiento y degasificación de la dacita. La presencia de magma máfico tanto en la erupción de 429 DC., como en la erupción de 1880, indica que este proceso es de importancia general en la historia eruptiva de la caldera Ilopango.

Ilopango Caldera in central El Salvador is filled by Ilopango Lake. In November and December of 1999, radon, thoron, carbon dioxide, and mercury soil gas concentrations were obtained at 106 points within the caldera, as well as carbon dioxide efflux. The spatial distribution of the concentrations of these gases and carbon dioxide efflux show that the values of Ilopango Caldera are within background levels of other active volcanoes of the world and El Salvador. However, several areas where anomalies of high radon, thoron, carbon dioxide, and carbon dioxide efflux coincide were identified in regions of dense faulting. Heavier carbon isotope values between −13‰ and −20‰ (overall range: −13.1‰ to −29.8‰) in the anomalous regions suggest mixing of minor amounts of volcanic gases with biogenic gases or the presence of C 4 plants. Degassing of carbon dioxide from the lake was calculated using a double boundary layer model. A total carbon dioxide efflux of 644 to 1111 t d −1 was calculated for the lake and the soils of the caldera. Considering the low soil degassing of carbon dioxide (0.3–3.9 g m −2 d −1 ), radon (1.2–108.5 pCi L −1 ), and mercury (0–0.016 mg m −3 ), Ilopango can be considered a quiescent caldera at the time of this survey.

San Miguel volcano in eastern El Salvador is one of the most active volcanoes in Central America. During the last 250 yr, it has erupted at least 28 times. The city of San Miguel (more than 300,000 inhabitants) is located 10 km from the summit. An investigation of the concentration of diffuse soil gases—radon, carbon dioxide, and mercury—and soil fluxes of carbon dioxide was done in December 1999–January 2000. Radon ( 222 Rn) concentrations ranged from 2 to 833 pCi/L (picocuries per liter) with an average of 110 pCi/L. Thoron ( 220 Rn) concentrations ranged from 20 to 2178 pCi/L with an average of 356 pCi/L. These are similar to concentrations measured at other erupting volcanoes of the world. Carbon dioxide concentrations and fluxes at San Miguel are low, with fluxes ranging from less than 0.1 to 5.0 g m −2 d −1 , with an average of 1.5 g m −2 d −1 . These fluxes are within the background levels of San Salvador volcano and the Santa Ana–Izalco–Coatepeque volcanic complex, and also compare with values found at Ilopango Caldera in El Salvador. Mercury soil gas concentrations were also low, with values ranging from zero to 56 µg m −3 , with an average of 2 µg m −3 . Carbon isotopic compositions indicate that the soil carbon dioxide is predominantly biogenic. The concentrations of the investigated gases as well as the flux of carbon dioxide are generally lower at the higher elevations of the volcanic edifice and higher at the lower elevations, close to NNW-trending faults, and to contacts between the different rock units. The low fluxes of carbon dioxide throughout the soils of San Miguel volcano are possibly due to low permeability of the volcanic cover, which is thicker at the higher elevations, and to ready degassing through the open volcanic pipe. This low flux is also consistent with the reported small but frequent historical eruptions of this volcano and its low SO 2 fluxes.

San Miguel volcano in eastern El Salvador is a classic composite cone, symmetrical and concave upwards. Its summit crater exceeds 344 m in depth and consists of several nested craters with nearly vertical walls. The inner crater has grown in depth and size since it was first described in 1924. The dominant eruptive product at San Miguel has been lava flows. Spatter and scoria cones commonly occur at flank vents that erupted historic lava flows. Lava flows erupted from flank vents on several occasions between 1699 and 1867, traveling as far as 8 km from their vents. During this time interval, the elevation of flank vents increased. All subsequent activity has been minor Strombolian and ash eruptions in the summit crater. Occasionally, scoria fall deposits, pyroclastic flow deposits, and phreatomagmatic ashes have been produced, but no substantial explosive event has been positively linked to San Miguel. Few lahar deposits have been identified as a result of the preponderance of lavas exposed on the upper and middle slopes. Geochemical analyses of lavas, tephras, and block and ash flow deposits erupted from San Miguel indicate that the majority of activity has been mafic in character, ranging between 51 and 53 wt% SiO 2 . Historic flank lavas plot at the mafic end of the chemical range and are basaltic. The most evolved flank lavas are basaltic andesites and comprise two chemically distinct subsets, distinguished mostly by the presence or absence of phenocrystic magnetite and their V and Al 2 O 3 contents. They occur only on the eastern flank of the volcano and appear to represent some of the oldest exposed lavas. A stratigraphic sequence of 22 crater lavas has the most restricted compositional range and exhibits two chemical trends. These trends may represent rapid-fire eruptions, with little to no intervening reposes, from two distinct batches of magma. Overall, the development of San Miguel volcano is fairly simple and consists of two evolutionary stages. The first stage consisted of eruption of lavas from a central vent and growth of the symmetrical cone. Shallowing of the subvolcanic magma chamber may have been associated with this stage. This was followed by a significant change in the subvolcanic plumbing system characterized by flank eruptions and onset of the second growth stage. Magma draining laterally from the magma chamber to the flank vents led to the collapse of the summit region. Modification of the summit crater is an active process that continues today.

A diffuse CO 2 and 222 Rn degassing survey was carried out at San Salvador volcano in February 1999. The goal of this study was to evaluate the spatial distribution of diffuse degassing rates of both gas species and its utility for San Salvador volcano hazard forecasting. Three hundred eighty-eight measurements of CO 2 efflux and 380 of soil gas 222 Rn were performed covering most of San Salvador volcano, but taking into consideration geological and structural characteristics of the study area. CO 2 efflux and soil gas 222 Rn data showed a wide range of values up to 760 g m −2 d −1 and 284 pCi L −1 , respectively. Most of the study area showed CO 2 efflux background values (B = 12.6 g m −2 d −1 ). Anomalously high CO 2 efflux data (> 9 × B) were mainly detected outside El Boquerón crater, suggesting a close spatial correlation with major faults and complex fracture/fault systems. Relatively high 222 Rn emissions (>140 pCi L −1 ) were also detected outside El Boquerón crater and located parallel to the NW fissure related to the 1917 eruption. The total diffuse CO 2 output of the volcano was estimated at ∼4000 t d −1 for an area of 266 km 2 . Carbon isotope signatures of the soil CO 2 ranged from −6.9‰ to −35.4‰, suggesting a mixing between different carbon reservoirs. Spatial distribution of the carbon isotope ratios showed that most of the isotopically positive carbon emission is actually occurring at El Boquerón crater. These results suggest that monitoring spatial and temporal variations of diffuse degassing will be a potential geochemical tool for the San Salvador volcanic surveillance program.