Special Paper 498 contains 12 new scientific papers, assembled as part of an NSF-sponsored workshop in 2011. The work highlights study of persistently active volcanoes and their hazards, mostly in Central America. Such volcanoes are termed “open vents” by volcanologists, and they offer the chance to study active processes. Insight into how volcanoes work and how hazards might be mitigated are the goals of the work. Overall, the volume presents insight into hazards infrastructure collaborations and development for geoscientists and students.

After 200 yr of repose, Pacaya Volcano resumed Strombolian activity in 1961 and has remained active until the time of this writing (2013). A three-dimensional map of 50 yr of nearly continuous activity of Pacaya depicts an accumulation of homogeneous, crystal-rich high-Al basalt on the west side of a preexisting cone. The material erupted is loose and welded spatter, volcanic ash, and 249 pahoehoe and a‘a lava flows, most of which were extruded in a few days, and most have extended less than 2 km in length from vents near the 2500-m-high summit down slopes of 20°–33°. The configuration of lava flows makes up a rigid, web-like network that welds the asymmetrical, steep western slope of an expanded Pacaya cone. The vents have fed the lava flows, forming a sieve-like pattern where lava leaks out. The cone contains a complex network of intrusive feeders, which fill and empty with lava, degas, and drain back. The volcano has shown explosive lava fountaining and effusive periods of activity and often exhibits both, as summit eruptions occur while lava drains from the cone. Lava flows and pyroclastic units from collapse-related avalanches and tephra fall tend to alternate. The overall length of lavas is limited, so that inhabited areas below the cone on most flanks are unlikely to be reached by flows, although topographic barriers, which blocked the flow of lava to the closest villages north of Pacaya, are now filled, so that lavas of moderate length (~2 km) could reach towns to the north under some conditions. The volcano is known to have experienced catastrophic explosive collapse in the last few thousand years. The current cone itself may be unstable because the new material has mostly asymmetrically loaded the west side of an old cone, and collapse to the west may be more likely because of mass imbalance and because of persistent activity that opens paths and accumulates on that side. Collapse to the west would threaten significant populations. Pacaya's past eruptions lasted centuries, with repose intervals of similar length, so the current activity may continue for another century or more. Overall, Pacaya is a complex of overlapping basaltic cones, and its pattern of activity provides insight into the early stages of composite cones such as nearby Agua, Fuego, Atitlán, and Santa María, all larger and older cones on the volcanic front of Guatemala with Pacaya.

The crater lake at Santa Ana Volcano (El Salvador) was monitored during 1992–1993 and 2002–2007. Crater lake chemistry was generally similar until the 2005 eruption. Acidification of the hydrothermal system by condensing magmatic gases yielded fluids that sustained a cool acid sulfate-chloride lake roughly 200 m in diameter (temperature = 16–28 °C, pH = 0.7–2.0, SO 4 2− = 4500–14,000 mg/L, Cl − = 1100–9200 mg/L, total dissolved solids [TDS] = 7000–25,000 mg/L). The phreatomagmatic eruption Volcanic Explosivity Index (VEI) 3 of October 2005 modified the summit crater morphology, leading to physical, thermal, and chemical changes in the lake over the next few years. The lake became hotter and more acidic, with variable chemistry and color (temperature = 24–66 °C, pH = 0.4–1.3, SO 4 2− = 2500–9800 mg/L, Cl − = 3200–22,000 mg/L, TDS = 10,000–36,000 mg/L, turquoise-gray-yellow color). The SO 4 2− /Cl − ratio dropped below 1, indicating an increase in the rate of volcanic gas input and coincident S depletion by abundant precipitation of native sulfur and secondary S-bearing minerals (alunite, gypsum, iron sulfides, and anhydrite). An increase in rare earth element (REE) concentrations in lake waters indicated leaching of the newly intruded magma. The eruption likely enhanced permeability in the edifice, further increasing the amount of available fresh wall rock to react with acidic fluids, and the concentration of rock-forming elements in the lake increased fivefold to a maximum of 90 g rock dissolved per kg water. The magma continued to degas through the lake bottom at the drowned eruptive vent, providing a large, direct gas input into the lake. Direct gas discharge into the lake led to sulfur saturation and formation of hollow sulfur spherules by percolation of gas bubbles through the molten sulfur bottom layer. Increased heat input into the lake (8–830 MW, equivalent SO 2 flux of 16–1600 t/d) led to enhanced evaporation and highly variable lake mass. Consequently, on three occasions during 2006 and 2007, the lake area diminished to 70% of its former size.

Arenal Volcano is a small (~1750 m above sea level, ~10 km 3 ) stratovolcano that continuously erupted between July 1968 and October 2010. During this long-lasting eruption (over 42 yr), a large volume of material—~5.6 × 10 8 m 3 of dense rock equivalent—has been extruded and has produced a thick and extended lava field, mainly on the western flank of the edifice. Measurements of ground deformation obtained using a network of dry-tilt stations are presented for the period 1986–2000. They show a continuous subsidence of the volcano with maximal amplitude on the western side. The load effect of the lava field is calculated and explains the largest part of the observed tilts. Once the data are corrected by this load effect, pressure source models are not supported by the observations and by quality criteria on the models. Although the dry-tilt data from Arenal Volcano give limited constraints on the deformation models, they are representative of a long period of activity that cannot be recovered by other means. Moreover, the corresponding interpretative model is consistent with results obtained by geotechnical studies and modern ground deformation methods like interferometric synthetic aperture radar (InSAR).

We test the suitability of short-occupation differential and absolute positioning methods using data from a 13 station global positioning system (GPS) network that spans the Santa Ana Volcano and Coatepeque caldera of western El Salvador for monitoring intereruptive activity and tectonic movements near these potentially hazardous features. Data spanning a 1 yr period from 12 GPS benchmarks located 1.9 km to 9.7 km from a continuous reference GPS station were processed with Trimble differential processing software to determine the repeatabilities and hence precisions of the differential station coordinates. For observation sessions spanning 20 min, the coordinates of the benchmark closest to our reference station are repeatable to within 4–6 mm in the horizontal component of the baseline between the two sites and 12–13 mm for the vertical component. In contrast, the horizontal and vertical repeatabilities at the site farthest from the reference station are 7–11 mm and 30–34 mm, respectively. These results suggest that any horizontal movement of the benchmarks larger than ~10 mm relative to the reference site, or vertical movement larger than 10–30 mm (depending on the baseline distance) should be detectable. Five sites adjacent to and within Coatepeque caldera moved upward at rates of 180–470 mm/yr relative to the reference site from February to July 2009. In contrast, no uplift pattern during this period was observed for the other network sites, suggesting an uplift source beneath the caldera. No increase in microseismicity coincided with the transient inflation event, and no other possibly corroborating observations from the caldera are available. The cause of the uplift is thus unknown. Our results suggest that differential GPS with occupation times of 20 min or more is a useful monitoring tool at subtropical volcanoes and calderas for networks with baselines that are shorter than ~10 km. Absolute positioning as an alternative GPS processing method gives precisions of ~10 mm (95%) in the absolute station coordinate estimates for occupations as short as 3 h, i.e., sufficiently precise for monitoring volcano deformation. This new monitoring strategy, whereby one dual-frequency receiver is used for short benchmark occupations without any need for a reference station, is simpler than that required for differential monitoring and minimizes tropospheric water vapor as a source of noise in station position estimates.

Concepción is currently the most active composite volcano in Nicaragua. Ash explosions of small to moderate size (volcano explosivity index 1–2) have occurred on a regular basis. Gravity data collected on and around the volcano between 2007 and 2010 confirm that a younger cone is built atop an older truncated edifice of denser material, predominantly lavas. The bulk density of the volcanic cone is 1764 kg m −3 (with an uncertainty of at least ±111 kg m −3 ), derived from gravity data. This estimated bulk density is significantly lower than densities (e.g., 2500 kg m −3 ) used in previous models of gravitational spreading of this volcano and suggests that the gravitational load of the edifice may be much lower than previously thought. The gravity data also revealed the existence of a possible northwest-southeast–oriented normal fault (parallel to the subduction zone). Episodic geodetic data gathered with dual-frequency global positioning system (GPS) instruments at five sites located around the volcano's base show no significant change in baseline length during 8 yr and 2 yr of observations along separate baselines. Structures deformed after the Tierra Blanca Plinian eruption ca. 19 ka, which significantly altered the form and bulk density of the volcano, may be due to the spreading of the volcano, but may also be related to volcano loading, magmatic intrusions and their subsequent evolution, and other volcano-tectonic processes, or a combination of any of these factors. A joint interpretation of our gravity and geodetic GPS data of Concepción suggests that this volcano is not spreading in a continuous fashion; if it is episodically spreading, it is driven by magma intrusion rather than gravity. These results have important implications for volcanic hazards associated with Concepción Volcano. Although during the last 15 yr tephra fallout and volcanic debris flows (lahars) have been the pervasive hazards at this volcano, earthquakes from an eventual slip of the fault on the east-northeast side of the volcano (delineated from our gravity measurements) should be considered as another important hazard, which may severely damage the infrastructures in the island, and conceivably trigger a volcano flank collapse.

Since it reactivated in 1994, Popocatépetl Volcano has undergone cycles of formation and destruction of several lava domes. This surface activity is generally associated with increasing seismic activity before the explosions that destroy the domes. We carried out a comprehensive analysis of seismic records from November 2002 to February 2003 in order to identify precursors of a series of explosive events. We obtained daily numbers of volcano-tectonic earthquakes and long-period events, as well as daily tremor duration. Spectral features of the long-period events and tremors were calculated, and high-frequency precursory signals of the long-period events were studied. No clear variations of these characteristics of the seismicity could be detected before the eruptions. Real-time Seismic Amplitude Measurements (RSAM) calculations show that, besides small fluctuations related to the explosions, the rate of seismic energy released was quite stable during the studied period. Minor short-lived variations of RSAM levels were observed before only five of eighteen eruptions, with no accelerating release of energy. It is thus quite difficult to identify reliable seismic precursors during the eruptive sequence. This situation is probably related to the open state of the system and has important implications for future risk assessment regarding this volcano.

Fuego Volcano (14°29′N, 90°53′W, 3800 m) is the southernmost vent of the north-south–trending Fuego-Acatenango volcanic complex. A basaltic-andesite stratovolcano, Fuego has had more than 60 subplinian eruptions since A.D. 1524, making it one of the most active volcanoes in the world. Since 1999, Fuego has exhibited continuous low-level activity, which alternates between periods of lava effusion with Strombolian explosions and periods of discrete explosions with no lava effusion. We analyzed explosions recorded on a broadband seismometer and infrasonic microphones in June and July 2008. The explosions were identified through a combination of visual field observations and the examination of infrasound records. Acoustic waveform cross-correlation indicated a highly repetitive source appropriate for investigating temporal variations in the wave field. The primary focus of this study is a time period from 8 to 27 June 2008, which included the emergence of a new lava flow. Using seismic coda wave interferometry analysis of 159 well-recorded explosions, we detected short-term relative changes in the velocity structure ranging from −0.23% to 0.61%. This rapid variation may indicate minor fluctuations in volatile content. Variations in seismic and acoustic wave arrival time differences, which might result from changes in source depth, are attributed to wind speed variations.

Rapid characterization of tephra from ongoing explosive eruptions can provide valuable insights into eruptive mechanisms, especially when integrated with data from other monitoring systems. Here we gain perspective on Stromboli's eruptive processes by linking the characteristics of ash collected in real-time with videos of each explosion. A 3 day, multifaceted field campaign at Stromboli was undertaken by Italy's Istituto Nazionale di Geofisica e Vulcanologia in October 2009. At this time, activity was at a moderately intense level, with the occurrence of an average of 4–5 explosions per hour at each of the SW and NE craters. Eight ash samples were analyzed using binocular and scanning electron microscopes to gain data on the components, grain size and morphology distributions, and surface chemistry of ash particles within each sample. Monitoring video of each explosion enabled an estimation of the duration and height of each sampled explosion. In each sample, the proportion of fluidal, glassy sideromelane (as opposed to blocky, microcrystalline tachylite plus lithics), the degree of “chemical freshness” (as opposed to alteration), and the average size of particles appear to correlate with the explosion “type” described in previous studies, and the maximum launch height of the corresponding explosion. Our observations suggest that more violent explosions (i.e., those driven by the liberation of larger and/or more pressurized gas volumes) can be associated with type 2a conditions and the fragmentation of hot and low-viscosity magma, while weaker type 2b explosions erupt predominantly ash-sized particles derived from the fragmentation of colder, more outgassed magma and passive integration of lithic wall debris. The formation of fluidal sideromelane ash particles (up to Pele's hair) requires the aerodynamic deformation of a relatively low-viscosity magma and demonstrates unequivocally that ash at Stromboli is not derived entirely from wall rock and/or brittle fragmentation of stagnant magma. We suggest that this ash-sized material forms through rapid acceleration and breakup of larger magma fragments, as supported by evidence from high-speed video of two of the sampled explosions.

Geologic mapping at the base of Volcán Barú, Panama, characterizes two large andesitic volcanic debris avalanche deposits attributed to sector collapse at Volcán Barú. The older Caisán debris avalanche deposit is at or beyond the radiocarbon dating range, >43,500 yr B.P., whereas the younger Barriles debris avalanche deposit is constrained by two radiocarbon ages that are ca. 9000 yr B.P. The total runout length of the Caisán deposit was ~50 km, covering nearly 1200 km 2 . The Barriles deposit extended to ~45 km and covered >990 km 2 , overlapping most of the Caisán. Over 4000 hummocks from these deposits were digitized, and statistical analysis of hummock location and geometry depicts flow patterns of highly fragmented material affected by underlying topography and also helps to define the shorter runout limit of the Barriles deposit. The Barriles and Caisán deposits are primarily unconfined deposits that are among the world's most voluminous subaerial debris avalanche deposits. Two different geospatial procedures, utilizing deposit thicknesses and edifice reconstruction, yield calculated volumes ~30 km 3 and larger for both deposits. Subaerial deposits of comparable scale include those from Mount Shasta, Socompa, and Shiveluch. Currently, the modern edifice is 200–400 m lower than the estimated precollapse Barriles and Caisán summits, and only 16%–25% of the former edifice has been replaced since the last failure. The ~10 km 3 postcollapse lava-dome complex, however, implies a Holocene magma production rate of 1.1 km 3 /k.y., comparable to elevated eruptive pulses documented at other stratovolcanoes, underscoring the importance of hazards assessment and monitoring of this active volcano.

Hazards in monogenetic volcanic fields include processes and events occurring prior to, during, and after an eruption. This contribution identifies hazards resulting from processes occurring prior to and after a volcanic eruption. From recent experiences in the Chichinautzin volcanic field, hazardous events associated with reports of potential impending eruptions have turned out to be three types of false alarms: fires or gas explosions in sanitary landfills, underground fires, and anthropogenic lava flows. Typically, people who live at monogenetic volcanic fields know that an eruption is a likely event, so when they observe deformation of the ground, heat flow, and explosions, they report these anomalous events to the authorities as volcanic. A methodology should be established to cope with reports of new volcanic activity and to handle the outcome, which could be volcanic or nonvolcanic hazards. The hazards related to events occurring after an eruption include the planning of cities and villages around tube systems, building hazards over lava tubes, pollution due to sewage release in lava tube systems, with consequences to public health and the environment, and endangering threatened species that live in the volcanic systems after the eruptions. Here, we propose a view of volcanic hazards that has not been made before and is distinct from the usual hazards evaluation during eruptions.

Hudson Volcano is one of the most active volcanoes in the southernmost Southern Andean volcanic zone, characterized by an ice-filled caldera 10 km in diameter. Tephrochronological studies indicate records of explosive activity from the late Pleistocene to historical times. In fact, the last large eruption occurred in August 1991 and is considered to be one of the largest eruptions of the twentieth century. The volcano is located in a remote and roadless region of the Patagonian Andes, which means that numerical models play an important role in assessing volcanic hazards at Hudson. In particular, these models are used to identify areas susceptible to be impacted by lahar flows and tephra fallout. In addition, a proximal-hazard zone was built using the energy cone model, which is useful when little or no prior geologic data are available. Lahar-inundation hazard zones were delineated using the LAHARZ model, based on empirical relationships. Several volumes were considered because of the range of potential lahar-initiating events, such as ice melting or mobilization of loose pyroclasts. Simulations indicate that valleys located west of the volcano are likely to be inundated by lahars, even small-volume lahars triggered by small eruptions, as have been recorded during historical episodes. In contrast, only large events would likely affect main populated settlements located farther west from the volcano. Tephra-fall deposits were simulated with an advection-diffusion model, Tephra2, employing wind data derived from atmospheric global data sets. Both spatial distribution of deposits and thickness derived from the August 1991 eruption were satisfactorily validated. Three eruptive scenarios were selected according to the geological record of the volcano. Results of simulations are outlined as probabilistic maps of mass accumulation on the surface and also as exceedance probability curves for selected localities. This analysis shows that regions east of the volcano are more vulnerable to tephra fallout throughout the year, and therefore no major interseasonal variability is recognized. However, the arrival of weather fronts, common during autumn and winter, could trigger tropospheric wind shifts, which may increase the chance of meridional (north-south) transport of pyroclasts. Finally, according to available tephrochronological data, the occurrence of a large eruption was estimated, indicating 10%–20% likelihood of an eruption ≥VEI 4 (where VEI is volcanic explosivity index) during the next 100 yr.

Boquerón Volcano, formed on the old San Salvador Volcano, is the youngest and active central edifice of the San Salvador volcanic complex, which also includes 25 secondary vents. The San Salvador volcanic complex is located in the vicinity of the San Salvador metropolitan area and is considered one of the most hazardous volcanic centers in El Salvador and Central America. Boquerón Volcano has a long record of effusive and explosive eruptions spanning 36,000 yr; the most recent eruption was in 1917. We reviewed and updated its eruptive history through detailed fieldwork, allowing the recognition of up to 25 different eruptions. Lava flows, ash-fall, and ballistic projectile deposits produced by Strombolian or violent-Strombolian eruptions are the most recurrent events preserved in the stratigraphic record of Boquerón Volcano. Pyroclastic-flow, and especially pyroclastic-surge, deposits are also present, indicative of explosive subplinian and Plinian eruptions, some of which had significant phreatomagmatic components. We define three hazard scenarios regarding ash fall, ballistic projectiles, and pyroclastic density currents for Boquerón Volcano and constrain them using its documented explosive eruptive history, fieldwork, and computer simulations. Each scenario is characterized by a likelihood of occurrence (high, medium, low), assigned to eruptive events of small, intermediate, or large magnitude, which are mainly characterized for the areal distribution of the related volcanic products. Resulting hazard maps show areas likely to be affected by future eruptions, enabling decision makers and the general public to consider volcanic hazards in land development and risk mitigation planning.