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This book provides a comprehensive description of the volcanological, petrological and geochemical features of the Poás Volcano (Costa Rica), one of the most active volcanic systems in Central America and part of the Central America Volcanic Arc (CAVA). Poás Volcano hosts a unique sulfur lake, which actually is one of the world's most acidic lakes, and has experienced molten sulfur eruptions. Past investigations, current monitoring activities and planned programs of investigation into lessening of the volcanic hazard are reported here. Specific sections of the monograph will be devoted to the impact of this volcano on the social, agricultural and industrial activities in the area. Legends and popular traditions related to this volcano will be described in the last chapter to round up a complete scientific review on this unique volcanic system.

Inhaltsverzeichnis

Frontmatter

Overview of the Tectonics and Geodynamics of Costa Rica

Abstract
Although Costa Rica is a relatively small region along the Central American Volcanic Arc (CAVA), its fascinating geology records several interesting examples of recent arc evolution. The forearc at present is in a state of subduction erosion, ranging from ‘moderate’ long-term rates of ~100 km3/km/Ma beneath Nicoya Peninsula to ‘extreme’ short-duration peaks of ~1,000 km3/km/Ma beneath Osa Peninsula. The margin is currently both seismogenic and tsunamogenic, with seismicity in the Osa Peninsula nucleating along one of Earth’s shallowest seismogenic plate interfaces. In Costa Rica, arc volcanism has created much larger volcanic edifices than it has northward along the CAVA. Forearc deformation is ongoing and active, and associated with large-scale erosion and sediment transport towards the trench that is currently being almost entirely trapped in forearc basins prior to the trench axis. Margin evolution is also strongly linked to documented spatial and temporal variations in the incoming Cocos and Nazca Plates. These conditions have had significant consequences for the geochemical evolution of the CAVA in Costa Rica, so that Costa Rica’s active geology records one of Earth’s most diverse and interesting volcanic arcs.
Paola Vannucchi, Jason P. Morgan

Geochemical and Geochronological Characterisation of the Poas Stratovolcano Stratigraphy

Abstract
In this chapter the stratigraphy of Poás volcano by using geological, petrographical, geochronological and geochemical analyses on the volcanic products erupted during the last 600 ka is defined. The northern flank of Poás consists of the following Units and Members: Río Sarapiquí, La Paz, Puente de Mulas Member (possibly from another source, though interdigitated with Poás products), Río Cuarto Lavas, Von Frantzius, Congo, Bosque Alegre and Laguna Kopper. The products on the southern flank are the Colima Formation, La Paz, Tiribí, Achiote, Poasito, Sabana Redonda and Poás Lapilli Tuff. The central part of the volcano is made up of the Poás Summit Unit, which includes the Main and Botos craters. Rock composition varies from basalts to dacites. During the last 600 ka the content of K2O and other oxides (e.g. TiO2 and P2O5) and trace elements (e.g. Zr, Ba) has varied significantly through time, implying the presence of two geochemical end-members since the beginning of the magmatic activity at Poás: (1) the Sabana Redonda Geochemical Component (TiO2 > 1%) enriched in HSFE and other trace elements, present in La Paz Andesites, Lavas Río Cuarto, Poasito, Sabana Redonda, Poás Lapilli Tuff and Botos crater lavas and (2) The Von Frantzius Geochemical Component (TiO2 < 0.8%) that was recognized in the lavas from the Main Crater, Von Frantzius, Achiote, Bosque Alegre, Congo and Botos. Lavas related to both magmatic components have coexisted over time as indicated by those found at Botos and the Main Crater. Units possibly related to a common vent, i.e. La Paz, Achiote and Main Crater, the percentages of K2O and TiO2 decreased through time.
Paulo Ruiz, Sara Mana, Esteban Gazel, Gerardo J. Soto, Michael J. Carr, Guillermo E. Alvarado

The Extraordinary Sulfur Volcanism of Poás from 1828 to 2018

Abstract
This chapter is arguably the most complete compilation of sulfur volcanism of any given volcano on Earth: Poás. Sulfur volcanism at Poás is described in historical literature since 1828, and in scientific literature since the 1960’s. We first classify the various manifestations of sulfur volcanism at crater lake bearing volcanoes (subaerial and sub-lacustrine sulfur pools, sulfur spherules, flows, cones/hornitos, and sweat, and pyroclastic and burning sulfur), based on work by Japanese pioneers in the early 1900s. Their first observations and models have passed the test of time and still stand as theories today. Comparing the sulfur volcanism at Poás with that in other (55) volcanoes, it is honest to say that only White Island (New Zealand) and Kawah Ijen (Indonesia) are the only ones comparable with Poás, being the most dynamic of them all.
Raúl Alberto Mora Amador, Dmitri Rouwet, Priscilla Vargas, Clive Oppenheimer

Coseismic Landslide Susceptibility Analysis Using LiDAR Data PGA Attenuation and GIS: The Case of Poás Volcano, Costa Rica, Central America

Abstract
A landslide susceptibility model for Poás volcano was created in response to the most recent event that triggered landslides in the area (the Mw 6.2 Cinchona earthquake, which occurred on the 8th of January, 2009). This earthquake was the sixth event related to destructive landslides in the last 250 yr in this area and it severely affected important infrastructures. This chapter refers to a study, which consisted of three phases, as follows: (1) creation of a post-Cinchona earthquake landslide catalog, which was done manually based on a set of high resolution orthophotos and LiDAR data and it includes 4,846 landslides; (2) a landslide susceptibility model, based on the Mora-Vahrson-Mora method, the data from our landslide inventory, and a new modeling of earthquake triggering indicators based on the attenuation of the peak ground acceleration of the event, and (3) an evaluation of the methodology used, which for the Cinchona case resulted in an overlap of the actual landslides and the higher susceptibility zones of ~97%. Based on our new methodology, four landslide susceptibility models were simulated: the Cinchona earthquake, the Mw 5.5 Sarchí earthquake 1912, and two hypothetical earthquakes: one on the Angel fault (Mw 6.0) and the other one on the San Miguel fault (Mw 7.0). The Toro and Sarapiquí river canyons, the non-vegetated corridor located west from the main crater of Poás and the areas where the La Paz Andesites Unit are located are always the zones with the highest susceptibility to slide values. Meanwhile, the northern part of the study area, where the Río Cuarto Lavas unit outcrops, always presented the lowest susceptibility values due to both the low slope angles and weathering level of its rocks.
Paulo Ruiz, Michael J. Carr, Guillermo E. Alvarado, Gerardo J. Soto, Sara Mana, Mark D. Feigenson, Luis F. Sáenz

Seismicity of Poás Volcano, Costa Rica

Abstract
This chapter summarizes the results of previous studies on the seismicity of Poás volcano and shows its behavior from 1980 to 2006, the last year of analogical recording at the seismic station of the Red Sismológica Nacional (RSN: ICE-UCR) located on the summit of the volcano. A new estimate of the signal frequency for those events occurring between 2007 and 2015 is also shown. The overview is based on the need to understand and classify the seismic signals of the Poás volcano according to the available records, modern instrumentations, and current knowledge about volcanic seismology. This will help to improve the management of volcanic emergencies and interpret changes in seismic activity in the volcano. To carry out the work, we first revised previous publications related to the seismicity of Poás volcano. Then, we analyzed seismic records of the seismic station VPS2 of the Red Sismológica Nacional (RSN: ICE-UCR) and other data generated by temporary seismic networks that were installed in the volcano. Three seismic signals have been clearly identified at Poás volcano: low-frequency events, tremors, and tectonic earthquakes. The most typical ones are low-frequency signals. The low-frequency events occur inside and around the main crater, at shallow depths, in response to vapor pressure. Tremors sporadically occur at Poás before and during the period of perturbation inside the volcano as a consequence of degassing. Swarms of small tectonic earthquakes (frequency = 1–2.5 Hz) also occur within the volcanic area in response to movements in local faults. The seismicity of Poás volcano, especially the low-frequency ones, increases before each eruptive cycle.
Mario Fernández-Arce, Raúl Alberto Mora Amador

Diffuse CO2 Degassing and Thermal Energy Release from Poás Volcano, Costa Rica

Abstract
During the period 2000–2003 four soil CO2 efflux surveys were carried out at Poás volcano (Costa Rica) to investigate the spatial distribution and evaluate the diffuse CO2 emission as well as its associated thermal energy. Inspection of soil CO2 efflux maps showed that the highest values were always identified inside the Main Crater of Poás, being the 2002 survey the one with the highest number of anomalous observed values. The spatial distribution of soil CO2 efflux and the δ13C–CO2 values in soil gas samples showed a positive correlation with the main volcanic-structural features of the area. Main soil CO2 efflux anomalies were identified close to fumaroles, where several acidic hot springs and soils with high permeability were recognized. Temporal evolution of diffuse CO2 emissions showed the lowest emission rate in 2000 (164 ± 15 t d−1), followed by a significant increase inside the active crater during 2001 and 2002 (423 ± 54 and 537 ± 69 t d−1, respectively) and with a relatively constant value in 2003 (542 ± 63 t d−1). These data correlated with the observed changes in the δ13C–CO2 mean value of collected soil gases. To estimate the thermal energy release associated with the diffuse CO2 degassing, we considered the diffuse CO2 emission released from the active crater as the most representative of a deep-seated source. Calculated thermal energy released through soil was estimated in 255, 548 and 831 MW for 2000, 2001 and 2003 surveys, respectively. Temporal variations of the diffuse CO2 degassing and thermal energy release also showed a good correlation with the δ13C–CO2 values and 3He/4He ratios measured in the fumarolic discharges of Poás during the same period, with a significant mantle-derived contribution. These observations evidenced the occurrence of changes in the shallow magmatic-hydrothermal system of Poás that were likely related to a potential magmatic intrusion during the period 2000–2003.
Gladys V. Melián, Nemesio M. Pérez, Raúl Alberto Mora Amador, Pedro A. Hernández, Carlos Ramírez, Hirochicka Sumino, Guillermo E. Alvarado, Mario Fernández

Behaviour of Polythionates in the Acid Lake of Poás Volcano: Insights into Changes in the Magmatic-Hydrothermal Regime and Subaqueous Input of Volatiles

Abstract
In this chapter, we document an extensive record of concentrations and speciation of polythionates (PTs: S4O62−, S5O62−, and S6O62−), which form in the warm (21–60 °C) and hyper-acidic (pH < 1.8) waters of the crater lake of Poás volcano (Costa Rica) through interaction with gaseous SO2 and H2S of magmatic origin. Our data set, together with earlier published results, covers the period 1980–2006 during which lake properties and behavior were marked by significant variations. Distinct stages of activity can be defined when combining PT distributions with geochemical, geophysical and field observations. Between 1985 and mid-1987, when fumarolic outgassing was centered on-shore, the total concentration of PTs in the lake was consistently high (up to 4,200 mg/kg). Mid-1987 was the start of a 7-year period of vigorous fumarolic activity with intermittent phreatic eruptions from the lake, which then dried out. Concentrations of PTs remained below or close to detection limits throughout this period. After mid-1994, when a new lake formed and fumarolic outgassing shifted to the dome, the total PT concentrations returned to relatively stable intermediate levels (up to 2,800 mg/kg) marking more quiescent conditions. Since early 1995, numerous weak fumarole vents started, opening up at several other locations in the crater area. During short intervals (November 2001–May 2002 and October 2003–March 2005), PTs virtually disappeared. After April 2005, PTs re-appeared in large amounts (up to more than 3,000 mg/kg) until February 2006, one month before the onset of the March 2006–2017 cycle of phreatic eruptions, when concentrations dropped and remained below 100 mg/kg. The observed behavior of PTs records changes in the input and SO2/H2S ratios of subaqueous fumaroles. The prevailing distribution of PTs is S4O62− > S5O62− > S6O62−, which is common for periods when total PT concentrations and SO2/H2S ratios of the gas influx into the lake are relatively high. PTs are virtually absent as a consequence of thermal or sulphitolytic breakdown during periods of strong fumarolic outgassing in response to shallow intrusion of fresh magma or fracturing of the solid envelope around a pre-existing body of cooling magma. They are also low in abundance or undetected during quiescent periods when subaqueous fumarolic output is weak and has low SO2/H2S ratios, resulting in a concentration sequence S5O62− > S4O62− > S6O62−. The onset of phreatic eruptions are preceded by an increase in PT concentrations, accompanied by a change in the dominance from penta- to tetrathionate, and followed by a sharp drop in total PT content, up to several months before. Periods of phreatic eruptive activity that started in 1987 and 2006 followed these PT signals of increased input of sulfur-rich gas, in both cases possibly in response to shallow emplacement of fresh magma or hydrofracturing.
María Martínez-Cruz, Manfred J. van Bergen, Bokuichiro Takano, Erick Fernández-Soto, Jorge Barquero-Hernández

Geophysical and Geochemical Precursors to Changes in Activity at Poás Volcano

Abstract
Acidic crater lakes at persistently active volcanoes act as both a moderator and an index of volcanic processes. Poás exhibits cyclic behavior characterized by calm periods alternating with periods of phreatic eruptions that last 2–10 years, which roughly correspond to liquid or vapor dominance. However, what causes the system to move on from one phase to the other remains unclear. By integrating the insights gained from different methods of monitoring, a view of the physical and chemical changes occurring before, during and after alterations in activity can be gleaned.
Hazel Rymer, María Martínez, Jorge Brenes, Glyn Williams-Jones, Andrea Borgia

39 Years of Geochemical Monitoring of Laguna Caliente Crater Lake, Poás: Patterns from the Past as Keys for the Future

Abstract
Since 1978 water chemistry of the Laguna Caliente crater lake has been used to monitor volcanic activity at Poás, Costa Rica, making it arguably the best studied hyper-acidic crater lake on Earth. During these 39 years, three phases of unrest occurred, manifested through frequent phreatic eruptions, with each a duration of several years to over a decade (1978–1980, 1986–1996, 2006–2016). We here present a novel technique to deal with the long time series of the chemical composition of water of Laguna Caliente, independent on previous deterministic research and resulting conceptual models. Common patterns of chemical parameters in relation with phreatic eruptive activity for the period 1978–September 2014 are sought, applying the objective statistical method of Pattern Recognition. This resulted in the definition of the strongest precursory signals and their respective thresholds. Numerical outcomes often confirm findings based on geochemical models (e.g. SO4, SO4/Cl and pH are strong monitoring parameters). However, some surprising parameters (opposite behavior of Mg/Cl ratios, decreases in Ca and Mg concentrations, increasing Al/Mg ratios) still need a geochemical explanation and should be a focus for future research strategies. The obtained parameters and thresholds were retrospectively applied for the “test period” of the Pattern Recognition method (November 2014–February 2016). This test provided hints that suggested that eruptive activity at Poás was not yet over, despite apparent quiescence in early 2016. Indeed, after new phreatic eruptions since May 2016, the 2006–2016 phreatic eruptive cycle culminated in phreatomagmatic activity in April 2017. We conclude that evaluating time series of chemical composition of crater lakes framed in the Pattern Recognition method can be a useful monitoring approach. Moreover, increased sampling frequency can provide more details and more adequate prediction of phreatic activity at Poás. Comparing Laguna Caliente with other two well monitored acidic crater lakes (Ruapehu Crater Lake, New Zealand and Yugama, Kusatsu-Shirane, Japan) Poás results unique in many ways and undoubtedly the most active crater lake of the three during the past four decades.
Dmitri Rouwet, Raúl Alberto Mora Amador, Laura Sandri, Carlos Ramírez-Umaña, Gino González, Giovannella Pecoraino, Bruno Capaccioni

The Last Eighteen Years (1998–2014) of Fumarolic Degassing at the Poás Volcano (Costa Rica) and Renewal Activity

Abstract
This chapter reviews the geochemical and isotopic data from the fumarolic gas discharges collected in a discontinuous mode from 1998 to 2014 at Poás volcano. During this period, the “Tico” volcano experienced a renewed phreatic activity that started in 2006 after a couple of decades of relative quiescence. In January 2009, a 6.2 Mw earthquake hit the village of Cinchona, which is located a 4 km to the east of Poás. As the phreatic activity kept evolving, the hyperacidic lake (“Laguna Caliente”) dried out and the high-temperature fumaroles that previously were likely entering the lake were revealed, though not accessible. The pyroclastic dome that formed in the early fifties was destroyed at the beginning of 2017 by several relatively small phreatomagmatic (strombolian and vulcanian type) small-size eruptions. The risk of sudden phreatic and phreato-magmatic events prevented the direct sampling of the fumaroles and as a consequence, no geochemical data were sampled in the last three years. Nevertheless, interesting hints were recorded by the gas geochemistry before the 2006 phreatic activity and the 2009 Cinchona seismic events, mainly based on the temporal variations of the H2S/SO2, H2/H2O, H2/Ar, CO/CO2, CH4/CO2 and HCl/HF ratios. However, in most cases the geochemical record is not complete since the gas discharging vents migrated or stopped their activity and new fumaroles formed up to the recent visual observations.
Orlando Vaselli, Franco Tassi, Tobias P. Fischer, Daniele Tardani, Erick Fernández, María del Mar Martínez, Marteen J. de Moor, Giulio Bini

Volcanic Hazard Assessment of Poás (Costa Rica) Based on the 1834, 1910, 1953–1955 and 2017 Historical Eruptions

Abstract
Poás is a complex stratovolcano with an altitude of 2,708 m a.s.l., located in the Cordillera Volcánica Central of Costa Rica. Prior to 2017, the last three historical eruptions occurred on 7th of February 1834, between January and May 1910 and during the period 1953–1955. Very few information exists on the 1834 eruption. The only references state that: it was an important event; ash reached >53 km W–SW of Poás, and it harmed the grasslands around the volcano. Related deposits of this eruption suggest phreatic activity, which launched bombs and blocks. Moreover, there is evidence of pyroclastic flow deposits near the crater. The 1910 eruption is better described. Despite the fact that ash fall is only reported near the volcano, a volume of the deposit of 1.6 × 107 m3 was estimated. Deposits of the eruption are white in color with many hydrothermally altered, and minor presence of juvenile fragments (vesicular lapilli). The eruption is classified as vulcanian, with deposits of ash fall and pyroclastic flows close to the crater. A Volcano Explosivity Index 3 (VEI 3) is estimated. The eruption affected agriculture. The 1953–1955 eruptions had a longer duration. Various ash fall deposits at several sites were reported. Deposits of this eruption, easily distinguished in the field, are black scoria lapilli, bombs with, sometimes fusiform, bread crust textures. In the eastern sector of the crater bombs can reach meters in size; such large bombs near the eruption centre at one side suggest the inclination of the eruptive conduct, or an asymmetrical vent-crater system. Inside the crater a 40 m-high dome and a lava flow were extruded during the eruption. Towards the eastern side of the current Laguna Caliente crater lake, relicts of a 8.5 m thick lava pool are found. During the entire eruptive episode, the acid lake presumably lacked. The eruption is described to be of a mixed type: strombolian, phreatomagmatic, vulcanian and dome extrusion eruptions. Considering the characteristics of this eruption, the height of the eruption column, ejected volume (2.1 × 107 m3), and its presumed duration, a VEI 3 is estimated. The eruptions damaged agricultural activity (including cattle), and forced the spontaneous evacuation of some people. In April 2017 magmatic eruptions followed a decade-long period of intense phreatic activity. These eruptions destroyed the 1953–1955 Dome and led to the complete dry out of Laguna Caliente. Pyroclastic cones and sulfur volcanism manifested at the bottom of the former crater lake bottom. The 2017 eruption severely affected touristic activities at and near Poás, with an estimated economic loss of 20 million dollars. By May–August 2018 Laguna Caliente reappeared. The volcanic hazards related to the three studied historical eruptions are: pyroclastic flows (at least 1 km from the eruptive centre, including reaching the current mirador sector), ballistics (bomb ejections up to 2 km from the emission centre), dispersion and fall of pyroclasts (tens of kms), gas emission and acid rain, dispersed by WSW dominant winds, and lahars in most of the river canyons SW of the volcano.
Raúl Alberto Mora Amador, Dmitri Rouwet, Gino González, Priscilla Vargas, Carlos Ramírez

History, Legends, Customs and Traditions of Poás Volcano, Costa Rica

Abstract
This chapter compiles known information on the history, legends and traditions regarding Laguna Caliente of Poás volcano. We describe the evolution since the first ascends in the 16th century, until the massive incurrence of touristic activities. The exciting “Legend of Rualdo” on a bird that sacrificed its sweet song to save its pretty maiden, who was going to be sacrificed to stop the fury of the volcano, is described. The “Legend of Rualdo” narrates supernatural facts and explains the formation of Laguna Caliente thanks to a pact between a bird and the powerful volcano. Moreover, the origin of the yearly tradition of climbing Poás volcano on March 19th, the day of the patron Saint of San José, is documented. Legends and traditions can be useful tools to decipher past activity of volcanoes, in this case Poás.
Raúl Alberto Mora Amador, Mario Fernández, Dmitri Rouwet

Poás Volcano Biodiversity

Abstract
Stablished in 1971, Poás Volcano was the first Costa Rica National Park. Rich in biodiversity and landscapes due to the physiognomic characteristics and its outstanding attraction it has as an active volcano, Poás is the main protected area of the country and the Central American region in terms of touristic visitation. As protected area it is also important its contribution to the preservation of the Cordillera Volcánica Central native forest and the role that forest coverage plays, in the generation of water for human consumption.
Frank González, Carolina Seas, Zaidett Barrientos, Sergio Gabriel Quesada-Acuña, Raúl Alberto Mora Amador
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