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Irazú is one of the largest and most active volcanoes in Costa Rica. We present the tephro-chronostratigraphy of the last 2.6 ka of the Irazú volcano based on detailed field work and C14 radiometric dating, as well as a revision of the geological and historical records. In the stratigraphic record we identified at least 30 tephra units. Eight of them corresponding to the historical period (i.e., after 1700 A.D.), separated by repose periods of different durations. The distribution of the deposits, the volcanic morphologies (craters and pyroclastic cones) and the radiometric ages indicate that most of this recent eruptive activity has occurred from the summit of Irazú along an E–W fissure (~ 4 km long). Toward the west of the summit, near the Sapper hill may be the source of the oldest eruptions at 200 A.D., while the La Laguna cone, located to the east of the summit, could have formed around 1540 A.D., and Main Crater to the west could have formed around sixteenth–seventeenth century. Since then, the historical eruptions (i.e., 1723–1724, 1917–1921, 1924, 1928, 1930, 1933, 1939–1940 and 1963–1965) have been sourced from this crater, but not all of them are registered in the stratigraphy. The eruption frequency of Irazú during this period ranges from 23 to 100 years, with a major event about every 80 years. Irazu's eruptions have been mainly phreatomagmatic and Strombolian, including some phreatic explosions. We present a detailed tephro-chronostratigraphy that will help to building temporal analysis for hazard assessment and risk management plans to face future eruptions at Irazú. [ABSTRACT FROM AUTHOR]

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The spatiotemporal distribution of volcanic activity poses a significant challenge to risk mitigation measures, as it is still largely unexplored in most of the volcanic systems. In this study, we re-assess the deformation observed by leveling surveys covering the nationwide tragedy 1963–1965 Irazú eruption in Costa Rica with a state-of-the-art analytical source inversion model. We combine the analytical model results with recent geophysical, geochemical, and petrology data to build a geological model of Irazú and its 10 km-distant Turrialba volcano. Based on the leveling survey, the source inversion model finds a reservoir between 5 and 7 km below the Irazú crater which is deeper than previously published. We also confirm that the source location is on top of the mid-crustal reservoir that was feeding Turrialba between the 2010–2022 eruptions. Using previous seismic tomography, gravity, petrology, and geochemistry study of Turrialba and Irazú, as well as other studies conducted on nearby volcanoes worldwide, we find that Irazú and Turrialba volcanoes likely share a mid-crustal plumbing system which could suggesting that their plumbing systems are interconnected with each other. These findings have important implications on the spatiotemporal distribution of the volcanic activity and for the 2.8 million inhabitants settled within a 50 km radius. Observations during recent episodes indicates that inflation beneath Irazú has the potential to trigger eruptive activity at either Irazú or Turrialba. While further analysis is required to assess the tectonic control on volcanic activity, tectonic processes may shape both short- and long-term volcanic activity. These results have global implications for risk mitigation measures for nearby volcanoes. [Display omitted] • 1958–64 data suggests a 5–7 km deep reservoir below Irazú during 1963–65 eruption. • This reservoir is on top of the reservoir that feeds the 2010–2022 Turrialba eruption. • Irazú and Turrialba might share a mid-crustal reservoir. • Spatiotemporal distribution activity of ITVS might be controlled by tectonics. [ABSTRACT FROM AUTHOR]

Abstract: Costa Rica is located geographically in the southern part of the Central American Volcanic Front, a zone where interaction between the Mesoamerican and South American cultures occurred in pre-Columbian times. Several volcanoes violently erupted during the Holocene, when the first nomadic human hunters and later settlers were present. Volcanic rocks were the most important geo-resource in making artifacts and as construction materials for pre-Columbian inhabitants. Some pottery products are believed to resemble smoking volcanoes, and the settlements around volcanoes would seem to indicate their influence on daily life. Undoubtedly, volcanic eruptions disrupted the life of early settlers, particularly in the vicinity of Arenal and Irazú volcanoes, where archaeological remains show transient effects and displacement caused by periodical eruptions, but later resilient occupations around the volcanoes. Most native languages are extinct, with the exception of those presently spoken in areas far away from active volcanoes, where no words are related to volcanic phenomena or structures. The preserved legends are ambiguous, suggesting that they were either produced during the early Spanish conquest or were altered following the pre-Columbian period. [Copyright &y& Elsevier]

Abstract: Typical records of volcanic tremor and explosion quakes at Arenal volcano are analyzed with a high-resolution time-frequency method. The main characteristics of these seismic signals are: (1) numerous regularly spaced spectral peaks including both odd and even overtones; (2) frequency gliding in the range [0.9–2] Hz of the fundamental peak; (3) frequency jumps with either positive or negative increments; (4) tremor episodes with two simultaneous systems of spectral peaks affected by independent frequency gliding; (5) progressive transitions between spasmodic tremor and harmonic tremor; (6) lack of clear and systematic relationship between the occurrence of explosions and tremor. Some examples of alternation between two states of oscillation characterized by different fundamental frequencies are also observed. Some tremor and explosion codas are characterized by acoustic and seismic waves with identical spectral content and frequency gliding, which suggests a common excitation process. We propose a source model for the tremor at Arenal in which intermittent gas flow through fractures produces repetitive pressure pulses. The repeating period of the pulses is stabilized by a feedback mechanism associated with standing or traveling waves in the magmatic conduit. The pressure pulses generate acoustic waves in the atmosphere and act as excitation of the interface waves in the conduit. When the repeating period of the pulses is stable enough, they produce regularly spaced spectral peaks by the Dirac comb effect and hence harmonic tremor. When the period stability is lost, because of failures in the feedback mechanism, the tremor becomes spasmodic. The proposed source model of tremor is similar to the sound emission process of a clarinet. Fractures in the solid or viscous layer capping the lava pool in the crater act as the clarinet reed, and the conduit filled with low velocity bubbly magma is equivalent to the pipe of the musical instrument. The frequency gliding is related to variations of the pressure in the conduit, which modify the gas fraction, the wave velocity and, possibly, the length of the resonator. Moreover, several observations suggest that two seismic sources, associated with two magmatic conduits, are active in Arenal volcano. They could explain in particular the apparent independence of tremor and explosions and the episodes of tremor displaying two simultaneous systems of spectral peaks. [Copyright &y& Elsevier]

After a century of quiescence, Turrialba, a Quaternary stratovolcano in the Costa Rican Volcanic Front, started a new eruptive cycle in 2010 that is ongoing to this date (July 3rd, 2019). Violent eruptions began in 2014, producing columns of ash that reached 4 km above the summit of the volcano. Here we provide new major and trace element data from fresh, glassy fragments (basaltic to rhyolitic) and mineralogical data for feldspars, olivines, and pyroxenes from ash fallout collected from 2016 and 2017 eruptions. We investigate the composition of the magma replenishing the system as well as the different juvenile components involved in the latter phase of this renewed eruption cycle. We find a range of mafic to felsic compositions in every eruptive event sampled. Our new geochemical data, coupled with Monte Carlo binary mixing simulations, indicate that magmas at Turrialba volcano are influenced by the input of a back-arc-type, strongly high field strength element-enriched (20–60 ppm) component generated by decompression melting from subduction-driven corner-flow. Thermodynamic modeling using MELTS software suggests that fractional crystallization alone fails to explain the compositional range of Turrialba's eruptive products. We instead propose that a complex, multi-stage magma mixing process produced renewed activity at Turrialba based on the trace element variability, the dominant intermediate magmatic compositions observed, and the results of our multi-element mixing simulations. This mixing process occurs upon injection of new mantle-derived melts into Turrialba's fractionated magmatic reservoirs and is governed by chaotic dynamics induced by gradients in temperature and viscosity. • We found compositionally heterogeneous fresh glass and minerals in Turrialba volcano's 2016 and 2017 ash fallout • Significant presence of a rhyolitic component and predominance of trachy-andesitic to trachy-dacitic compositions • We present evidence for the coexistence and coeval eruption of a back-arc and typical arc type geochemical signatures • Pervasiveness of mixing from source to surface is observed at Turrialba volcano suggesting a multi-stage process [ABSTRACT FROM AUTHOR]

The Poás volcano in Costa Rica has been studied as a Mars geochemical analog environment, since both the style of hydrothermal alteration present and the alteration mineralogy are consistent with Mars' relict hydrothermal systems. The site hosts an active volcano, with high-temperature fumaroles (up to 980°C) and an ultra-acidic lake. This lake, Laguna Caliente, is one of the most dynamic environments on Earth, with frequent phreatic eruptions, temperatures ranging from near-ambient to almost boiling, a pH range of −1 to 1.5, and a wide range of chemistries and redox potential. Martian acid-sulfate hydrothermal systems were likely similarly dynamic and equally challenging to life. The microbiology existing within Laguna Caliente was characterized for the first time, with sampling taking place in November, 2013. The diversity of the microbial community was surveyed via extraction of environmental DNA from fluid and sediment samples followed by Illumina sequencing of the 16S rRNA gene. The microbial diversity was limited to a single species of the bacterial genus Acidiphilium. This organism likely gets its energy from oxidation of reduced sulfur in the lake, including elemental sulfur. Given Mars' propensity for sulfur and acid-sulfate environments, this type of organism is of significant interest to the search for past or present life on the Red Planet. Key Words: Mars astrobiology—Acid-sulfate hydrothermal systems—Extremophiles—Acidic—High temperature—Acidiphilium bacteria. Astrobiology 18, 923–933. [ABSTRACT FROM AUTHOR]

Abstract: Pyroclastic flows represent the greatest volcanic hazard at Arenal volcano, Costa Rica, due to their recurrence, unpredictability, potential run outs, high velocities and short emplacement times. The main pyroclastic flow events occurring at Arenal during the present period of eruptive activity have been characterized and simulated using the geophysical flow model TITAN2D. The simulations performed, coupled with analyses of the evolution of Arenal concerning the topographic and morphological development of the volcanic edifice and the eruptive activity; provide insight into various temporal and spatial patterns of pyroclastic flow hazard. Increased pyroclastic flow frequency is shown to be related to the vertical growth rate of the active crater. The topographic evolution of the volcanic edifice and of the morphology of the lava field explain several observed spatial hazard patterns relating to flow directionality and run-out, and lava effusion rates are shown to be related to pyroclastic flow magnitude and volume. Identified patterns highlight the dynamism of pyroclastic flow hazard at Arenal, and its close relationship to the evolution of the volcanic edifice and of the eruptive activity. The simulations performed also draw attention to the sensitivity of pyroclastic flow emplacement to topographic features and to topographic change, highlighting the importance of up to date and accurate representations of the topography (DEMs) of the volcano for related hazard assessments. [Copyright &y& Elsevier]

Volcanoes change shape as they grow through eruption, intrusion, erosion, and deformation. To study volcano shape evolution we apply a comprehensive morphometric analysis to two contrasting arcs, Central America and the southern Central Andes. Using Shuttle Radar Topography Mission (SRTM) digital elevation models, we compute and define parameters for plan (ellipticity, irregularity) and profile (height/width, summit/basal width, slope) shape, as well as size (height, width, volume). We classify volcanoes as cones, sub-cones, and massifs, and recognize several evolutionary trends. Many cones grow to a critical height (∼1200 m) and volume (∼10 km3), after which most widen into sub-cones or massifs, but some grow into large cones. Large cones undergo sector collapse and/or gravitational spreading, without signifi cant morphometry change. Other smaller cones evolve by vent migration to elliptical subcones and massifs before reaching the critical height. The evolutionary trends can be related to magma flux, edifice strength, structure, and tectonics. In particular, trends may be controlled by two balancing factors: magma pressure versus lithostatic pressure, and conduit resistance versus edifice resistance. Morphometric analysis allows for the long-term state of individual or volcano groups to be assessed. Morphological trends can be integrated with geological, geophysical, and geochemical data to better define volcano evolution models. [ABSTRACT FROM AUTHOR]

Abstract: Irazú volcano, in Costa Rica, erupts magmas unusually enriched in incompatible trace elements (e.g., K, REE) relative to most other arc volcanoes worldwide. Previous studies place this enrichment in the mantle, with minimal inputs from the subducting slab. In order to test the subduction vs. mantle hypotheses, we present here the first published measurements of the pre-eruptive volatile content of Irazú magmas. Olivine-hosted melt inclusions from basaltic–andesite scoria from the 1723 eruption are volatile-rich, containing >3 wt.% H2O, >200 ppm CO2, >2500 ppm S, >2200 ppm Cl and >1800 ppm F. The average composition of the 1723 melt inclusions is very similar to that of the host scoria (SiO2 =54% SiO2), although inclusions include more mafic (48% SiO2) and felsic (57% SiO2) compositions. The 1723 melt inclusions have the same trace element characteristics (e.g., Ba/La) as the host scoria, ruling out exotic crustal or mantle sources. Together, the melt inclusions and their host olivines (Fo87–79) define a closed-system ascent path (150–20 MPa) of coupled degassing, crystallization, and cooling (1075–1045 °C). The maximum H2O measured in the melt inclusions and the shape of the degassing path together constrain the pre-eruptive H2O content to 3.2–3.5 wt.%, significantly higher than in ocean island basalts, but typical of arc magmas. The high H2O in Irazú melts, coupled with their high Cl/K2O, are inconsistent with enriched mantle with minimal slab fluid addition. We propose instead that subducting input is the dominant contributor to Irazú''s geochemical compositions. Galapagos-derived seamounts and volcaniclastics are currently entering the trench near Irazú, and provide to the Irazú source both volatiles (from seafloor hydration and chlorination) and ocean–island-type trace elements and isotopes. A few percent of subducted Galapagos volcanics added to MORB mantle can create Irazú compositions quantitatively, provided elements are further fractionated according to solute-rich liquid or melt–eclogite partition coefficients. Subduction of seamount chains may create high-K arc volcanism elsewhere, such as in the northern Marianas. [Copyright &y& Elsevier]

Abstract: We use the spatial autocorrelation (SPAC) method with improved inversion algorithms to estimate the Love and Rayleigh dispersion curves at two sites at the West and Northeast flanks of Arenal volcano, Costa Rica. At the West flank site, the Rayleigh waves phase velocities vary from 765 m s−1 at 1 Hz to 300 m s−1 at 12 Hz and those of Love waves between 780 and 295 m s−1 in the same frequency band. At the Northeast flank site, the Rayleigh wave velocities range from 1386 to 300 m s−1 and those of Love from 1983 to 315 m s−1. From dispersion curves we derive shallow (<400 m) P and S waves velocity models. 2D velocity models down to a depth of 150 m are also obtained by seismic refraction surveys along two radial profiles on the tephra apron at West and East flanks. They present strong vertical and lateral variations in the velocity and thickness of the layers. Strong variations in amplitude of the spectral peaks are observed for the seismic events along two radial arrays. These site effects are analysed using the H/V spectral ratio method and S-wave theoretical transfer functions. Results show that the wave amplifications are related to resonance effects of shallow structure (<150 m) and occur only where impedance contrast with the deeper layers is strong enough. In contrast, almost no site effect are detected at the Masaya shield volcano, Nicaragua, where the structure is more homogeneous and mainly composed of lava flows. When a resonance of the shallow layers occurs, the correlation coefficients between close stations increase at the corresponding frequency. The site effects may thus produce spurious results with the SPAC method. The H/V spectral ratio, used in complement of the SPAC method, can help detecting the site effects and testing the plane layer hypothesis. Furthermore, the theoretical transfer functions calculated for the estimated velocity models is also useful to validate the models. [Copyright &y& Elsevier]