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Poás volcano, Costa Rica: Geology of the summit region and spatial and temporal variations among the most recent lavas
Abstract
The recent eruptive history of Poás volcano is described here on the basis of field mapping of the summit area. This period, which may represent 10 to 20,000 years and less than 10% of the history of the volcano, began with caldera formation. Subsequent events included: composite cone construction, faulting and flank subsidence, flank fissure eruption, and multiple crater collapses. Local and regional structures have controlled the locations of summit vents and flank cinder cones.Chemical compositions of lavas of known stratigraphic position demonstrate temporal magmatic variation at Poás. Three similar felsic to mafic magmatic sequences occurred at the summit. The first and second sequences were separated by flank and summit eruptions of a distinct magma batch, enriched in TiO2 and P2O5. The three felsic to mafic sequences appear to be cyclical, and the repeated, similar variations are interpreted as progressive tapping of zoned magma bodies developed repeatedly from a common parent by similar crystal fractionation processes. The breaks between the three sequences correlate with shifts or major modifications of the eruptive center. The chemical composition of the most recent lava, erupted in 1954, indicates that the volcano is near the end of the present sequence.The lavas are calc-alkaline basalts and andesites that are similar to the mafic lavas that comprise the bulk of the Central American volcanic front. The samples can be divided into 3 spatial-temporal-geochemical groups. A summit group that represents the most recent activity; a TiO2-rich group that erupted primarily on the south flank about 7500 yr. B.P.; and an Al2O3-rich group that erupted primarily from vents north of the presently active Main Crater. Geochemical variations and projections into pseudoternary CMAS diagrams suggest a moderate pressure for the summit group, that is, a magma chamber at intracrustal to subcrustal depths. The Al2O3-rich group define smaller primary phase volumes for olivine and plagioclase, which suggests a deeper and or more water-rich magma chamber. The TiO2-rich group appears to be a batch of mixed magma with the summit group magma as one endmember and a less siliceous magma as the other.
... Fueron mapeadas por Thorpe & Francis (1981) y Prosser & Carr (1987) como dos calderas en nido, y se mencionan en Brown et al. (1987). Corresponden parcialmente con algunos lineamientos mostrados en Woodward & Clyde (1993). ...
... Con respecto a las estructuras cuspidales del volcán Poás, Prosser & Carr (1987) las han interpretado como los bordes de calderas cuspidales. Brown et al. (1987), basados en mediciones gravimétricas, encuentran materiales de baja densidad que parecen rellenar depresiones topográficas a profundidades de varios cientos de metros. ...
... Soto (1999) considera que las supuestas calderas estarían cubiertas por los depósitos de Von Frantzius, Botos y del cráter principal del volcán Poás, pero observa que el problema es que los depósitos de las erupciones plinianas o ignimbríticas cuya erupción originaría la caldera, no aparecen. Indica que la "lapilli tuff" que proponen Prosser & Carr (1987), podría ser depósitos de pómez del Barva. ya completó un ciclo de rupturas, aunque si esta falla se extiende más al noroeste como sugerimos, faltaría este sector de romperse. ...
... Actividad histórica Raccichini y Bennett (1977); Casertano et al. (1983); Alvarado (2000; Mora ( 2010) Geomorfología Prosser y Carr (1987); Soto (1999); Alvarado (2000); Gazel y Ruiz (2005); Ruiz et al. (2010Ruiz et al. ( , 2019a) Geología Prosser y Carr (1987); Cigolini et al. (1991); Soto (1999); Gazel y Ruiz (2005) A continuación, se desglosan y describen aspectos geomorfológicos, tectónicos y de vulcanología, base fundamental para el estudio de amenaza volcánica. Puesto que, para un estudio de amenaza volcánica, las unidades y estructuras más relevantes son aquellas del Holoceno o a lo sumo del Pleistoceno Superior Tardío, justo aquellas que represente un mayor potencial de repetirse a corto plazo, en el presente estudio se les da particular énfasis a dichas unidades estratigráficas y tectónicas. ...
... Actividad histórica Raccichini y Bennett (1977); Casertano et al. (1983); Alvarado (2000; Mora ( 2010) Geomorfología Prosser y Carr (1987); Soto (1999); Alvarado (2000); Gazel y Ruiz (2005); Ruiz et al. (2010Ruiz et al. ( , 2019a) Geología Prosser y Carr (1987); Cigolini et al. (1991); Soto (1999); Gazel y Ruiz (2005) A continuación, se desglosan y describen aspectos geomorfológicos, tectónicos y de vulcanología, base fundamental para el estudio de amenaza volcánica. Puesto que, para un estudio de amenaza volcánica, las unidades y estructuras más relevantes son aquellas del Holoceno o a lo sumo del Pleistoceno Superior Tardío, justo aquellas que represente un mayor potencial de repetirse a corto plazo, en el presente estudio se les da particular énfasis a dichas unidades estratigráficas y tectónicas. ...
... En la cima del Poás se ha propuesto la existencia de calderas desde por lo menos finales de la década de 1960, basado en fotografías aéreas, en particular debido a la presencia de rupturas en las pendientes que generan escarpes con formas arqueadas (Fig. 2, 3, 4 y 5). La mayoría de los autores propusieron dos calderas anidadas, pobremente preservadas: la más grande de unos 5-6 km de diámetro, mientras que la más pequeña e interna, posee una forma elíptica de unos 4 km en su eje mayor (R. Madrigal en Boza, 1968;Moore, 1974;Thorpe et al., 1981;Alvarado, 1989;Prosser y Carr, 1987. Para otros investigadores, estas estructuras podrías corresponder más bien con grábenes volcánicos arqueados con rumbo NNW (Soto, 1994;Alvarado 2000;Alvarado et al., 2000;Montero et al., 2010;Ruiz et al., 2010), o como estructuras volcanotectónicas complejas (Alvarado, 1984(Alvarado, , 1989. ...
... The age of the lake is not well established. It is younger than the Botos cone (8300 yr B.P., calibrated age after Prosser and Carr 1987), but it appears to have been present for several centuries based on the stratigraphy of the crater interior (Casertano et al. 1983;Cigolini et al. 1991), and also according to Amerindian legends (Zeledón 2007). Preliminary findings from radiocarbon dating of organic matter in lacustrine deposits suggest a calibrated age of at least 890 ± 40 yr B.P. (Mora-Amador et al. 2004). ...
... The lake deposits consist of alternating fineand coarse-grained laminations, containing either elemental sulfur, or poorly crystallized silica precipitates and gypsum (Brantley et al. 1987). Disseminated sulfur, and sulfur "pipes", veins and tubes, are present within the lake sediments (Prosser and Carr 1987). The sediments were precipitated by the disproportionation of H 2 S in the lake water. ...
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.
... The volcanic crater of Poás Volcano, Costa Rica ( Figure 1) has been characterized as a Martian analog due to its mineralogical consistencies with relict Martian hydrothermal systems Black et al., 2015;Black et al., 2016;Rodríguez and van Bergen, 2017;Black and Hynek, 2018). The Poás Volcano is a basaltic andesite stratovolcano in the Central Cordillera of Costa Rica (Prosser and Carr, 1987). Poás has been active throughout the Holocene; phreatic to phreatomagmatic eruptions are common even in times of quiescence, such as between 1955 and 2017. ...
Past acid-sulfate hydrothermal systems on Mars have promise in their ability to have hosted life for billions of years. One method for analyzing these systems is to study analog environments on Earth. To assess the astrobiological potential of Martian acid-sulfate hydrothermal systems, the crater lake of the active Poás Volcano, Laguna Caliente, was sampled in 2013 and 2017. Laguna Caliente presents an extremely dynamic terrestrial environment with near-ambient to boiling temperatures, pH fluctuations from −0.87 to 1.5, a wide range of chemistries and redox potential, and frequent phreatic-to-phreatomagmatic eruptions. Samples of lake fluid, sulfur clumps, and lake bottom sediment underwent 16S rRNA gene sequencing and metagenomic “shotgun” sequencing, which revealed this lake hosts an extremely low biodiversity of microorganisms dominated by Acidiphilium spp. Shotgun metagenomics of the samples suggests this community has numerous genetic adaptations that confer survival, including functional pathways to reduce the effects of toxic metals and numerous metabolic pathways utilizing a variety of simple and complex sugar molecules. The identification of these various metabolic pathways suggests adaptations related to carbon limited environments, fulfillment of high energy requirements, and survival in a hostile volcanic setting. The perseverance of life in Laguna Caliente indicates life on Mars could have thrived in analogous environments, stressing the need for the search for life in relict Martian acid-sulfate hydrothermal systems.
... Poás (10°11′26″N 84°13′56″W) is a stratovolcano in Costa Rica that belongs to the Central Volcanic Range (CVR; Ruíz et al., 2019). It is composed of various volcanic structures, such as the composite cones at its top, Von Frantzius, Botos, and the Main Crater, that represents the spot of the historical activity (Prosser and Carr, 1987;Mora-Amador et al., 2019a). The main hydrological features at Poás volcano include 1) a hyperacid crater lake, named Laguna Caliente, that fills the active Main Crater, 2) a freshwater lake hosted at Botos cone, called Laguna Botos, and 3) acid springs and streams at the north-western flank of the volcano, which are part of the Río Agrio drainage basin and that arguably represent the seepage out of Laguna Caliente Sanford et al., 1995). ...
Decades of geochemical monitoring at active crater lakes worldwide have confirmed that variations in major elements and physico-chemical parameters are useful to detect changes in volcanic activity. However, it is still arduous to identify precursors of single phreatic eruptions. During the unrest phase of 2009–2016, at least 679 phreatic eruptions occurred at the hyperacid and hypersaline crater lake Laguna Caliente of Poás volcano (Costa Rica). In this study, we investigate the temporal variations of Rare Earth Elements (REE) dissolved in Laguna Caliente in order to 1) scrutinize if they can be used as a new geochemical tool to monitor changes of phreatic activity at hyperacid crater lakes and 2) identify the geochemical processes responsible for the variations of REE concentrations in the lake. The total concentration of REE varies from 950 to 2,773 μg kg⁻¹. (La/Pr)N-local rock ratios range from 0.93 to 1.35, and Light REE over Heavy REE (LREE/HREE)N-local rock ratios vary from 0.71 to 0.95. These same parameters vary in relation to significant changes in phreatic activity; in particular, the (La/Pr)N-local rock ratio increases as phreatic activity increases, while that of (LREE/HREE)N-local rock decreases when phreatic activity increases. REE concentrations and their ratios were compared with the variations of major elements and physico-chemical parameters of the lake. Calcium versus (La/Pr)N-local rock and versus (LREE/HREE)N-local rock ratios show different trends compared to the other major elements (Na, K, Mg, Al, Fe, SO4, and Cl). Moreover, a higher loss of Ca (up to 2,835 ppm) in lake water was found with respect to the loss of Al, K, and Na. This loss of Ca is argued to be due to gypsum precipitation, a process corroborated by the mass balance calculation simulating the precipitation of gypsum and the contemporaneous removal of REE from the lake water. The observed relations between REE, changes in phreatic activity, and the parameters commonly used for the monitoring of hyperacid volcanic lakes encourage investigating more on the temporal and cause-effect relationship between REE dynamics and changes in phreatic activity at crater lake-bearing volcanoes.
... There are few petrographic and chemical analyses available of the Hule area (Fig. 6; Table 1). There are 4 analyses from the Hule intra-maaric cones and lavas (McBirney and Williams, 1965;Tournon, 1984;Prosser and Carr, 1987;Malavassi, 1991), and one from the juvenile andesitic pumice of the Hule tephras (Soto, 1999). There are eight other samples from this area, although without precise locations (Malavassi, 1991), of which five appear to be from the intra-maaric cone-lavas, one from the silica-rich andesites of Hule tephra, and two from the walls of the maar. ...
The Hule and Río Cuarto maars are respectively located 11 and 18 km northward of the active crater of Poás volcano, on the Caribbean side of the Central Volcanic Range of Costa Rica. They lie on the northern part of Poás volcano massif, along a N–S trending, ~ 27 km-long volcanic fracture crossing the Poás volcano. The volcanic products from Hule maar (2.3 km × 1.8 km, area ~ 3.5 km2) are mainly pyroclastic surges (poorly vesiculated andesites with very small plagioclases), silica-rich andesitic pumice flows, air-fall deposits, ballistic blocks, and reworked deposits that overlie the regional Pleistocene volcanic basement. They were produced during three main explosive phases. Two overlapping pyroclastic cones have developed within the Hule maar, and at least three lava fields are related to them (high-Al basalt to basaltic andesite). Another maar, Pata de Gallo (400 m across), is located less than 1 km off the SE rim of Hule. Río Cuarto is a nearly circular maar (700–850 m across) with a surface area of 0.33 km2. Río Cuarto products include surges, ballistics and air-fall tephra, produced during three main explosive phases. These deposits show a narrow fan oriented westward, according to westerly wind direction. They indicate a westerly-directed surge (first 2 km), followed by air-fall deposits (up to 5 km away). Radiocarbon dating has shown that Hule was formed ~ 6.2 ka ago and Pata de Gallo probably formed ~ 2.8 ka ago, while the intra-maar products could have ages of ~ 1.7 ka or ~ 0.7 ka, indicating that Hule is a polygenetic maar. There are no radiocarbon ages yet for dating the formation of Río Cuarto maar, but archaeological data suggest that it erupted between 3–4 ka ago. The volume of pyroclastic deposits associated to Hule maar is estimated to be 0.51–0.53 km3, from which ~ 20% is juvenile material, therefore 0.07–0.08 km3 of new dense rock equivalent (DRE) magma, after subtracting 20–30% of porosity. The tephra from Río Cuarto is estimated to be 4.4 × 107 m3, of which 0.008 m3 correspond to DRE magma. The Hule and Río Cuarto maars are occupied by lakes and, in the last decades, several lake-overturn events have taken place, with a repeat cycle of six to seven years. The main outcome of these events has been the mass death of fish accompanied by changes in the lake color. In these systems, the hazard related to the possible occurrence of Nyos-type gas eruptions can be considered negligible or very local, but significant for tourists who camp by the lakes.Research Highlights► Hule and Río Cuarto maars are Holocene structures (6.2 and 3-4 ka old respectively). ► Hule is a polygenetic maar and has the smaller Pata de Gallo maar associated. ► Hule pyroclastics are ~0.50 km3 in volume, ~20% juvenile material. ► Volume of Río Cuarto tephra is ~0.044 km3. ► Lake-overturn events occur every 6-7 years in Hule and Río Cuarto maar lakes.
... In Table 3 the most important geologic and geomorphologic features of each volcanic unit that was affected by the coseismic landslides are summarized. More stratigraphic details, geochemical and geochronological data of these volcanoes and their units can be found in Prosser and Carr (1987), Alvarado and Carr (1993), Soto (1999), Gazel and Ruiz (2005) ). Alvarado and Carr (1993) recognized for these volcanoes eight volcanic units, and their products correspond to basaltic and andesitic lavas, pyroclastic flows, volcanic breccias, lahars and volcanic alluvium. ...
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.
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.
El volcán Poás es uno de los más interesantes centros eruptivos de Costa Rica por la variedad y peculiaridad de sus manifestaciones volcánicas. Junto a los rasgos geológicos de la zona se describe, resumidamente, la estructura de la cal[1]dera activa, evidenciando una clara ciclicidad de largo período en la actividad del volcán. Se recolectan, además, las noticias bibliográficas sobre la actividad reciente, esto es, a partir del inicio del siglo pasado, clasificando las manifestaciones en cuatro categorías: a) fumarólica y/o solfatárica; b) plumas de barro; e) explosiones freáticas conocidas en la bibliografía como geyseriformes); d) erupciones freato-magmáticas y de lava. El análisis de los fenómenos particulares, observados en 1980-81 permite atribuir el paula[1]tino aumento de la temperatura de las fumarolas, de 92° e en diciembre de 1980 hasta 960° e de marzo a noviembre de 1981, a la subida y salida de los fluidos concentrados en la masa mag mática superficial, involucrada en la erupción de 1953, cuya fracturación, en la parte ya rígida, había sido demostrada por una crisis sísmica local que comenzó el 27 de julio de 1980 y que duró unos diez días. Finalmente, se anticipa la idea de que todas las complejas manifestaciones del volcán Poás pueden ser producidas por un mismo mecanismo eruptivo.
doi: https://doi.org/10.22201/igeof.00167169p.1983.22.3.865
Compositions of andesitic to dacitic melts in equilibrium with plagioclase, orthopyroxene, and clinopyroxene have been experimentally calibrated as a geohygrometer. Application to Melson's data on melt inclusions and matrix glasses from the 1980 1982 cycle of eruptions of Mount St. Helens reveals systematic dewatering trends---ca. 4% (1980) to 1% (1982) H2O in subliquidus (intratelluric) melt in pyroclastics and 2% 3% (1980) to 1% (1982) in subliquidus melt in dome-building dacites. Before each pyroclastic event, intratelluric crystallization achieved melt H2O contents of ca. 6% (at a depth of at least 8 km), a necessary condition for crystallization of amphibole. Dewatering trends indicate that individual magma blobs or ribbons then ascended to successively higher levels over the 3-yr cycle, from a minimum depth of 4 km in 1980 to 0.35 km in 1982, and pyroclastically erupted. Comparison of Mount St. Helens compositions to worldwide andesites and dacites suggests that near-liquidus arc magmas that crystallize plagioclase and pyroxenes contain 1% 2% H2O, on average, whereas magmas in continental margins contain appreciably more H2O.
The regional variation of physical and geochemical characteristics of
Central American volcanoes occurs in two fundamentally different
patterns. The first pattern is symmetrical about Nicaragua. Crustal
thickness, silica contents of mafic lavas and volcanic edifice heights
are lowest in Nicaragua and increase smoothly toward Costa Rica to the
south and Guatemala to the north. Magma density is maximum in Nicaragua
and decreases smoothly outward. The regional variation in crustal
thickness is just enough so that magma densities, calculated at
appropriate Moho pressures, are the same at the base of the crust
throughout the region. This is consistent with magma ponding at the base
of the crust. The bulk compositions of Central American basalts show the
same symmetrical variation. Suites of Nicaraguan basalts plotted in
pseudo-ternary CMAS projections indicate large olivine and plagioclase
primary-phase volumes. Toward Costa Rica and Guatemala the olivine and
plagioclase fields inferred from suites of basaltic lavas are smaller,
which is consistent with fractionation at increasing depth. The second
pattern is the segmentation of the volcanic front and the plate margin
in general. The segmentation strongly affects the spacing and size of
volcanic centers. At segment boundaries volcanic centers are generally
small and unusually widely spaced. Toward segment interiors volcano
spacing and size increase systematically. The LIL element contents of
lavas strongly reflect this pattern. For lavas with similar silica
contents the larger the volcano, the higher the LIL element contents.
The relationships between segmentation, volcano spacing and volcano size
are compatible with diapiric rise of magma accumulated in narrow ribbons
near the upper surface of the underthrust slab. The relationship between
volcano volume and LIL element content is qualitatively in agreement
with an open-system fractionation model.
Samples of 26 successive lavas from the high composite volcano of Santa María in southwestern Guatemala, demonstrate variations in magnetic declination and inclination which can be roughly-correlated with similar data from cores in the Gulf of Mexico and lake sediments near Tlapacoya, Mexico, and suggest that Santa María's constructive history goes back to approximately 30,000 years B.P. The basaltic andesites and associated pyroclastics, which make up this volcanic succession and comprise the latter 40% of the volcano's eruptive output, show systematic increases in , and Zr, and decreases in MgO and CaO, up the succession. These evolutionary changes anticipate the dacites which first appeared after a period of repose in Santa María, first as pyroclastic debris in the catastrophic flank eruption of 1902, and later as the endogenous dacite dome of Santiaguito which appeared in 1922 and has continued to grow to the present day. The increasing effects of crystal fractionation in the latter part of Santa María's history, as reflected in the compositional changes in the basaltic andesites and its culmination in the Santiaguito dacites, is thought to be due to the inhibiting effect which increasing cone height has had on the volcano's eruptive frequency through time, so progressively increasing the lengths of the repose periods when fractionation could proceed undisturbed in the top of the feeding magma column. Data from other high composite cones in Guatemala suggests that 3,500-4,200 meters represents a threshold cone height above which basaltic andesite is not erupted in this region, and it is believed that this limitation led to the historically documented dormancy of Santa María during which the Santiaguito dacites evolved. A peridotitic mantle source, rather than eclogite, for the basaltic andesites is consistent with their chemistry, provided that the primary liquids have been depleted in Ni and Cr by approximately 10% olivine and clinopyroxene fractionation.
The measurement of gravity over active volcanoes is usually aimed either
at the development of sub-surface structural models1-4 or the
investigation of temporal changes in gravity associated with magma
movements5-13. The positive gravity anomaly over Poás
volcano, Costa Rica4 indicates the presence of a cylindrical
pipe of denser material extending to several kilometres depth. Although
the volcano is active, this feature has remained apparently stable for
at least the past four years. However, our programme of high-precision
microgravity studies indicates changes with amplitudes of 120 µGal
at the crater rim and 50 µGal on the flanks with a periodicty of
~30 days. These data are interpreted in terms of either relative
movements of several metres between the magma body and the stable
volcanic edifice or of cyclic density change related to the degree of
vesiculation or crystallization of the magma: there is a possibility
that these variations are tidally induced. Such studies may have wider
relevance in analysing previously unrecognized components of
microgravity change and in the monitoring of active volcanoes.
Two sequences of alternating units of lavas and pyroclastic rocks from two volcanoes in Central America were sampled and analysed. The rocks from Izalco, the smaller, more mafic cone, show a continuous increase in SiO2, K2O, Zr and La, and a decrease in CaO, MgO, Co and Cr with time. These trends are shown to be consistent with progressive crystal fractionation. Atitlán, a larger and more silicic volcano, displays a discontinuous trend in which geochemical reversals are thought to reflect the development of a series of small bodies of zoned magma. The lavas of each zoned section show decreases in SiO2, K2O, Hf and Ba, and increases in CaO, MgO, Co and V from the bottom of the section to its top. The different trends at Izalco and Atitlán are believed to reflect the restrictions cone height places on the effects of crystal fractionation. As cone height increases, the intervals between eruptions progressively lengthen, allowing more time for crystal fractionation to proceed. The two volcanoes may represent an early and a later stage in the general evolutionary development of large composite cones. Despite the remarkable regularity of the alternating flows and pyroclastic eruptions, no systematic chemical variations were recognized between the two types of lava.
The230Th-238U radioactive disequilibrium method was applied to the study of recent volcanic rocks from Costa Rica. Most samples are from the Irazu volcano. Some samples were dated by internal isochrons using the (230Th/232Th)-(238U/232Th) diagram, others were studied only by whole rock analyses. The evolution of the parent magma may be followed by the initial (230Th/232Th)0 ratios of the rocks. A model involving a differentiating magma chamber that existed for 140,000 years under the Irazu volcano correlates well with the observations. Other volcanoes seem to be in earlier stages of their evolution. Continuing study may help to solve the tholeiitic to andesitic volcanism relationship.
Direct field evidence for links between volcanic rocks and contemporaneous subvolcanic intrusions are difficult to establish. Such links might, however, be inferred by making physical measurements on a youthful active volcano. Poás is a small composite volcano (elevation 2700 m, diameter 20 km) which has evolved to its present form by a sequence of caldera- and crater-forming episodes, possibly during the last 5 × 104 y. There has been a high level of historic activity from the active crater, which is 1 km in diameter and 300 m deep, and which contains a hot lake. Lavas and dominant pyroclastic rocks exposed in the active crater are calc-alkaline basaltic andesites and andesites. Poás volcano is therefore typical of those in island arcs and young continental margins. Gravity measurements show that the volcano has a regional negative Bouguer anomaly, of amplitudes 100-200 gu, upon which is superimposed a closed positive anomaly, in the active crater area, about 2 km in diameter and with a maximum amplitude of 100 gu. This is thought to indicate a 1 km radius cylinder of solid rock of basaltic andesite or andesite composition with a shallow upper surface and extending to a depth of several kilometers. The concept of a large shallow magma chamber, with a diameter similar to that of the volcano, appears not to be appropriate for this volcano.