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.
Keywords
1 Introduction
The society often forgets how vulnerable we are as humans, and simply procrastinates lessons from memories into the future. Beck (2000) describes how the concept of risk inverts the relationship between past, present and future. The cause of an experience in the present is often framed in the future, and is therefore inexistent, invented and fictitious. In volcanology, the reconstruction of the eruptive history of a volcano based on writings from the recent history intrinsically implies the concept of risk, as impressions of the observers strongly bias the scientific concept of hazard. Reinterpretation of past writings with scientific rigor is necessary to translate the risk concept into useful information for risk assessment and reduction at active volcanoes (Bretón et al. 2002; Cashman and Cronin 2008; Del Gaudio et al. 2010; Todesco et al. 2015; Caudron et al. 2015). Using the information from the geological record, compiling details from writings helps constructing the eruptive history and unrest signals of volcanoes (Phillipson et al. 2013). Such catalogues are the basis for probabilistic hazard assessment (e.g. BET, pattern recognition, stochastical analyses, Marzocchi et al. 2004; Marzocchi and Bebbington 2012), besides being highly insightful on how the volcano will behave in the future on the long-term (e.g. largest expected eruption).
Poás volcano, Costa Rica, is a complex stratovolcano with an altitude of 2,708 m a.s.l., with three main structures: the Main Crater, the Von Frantzius cone and Laguna Botos (Fig. 1). The Main Crater hosts the hyper-acidic and hot crater lake Laguna Caliente, rimmed to the south by a dome extruded in 1953. Laguna Caliente has a diameter of 280–300 m and a depth between 23 and 60 m, depending on the state of activity (Brown et al. 1989; Rowe et al. 1992a, b; Martínez et al. 2000; Rouwet et al. Chapter “39 Years of Geochemical Monitoring of Laguna Caliente Crater Lake, Poás: Patterns from the Past as Keys for the Future”). Poás is the most visited volcano in Central America with over 400,000 visitors a year.
This chapter compiles bibliographic data from ancient writings (newspaper articles, scientific reports) for the three major historical eruptions of Poás volcano (1834, 1910 and 1953–1955), covering a period before modern volcanology came up, and prior to the most recent phreatic eruption cycles of 1985–1994 and 2006–2016. The most recent “historical” activity reappraisal occurred at the moment of writing (April 2017). A geological survey of the deposits and stratigraphy related to these three eruptions aims to reconstruct the eruptive events. The information from ancient writings is extremely helpful to reconstruct the eruptive history of Poás generally characterized by low-volume eruptions whose deposits are largely weathered or even eroded from the geological record, introducing large uncertainties in the interpretation of the eruptions and the eruptive models of the 1834, 1910 and 1953–1955 eruptions. A major question is answered in this contribution in order to assess the volcanic hazard, deduced from the four eruptions in the recent, or very recent past: How will the next eruption be like? Models of the four historical eruptions are presented here. This chapter can be helpful in creating a hazard map, as being based on the interpretation of the ancient writings and the geological record, it can be a useful document for authorities to take risk-reducing measures in the short-term, and orient human activities near Poás on the long-term.
2 Methodology
The first step is a thorough search of bibliographic information on the major historical eruptions. Ancient writings (newspaper articles, scientific reports) are gathered at the libraries Carlos Monge, Federico Tinoco and the library of the Escuela Centroamericana de Geología (all at the Universidad de Costa Rica). Other information was collected at the Biblioteca Nacional de Costa Rica, the Biblioteca del Museo Nacional de Costa Rica and the offices of the Archivo Nacional. Moreover, an unpublished letter by a researcher that visited the area during the activity of the 1950s was used. Parts of the narrations from ancient writings are reported as such in this study (in italic), and literally translated from Spanish into English. The literal translation will often demonstrate that interpretations in a volcanological framework are not always straightforward.
Based on information derived from the data compilation of ancient writings, a geological survey of recent volcanic deposits around Poás was executed. As many deposits are highly eroded (yearly rainfall of 4,000 mm), geological descriptions resulted to be insufficient in some cases to totally meet the aims of the study. Therefore, a proper combination of all this information with the contemporary chronicles is the key for a correct reconstruction.
Based on the interpretations of stratigraphy and the ancient writings, we created maps of maximum dispersal of ash fall, ballistics, pyroclastic surges, lahars, and of gas dispersion. The location of bombs in the perimeter of the Main Crater is recorded by GPS.
A map based on the historical activity of Poás was drawn, as follows: (1) bibliographic study of the historical eruptions; (2) tephrostratigraphic survey of the volcanic deposits associated with the major historical eruptions, and (3) definition of the expected hazards and characterization of the expected eruptions based on hazard assessment of historical activity.
Ash volumes are estimated by multiplying the thickness of the deposits for reference outcrops of historical eruptions with the surface area covered by the related eruptive products.
3 Historical Volcanology of Poás
3.1 The 7 February 1834 Eruption
3.1.1 Pre-eruptive Stage
The only document available for the pre-eruptive stage was written by Von Frantzius (1861), who reported some observations made by Miguel Alfaro, who descended the crater in 1828:
… during the previous years Miguel Alfaro himself had seen a vapor column, which also threw stones upwards, and around those sulfur accumulations and in the volcanic ash a blue flame burned. In the surroundings ash also found conical masses of pure sulfur, with an altitude of 3 to 4 feet. The lake then was a lot smaller; nevertheless, the water roared with more power and appeared more acidic and hotter than now. It seemed that at that time the volcano would have been a lot more active in throwing ash, and that it went gradually slowing down
The description of Miguel Alfaro, six years before the 1834 eruption, can be interpreted as the occurrence of phreatic eruptions (explosion of vapor, mud or other non-incandescent or melted material), the formation of sulfur cones and sulfur combustion, that are related to exhalation temperatures >444 °C (the boiling T of sulfur), and the presence of high-temperature fumaroles. Alfaro’s comparison of the lake size with the 1861 situation suggests a drop in lake level. This type of activity is rather common at Poás (Fig. 2). Recently, in April 1989, May 2005, September 2009, and following the April 2017 eruptions (see Sect. 3.4) similar features of sulfur volcanism were observed: sulfur combustion, sulfur structures, and phreatic eruptions (Oppenheimer 1992; Mora-Amador and Ramírez 2008; Mora-Amador 2009; Mora Amador et al. Chapter “The Extraordinary Sulfur Volcanism of Poás from 1828 to 2018”). No information on seismic activity prior to the 1834 event exists.
3.1.2 Eruptive Stage
Peraldo and Rodríguez (2001) textually reported part of the Archivo Nacional Municipal, 778, document:
The news got to me that on Friday seven in the night there was an ash eruption of the los Votos volcano, unknown to the level that it is certain that in the area near Púas, like the site of Agualate, has been buried the graze lands leaving it without resources for the animals to provide for themselves and how in this site there was the cattle that belongs to the parish that was auctioned the day 2 of the current (year), I believe it is essential on the moment that I’ve known, to manifest to You this occurrence that in similar (…unreadable…) and to have lost at the end of the last auction with the possible promptness to tell me to who it has been auctioned to proceed with the delivery to find oneself in the case that soon resign to pay for the animals, and that it doesn’t result in loss of the parish or to who will be (…unreadable…) and that opportunely will dispose of them. Barva, 15 February 1834.
The report above is the only document mentioning the exact moment of the event: February 7, 1834 in the night. The key point is understanding the meaning of the word “aterrar” in Spanish. In the Nuevo Diccionario de Costarriqueñismos (Quesada 2007), the word “aterrar” means “obstruct” or “filled with ground”, although not necessarily with ground (“tierra”), hence, “aterro” in the context of the writing means obstruction caused by a ground avalanche. We conclude that the report refers to ash (and possibly lapilli) fall out, in some ranches near Poás. The corresponding areas at present should be Poásito, Vara Blanca and surroundings. From the report, the graze lands were “buried” to such an extent that animals cannot graze. Following Blong (1984), a thickness of 10–15 mm of ash caused the loss of graze land. Regardless of the terminology of “aterrado”, we infer a significant impact by ash and lapilli fall out with a thickness in the order of centimeters between 5 and 10 km around the emission centre. Von Frantzius (1861) is the first to write a scientific report on the 1834 eruption:
In 1834 a strong ash rain destroyed graze lands from the city of Alajuela on the south flanks of the volcano.
He continued:
…when in the year 1834 the ash rain occurred, the sky suddenly darkened, strong detonations were heard and the ash fell in southwestern direction until close to Esparza, at a distance of ten miles (diez leguas).
The Spanish word “legua” is an ancient unit to express the distance a person or a horse can cover within one hour. As such, a “legua” is a distance varying from 4 to 7 km. Hence, the distance described by Von Frantzius (1861), varies from 40 to 70 km from the emission centre (i.e. 55 km on average).
In 1963 Oersted mentioned:
… in the volcanic ashes dispersed in the surroundings, small pieces of native sulfur are found, and this substance should have been more abundant, as the crater was frequently visited by sulfur explorers. This volcano does not seem to be completely extinct; fact is that, in 1834, a strong eruption occurred accompanied by underground detonations and ashes vomited by the volcano launched up to 30 miles (millas) distance.
Oersted (1863) spoke about a distance of 30 miles (“millas”), but did not mention the direction the ash clouds travelled. The roman mile is 1,480 m. Based on this conversion, the author estimated an emission distance of 44.4 km from the emission centre. The distance between Poás and Esparza is 53.1 km, as such, the estimated distances by Von Frantzius (1861) is similar as the one reported by Oersted (1863). The underground detonations mentioned by Oersted (1863) can be interpreted as thunder or possibly earthquakes during the eruption (Fig. 3).
3.1.3 Effects of the Eruption
The report of the Archivo Nacional, Municipal, 778, mentioned how graze lands were “buried”, causing the mobilization of cattle to other places, due to the lack of food. During this period, there were very few villages near Poás. Basically, the area near the crater area was occupied by cattle ranches and damage was only reported for those ranches. No loss of human lives is reported.
3.1.4 Eruption Model
We can infer an initial phreatic phase, with the eruption of lake sediments and hydrothermalized material saturated in molten sulfur, after which a major phreatic event occurred that launched bombs and blocks to a distance still close to the crater (700 m from the possible emission site). Deposits show parallel lamination that represent ash fall out, as well as inclined lamination with wedging structures indicating pyroclastic surges in the sector close to the crater. A minimum column elevation of 5 km is estimated. Afterwards, the wind took care of transporting the fine material, following the information obtained until the town of Esparza, up to 55 km distance from Poás volcano, in southwestern direction.
3.2 The January–May 1910 Eruption
3.2.1 Pre-eruptive Stage
Leiva (1904) reported a lake water temperature of 42 °C, besides sporadic phreatic eruptions:
…this feature is generally pre-announced with agitation of the waters very near the centre of the lake. After a noise bursts while a dense column of whitish gases violently rises up to many hundreds of meters to open up into crests of brilliant white that dissolve by the wind or that otherwise fall as condensates of fine mists.
Rudín (1905) witnessed several phreatic eruptions (Fig. 4):
…these appeared without preliminary signs of any kind and consisted in a small bubble of mud, a bit off the centre of the lake, followed by a small column of water vapor.
Moreover, he described other mayor ones:
…when suddenly an immense black column rose from the lake that seemed boiling mud that, while falling back down, broke in thousands of fragments, giving the eruption a magnificent and impressive aspect. In a few seconds the black column was surrounded by water vapor, emanating from the column itself, followed by the release of the vapor mass that was expulsed from the chimney of the volcano, leading to the disappearance of everything enveloped in a cloud that completely filled the crater.
Rudín (1905) estimated a diameter of Laguna Caliente of 500, and 600 m height and 100 m in diameter of the column during the largest eruption. During the 2006–2016 phreatic eruption cycle, phreatic eruptions reaching 500 m in height (200 m above the crater rim) can be affected by the wind direction. In some of these recent cases, erupted ash reached the tourist “mirador”, Laguna Botos, and the visiting centre of the Parque Nacional Volcán Poás (PNVP hereafter).
The last report prior to the 1910 eruption was written by Leiva (1906), who observed during his visit that the level of Laguna Caliente had decreased. He reported continuous phreatic eruptions; moreover, some people living near reported, prior to his visit, flames produced by sulfur combustion (Fig. 2a).
3.2.2 Eruptive Stage
On 25 January 1910 in the afternoon, one of the most important historical eruptions of Poás started. Hereafter, a compilation of public news from newspapers is presented in chronological order, together with reports from a group of researchers sent by the president of the Republic of Costa Rica:
On Wednesday 26 January 1910, La República informs:
Eruption of Poás Volcano. Strong ash rain-very beautiful spectacle”. This information arrived at the Telegraph from Alajuela the previous night at 6:15 p.m., stating: “Poás Volcano in eruption, very beautiful spectacle in the cordillera, ash rain in abundance, people alarmed. To north side of the city, the sky seems reddened.
Brenes (1932) reported that the eruption was perfectly visible from the faraway province of Guanacaste. On Thursday 27 January 1910, La Prensa Libre entitled: “The eruption of Poás”, informed by Alberto Rudín:
We arrived at the volcano at eight a.m. In the “Lechería” (i.e. dairy farm) lots of sand, rain, small stones, up to the size of corn, in Potrero Pequeño mud and stones fell, up to a hundred weight. The crater and surroundings entirely covered by a layer of volcanic mud. The quantity of mud thrown out was tremendous as it entirely covered the crater, the northern hill and the surroundings of the west and south. Signs are obvious that it occurred in real streams. Lots of stones of five to forty cm that opened, while falling, holes of a yard deep.
The stones should have come from the interior, as they show molten sulfur. Today there was not even one eruption until 2 p.m. it seems to me there was still no reason for serious fear. The waters of all the rivers had a remarkable sour taste and the roads were found entirely covered by a layer of ash that seemed aluminum.
“Potrero Pequeño”, mentioned by Rudín is the current site of the entrance gate to the PNVP, which is also known locally as “Lechería”. This site is currently inhabited and known for commercial activities for tourists. Interpreting the news that this event showered more than 2 km distance from the emission centre, lapilli and decimeter-sized blocks generated impact craters of nearly a meter deep, while at a distance of 4–5 km, ash and lapilli fell. The description of the eruptive products suggests that much of the material originated from Laguna Caliente (mud, molten sulfur). Sulfur in the rock pores can be explained: at the bottom of Laguna Caliente subaqueous pools of molten sulfur form (Oppenheimer 1992; Takano et al. 1994; Rowe et al. 1992a, b; Christenson et al. 2010; Rouwet and Morrissey 2015; Mora Amador et al. Chapter “The Extraordinary Sulfur Volcanism of Poás from 1828 to 2018”). The presence of sulfur in pore spaces of many rocks suggest an emission temperature of at least 116 °C, the melting point of sulfur (Takano et al. 1994). More recently, Bennett and Raccichini (1978) and Francis et al. (1980) have reported the same feature for Poás, also observed since March 2006 following phreatic eruptions (Fig. 5).
The next day, on 28 January La Prensa Libre informed: “More ash rain”, detailing that:
This morning, between 9:30 and 10:30, a fine dust fell, not as the white ash such as on the 25th in the afternoon, but rather a rose ground, in small quantities; they were very small grains that reduced into fine dust when crushed. This fact was known because in a patio a billiard cloth was left to dry, that, when removed, it was noted that the small grains fell on it. It was shaken off and relocated with the other side up and some grains continued to fall.
On Saturday 29 January, Prensa Libre informed: “From Grecia, ash rain”, and detailed:
It was six in the afternoon the day before yesterday when, above the summit of Poás, a huge plume of smoke, gases and ashes that rose to prodigious heights was soon disintegrated into an ash rain that caused alarm among the inhabitants of the town. The streets, roofs and fields were whitened by this phenomenon that was never contemplated before. The course from the day before yesterday till today, without anything regrettable to occur, faded off fears that these natural features gave rise to, whereas the common people blamed it to the appearance of the comet Halley.
We can deduce that at least two eruptions were reported after the one of 25 January: (1) on the 27th of January early-morning minor ash fall in the centre of Alajuela, and (2) in the afternoon ash fall in Grecia during the same day, both of minor size and intensity than the first reported eruption.
An important detail was revealed on 2 February: “The ashes on the La Paz river, a tributary of the Sarapiquí, poisoned the water, causing the death of fish in this river.” Possibly the type of oblique eruptions, described by Casertano et al. (1983), or strongly wind-directed eruptions are responsible for the large quantities of ash and sulfur in these rivers, besides the ash fall transported by directed winds.
On the 4th of February 1910, Juan Rudín, Anastasio Alfaro, Gustavo Michaud and Alberto Rudín, presented a report entitled: “Great ash eruption of Volcán Poás, 25 January, 4 h. 45 m. p.m”. A transcription of their observations on the summit of the volcano, after their expedition on request by the President of the Republic, Cleto González Víquez, followed (Rudín et al. 1910):
On 25 January, shortly before five in the afternoon, an immense column could be observed from San José, apparently as a smoke plume, which actually was a mixture of water and ashes that rose above the summit of Volcán Poás, up to a height that we could estimate as 4000 meters, and that consequently, through evaporation, extended towards the flanks and upwards to the prodigious heights of approximately 8000 meters… The column initially appeared to be of an intense grey color, whereas afterwards, due to the evaporation, an immense cloud of a light grey dye extended and slowly changed its nuances. The apparent shape was the one of a giant fungus, or maybe better, of an unleafed cauliflower, of colossal dimension, wide at the top and reposing on a relatively tight base.
When moved by higher winds, the cloud extended for the entire Meseta Central, and produced the reported ash rains between 6 and 8 in the evening of 25 January. After posterior observation made in San José, in San Pedro de Poás and on the summit of the volcano, it is not exaggerated to assure that the quantity of ash thrown out by Poás in the afternoon of the 25th can be estimated to be 800,000 cubic meters, with a mass of 640,000 tons, that is a sufficient amount to cover la Sabana (cfr. an area in San José, now a park) with a 1-kilometer thick layer. Commissioned by the Ministry, we left this city the 28th in the morning and could observe that the quantity of ash increased progressively until covering the cultivated lands in the region of San Joaquín de Heredia, with a fine grey-colored layer as if the coffee plantations were intentionally covered with the famous fertilizer Albert. On the side of the roads of Alajuela and its surroundings there were rests of ashes that gave a precious aspect in the shadows, as if there were aluminum rocks. The appearance between Alajuela and the river of Poás seems uniform, full of small tributaries that accidentally search their way between highs and lows until the river banks.
Along the entire trajectory the fall of small stones in sufficient abundance can be noted, and following on what the people living at La Lechería told, these falling small stones gave the impression of a heavy hailstorm; some of those stones reached a diameter of one to two centimeters. Higher up on the mountain small stones of higher magnitude are disseminated, that often remained, together with the ashes, on leafs of trees and bushes, and that fell on the soil when shaken by the wind or intentionally by hands.
Close to the second ranch, some pieces of stones of three to four centimeters size, generally light and porous, were found. In the proximity of the crater some stones fell that increased in size when nearing the crater rim. In the morning of the 26th, it was entirely covered by a uniform layer of ash that gave the weird appearance for those people used to admire its nuances. Afterwards, water and wind cleaned some sites, showing spots of other colors that break the monotony of the whole. The first impression that one had when reaching the crater is that the lake at the bottom became bigger, without being possible to say how much and from which side. Moreover, some considerable collapses occurred on the north side, having enlarged the Laguna Caliente towards this side in estimable way.
The entire surroundings of the crater received ash in the form of mud, as signs are shown that this occurred on the branches covering them also on the lower parts.
To a distance that varies from 150 to 200 meters from the upper rim of the crater, numerous stones had fallen, of which its origin will be explained hereafter: they are present in all sizes, from 5 to 48 cm in thickness, some light but the major part with a solid and heavy consistency. Almost all of them had fallen with their direction inclined towards the exterior, as such describing a parabola in their trajectory, as can be proven by the direction of the holes in which they are buried; they also should have to be fallen from high elevation, as the big ones staved into the soil for more than a meter depth, and as they kept their power to cut roots and easily break branches thicker than a human arm. The holes are most abundant on the SW side, while at the SE, towards the cold lake, they become less numerous, until complete disappearance in this direction.
In another report entitled “some new data on Volcán Poás”, Alberto Rudín (1910) details:
Already going down the playones, we could notice that the amount of volcanic mud thrown out in this area was a lot more than other areas received: soil, branches, and three leafs are completely covered, forming a layer of nearly a decimeter. I have to advise that because of the thick fog that day we got lost and went down a lot more than we generally do, until close to river Ángel, after having crossed the myrtle bushes. These are literally laying on the soil, squeezed by the weight of the ash and the mud that covers them; apparently they died as a consequence of the large amounts of acids they received. In this entire trajectory only very few stones had fallen. In the eastern playones, in addition to mud, a true rain of stones of all sizes has fallen; but contrary to what is observed on the south side, they shouldn’t have fallen from very high, the majority settled on the soil, whereas only some opened a rather large gap.
As on the south side, the stones must have fallen while hot, as sulfur that impregnated them poured out in melted state falling on the soil and sometimes forming precious stalactites and stalagmites and even perfect columns, while other times, probably when the rocks fell being hotter, or with more sulfur in them, it occurred at the surface that large plates formed that were similar to palm trees. The amount of this deposit that could be collected in the available short moments was enough to instigate several residents of San Pedro to create a commercial business. For this reason, at this moment it is very difficult to still find those stalactites of regular shape that remained intact.
In the northern playones the amount of mud that fell was that voluminous that it is absolutely impossible to verify if stones also fell. The thickness of the layer, measured in the sites where it could not have been accumulated by the action of water, reaches in many sites up to half a meter.
We also descended the crater from the west side following a rope that started at the arrival. This descent is maybe even worse than from the front side, and surely a lot longer, as it has many difficult and dangerous passages. In this area very few stones fell, and in spite of the ash being present, there is no sign of mud. As noted, mud is distributed very irregularly; it fell abundantly on the north side, a little less on the east side, little in the south and nothing in the west.
Analyzing the historical photograph taken during the eruption from San José (Fig. 6), the height of the ash and gas cloud above Poás volcano is estimated at a little more than 3,000 m. There is a possibility that afterwards the ash column became larger because of later events and because of the wind forces being able to transport the ash to the heights as reported by Rudín et al. (1910). Nevertheless, in the photograph it is noted that the fungus starts to collapse due to the decoupling of the ash from the gas motion, and ash starts to fall.
3.2.3 Effects of the Eruption
On Wednesday 9 February, República reported dead fish in Sarapiquí River:
On the 26th of last January, all day long immense amounts of fish descended from Sarapiquí river, some were already dead others were dying, of all types and colours, small and large ones. The neighbors of the river collected large quantities of dying Joturos. The river banks were invaded by many death animals. Until Sunday this gave off an unpleasant smell. Some people say the water of Sarapiquí is sour: I neither wanted to taste it, nor did I go near as the stink was unbearable. It is thought that the sis due to the fact that maybe the lake of Poás flooded into Los Ángeles driver and from there into Sarapiquí.
The first of February 1910, La Prensa Libre informed: “The ash rain”, reporting on damages due to the eruption:
The eruption of Poás volcano, that brought ash rains, resulted in the fact that some ranches became useless as the cattle stopped grazing, because the grassland remained covered with a layer of fine ash, leaving an acre and sour taste. In the gardens of this city, the flowers that received this ash were burned.
La República informed on the second of February: “The ash of Poás turns out to be poisonous” and detailed on how the ash falling into La Paz river, a tributary of Sarapiquí, poisoned the waters causing the death of fish.
Rudín et al. (1910) reported a total destruction of the vegetation near the active crater. Neither deaths, nor damage to infrastructure were reported.
3.2.4 Deposits Related to the 1910 Eruption
Field work indicates that the deposits related to the 1910 eruption are characterized by whitish-grayish material composed of ash with lithic fragments, occasional juvenile scoria with a diameter up to 2 cm (especially on the rim of the active crater), as well as andesitic lapilli with a diameter up to 5 cm, with both punctual and floating contacts; the contacts of the upper and lower layers are irregular (Fig. 7). Only inside the crater and in the proximal area an inverse grading could be observed, with less ash and more lithics at the punctual contact. A larger thickness is noted to the north, east and south of the active crater, whereas thickness decreases towards the west. The maximum thickness reaches 85 cm in the southeastern sector. Occasionally, at less than two kilometers from the active crater, it is difficult to distinguish these deposits. We observed light grey ashes with parallel lamination and diffuse inclination, and lateral thinning with desiccation cracks (Fig. 8).
In general, a small layer of red-brown soil overlies these deposits. Despite the short distance from the emission centre, there is no regularity on the layer thicknesses. That is why it is thought that the events in this period were oblique eruptions, as described by Rudín et al. (1910) and Casertano et al. (1983).
3.2.5 Eruption Model
A magma rise generated the heating of the magmatic-hydrothermal system and triggered phreatic eruptions accompanied by high-temperature fumaroles with sulfur combustion. It should be stated that the 1910 deposits show fragments of scoracious rocks, hence probably indicating a juvenile origin, and a transition from phreatic to phreatomagmatic activity.
The deposits evidence different eruptive events, with fall out and pyroclastic surge material close to the crater area. In one outcrop (column 186) up to five different events can be described, with parallel and inclined lamination, and lateral wedging).
Based on the column height, on the field data gathered and on the estimated volume of the erupted ash column into the atmosphere, the 1910 eruption represents a Volcanic Explosivity Index of 2 (VEI, Newhall and Self 1982).
Figure 9 compiles the pre- and syn-eruptive phases of the 1910 eruption, based on information from literature and field data from the period 1889-May 1910. It is worth noting that prior and during the eruption the Laguna Caliente crater lake did not disappear.
3.3 The 1953–1955 Eruptions
3.3.1 Pre-eruptive Stage
Between 1934 and 1950, Hantke (1951, 1953) reported weak fumarolic activity in the main crater. Specifically for 4 February 1947 he observed a rather low lake level for Laguna Caliente (Fig. 10). Fernández (1961) is the one to describe the first drastic changes at Poás, starting in 1952.
Monestel (1953) mentioned that Dr. Frank Turnes of Abott Laboratorios witnessed a fall out event when he walked near Laguna Botos on 18 June 1952:
…Being close to the cold water lake, they were surprised by tremors, of which the “biggest” one he estimated to be IV on the modified scale [unreadable] and later they were wetted by an eruption of hot water and grey mud.
He added the following:
On the 3th of March 1953 he witnessed an event… at 3:30 in the afternoon, on a clear day, calm wind and ambient temperature of 19 °C. The eruption was of short duration and the intercrateric lake -calm in the morning when he arrived with Dr. Turner-, dropped its water level. He said it was strange that the blue-green color of the lake, that in other occasions (1952) he’d seen grey, had changed into reddish as of oxidation. He informed Don Fausto that the first day of March, when he descended to study the plants, from Bajos del Toro, wading through Desagüe river, when starting the rough job, and passing the waterfall, he noted the prevalence of waters contaminated by volcanic materials, grayish-yellows, with a strong smell of sulfur. The river dragged masses of grey clays with volcanic ash, plastic sands as well as some tree trunks [unreadable] and chunks of black and grey very porous and light rocks.
In this note (Monestel 1953), refers to possibly mud flows in Desagüe river that have been confirmed by inhabitants of Bajos del Toro.
Before the 16th of May 1953 eruptions, at least three possibly phreatic eruptions were reported: 18 June and 9 September 1952 and another one on 2 March 1953. Some of them were preceded by seismic movements. These eruptions should have had an altitude of at least 300 m, in order to make the ashes reach Laguna Botos, 300 m higher in elevation; moreover, the felt earthquakes could be related to magma ascent.
3.3.2 Eruptive Stage
The third important eruptive activity at Poás in historical times started between 15 and 16 May 1953. By 20 May 1953, the newspaper La Nación mentioned ash fall in Vara Blanca and surroundings. They reported that since Monday 18 at mid-day ash was falling, with a thickness up to a “quarter”, affecting the people, crops and cattle. It is mentioned that the cattle cannot use the pasture, get fed and drink water, as the ash covered the terrain with an “immense grayish sheet that releases a strong sulfur smell”.
The same day they published that the river waters came down contaminated with ash smelling of volcanic gases that cause dizziness and vomit to whom inhales them. As this event was immediately recognized as harmful, it can be interpreted as directed, oblique eruptions that Poás launched towards the headwaters of Ángel river, apart from the ash fall.
On Friday 22 May, La Nación reported a “New and big eruption of Poás”, and that the entire city of Grecia woke up covered by a thin layer of ash, and that the eruption was easily visible. They continued that the Jefe Político, Sr. Ricardo Barquero, informed that the big eruption was accompanied by ash rain, and mentioned that the day before the city woke up covered by a thin ash layer that covered roofs and houses, trees, streets, etc., which caused unrest in the community. They explained:
The eruption was seen during almost every hour of the mourning. Yesterday in the capital, at about seven thirty in the mourning a tremor of very low intensity and short duration was felt.
On 24 May 1953, La Nación published that volcanic activity increased:
In the early morning, the neighbors in the surroundings of the volcano evacuated their homes in a hurry, helped by workers of the Ministerio de Obras Públicas that are building the road to Poás. The neighbors only grabbed the essentials to pass these days while quiet returns. At three in the mourning three strong tremors could be felt at regular intervals that created a nervous tension to all the people in the area. Meanwhile, the rain of ash and volcanic material continued, thrown out by the crater, being dispersed by the wind towards all the cardinal points.
On 25 May 1953, Yeudi Monestel, journalist of La Prensa Libre, sent a letter to Dr. César Dóndoli (founder of the Central American School of Geology, Universidad de Costa Rica), explaining what he observed during his visit to Poás. In a modified sketch (Fig. 11) he shows how far the volcanic material reached in the entire area. To continue some parts of this document:
…in the row before “Potrero del Hotel”, the guide Nazario Arias Piedra showed us on the right hand side of the gauge two impact craters:
One of three meters in diameter and another one of only 1.3 meters, in which at their bottom two enormous rocks had been embedded, one of them angular with a weight of at least a ton. One of the crater - the one with the largest rock- was 60 cm deep.
Monestel (1953) mentioned that from “Potrero del Hotel” on, a coarser and more sandy material was observed, with many chunks of porous grayish lavas. His sketch (Fig. 11) clearly shows where the ash, and material with a larger diameter-lapilli-type, bombs and blocks- fell. Again, like in the 1910 eruption, they mention that the Ranch is a site where coarser material fell. It is worth noting that they mentioned the falling of large-size bombs and blocks in the row before “Potrero del Hotel”.
On the other hand, following the report, the ash fall continued during days and weeks, affecting graze fields and, hence, cattle, causing illness and ulcerous to the animals.
On Thursday 28 May 1953, La Nación published:
Rumbles and smoke from Poás volcano yesterday. They report on the areas where ash fell that weighs the critical situation presented in the first days, luckily no animal had died, moreover they informed that the region of Sarapiquí was notified on the rumbles of Poás volcano with regular intervals and that a thick smoke column rose from its crater that can be observed from many kilometres distance.
On 6 June 1953, La Nación published: “Ash rain in Río Cuarto”. In the article it is mentioned that an eruption of smoke and ash on 5 June covered the region of San Carlos with a layer of volcanic detritus. The article continued:
The police officer of Grecia, Sir Ricardo Barquero, sent yesterday late a telegram to the Ministerio de Gobernación, reckoning on the alarming information that he received from Río Cuarto, in relation to the eruptions of Poás volcano since several weeks. The mentioned official said that the local authority reported him that yesterday at nine o’clock a heavy ash fall took place that covered practically every pasture in the Río Cuarto region, and that this mainly affected the cattle.
The 5 June eruption was particularly important, because it could be observed from considerable distance. In Fig. 12 the photograph published by La Nación on 6 June 1953, shows an eruption with a plume of ash and gas from a distance, but, unfortunately, they did not specify if the photograph was taken from Río Cuarto, from Grecia or from the Central Valley, San José.
La Nación published the following on 6 June 1953: “On Thursday of Corpus Cristi (cfr. 40 days after the resurrection), the eruption of Poás volcano was gigantic, rising up to twenty-three thousand feet height.” This is equivalent to 7,000 m height. Nevertheless, following the analysis of Fig. 12, assuming it was taken from (1) San José, the eruption height was approximately of 3,400 m, and (2) Río Cuarto, it is estimated at 4,800 m height.
On Wednesday 10 June 1953, the same newspaper informed: “Poás swallowed the immense lake in its crater”. This particularly long article commented how “the geyser of Poás, the largest in America” had disappeared. Moreover it added how active the lake was a few weeks before, and how the recent activity managed to make it disappear. It described how the “columns of white smoke, above the summit” were visible and how they transformed into “immense black cauliflowers. Terribly black like a wolf’s mouth.”
The black and dark colorations might be related to juvenile material of magma liberated during the event.
On 12 June 1953, La Nación published: “We could see the fury of the giant”. This article dealt with a chronicle sent to the newspaper’s editorial, by Sir Rafael Sanabria B. and don José Manuel Salazar, narrating their visit to the “giant” on 24 May 1953. In this article they mentioned that they were informed that the day before (Saturday 23 May)…
…the imposing eruptions, how they threw out sandy ash and smoke up to high elevations, and how they launched incandescent stones of big dimension that fell towards the rim of the crater, from its bottom, during the moments when the base of the enormous fungus formed by the eruptions remembered atomic explosions, red lightning, impossible to photograph, very quickly left the crater. Preceded by thunderous rumbles, that asked for huge respect, thick clouds of sandy ash and sulfurous vapors, that to our great luck, if not, we wouldn’t be able to witness this marvellous show, were pushed to the north by the wind. It was about 11 o’clock in the morning when we had the opportunity to see the largest eruption and we felt how the soil, without reaching tremor.
The earlier mentioned incandescent rocks correspond to bombs, which are made of semi-molten material that was thrown out of the crater at near-magmatic temperatures, whereas the shapes at the base of the fungus formed by the eruptions, could correspond to pyroclastic surges that advanced at the higher parts and perimeter of the active crater, or to a rapidly ascending plume.
Afterwards, for several weeks no further news on the activity of Poás volcano could be found; it is until 10 July 1953 that La Nación, published “Volcanic activity resumes”, an article in which they mentioned that on 9 July of this year, a spectacular eruption of Poás volcano could be observed from the capital. The articles mentioned:
This eruption drew the attention as it had the peculiarity that the smoke was elevated above the colossus having a black opaque color, contrary to the regularly clear color. The eruption was produced by large puffs, assuming they were the result of internal explosions of the volcano. The show was imposing.
Bullard (1956) in his article “Volcanic activity in Costa Rica and Nicaragua in 1954” mentioned that on 19 November 1953 the eruption was particularly violent, and launched a large quantity of pyroclastic material. This activity resulted in: “on the crater rim scorias and bomb-shaped fragments, with the size of a human head, are abundant.”
The phenomenon described between 14 and 19 November 1953 is related to the launching of juvenile pyroclastic material, lapilli and bombs. It is possible that during this phase the major parts of the dome south of Laguna Caliente were emplaced.
On 2 June 1957, Burgos mentioned in La República that in April 1955:
At a few meters in front of our amazed pupils the colossus vomited fire, large whirlwinds of smoke, ash, launching up scorias and incandescent material, in the middle of subterraneous detonations and frightening roars. The majority of it fell noisily in the crater itself, and the rest on the playones. Its flanks gave way to an inflamed material that flowed, illuminating everything, while the earth trembled.
Burgos also referred to Strombolian activity. It is possible that he was also talking about a lava pool or lively red cracks of the recently emplaced dome. Describing that “the flanks opened”, it could be related to the behavior of lava accumulated in a depression.
Dóndoli (1965) mentioned that during 1956 the volcano resumed its normal activity of intense fumarolic emissions, with intermittent throw outs of ashes and sands. He noted that in the surroundings of the cone at the bottom, cake-shaped material of scoriaceous lava was launched by the volcano. Moreover, he indicated that Laguna Caliente was reformed in the crater during periods of intense rainfall, but that it easily evaporated while generating the expulsion of mud. This feature was repeated in various occasions between 1956 and 1965. We can deduce that for the duration of an entire decade Laguna Caliente was formed and disappeared, suggesting a strong relation with the rainy periods and with variations in volcanic activity. Generally speaking, at least phreatic eruptions must have occurred during this entire period.
Vargas (1967) mentioned that in the Sarchí, Toro Amarillo and Tres Amigos rivers, during the period 1953–1955, repeated mud fronts were observed.
3.3.3 Effects of the Eruption
On 18 July 1953 Prensa Libre informed: “Hundreds of cattle in danger.” The article continued with a report by R. Calderón Monge, political chief:
Dense layers of, firstly, ash, and later on, black dust, launched by Poás volcano left in trouble the ranchers of the higher parts of San Pedro de la Unión and La Luisa, and especially those of the district of Toro Amarillo. It will become worse if the summertime of the last couple of days will continue, as the volcanic material will not be washed away and the cattle will perish of hunger. Yesterday I arrived at Trojas, in the way to Toro Amarillo and I’ll tell you that the roads where it didn’t rain became black colored. I think that the Ministerio de Agricultura should take measures to save hundreds of cattle.
On Friday 15 January 1954, the newspaper La República published: “Large losses for coffee plantations due to the eruption of Poás volcano.” In this article, the Chief of the Coffee Section of the Ministerio de Agricultura e Industria informed on the state in which the coffee plantations remained in the areas of San Miguel and San Roque de Grecia, as a consequence of the strong eruptions of Poás. It also informed that the ashes burnt the leaves of the coffee trees, impairing its future flourishing, destroying its vivariums and leaving the trees that serve as cover completely leafless.
3.3.4 Deposits Related to the 1953–1955 Eruption
The 1953–1955 eruption can be recognized in the field by the presence of a layer of black scoria of basaltic-andesitic composition, made of ash and cauliform or fusiform bombs, with a maximum size of 32 cm. This layer is exposed inside the crater, and on the north and east of the crater rim (Fig. 13).
In 1953 the crater lake completely disappeared and a dome started to grow reaching 40 m in height. This dome is the one that still degasses/erupts at present although it was destroyed in April 2017, now the locus of major magmatic eruptions (see Sect. 3.4). The sequence of layers of the eruptive products of the 1953–1955 eruptions is composed of lapilli of 0.5–4 cm, with a clast-supported contact. Moreover, there are fusiform bombs (Figs. 13 and 14) and andesitic blocks of variable dimensions (between 10 and 160 cm), with afanitic and afanitic-porfiritic textures and with some mm-size crystals of dark grey plagioclase.
The thickest deposits (~50 cm) are found towards the south inside the crater. These appear as dark smudges or patches of several decimeters to meters on the inner crater walls. Towards the north, this layer is almost completely absent, except for some centimetre-sized bombs. Towards the east side of the crater occurs the highest amount of bombs, with centimeter to meter sizes. Porous black-grey colored blocks and bombs with a diameter of decimeters to meters, of mainly andesitic-basaltic composition, and with afanitic-porfiritic textures and some fusiform textures, are recognized.
3.3.5 Dome Emplacement, Lava Flow and Lava Pool
Bullard (1956) mentioned that as a result of the 1953 eruption a cone of ash and pyroclastic products started to form in the place previously occupied by the lake.
During their field exploration, Racicchini and Bennett (1978) determined that the pyroclastic material covered a merely compact, though fractured, igneous mass. They suggested this was related to an extrusion of a viscous melt, that ended with the formation of an endogenous dome, and that during the 1953 eruption part of this dome was buried and part was destroyed. Francis et al. (1980) and Casertano et al. (1983) described the 1953 activity as eruptive episodes of incandescent pyroclastic products (Strombolian eruptions) with lava effusion inside the crater. They estimated that the dome was emplaced on 9 November 1953.
A massive lava is observed in the field, manifesting fracture cooling during the emplacement of the dome. Out of these fractures fumaroles exhale high-temperature gases that in various occasions have reached nearly 1,000 °C. In the upper part and south end of the dome a decimeter-thick layer of hydrothermalized bombs, blocks, lapilli and ash cover the dome structure.
The north face of the dome is uncovered, and when gases are transported vertically by the wind, a massive lava-like structure can be clearly distinguished. At the base of this structure a colluvium of eruptive products is present, derived from the erosion and landslides of these rocks.
At the east and south parts at the shores of Laguna Caliente, a massive, fractured, andesitic lava flow is exposed. When its interior crops out it has a dark grey color and a cap of whitish weathering. The maximum thickness of the flow is 8.5 m (Figs. 15 and 16). It is possible that in this site a lava pool was formed, filling a preexisting depression prior to the emission of the flow.
Analyzing the historical information of the dome emplacement together with the lava flow and lava pool, the information that we gathered suggest that this activity occurred in three episodes:
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(1)
At the end of May 1953, intense eruptions produced incandescent pyroclasts;
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(2)
At the beginning of November 1953, eruptions of pyroclasts are reported. Eyewitnesses described reddish radiance seen from San Miguel de Grecia, Poasito and Vara Blanca;
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(3)
In April 1955, when the journalist Vargas (1967) witnessed a natural spectacle: “Its flanks gave way to an inflamed material that flew, illuminating everything, while the soil rumbled.” It is possible that Vargas referred to the lava flow emplacement at the crater bottom.
3.3.6 Fall Out of Bombs
Bombs with a diameter larger than half a meter were found in the upper part of the crater (“terrazas” sector) and were localized with GPS. A higher concentration of bombs was found in the eastern sector. This bomb distribution may indicate the conduit through which the 1953–1955 eruptions were generated was tilted towards the east (Fig. 17).
3.3.7 Eruptive Model
In the 1940s, when fumarolic activity was constant, an increase in heating of Laguna Caliente was observed. In 1952, the first phreatic eruptions were reported, with falling ash and mud in the immediate surrounding of the crater (close to Laguna Botos), accompanied by earthquakes perceived by people living in the surroundings. It is possible that at the beginning of 1952 a magma started to rise causing the complete dry-out of Laguna Caliente, followed by eruptions of juvenile material, block to ash particles.
Afterwards the dome and lava emplacement took place, reaching a thickness of circa 40 m. The most important eruptions of juvenile material were reported at the end of March, June, November and December 1953. In 1954 no Strombolian eruptions were reported. The last Strombolian-type eruption was reported in April 1955. However, pyroclastic surges that affected the crater area during this period cannot be excluded, although field evidence is absent.
This eruption was complex because it involved four types of activities: Strombolian, phreatomagmatic, dome-extrusion, Vulcanian. Considering the characteristics of the eruptions, the eruption column height and ejected volume a VEI of 3 is estimated (Newhall and Self 1982).
Figure 18 provides a summary of the phases described prior to and during the 1953–1955 eruptions. The figure shows that Laguna Caliente was subjected to dry-out, disappearance and ephemeral existence during a period of two decades. This period of time was the most unstable phase of Laguna Caliente during historical times. Eruptive processes that transient from phreatomagmatic explosions to lava flows had a duration of two years (May 1953–April 1955). These events were sometimes accompanied by pyroclastic surges. It is quite possible that phreatic eruptions occurred during the entire eruptive period.
3.4 The 2017–2018 Eruptions
3.4.1 Pre-eruptive Stage
Between 1986 and 1994 Poás manifested phreatic activity and the dry-out of Laguna Caliente (Mora Amador et al. Chapter “The Extraordinary Sulfur Volcanism of Poás from 1828 to 2018”; Martínez et al. Chapter “Behaviour of Polythionates in the Acid Lake of Poás Volcano: Insights into Changes in the Magmatic-Hydrothermal Regime and Subaqueous Input of Volatiles”; Rouwet et al. Chapter “39 Years of Geochemical Monitoring of Laguna Caliente Crater Lake, Poás: Patterns from the Past as Keys for the Future”; Vaselli et al. Chapter “The Last Eighteen Years (1998–2014) of Fumarolic Degassing at the Poás Volcano (Costa Rica) and Renewal Activity”), followed by more or less a decade of quiescence. In 1998, five new fumarolic fields were observed in the southeast inside the crater with temperatures between 70 and 140 °C, and the liberation of gases on the north face of the 1953 dome. Since 2000, fumarolic activity has migrated towards the east of the crater, where gases escaped from at least four sites. One of them was very vigorous and named Fumarola Naranja (see Mora Amador et al. Chapter “The Extraordinary Sulfur Volcanism of Poás from 1828 to 2018”). In January 2005 Laguna Caliente reached its maximum level since 1950. Since February-March 2005 Laguna Caliente manifested changes in color and temperature (Rouwet et al. 2016). In May 2005 an 82 m-long sulfur flow was emplaced from Fumarola Naranja; contemporaneously, sulfur slick floated on the surface of Laguna Caliente, originating from the sub-lacustrine molten sulfur pool (Mora-Amador and Ramírez 2008; Mora Amador et al. Chapter “The Extraordinary Sulfur Volcanism of Poás from 1828 to 2018”). On 25 and 26 December 2005 tailed floating sulfur spherules were observed, often a precursory signal for phreatic activity on crater lake bearing volcanoes (Takano et al. 1994; Mora et al. this issue). During the same period, low amplitude seismicity increased (Fernández and Mora Amador Chapter “Seismicity of Poás Volcano, Costa Rica”). On 24 March 2006 the first phreatic eruptions occurred after almost 12 years of quiescence.
This resumed phreatic activity culminated during the next 10 years, converting Poás’ Laguna Caliente into the most active crater lake on Earth (Fischer et al. 2015; de Moor et al. 2016; Rouwet et al. 2016) (Fig. 19a). Incandescence of the dome fumaroles (Fig. 19b) and floating sulfur on Laguna Caliente were common features (Fig. 19c). The gas geochemistry of the Fumarola Naranja, Dome fumaroles and gas plume liberated by the 1953 dome and Laguna Caliente are reported by Hilton et al. (2010), Fischer et al. (2015), de Moor et al. (2016), and Vaselli et al. (Chapter “The Last Eighteen Years (1998–2014) of Fumarolic Degassing at the Poás Volcano (Costa Rica) and Renewal Activity”). A detailed analysis of the changes in water chemistry of Laguna Caliente are documented by Rouwet et al. (2016) (for the period 2005–2010) and Rouwet et al. (Chapter “39 Years of Geochemical Monitoring of Laguna Caliente Crater Lake, Poás: Patterns from the Past as Keys for the Future”) (for the period 2010–2016). Despite being a decade-long period of phreatic eruptive activity, similar as the 1986–1994 period, within the context of this study the 2006–2016 period is considered a pre-eruptive stage. By October 2016, phreatic activity finished and Poás was apparently set for a new period of quiescence. This was also suggested by the observations made in January–February 2017: the absence of sulfur spherules (i.e. absence of sub-lacustrine molten sulfur), low degassing pressures from the dome fumaroles.
3.4.2 Eruptive Stage
After six months of apparent calmness, dynamic changes have occurred since early April 2017. We here report:
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(1)
On 1 April 2017 a new fumarole appeared SW of the active fumarolic field on the 1953–1955 dome in a tephra deposit (possibly an old pyroclastic cone). The gas escape during one day.
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On 7 April 2017 renewed activity is reported: a geyser-like spring opened in the southern sector of the crater at an outcrop of Holocene tephra deposits (Fig. 19d, e). From this boiling spring (approximately 90 °C, at crater height), thousands of cubic meters of hot acid (pH = 0.5) water was liberated, forming a 400 m-long creek eventually draining into Laguna Caliente. The following days intense exhalative degassing activity occurred from Laguna Caliente, covered with sulfur slick. Black spherules with a diameter of 1.5 mm were observed (Fig. 19f).
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On 12 April (5 pm) the first major phreatomagmatic eruption occurred (Fig. 20a). The tourist mirador was covered in ash and mud (Fig. 20b), and the dome was broken and fractured (15 m long fractures, Fig. 20c). By 12 April (mid-day), only fine mud and gas exhaled from the spring, without a clear water discharge (Fig. 20d). Moreover, part of the northern wall of the dome, at the locus of the 1st of April new fumarole exhalation, was destroyed by an explosion. An eruptive plume of 700 m was observed, while Laguna Caliente exhibited constant vigorous degassing.
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On 14 and 15 April Poás generated eruptions with an eruptive column of more than 5 km (Fig. 21a), launching bombs and blocks of more than 50 kg, and ash until the tourist mirador (Fig. 21b–d). The dome was almost completely destroyed.
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After the 14 April eruption, at the site previously occupied by the dome, a scoria cone was formed, up to a pyroclastic cone of at least 40 m height, from where small Strombolian eruptions occurred with columns heights of 200–1,000 m. A N–S trending fracture, starting from the previous playón until the intracrater is evident.
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During 21 and 22 April 2017 multiple eruptions occurred. Shock waves could be perceived up to a distance of 2 km. The gas and ash plume rose up to >1 km height.
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On 22 April the major eruption of this cycle occurred at 7 pm (Fig. 22a), generating a pyroclastic flow emplaced inside the active crater, with a ash cloud reaching the tourist mirador (southern sector) and bombs reaching the eastern “terrazas” (Fig. 22b). The eruption launched bombs and blocks up to 1.5 km from the vent (tourist centre parking lot). Inside the active crater and on the “terrazas” sector 1–8 m sized bombs were deposited (Fig. 22c, d). Moreover, the pyroclastic surge destroyed vegetation of the Botos forest (southeast sector) (Fig. 22e).
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From May till September 2017 sporadic eruptions of gas, ash and pyroclasts occurred (Fig. 23a). These eruptions were rather small and the majority of the eruptive material remained inside the crater. Different centers of activity can be recognized inside the crater (Fig. 23): (a) the main pyroclastic cones, (b) the small sulfur volcano, (c) a small gas exit structurally similar to a small “castle”, and (d) a diffuse fumarolic field towards the west. In August 2017 eruptions of ash, aerosols and brown colored gases from the pyroclastic cone were frequent, together with yellow colored gases from the small sulfur volcano.
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On 8 January 2018 Carlos Cordero, Park Ranger, reports on the formation of a small lake in the NE sector if the intracrater (i.e. previously dried out Laguna Caliente lake basin). This lake had a celestial color and a diameter of approximately 50 m.
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On 20 January 2018, Cordero observed the appearance of a larger lake, arguably formed earlier but unobserved due to bad weather conditions (Fig. 23b). The newly formed lake locally reached depths of 15–20 m. Laguna Caliente officially reappeared, with a temperature of 40 °C and a pH near 1. The formation of the lake occurred in 12 days. Intense degassing occurred from the entire lake surface. Moreover, it is possible to observe mud deposits and floating sulfur on the lake surface forming strings. In the eastern sector of the intracrater a fumarolic field led to the emplacement of small molten sulfur deposits and liberated gas at high pressure.
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(11)
In the eastern sector of the new Laguna Caliente a 15 m long fumarolic field is observed, in which small amounts of molten sulfur was emplaced, two boiling pools were present and gas exhaled at high pressures.
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Laguna Caliente dried out again by the third week of February 2018. In March drone footage documented minor eruptive activity from three main vents at the former lake bottom (Fig. 23c).
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During April 2018, exhalative activity has continued. Monitoring authorities reported very low seismic activity. In May 2018, minor phreatic eruptions occurred from the new pyroclastic cones. Due to the start of the rainy season, small lagoons were formed in the northern sector of the former crater lake basin. The activity of the sulfur cones increased, and temperatures above 100 °C were measured.
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Between June and August 2018 (i.e. rainy season), the crater lake basin was re-filled almost entirely, manifesting evaporative degassing from the lake surface. The lake level rose several meters, to almost completely cover the relicts of the pyroclastic cones.
3.4.3 Effects of the Eruption
The PNVP is one of the most visited parks in Costa Rica, and Poás is the most visited volcano in Central America (Mora Amador et al. Chapter “History, legends, customs and traditions of Poás volcano, Costa Rica”). Due to the closing of the park since the 12 April 2017 eruption until August 2018 (the moment of writing) the economic loss is estimated at, at least, 20 million dollars. This estimate considers (1) the drop in economic activities of the surrounding people that generally thrive from national and international tourism, (2) the damage to the infrastructure of PNVP (Fig. 24), and (3) the direct loss due to the absence of the entrance fee for this prolonged period. Ash fall and acid gases mainly affected the areas of the PNVP. Moreover, the infrastructures of the PNVP were recently renovated and were planned to be inaugurated on 28 April 2017.
At least three lahars were registered in the surrounding rivers Ríos Agrio and Desagüe (Western flank of Poás), with the most important one following the 12 April 2017 eruption. Touristic activity in this area was suspended for several weeks. The impact of volcanic bombs and blocks, ash fall, the pyroclastic flow of 22 April 2017 and acid gases seriously damaged the mirador, and the tourist service areas (Fig. 24a–e), as well as the trails to Laguna Botos (southeast sector) (Fig. 24f).
3.4.4 Eruptive Model
The magmatic eruptive stage was preceded by almost 11 years (March 2006–October 2016) of phreatic eruptions of variable sizes and explosivities (up to eruption columns of approx. 1 km). During this pre-eruptive stage sulfur volcanism manifested in its various forms: molten sulfur pools and spherules, sulfur flows and pyroclasts (Mora Amador et al. this issue).
A relatively sudden increase (early April 2017) in the temperature of the magmatic-hydrothermal system, due to a new magma emplacement, fractured the crater floor and destroyed the 1953–1955 Dome. Afterwards, during the various eruptive pulses small ash cones were formed and posteriorly destroyed, and eventually built up to larger pyroclastic cones. The eruptive activity logically led to the dry out of Laguna Caliente by July 2017, mainly caused by the temperatures above boiling point for the entire lake basin. Since October 2017, exhalation temperatures and seismicity have declined. In January 2018, Laguna Caliente reappeared, disappeared four weeks later, and returned again between May and August 2018 (rainy season) with a maximum temperature of 62 °C.
4 Volcanic Hazards of Poás
Prosser and Carr (1987), Paniagua and Soto (1988), Soto and Paniagua (1992), Alvarado et al. (2000, 2007) made a review of the volcanic hazards related to Poás volcano. A new map of Poás, based on the historical record and field observations (from 1995 to 2010) of the 1834, 1910, 1953–1955 and 2017 eruptions, is proposed in this study.
Major hazards related to the historical eruptions of Poás volcano are pyroclastic surges, fall out of ballistics and tephra, lahars, and degassing of acidic gases, generating acid rain. In the map (Fig. 25) the largest polygon corresponds to the sites where ash fall (with fragments <2 mm) was reported during the first three studied eruptions. A yellow circle with a radius of 6.6 km around the active crater indicates the area where the fall out of lapilli and volcanic material with a grain-size between 2 mm and 6.4 cm is reported. The pink circle, with a radius of 2.5 km around the active crater corresponds to the sites where fall out of material with fragments larger than 6.4 cm occurred. Moreover, a circle with a radius of 1 km around the emission center, indicates the area affected by pyroclastic surges.
This new map is of highly practical use as during weekends or holidays, up to 100 people can contemporaneously visit the “mirador” at the crater rim. Details of the areas that could be affected within the PNVP are presented in Fig. 26.
4.1 Pyroclastic Surges
Paniagua and Soto (1988) mentioned that pyroclastic surges produced by explosive blasts during phreatomagmatic cycles could occur, but that these will possibly only affect the inner crater area. Nevertheless, during field-work, deposits related to pyroclastic surges were observed not only in the perimeter of the active crater (in the south, i.e. tourist mirador sector of the active crater and, in the southeast, i.e. mirador of Laguna Botos), but also along the trail towards Laguna Botos, and the eastern sector (i.e. “terrazas” sector). These deposits were emplaced during the first three studied eruptions (1834, 1910 and 1953–1955), with a maximum runout of 1 km from the emission centre (Figs. 25 and 26).
4.2 Ballistics
Paniagua and Soto (1988) delimited the fall out of the prehistorical and historical pyroclastic deposits to a zone of maximum hazard within 8 km around Poás (mainly blocks from phreatic explosions, with a diameter larger than 30 cm). During the 1834, 1910 and 1953–1955 eruptions ballistic projections reportedly reached little more than 2 km from the emission centre. In Fig. 11 it is observed that material >6.4 cm fell at the site Potrero, where the old hold was located, and where at present the entrance of the PNVP is located. Other sites where a large quantity of bombs and blocks are found is in the eastern and northern sector of the active crater (Fig. 17).
Figure 17 shows the perimetrical area of the active crater that can be affected by bombs with dimensions of at least half a meter. More than two hundred such bombs were distinguished with the highest concentration of those towards the east part of the crater. A person standing at the tourist “mirador” at the southern rim of the active crater could be hurt by hot bombs smaller than half a meter.
Falling bombs are very dangerous for people and can seriously damage infrastructure and human development. In the case of Poás, the infrastructure of the PNVP could be damaged (visitors centre, buildings, park entrance and access roads), as it effectively was damaged during the 2017 eruption. It is worth noting that practically all the touristic trails within the PNVP are located within the range of possible projections of bombs and blocks.
Lapilli fall out can also damage windows and other vulnerable structures. Its accumulation on roofs can cause their collapse. Lapilli fall out could affect villages towards the south of the volcano, such as Poasito, Altura and some dairy farms located in this sector, as well as some restaurants and tourist centers. The communities of Isla Bonita, Cinchona and Vara Blanca could be affected by this phenomenon, if a change in wind direction occurs at the moment of the eruption. Towards the north and west, within this range, there are no settlements (Fig. 25).
With respect to the direct impact on human lives, the probability of an impact is low, especially compared to the impact of pyroclastic density currents. Evidently, the direct impact of a bomb will cause injury or even death. Nevertheless, the distribution density of the bombs at a certain distance from the volcano is low enough; hence, the probability of direct impact will be low. Wearing a helmet offers the sufficient protection for lapilli fall out. Bomb impacts in the vicinity of the crater are a common phenomenon at volcanoes with moderate activity. This process causes deadly victims among tourists worldwide almost every year, due to the fact that they approach the volcano too much seeking to be near an eruption. This situation is particularly worrisome in the case an eruption occurs during the visiting hours of PNVP.
4.3 Dispersion and Fall Out of Pyroclasts
Ortiz and Araña (1996) mentioned that, of all the volcanic hazards in the world, dispersion and fall out of pyroclastics are those that affect the largest area. The fine material is dragged into the convective column up to high elevations, where it is dispersed due to the combined effect of diffusion and wind transportation. Ash particles are tremendously abrasive, for which they can damage all types of machinery, from airplane engines to printers or hard discs, etc. Motor vehicles suffer significant damage when driving on ash-covered roads.
The fine material rapidly decreases the filtering capacity if soils, tap pipes and blinds water flow paths, hence considerably increasing the probability of flooding, and possibly triggering mud flows. As the ash is extremely fine, it easily penetrates into clean rooms, such as surgery rooms, pharmaceutical, precision, mechanical, optical, and industrial alimentation laboratories.
4.4 Volcanic Gases and Acid Rain
Volcanic gases generally cause problems near eruptive vents or fumarolic fields, as they rapidly dilute into the atmosphere till concentrations below the toxicity threshold (Ortiz and Araña 1996). Nevertheless, in some circumstances they result toxic or even lethal. Some of the gases, like CO2, are denser than the air. Hence, they tend to accumulate in depressions, such as valleys and canyons. Moreover, CO2 is one of the major species in volcanic gases, as such, it can reach high concentrations and move downwards like a dense flow, asphyxiating every living being along its track. In the case of Poás, this problem can manifest when the wind direction is north-south, creating curtains of gases, especially SO2, besides the odorless CO2, at the tourist mirador and at the trails of the PNVP. The smell of gases can be strongly perceived within a radius of 2.5 km from the emission centre, and even weakly until 6.5 km.
Another hazard factor is constituted by acid rain, produced when water drops condense on volcanic aerosols that act as nucleus. Paniagua and Soto (1988) mentioned that the dominant wind from the WSW and the Cordillera Central, push these gases forward, for which these flanks of Poás are characterized by “burned” vegetation and acidic water in streams (e.g. Río Desagüe). This situation causes the dying of vegetation or inhibits its growth at those sites continuously exposed to acid rain. Roofs with metal covers suffer important corrosion. The loss of electric or electronic components, engines, etc, caused by volcanic gases and derivatives can be considerable.
At Poás, the grade of harm by acid rain largely depends on the wind direction. As such, the preferential sector of harm is NE–NW. Communities such as Bajos del Toro, San Miguel de Grecia, San Juan del Norte, Trojas and San Luís are directly affected by this phenomenon.
The last severe crisis caused by acid rain occurred in 1994, when Laguna Caliente disappeared for the last time. Mora (1997) carried out a survey in the areas affected by the gases, with the results summarized.
4.5 Lahars
The presence of a crater lake in the summit of a volcano, is a primary factor for the production of lahars, providing the necessary amount of water, although they can also be triggered by intense rain fall, frequent in the tropics (Carrivick et al. 2009; Procter et al. 2010; Rouwet et al. 2014b; Manville 2015 and references therein). Lahars are reported to have occurred in the riverbeds originating at Poás’ summit area, such as (1) La Paz, Ángel, that on their turn are drained by Sarapiquí river, and (2) Río Desagüe, Anonos, and Agrío that are drained by Río Toro, and (3) the rivers located on the south flank of Poás: Sarchí, Achiote, Prendas and Poásito rivers. Figure 25 indicates the rivers that have presented similar events, or that could present them in the future.
5 Summary
Due to the heavy rain regime at the higher elevations of Poás volcano (approximately 4,000 mm/year) a lot of the fall out material, ash and lapilli has been eroded. This is why the in situ geological record is not sufficient to create adequate hazard maps based on historical activity of Poás. Hence, the historical record (news papers, journals, letters, technical reports, etc) becomes a valuable tool to line out and characterize recent eruptive activity.
Major volcanic hazards related to the historical eruptions of 1834, 1910, 1953–1955 and most recently in 2017 are: pyroclastic surges, ballistics projectiles, tephra fall out, lahars, earthquakes, acid gases and rain.
Very few information exists on the 1834 eruption. Nevertheless, a phase of clear precursory signals occurred: phreatic eruptions and an increase in fumarolic activity. The main event of this eruption was phreatomagmatic that launched bombs within a diameter close to the active crater. Ash fall was reported until Esparza (>50 km W of Poás) while pyroclastic surges occurred near the crater rim. The eruption is catalogued as Vulcanian.
The 1910 eruption was detailed by Rudín et al. (1910). This eruption has premonitory signals through phreatic eruptions and an increase in the crater lake temperature. Besides the 25 January 1910 main eruption, at least two eruptions occurred afterwards. Analyzing the bibliographic information and volcanic deposits, we conclude that this eruption was phreatomagmatic. In the eruptive products some juvenile fragments were detected, although hydrothermalized lithics were most abundant, immersed in a matrix of fine and coarse ash. There is evidence of pyroclastic surges near the active crater. It is possible that the differences in thickness of the deposits of this eruption are due to the explanation by Casertano et al. (1983): this eruption occurred with an oblique eruptive centre. These evidences, together with the estimated column height (3–4 km), suggest the 1910 eruption was a Vulcanian with a VEI 2.
The eruptive period of 1953–1955 had the longest duration of the three studied historical eruptions. This eruption was anticipated by phreatic eruptions, easily perceived earthquakes and the evaporation of Laguna Caliente crater lake due to increased activity. Since May 1953, several Strombolian eruptions took place. The crater lake dried out totally and a dome was emplaced in the centre of the crater. During this phase, Vulcanian, Strombolian and dome extrusion eruptions occurred. The VEI for this eruption is estimated at 3.
The largest area affected by ash fall resulted from the 1834 eruption, with 1469 km2; in second place, the 1953–1955 eruption that affected 1,212 km2 and, at last, the 1910 eruption that affected only an area of 432.6 km2.
Currently, the cantons of Alajuela, Grecia and San Pedro de Poás are potentially the ones to be most affected by future volcanic activity similar to the one described in this study. Medical hospitals and clinics, fire department buildings, educational centers in these cantons deserve major attention and should be prepared to cope with cases of asthma, dermatitis, ocular problems and burnings following volcanic activity.
At the moment of writing this chapter, the fourth historical eruption of Poás started, after a phreatic phase that started in March 2006. Major and ongoing magmatic activity was preceded by dynamical changes inside the active crater with similar dynamics as during the three previous historical eruptions. As of August 2018, the 2017 eruption is not the major “historical” eruption of the four described here.
The here produced map for the medium to short term activity of Poás volcanic, based on the 1834, 1910, 1953–1955 and 2017 eruptions, is a tool that will orientate authorities in their decision making process and control human development, following restricted and regulated land use, create adequate facilities for human activity, promoting development while respecting the peculiarity of the natural site.
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Mora Amador, R.A., Rouwet, D., González, G., Vargas, P., Ramírez, C. (2019). Volcanic Hazard Assessment of Poás (Costa Rica) Based on the 1834, 1910, 1953–1955 and 2017 Historical Eruptions. In: Tassi, F., Vaselli, O., Mora Amador, R. (eds) Poás Volcano. Active Volcanoes of the World. Springer, Cham. https://doi.org/10.1007/978-3-319-02156-0_11
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