OVSICORI-UNA reported that a phreatic eruption at Poás was recorded at 1120 on 27 October. A phreatic eruption later that day (at 1757) ejected water, sulfur-rich sediments, and rock fragments out of the lake. The ejecta landed on the S and SW edges of the crater floor. According to a news article, local residents heard a loud rumble at about 0100 on 28 October; a phreatic eruption ejected sediment 500 m above the lake, and produced ashfall several hundreds of meters away.

24 October 2012            Back to Top

OVSICORI-UNA reported that a phreatic eruption at Poás was recorded at 1120 on 27 October. A phreatic eruption later that day (at 1757) ejected water, sulfur-rich sediments, and rock fragments out of the lake. The ejecta landed on the S and SW edges of the crater floor. According to a news article, local residents heard a loud rumble at about 0100 on 28 October; a phreatic eruption ejected sediment 500 m above the lake, and produced ashfall several hundreds of meters away.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA) , Tico Times

6 June 2012            Back to Top

OVSICORI-UNA reported that phreatic eruptions at Poás occurred on 6, 15, 20 and 26 May. The eruption on 15 May was preceded by about 6 hours of very-low amplitude harmonic tremor. Administrators of the Poás Volcano National Park witnessed the eruption and reported that sediment, water, rock fragments, and plumes were ejected 500 m above the lake surface. The level of the lake dropped ~0.9 m between 8 and 29 May.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)

28 December 2011            Back to Top

OVSICORI-UNA reported that fieldwork at Poás on 16 December showed new geyser activity from a vent on the N flank of the lava dome that formed in 2011. A water-and-mud fountain rose 5-6 m high, flowed into the lake, and built a terrace along the S shore. Gas-and-steam plumes rose from the lava dome.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)

9 November 2011            Back to Top

OVSICORI-UNA reported that during October fumarolic activity at Poás continued with bluish gas plumes rising from the lava dome; plumes rose more than 1 km and were reported by residents in Valle Central. Towards the end of the month, the fumarolic activity as well as incandescence from the lava dome decreased. The new craters at the N base of the dome united into a crater that was 25 m long and 7-10 m wide. Phreatic activity continued to occur from Laguna Caliente, the summit lake. The lake was 55 degrees Celsius, and the level had risen 22 cm between 14 September and 27 October.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)

31 August 2011            Back to Top

On 31 August OVSICORI-UNA reported that since mid-July incandescence was visible during the day on the lava dome of Poás, which had not occurred since 1981, and was the result of changes in activity that had started several months before. OVSICORI-UNA speculated that the changes could be either from recent magma intrusion or a change in the hydrothermal plumbing system and noted the need to further analyze data from seismic, deformation, geochemical, and field measurements.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)

27 July 2011            Back to Top

OVSICORI-UNA reported that on 23 July a group of observatory scientists visited Poás to document changes that had occurred there during the previous weeks. They noted that the subtle, semicircular scarp observed a few months earlier had rapidly progressed to a sharp scarp on the SE shore of Laguna Caliente. The 60-m-wide, 2.5-m-high scarp degassed and geyser activity was observed on the W end, next to the steaming lake. In an area about 40 m above the surface of the lake where there were fractured rocks and vigorous gas venting, incandescence emanated from the lava dome and a temperature of 670 degrees Celsius was measured.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)

25 May 2011            Back to Top

According to a news article, a team of geologists and volcanologists from the Universidad de Costa Rica (UCR) visited Poás on 25 May and observed 18 phreatic eruptions from Laguna Caliente (the summit lake) in a three-hour period.

Sources: Inside Costa Rica

4 May 2011            Back to Top

OVSICORI-UNA reported that during April sporadic phreatic eruptions occurred from the central part of Laguna Caliente, a summit lake of Poás. Gas plumes rose a few meters to several tens of meters above the lake surface. Solid dark material ejected from the lake fell back into it, causing small surges. The temperature of the lava dome was 560 degrees Celsius at accessible areas. Bluish fumarolic plumes rose from the dome; a gas plume rose 1 km and drifted NE, E, and SE.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)

9 February 2011            Back to Top

OVSICORI-UNA reported that on 2 February a small phreatic eruption occurred from the central part of Laguna Caliente, a summit lake of Poás. Solid dark material was ejected 6 m high and fell back into the lake. Before, during, and after the eruption a concentric area of the lake surface about 15 m in diameter was disturbed by a convecting cell. Bubbles of gas rose from the agitated area and upwelling sediments were observed.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)

15 December 2010            Back to Top

OVSICORI-UNA reported that during November several phreatic eruptions from the central part of Laguna Caliente, a summit lake of Poás, ejected material that fell back into the lake, occasionally producing small waves. Dark-colored gas plumes rose a few meters to several tens of meters above the lake surface. The temperature of the lava dome was 630 degrees Celsius at accessible areas. Fumarolic plumes from the dome rose 1 km and drifted W and SW. Some foliage on the SW flank, about 3.5 km from the crater, showed signs of damage from gases.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)

6 October 2010            Back to Top

OVSICORI-UNA reported that during September several phreatic eruptions from the central part of Laguna Caliente, a summit lake of Poás, ejected material that fell back into the lake. Gas plumes rose a few meters to several tens of meters above the water surface. The temperature of the lava dome was 810 degrees Celsius at accessible areas.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)

24 February 2010            Back to Top

OVSICORI-UNA reported that on 23 February a phreatic eruption from Laguna Caliente, a summit lake of Poás, ejected gray and sulfur-scented sediment onto the southern flanks. The event was seen by a scientist collecting samples in the low part of the crater and by visitors at the viewpoint area. Seismographs showed no precursory signals.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)

23 December 2009            Back to Top

OVSICORI-UNA reported that on 25 December a phreatic eruption from Laguna Caliente, a summit lake of Poás, was seen by visitors standing at a viewpoint to the S of the crater. Lake water mixed with sediment and blocks was ejected 550-600 m above Laguna Caliente and fell in the vicinity of the lake, within the crater. The amount of steam emitted from lava-dome fumaroles, at the S edge of the lake, increased subsequent to the eruption.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)

14 January 2009            Back to Top

OVSICORI-UNA reported that on 8 January an M 6.2 earthquake occurred about 10 km E of Poás at a depth of 6 km. About 20 people were killed and dozens were missing due to faulting and landslides.

Scientists performing geophysical measurements at the crater on 8 January observed numerous landslides and rockfalls from the inner walls around the active crater, and in the N sector of the main crater. New fractures opened on the E rim and fumarolic activity, from a pyroclastic dome S of the lake in the active crater, increased. Over 1,500 aftershocks were recorded during 8-9 January. On 12 January, scientists observed the rise of black sediment in the crater lake, and a phreatic eruption that ejected sediment and water about 15 m in height, within the crater. The area of the eruption was about 50 m in diameter.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)

16 January 2008            Back to Top

According to a news article, an eruption of volcanic material and gases from Poás rose to an altitude of 2.9 km (9,500 ft) a.s.l. on 13 January. The article also stated that a report from OVSICORI-UNA on 22 January revealed that an eruption of gas propelled material that did not fall beyond the cone. Authorities evacuated 20 people in the vicinity of the eruption.

Sources: La Nacion

5 September 2007            Back to Top

OVSICORI-UNA reported that during August, Laguna Caliente, a summit lake of Poás, was greenish-gray in color, exhibited convection cells in the lake's center, and contained dark gray floating particles. Subaqueous fumaroles at the center of the lake released gases that spread and covered the entire surface of the lake. The level of the lake had dropped 59 cm with respect to its level in July, and it had a temperature of 58 degrees Celsius. Fumarolic activity from a pyroclastic cone produced gas plumes that rose 400 m above the floor of the crater. Points of gas discharge were noted from the N and NW crater walls, the terrace, and the NE edge of the crater. Fumaroles in contact with the lake and the NE wall produced sulfur particles that floated in the lake. Emissions from the SE and NE crater walls were very low in volume.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)

18 October 2006            Back to Top

OVSICORI-UNA reported that during September, Laguna Caliente, a summit lake of Poás, was mostly gray in color and produced gas columns that reached the crater rim. The level of the lake had dropped 5 cm with respect to August measurements and had a temperature of 46 degrees Celsius. On 25 October, a phreatic eruption produced a plume that drifted 12 km SW of the crater. Fumarolic activity from a pyroclastic cone on the floor of the crater produced gas plumes that drifted W and SW. New points of gas discharge were noted from the crater floor, the SE and NE crater walls, the N terrace, and the NE edge of the crater.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)

20 September 2006            Back to Top

On 26 September at least two phreatic eruptions occurred at Poás. One during the night of 25 September reached a height of at least 350 m above the warm acid lake, depositing rock fragments, mud, and water in the southern part of the inner crater and outside the western part of the crater. Material ejected that night reached Trojas de Sarchí, almost 10 km SW. A smaller eruption on the morning of 26 September was limited to the area of the lake, and formed green-yellow semicircular zones of mud and sulfur up to 75 m in diameter.

Sources: Red Sismologica Nacional (RSN: UCR-ICE), Universidad de Costa Rica and Instituto Costarricense de Electricidad

6 September 2006            Back to Top

OVSICORI-UNA reported that during August, Laguna Caliente, a summit lake of Poás, was gray in color and produced gas columns that reached the crater rim. The level of the lake had dropped 47 cm with respect to July measurements and had a temperature of 41 degrees Celsius. Fumarolic activity from a pyroclastic cone on the floor of the crater produced gas plumes that drifted W and SW. New points of gas discharge were noted from the crater floor, the SE and NE crater walls, the N terrace, and the NE edge of the crater.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)

5 April 2006            Back to Top

On 1 April, clear weather allowed OVSICORI-UNA staff to visit Poás for the first time since explosions occurred during 24-26 March. They confirmed that Laguna Caliente at the volcano's summit had widened, and that sediments and blocks from the lake's bottom and surrounding walls were ejected during the explosions. The lava dome's (or pyroclastic cone's) N wall was greatly fractured and a 40 x 7 x 8 m segment of the wall was gone. In addition, a 40 x 4 x 6 m chunk of the SE wall of the lake was missing and must have been destroyed during the explosions. They also found that the lake was light gray due to large quantities of suspended sediments, had a temperature of 54 degrees Celsius, had a pH of 0.63, and the lake level had decreased in comparison to the level before the explosions.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)

29 March 2006            Back to Top

Following phreatic eruptions that began at Poás on 24 March, seismicity decreased at the volcano by 27 March. In addition, harmonic tremor that was recorded during the heightened activity ceased. As of the 27th, no significant deformation was recorded at the volcano and abundant evaporation continued at Caliente Lake.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)

22 March 2006            Back to Top

On 24 March around noon, small phreatic eruptions began at Poás. The eruptions originated from the bottom of the volcano's Caliente Lake. Witnesses described a sudden emission of water and sediments S of the lake. Roaring was heard in a nearby tourist area and weak earthquakes were felt.

OVSICORI-UNA visited the E side of the volcano on 25 March and confirmed that water, blocks, and sediments from the bottom of the lake had been ejected. Several dozens of impact craters were seen with diameters between 15 and 60 cm, extending E as far as 700 m. Blocks were found that ranged in diameter from a few to 50 cm wide. During 22-27 March, harmonic tremor was recorded. On the 27th, there was a reduction in seismicity and it returned to normal levels. No deformation was measured at the volcano. A news article reported that the area around the volcano was closed to visitors. Poás last erupted in 1994.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA) , Prensa Latina

2 February 2005            Back to Top

A field team from OVSICORI-UNA visited Poás on 25 January and found that the level of the volcano's hot acidic crater lake had risen in comparison to the previous month. Intense and sustained rainfall during the previous months caused the water level to increase by ~4 m. The area of the lake increased by ~20%. Flooding occurred in relatively flat areas to the N, E, and SE. Water reached about 150 m towards the SE of the lake. Scattered fumaroles and hot spots at the N base of the lava dome were flooded. Increased steaming was visible from the National Park. The lake temperature remained at 22 degrees C, with hot spots near the rim reaching up to 80 degrees C. OVSICORI-UNA noted that in the past an increase in lake level during a rainy period has been followed by a decrease during the drier months of February to April.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)

Below is a summary of eruption dates and Volcanic Explosivity Indices (VEI).

The following references are the sources used for data regarding this volcano. References are linked directly to our volcano data file. Discussion of another volcano or eruption (sometimes far from the one that is the subject of the manuscript) may produce a citation that is not at all apparent from the title. Additional discussion of data sources can be found under Volcano Data Criteria.

Alvarado G E, 1989. {Los Volcanes de Costa Rica}. San Jose, Costa Rica: Universidad Estatal a Distancia, 175 p

Alvarado G E, 2000. {Volcanes de Costa Rica: su geologia, historia y riqueza natural}. San Jose, Costa Rica: EUNED, 269 p

Alvarado-Induni G E, 2005. {Costa Rica, Land of Volcanoes}. San Jose, Costa Rica: EUNID, 306 p

Barquero-H J, 1998a. {Volcan Poas}. San Jose, Costa Rica: privately published, 42 p

Barquero-H J, Saenz-R R, 1987. Aparatos volcanicos de Costa Rica. {Heredia, Costa Rica: OVSICORI-UNA}, 1:750,000 map and volcano list

Carr M J, 1984. Symmetrical and segmented variation of physical and geochemical characterisitics of the Central American volcanic front. {J Volc Geotherm Res}, 20: 231-252

Casadevall T J, Rose W I Jr, Fuller W H, Hunt W H, Hart M A, Moyers J L, Woods D C, Chuan R L, Friend J, 1984. Sulfur dioxide and particles in quiescent volcanic plumes from Poas, Arenal, and Colima volcanos, Costa Rica and Mexico. {J Geophys Res}, 89: 9633-9641

Casertano L, Borgia A, Cigolini C, 1983. El Volcan Poas, Costa Rica: cronologia y caracteristicas de la actividad. {Geof Internac}, 22: 215-236

Casertano L, Borgia A, Cigolini C, Morales L D, Montero W, Gomez M, Fernandez J F, 1987. An integraded dynamic model for the volcanic activity at Poas Volcano, Costa Rica. {Bull Volc}, 49: 588-598

Cigolini C, Kudo A M, Brookins D G, Ward D, 1991. The petrology of Poas Volcano lavas; basalt-andesite relationship and their petrogenesis with the magmatic arc of Costa Rica. {J Volc Geotherm Res}, 48(3-4): 367-384

Horn S P, 2001. The age of the Laguna Hule explosion crater, Costa Rica, and the timing of subsequent tephra eruptions; evidence from lake sediments. {Rev Geol Amer Central}, 24: 57-65

IAVCEI, 1973-80. Post-Miocene Volcanoes of the World. {IAVCEI Data Sheets, Rome: Internatl Assoc Volc Chemistry Earth's Interior}.

Martinez M, Fernandez E, Valdes J, Barboza V, van der Laat R, Duarte E, Malavassi E, Sandoval L, Barquero J, Marino T, 2000. Chemical evolution and volcanic activity of the active crater lake of Poas Volcano, Costa Rica, 1993-1997. {J Volc Geotherm Res}, 97: 127-141

Mooser F, Meyer-Abich H, McBirney A R, 1958. Central America. {Catalog of Active Volcanoes of the World and Solfatara Fields}, Rome: IAVCEI, 6: 1-146

Prosser J T, 1985. Geology and medium-term temporal magmatic variation found at the summit region of Poas volcano, Costa Rica. {Bol Vulc Univ Nac Costa Rica}, 15: 21-39

Raccichini S, Bennett F D, 1977. Nuevos aspectos de las erupciones del Volcan Poas. {Rev Geog Amer Central}, 5-6: 37-53

Rowe G L Jr, Brantley S L, Fernandez J F, Borgia A, 1995. The chemical and hydrologic structure of Poas Volcano, Costa Rica. {J Volc Geotherm Res}, 64: 233-267

Rowe G L Jr, Brantley S L, Fernandez M, Fernandez J F, Borgia A, Barquero J, 1992. Fluid-volcano interaction in an active stratovolcano; the crater lake system of Poas Volcano, Costa Rica. {J Volc Geotherm Res}, 49: 23-51

Rowe G L Jr, Ohsawa S, Takano B, Brantley S L, Fernandez J F, Barquero J, 1992. Using crater lake chemistry to predict volcanic activity at Poas Volcano, Costa Rica. {Bull Volc}, 54: 494-503

Rymer H, Cassidy J, Locke C A, Barboza M V, Barquero J, Brenes J, van der Laat R, 2000. Geophysical studies of the recent 15-year eruptive cycle at Poas Volcano, Costa Rica. {J Volc Geotherm Res}, 97: 425-442

Sanford W E, Konikow L F, Rowe G L Jr, Brantley S L, 1995. Groundwater transport of crater-lake brine at Poas Volcano, Costa Rica. {J Volc Geotherm Res}, 64: 269-293

Sapper K, 1925. {The Volcanoes of Central America}. Halle: Verlag Max Niemeyer, 144 p

Stine C M, Banks N G, 1991. Costa Rica volcano profile. {U S Geol Surv Open-File Rpt}, 67 p

Thorpe R S, Locke C A, Brown G C, Francis P W, Randal M, 1981. Magma chamber below Poas volcano, Costa Rica. {J Geol Soc London}, 138: 367-373

Viramonte J G, Navarro Collado M, Malavasi Rojas E, 1997. Nicaragua-Costa Rica Quaternary volcanic chain. IAVCEI General Assembly, Puerto Vallarta, Mexico, January 19-24, 1997, Fieldtrip Guidebook, 17 p

The broad, well-vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano, which is one of Costa Rica's most prominent natural landmarks, are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the 2708-m-high complex stratovolcano extends to the lower northern flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, is cold and clear and last erupted about 7500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since the first historical eruption was reported in 1828. Poás eruptions often include geyser-like ejections of crater-lake water.