Volcano profile · Central Costa Rica
Poás
Costa Rica's active crater-lake volcano, where volcanic heat, gas and groundwater combine to produce sudden hydrothermal explosions.
Latest activity and alert level
Check the live status hub
Poás can change rapidly as heat, gas and water redistribute beneath the active crater. View the Volcoholics volcano directory for the latest verified official position from OVSICORI-UNA.
View latest Poás status →Costa Rica’s acid-lake volcano
A broad volcanic complex with a restless crater
Poás rises to about 2,697 metres in Costa Rica’s Central Volcanic Range and contains several craters aligned roughly north to south.
The active summit crater hosts Laguna Caliente, a geothermally heated lake whose size, colour and depth can change dramatically. It is among the world’s most acidic natural lakes and has repeatedly produced phreatic and phreatomagmatic eruptions.
Poás is therefore a volcano where water is part of the eruptive system. Heat and gas from below interact with shallow groundwater and lake water, sometimes building pressure rapidly enough to fragment rock, mud and water without a large lava eruption.
2,697 mSummit elevation
StratovolcanoVolcano type
Active crater lakeLaguna Caliente
OVSICORI-UNAOfficial monitoring
Poás quick facts
- Volcano type
- Complex stratovolcano
- Location
- Alajuela Province, Costa Rica
- Active crater lake
- Laguna Caliente
- Other summit lake
- Lake Botos
- Official observatory
- OVSICORI-UNA
Activity and behaviour
Hydrothermal unrest above a shallow volcanic system
Evergreen profile
How Poás behaves
Poás is known for vigorous degassing, crater-lake changes and sudden phreatic or phreatomagmatic explosions. The shallow hydrothermal system can shift rapidly as heat, gas and water interact.
Check the latest verified status →Characteristic processHydrothermal explosionsSteam pressure can fragment water, mud and rock
Defining featureAcid crater lakeLaguna Caliente changes with heat and rainfall
Persistent signalVolcanic gasStrong fumarolic degassing is central to the system
Proximal hazardBallistics and ashExplosions can affect the crater rim rapidly
Official sourceOVSICORI-UNAAuthoritative monitoring and scientific reporting
Access depends on volcanic conditions
A viewpoint beside an active hydrothermal system
Unlike a broad permanent exclusion radius, access around Poás is managed through Poás Volcano National Park and can change with volcanic and atmospheric conditions. The active crater can produce sudden explosions, ash, ballistic fragments and hazardous gas.
Laguna Caliente
The lake is part of the volcano
The lake is not scenery sitting on top of the volcano — it is part of the active system.
Laguna Caliente occupies the active crater and receives heat and volcanic gases from below. Its water level, temperature, colour and chemistry respond to changes in rainfall, evaporation, degassing and hydrothermal activity.
At times the lake has shrunk dramatically or disappeared, exposing intensely fumarolic crater-floor areas. At other times it has refilled and become the source of geyser-like phreatic explosions.
Highly acidicOne of the most acidic natural crater lakes known.
Geothermally heatedHeat and gases continually enter from below.
Variable levelRainfall and heat can alter lake volume quickly.
Eruptive sourcePhreatic explosions have repeatedly erupted through the lake.
How Poás works
Pressure builds in water, gas and altered rock
Heat and volcanic gas rise into a shallow hydrothermal system. Groundwater and crater-lake water circulate through hot, chemically altered rock. If pressure rises faster than it can escape, sudden steam-driven fragmentation can produce an explosion.
This is why Poás does not need a large lava flow to become dangerous. A relatively small volume of rapidly expanding steam can accelerate rock, mud and ash from the crater.
Monitoring
Watching chemistry, heat and earthquakes together
OVSICORI-UNA combines geophysical, geochemical and visual observations because Poás can change through both magmatic and hydrothermal processes.
A volcano through time
Poás eruption timeline
1828
Historical eruptive record begins
Reports from the 19th century establish Poás as a persistently active crater-lake volcano.
Major explosion1910
Powerful phreatic eruption
A strong explosive event ejected material from the active crater and remains one of Poás’s best-known historical eruptions.
1953–1955
Sustained phreatomagmatic activity
A prolonged eruptive episode built a lava dome and repeatedly disturbed the active crater system.
1987–1990
Lake decline and repeated phreatic explosions
Laguna Caliente progressively shrank, then disappeared, while explosive hydrothermal activity and strong fumaroles continued.
1994
Renewed ash and gas activity
Explosions, ash emissions and intense degassing affected the crater and surrounding agricultural areas.
Modern eruption2017
Powerful explosions close the national park
A major explosive sequence produced ash, altered rock, gas and debris, leading to prolonged closure of the crater viewing area.
Hazards
Poás can be dangerous without producing lava
Rapid steam expansion can eject water, mud, ash and rock with little warning.
Large fragments can be thrown around the active crater during stronger explosions.
Acidic gases can affect breathing, vegetation and infrastructure downwind.
Fine ash can affect nearby communities, agriculture and aviation.
Volcanic gases can combine with atmospheric moisture and damage vegetation or equipment.
Hydrothermal alteration weakens rock around the active crater and fumarolic areas.
Myths versus reality
Poás challenges the usual picture of an eruption
Steam-driven explosions, ash and gas can create serious hazards without visible lava.
The lake sits directly within an active hydrothermal system and can itself become the source of explosive activity.
At close range, even a short phreatic explosion can eject dangerous ballistic material.
Volcoholics insight
Water can be every bit as explosive as magma.
Poás demonstrates that volcanic danger is not measured only by flowing lava. Heat transferred into groundwater and the crater lake can build explosive steam pressure capable of fragmenting rock and generating hazardous eruptions.
Questions answered
Poás explained
Where can I find Poás’s latest official status?
Use the Volcoholics volcano directory for the latest verified summary, then follow OVSICORI-UNA and Costa Rican authorities for authoritative monitoring and access information.
What is Laguna Caliente?
Laguna Caliente is the hot, highly acidic lake occupying Poás’s active crater.
Why does the crater lake sometimes disappear?
Strong heat and evaporation can exceed rainfall and groundwater supply, causing the lake to shrink or vanish during periods of intense hydrothermal activity.
What is a phreatic eruption?
A steam-driven explosion caused when water is rapidly heated and pressurised, fragmenting surrounding rock without necessarily erupting new magma.
Who monitors Poás?
OVSICORI-UNA, the Volcanological and Seismological Observatory of Costa Rica at the National University, provides official scientific monitoring.
Official science, made readable
Built from the agencies studying Poás
This evergreen profile uses OVSICORI-UNA publications and Smithsonian Global Volcanism Program records as its factual basis. Geology, eruption style, hazards, monitoring and historical eruptions are presented separately from today’s operational status.
OVSICORI-UNAOfficial Costa Rican volcano monitoring, scientific observations and activity reporting.
Smithsonian Global Volcanism ProgramLong-term eruption chronology, morphology and geological reference information for Poás.