🌋VolcanoAtlas

Poás

Costa Rica’s Acid Lake Volcano

Elevation

2,697 m

Last Eruption

2025

Type

Stratovolcano

Country

Costa Rica

Location

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Volcanic Hazards & Risk Assessment

Primary Hazards

  • Pyroclastic flows
  • Lava flows
  • Volcanic bombs and ballistics
  • Lahars and mudflows

Risk Level

Population at RiskLow
Infrastructure RiskHigh
Aviation RiskSignificant

Geological Composition & Structure

Rock Types

Primary
Unknown
Silica Content
Varied composition

Tectonic Setting

Unknown
Intraplate setting with hotspot or regional volcanic activity.

Age & Formation

Epoch
Unknown
Evidence
Unknown

Eruption Statistics & Analysis

MetricValueGlobal RankingSignificance
Total Recorded EruptionsUnknownLowModerately active volcano
Maximum VEIVEI UnknownMinorLocal impact potential
Recent Activity1 years agoVery RecentCurrently active

Monitoring & Alert Status

Monitoring Networks

Global Volcanism Program
International eruption database

Current Status

Active
Recent volcanic activity detected. Continuous monitoring in place.

Authority Sources

Smithsonian GVP
Official volcano database

Related Volcanoes

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rocky mountain photography

Carlos Umaña

via Unsplash

a blue lake surrounded by lush green trees

Javier Huerta Pérez

via Unsplash

brown and gray rock formation near blue sea under white clouds during daytime

Chalo Garcia

via Unsplash

aerial view of lake in the middle of mountains during daytime

Manfred Madrigal

via Unsplash

aerial photography of gray mountain with smoke

Alejandro Leitón

via Unsplash

a view of a snow covered mountain range

César Badilla Miranda

via Unsplash

Other Volcanoes in Costa Rica

Interesting Facts

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Laguna Caliente in Poás’s active crater has a pH near zero, making it one of the most acidic natural lakes on Earth — roughly equivalent to battery acid.

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Poás has produced 65 recorded eruptions, making it Costa Rica’s most frequently erupting volcano by a wide margin.

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The volcano’s active crater contains Laguna Caliente while a separate dormant crater just 600 m to the south holds the serene, cold-water Botos lake — two dramatically different lakes on one summit.

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Before the 2017 eruption, Poás Volcano National Park received approximately 400,000 visitors per year, making it one of the most visited active volcano sites in the Americas.

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Poás’s VEI has never exceeded 2 in its entire recorded history — it is one of the world’s most active volcanoes by eruption count while being among the least explosive.

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The April 2017 eruption forced the closure of the national park for nearly two years, resulting in an estimated $10 million in lost tourism revenue.

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Since 2019, visitors to the crater overlook are limited to timed 20-minute sessions and must shelter in purpose-built bunkers if gas levels spike.

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Acid rain from Poás’s SO₂ emissions damages coffee crops, strawberries, and ornamental plants on the volcano’s agricultural slopes.

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Poás is located just 37 km from San José, Costa Rica’s capital city of ~350,000 people — one of the shortest distances between an actively erupting volcano and a national capital.

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The volcano’s Botos crater lake last erupted approximately 7,500 years ago and is now a tranquil rainwater lake surrounded by cloud forest, popular with birdwatchers.

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The 20th century alone saw 35 eruptions from Poás — an average of one eruption every 2.9 years.

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Poás’s characteristic eruption style involves geyser-like ejections of superheated, hyperacidic lake water to heights of 50–500 m with little or no warning.

Frequently Asked Questions

Is Poás Volcano still active?
Poás is one of the most active volcanoes in Costa Rica and Central America. It most recently erupted in January 2025, continuing a pattern of frequent phreatic (steam-driven) eruptions from its crater lake, Laguna Caliente. The volcano has produced 65 documented eruptions, with 13 events in the 21st century alone. Poás is in a state of persistent low-level unrest, with continuous fumarolic gas emissions, elevated lake temperatures, and intermittent phreatic explosions. Two monitoring agencies — OVSICORI-UNA and RSN — maintain continuous surveillance of seismicity, gas emissions, and crater lake conditions.
Can you visit Poás Volcano?
Yes, Poás Volcano National Park is open to visitors but operates under a controlled-access system implemented after the 2017 eruption. Visitors must purchase timed-entry tickets in advance through the SINAC website. Visits to the crater overlook are limited to 20-minute sessions, and the park closes when volcanic gas concentrations exceed safe levels. A paved road leads from San José (approximately 37 km, 1–1.5 hours by car) to a parking area near the summit. The best time to visit is early morning (park opens at 7:00 AM) before clouds obscure the crater. Entry costs approximately $15 USD for non-residents. Protective shelters are available at the viewpoint.
What is Laguna Caliente?
Laguna Caliente (‘Hot Lake’) is the hyperacidic crater lake that occupies Poás’s active summit crater. It is one of the most acidic natural bodies of water on Earth, with a pH frequently measured near zero — roughly equivalent to battery acid. The lake’s extreme acidity results from intense fumarolic activity on the crater floor that continuously injects sulfur dioxide, hydrogen sulfide, and hydrochloric acid into the water. The lake’s color shifts between vivid turquoise, emerald green, and milky gray depending on sulfur content, temperature, and suspended particles. Phreatic eruptions from Laguna Caliente produce geyser-like ejections of superheated, acidic water.
Is Poás Volcano dangerous?
Poás presents moderate danger, primarily from sudden phreatic eruptions and volcanic gas emissions. Phreatic explosions can eject rocks and scalding acidic water hundreds of meters from the crater with little warning — the 2017 eruption ejected ballistic blocks into areas previously open to tourists. SO₂ gas emissions create respiratory hazards near the crater and generate acid rain affecting surrounding agricultural communities. However, Poás has never produced a VEI 3 or larger eruption in its recorded history, and the post-2017 safety measures (timed visits, shelters, gas monitoring) significantly reduce visitor risk. The greater San José area is not at direct risk from Poás’s typical eruption style.
How tall is Poás Volcano?
Poás rises to 2,697 m (8,848 ft) above sea level, making it the second-tallest active volcano in Costa Rica’s Central Volcanic Range after Irazú (3,432 m). The summit is easily accessible by vehicle via a paved road, making Poás one of the highest points in Costa Rica that can be reached without hiking. The active crater sits near the summit at approximately 2,600 m elevation, with the crater floor roughly 40 m below the rim. The volcano’s broad edifice covers approximately 300 km².
What type of volcano is Poás?
Poás is classified as a complex stratovolcano (composite volcano) with three craters aligned along a N–S structural fissure. It produces andesitic to basaltic-andesitic magma, typical of subduction zone volcanism along the Central American Volcanic Arc. Poás’s eruption style is dominated by phreatic (steam-driven) and phreatomagmatic explosions from its crater lake, rather than the effusive lava flows or large explosive Plinian eruptions characteristic of some other stratovolcanoes. The volcano’s persistent hydrothermal system and hyperacidic crater lake make it more comparable to volcanic systems like Kawah Ijen (Indonesia) or White Island (New Zealand) than to typical explosive stratovolcanoes.
When did Poás last erupt?
Poás’s most recent eruption occurred in January 2025, involving phreatic activity from Laguna Caliente. Prior to that, eruptions occurred in December 2023–January 2024, August 2023, April 2022, and February 2019. The most significant recent eruption was the April 2017 phreatomagmatic event, which produced a 3 km eruption column, ejected ballistic blocks, and forced the closure of the national park for nearly two years. Poás erupts very frequently, with 65 recorded eruptions and an average recurrence interval of approximately 3 years since 1828.
How far is Poás from San José?
Poás Volcano is approximately 37 km (23 miles) northwest of San José, the capital of Costa Rica. The drive takes approximately 1–1.5 hours via well-maintained paved roads that climb through coffee plantations and cloud forest. This proximity makes Poás one of the closest active volcanoes to a national capital anywhere in the world. The nearest town is Poasito, on the volcano’s southern slope, and the city of Alajuela (population ~43,000) lies approximately 28 km to the south.
What is the best time to visit Poás?
The best time to visit Poás is early morning, as soon as the park opens at 7:00 AM. Clouds typically move in by late morning (10:00–11:00 AM), obscuring the crater from view. The dry season (December–April) generally offers better visibility than the wet season (May–November), though clear mornings can occur year-round. Book tickets in advance through the SINAC website, as entry is limited to timed sessions. Check OVSICORI for current volcanic activity status before visiting, as the park closes during periods of elevated gas emissions or eruptive activity.
Why is Poás’s crater lake so acidic?
Laguna Caliente’s extreme acidity (pH near zero) results from the continuous injection of volcanic gases into the lake water from intense fumarolic vents on the crater floor. These vents release sulfur dioxide (SO₂), hydrogen sulfide (H₂S), hydrochloric acid (HCl), and hydrofluoric acid (HF), which dissolve into the lake and react with water to form sulfuric acid, hydrochloric acid, and other corrosive compounds. The lake essentially functions as a natural acid bath heated from below by magmatic heat. Its extreme chemistry makes it inhospitable to all known life forms and capable of dissolving most metals and rocks.