Poás Volcano near San Jose, Costa Rica, reawakens

By David Jacobson, Temblor

Poás Volcano in Costa Rica has been experiencing unusual activity since April, which has forced the closure of Poás Volcano National Park, which draws up to 300,000 people per year. (Photo from: Lonely Planet)


Since April, Poás volcano in Costa Rica has been experiencing significant unrest, which has included several small eruptions, that has forced the closure of Poás Volcano National Park. Even though it is one of the most active volcanoes in Costa Rica, the current activity is considered “out of the ordinary” by to Professor J. Maarten de Moor (Observatorio Vulcanologico y Sismologico de Costa Rica). Because of this we thought we’d take a closer look at this volcano, which is visited by over 300,000 people per year.

This Google Earth image shows the craters of Poas Volcano. This also shows Laguna Botos, the lake in a crater that last erupted 7,500 years ago, and Laguna Caliente, one of the most acidic lakes in the world.


Poás is a stratovolcano approximately 20 km from the city of San Jose, the capital of Costa Rica, and home to a little over 300,000 people. Stratovolcanoes, also known as composite volcanoes, are formed by layers of lava, tephra, pumice, and ash. This type of volcano typically has steep slopes and will periodically undergo explosive and effusive eruptions. In the United States, Mt. St. Helens, and Mt. Shasta are examples of stratovolcanoes.

In terms of its eruptive history, Poás most recently underwent a period of phreatic activity, beginning in 2006, and commencing in 2014. The largest of these eruptions created columns greater than 300 m high. A phreatic eruption is a steam eruption, which means that no lava is expelled. These happen as groundwater quickly turns to steam as it is heated by magma. Such events can either be precursors to larger eruptions or can stand along.

While those eruptions, which occurred on-and-off for a period of eight years were purely steam eruptions, the activity that started in April has also involved magma. From communication with Prof. de Moor, we learned that, “the last time that Poás had a phreatomagmatic eruption similar to this was in 1953, and before that in 1910.” It is because of this that he deemed the activity out of the ordinary. Furthermore, by examining the eruptive history of Poás on the Smithsonian Global Volcanism Program website, we learned that there have been several other eruptions prior to 1910, none of which exceeded a volcanic explosivity index (VEI) of 2. This scale goes from 0 to 8, with 0 representing a very minor Hawaiian-like eruption, while an 8 would be an eruption of a supervolcano like Yellowstone. Therefore, eruptions of Poás appear to never be very large in scale.

The video below shows some of the recent activity at Poás Volcano from the Observatorio Vulcanológico y Sismológico de Costa Rica (OVSICORI)


Another aspect of Poás that makes it unique are its two crater lakes. Prof. de Moor pointed out to us that “Poás is quite a special volcano because of its very well-developed hydrothermal system and acid crater lake (now dried up by the magmatic heat.” The acid crater lake, Laguna Caliente is one of the most naturally-acidic lakes in the world, with a pH near zero (Battery acid also has a pH of zero). The other lake, Botos, fills an old crater that last erupted 7,500 years ago.

Crater lakes such as this are often used as an indicator of volcanic unrest. For example, at volcanoes in both New Zealand and Colombia, increases in the amount of gas emitted at crater lakes preceded larger eruptions. Therefore, they are closely monitored. In this instance however, the lake has completely evaporated, indicating a significant temperature increase.

The video below, from OVSICORI shows night activity at Poás Volcano


Even though the recent eruptions have not been large enough to physically harm people, they have forced the closure of Poás Volcano National Park. According to Prof. de Moor the park is “visited by >300,000 tourists per year, and the closing of the national park due to volcanic activity has a strong impact on the local economy. Based on information from a Costa Rican guide website, the park has been closed since April 13, and that due to another large eruption on June 6, will remain closed indefinitely. This will likely have a significant impact on the economy as we head into the peak tourist season. Should any larger eruptions occur at Poás, we will be sure to update this post.

The recent activity at Poás Volcano is Costa Rica is considered “out of the ordinary” by Prof. de Moor. This is because phreatomagmatic eruptions had not occurred since 1953, and before that 1910.


Personal communication with Professor J. Maarten de Moor (Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional, Heredia, Costa Rica)

J.M. de Moor, A. Aiuppa, J. Pacheco, G. Avard, C. Kern, M. Liuzzo, M. Martínez, G. Giudice, T.P. Fischer, Short-period volcanic gas precursors to phreatic eruptions: Insights from Poás Volcano, Costa Rica, Earth and Planetary Science Letters 442 (2016) 218–227

Observatorio Vulcanológico y Sismológico de Costa Rica (OVSICORI)

Smithsonian Institution (National Museum of Natural History) – Global Volcanism Program