Since January 5, the Poás volcano began a new activity that has made experts follow it closely and even generate warnings for the population that visits the National Park.
Journalists spoke with volcanologist María Martínez, from the Volcanological and Seismological Observatory (Ovsicori), who explained the important changes they have observed.
“The volcano’s lake dropped a lot, almost to the point of disappearing in 2024. From August, with the rains, it had a recovery thanks to constant rains until January. So, one would believe that as the underground hydrothermal system grows, the water cools, but that is not the case; the temperature increased from December 17 to January by 9°C,” said Martínez.
“According to the measurements, it went from 46°C to 55°C; without a doubt, it is a very important increase that comes from the heat inside the volcano through the lake and reaches the surface. there is light in the crater, you can see the lake with a lot of strong evaporation on the surface and a column of steam”.
Globules
“We have had constant globules during the month; between Tuesday and Wednesday alone we have recorded four,” she added. Globules are a phenomenon associated with an intense release of gas that raises the body of water and the sediments of the lake. The most important so far was on January 5, which threw materials, including rocks, in a radius of 100 meters outside the lake.
“The Ovsicori teams collected these rocks and we can say that they are old rocks, not new lava fragments; they were torn off by the injection of fluids from the lake into the environment,” the expert clarified. Fortunately, these sediments have fallen into the crater, because in 2017, when there was magmatic activity, the rocks reached the visitor building”.
“During this week, it has been relatively calm, but it is a volcano that has a very changing hydrothermal system. More eruptions and even phreatic eruptions could be expected, which are more energetic gas and steam eruptions, and are typical of this volcano,” she said. The expert also warned that these eruptions can occur suddenly, without early signs.
Other activities
Another point that draws attention in the activity that the volcano is showing is a deformation in the crust; it widened approximately 4 centimeters. “We want to see if the deformation is due to the push of deep fluids towards the surface; it could be water, gases or even magma. We are monitoring the GPS signals, which are very sensitive,” said Martínez.
According to Ovsicori, the activity during this month shows that, on some days, there are important peaks of carbon dioxide and sulfur dioxide gases, in addition to a lot of seismic activity.
“These gases seem to come from deep levels of the volcano; it is not continuous, but in the form of pulses. When a volcano erupts, the emission of gases and ash is continuous,” she said.
From 2020 until mid-2024, the volcano remained relatively calm. In the first half of the previous year, there were several eruptions similar to this one; the difference in 2025 is that there is more gas emission.
What can happen?
The scenarios of what can happen in the next few days or weeks with the volcano are very relative. The expert highlights the importance of monitoring and observing the different changes.
There may be a small rise of magma to the surface, which could explain the gas flows, higher temperatures, seismicity and a slight widening,” Martínez said. The other possibility is that it is just water and gases modifying the crust, or the volcano’s activity could decrease and it could enter a relative calm.
