Costa Rica · ENVIRONMENT
Key Facts
- —What happened Two phreatic eruptions hit Rincón de la Vieja on 23 August 2026, the larger at 7:38 a.m.
- —How big The 11 August pair reached about 1,000 metres above the summit, with magnitude readings of 4.1 and 4.5.
- —The catch The 1,000-metre plume estimate applies only to 11 August, not to the 23 August events.
- —Who it hits The volcano sits in the Guanacaste mountain range in north-western Costa Rica, near a national park.
- —What comes next OVSICORI-UNA keeps monitoring and publishing readings, and has issued no forecast of what follows.
Costa Rica’s volcano observatory recorded steam-driven blasts all August. The latest pair struck on the morning of 23 August.
Rincón de la Vieja eruptions continued in Costa Rica through August 2026, according to the country’s volcano observatory. The latest activity occurred on the morning of Sunday 23 August, when two phreatic eruptions were recorded.
Latest Eruptions on 23 August
However, OVSICORI-UNA reported two phreatic eruptions on Sunday 23 August 2026. The Rincón de la Vieja eruptions included one at 7:38 a.m. and a smaller one at 00:30.
The observatory published a dated bulletin about these events. Still, no further details on the Rincón de la Vieja eruptions have been released.
The observatory’s report described both events as phreatic, meaning steam-driven blasts. However, it did not provide any plume height estimate for this date, leaving Rincón de la Vieja eruptions unquantified.
However, the observatory’s bulletin for that day did not include a plume height estimate. Meanwhile, the smaller eruption at 00:30 was noted as less energetic.
The 11 August Pair
Earlier in the month, on Tuesday 11 August 2026, OVSICORI-UNA recorded two energetic phreatic eruptions. The first occurred at 5:17 p.m. and the second at 5:25 p.m., according to the observatory’s report published the next day.
In addition, the observatory attached magnitude readings of 4.1 and 4.5, respectively. These eruptions were detected by seismographs and infrasound acoustic sensors.
Meanwhile, the observatory published its findings the following day, Wednesday 12 August. In addition, the observatory estimated these steam and gas eruptions reached about 1,000 metres above the summit.
This estimate applies only to the 11 August events, not to later ones.
Weekly Count in Early August
In its official bulletin of Friday 7 August 2026, OVSICORI-UNA reported that 14 eruptions were recorded during that week. This count applies only to the week covered by that bulletin.
The same bulletin noted that after one particular eruption, no further eruptions had been recorded and background tremor decreased. However, this was only true as of 7 August, since eruptions resumed on 11 and 23 August.
The bulletin’s count of 14 eruptions covered that specific week only. In addition, it noted that background tremor decreased after one particular event.
However, the same bulletin noted that after one eruption, no further eruptions had been recorded and background tremor decreased. This was true only as of 7 August, since eruptions resumed on 11 and 23 August.
Plume Height Estimate
For example, OVSICORI-UNA estimated steam and gas plumes reached about 1,000 metres above the summit on 11 August. This estimate is measured above the summit, not above sea level.
The observatory uses seismographs and infrasound acoustic sensors to detect eruptions, even when they are not visible. Infrasound sensors record sound waves below the range of human hearing.
This estimate applies only to the 11 August eruptions, not to later events. However, the observatory did not specify the exact scale used for the magnitude readings.
In addition, the observatory’s estimate of 1,000 metres is measured above the summit, not above sea level. This figure belongs solely to the 11 August eruptions, as stated in the report.
Understanding Phreatic Eruptions
A phreatic eruption, known as ‘erupción freática’ in Spanish, is therefore a steam-driven blast. Groundwater heated by the volcano flashes to steam, throwing out water, gas, and shattered old rock.
Unlike other volcanic activity, phreatic eruptions do not involve fresh molten rock reaching the surface. This type of eruption is common at Rincón de la Vieja, an active volcano in the Guanacaste mountain range.
Such eruptions are common at Rincón de la Vieja, an active volcano in Guanacaste. However, they do not involve fresh molten rock reaching the surface.
However, these steam-driven blasts do not involve fresh molten rock reaching the surface. As a result, they are distinct from other volcanic activity at Rincón de la Vieja.
Monitoring Background Tremor
Meanwhile, background tremor, or ‘tremor de fondo’, is the continuous low-level shaking recorded on a volcano. Observatories track whether it rises or falls as one sign of what the volcano is doing.
OVSICORI-UNA reported that after an eruption in early August, background tremor decreased. However, this was a temporary observation, as eruptions were recorded again later in the month.
The decrease in background tremor was recorded as of 7 August 2026. However, eruptions resumed on 11 and 23 August, showing the lull was temporary.
Meanwhile, the observatory reported that background tremor decreased after an eruption in early August. However, this was a temporary observation, as eruptions resumed later in the month.
The Role of OVSICORI-UNA
OVSICORI-UNA is the volcanological and seismological observatory of the Universidad Nacional, Costa Rica’s national university. It is the state body that monitors the country’s volcanoes and publishes daily and weekly bulletins.
In fact, the observatory’s reports are the primary source of information about the eruptions. All eruption facts in this article trace back to OVSICORI-UNA.
This institution publishes daily and weekly bulletins on volcanic activity. As a result, all eruption facts in this article trace back to its official reports.
In addition, OVSICORI-UNA is the state body that monitors Costa Rica’s volcanoes and publishes bulletins. As a result, all eruption facts in this article trace back to its official reports.
Rincón de la Vieja Eruptions in Context
Rincón de la Vieja is an active volcano in the Guanacaste mountain range in north-western Costa Rica. It gives its name to the national park around it.
Moreover, the recent eruptions are part of a series of activity recorded throughout August 2026. OVSICORI-UNA has not provided any forecast or assessment of what might happen next.
The volcano sits in the Guanacaste mountain range in north-western Costa Rica. Meanwhile, it gives its name to the national park around it.
However, the recent eruptions are part of a series of activity recorded throughout August 2026. Meanwhile, the observatory has not provided any forecast or assessment of future activity.
What We Know So Far
In short, OVSICORI-UNA recorded 14 eruptions in the 7 August bulletin week, two on 11 August, and two on 23 August 2026. The observatory’s reports provide the only verified details about these events.
The observatory keeps watch on the volcano and publishes each reading. Its bulletins describe the eruptions themselves and nothing beyond them.
The observatory has not issued any forecast about future activity. However, it continues to monitor the volcano’s ongoing eruptions.
The observatory keeps a continuous watch and reports each event as it happens. Its bulletins stay with the readings themselves.
Frequently Asked Questions
What is a phreatic eruption?
A phreatic eruption is a steam-driven blast caused by groundwater heated by the volcano flashing to steam. It throws out water, gas, and shattered old rock, but does not involve fresh molten rock.
How many eruptions occurred at Rincón de la Vieja in August 2026?
OVSICORI-UNA recorded a series of eruptions throughout August. The observatory reported 14 eruptions in the week covered by its 7 August bulletin.
What does OVSICORI-UNA stand for?
OVSICORI-UNA is the Observatorio Vulcanológico y Sismológico de Costa Rica, the volcanological and seismological observatory of the Universidad Nacional. It is the state body that monitors Costa Rica’s volcanoes.
How does OVSICORI-UNA detect eruptions?
The observatory uses seismographs and infrasound acoustic sensors to detect eruptions. Infrasound sensors record sound waves below human hearing, which can signal an eruption even when it is not visible.
Connected Coverage
Sources: OVSICORI-UNA, Universidad Nacional de Costa Rica.
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