Rincón de la Vieja is erupting less often and with less force than it did over the summer. Scientists say that does not yet mean the volcano is calming down. The Volcanological and Seismological Observatory of Costa Rica (OVSICORI-UNA) said activity has been trending downward since mid-September. It recorded 18 eruptive events last week, most of them small, and none reached acoustic magnitude 4, the level the observatory classifies as moderate.
That is a sharp drop from the stretch between July 2 and mid-September, when OVSICORI counted 249 phreatic eruptions and nine lahars, the flows of mud and volcanic debris that race down river channels. The observatory pointed to three changes. Acoustic pulses from small eruptions have decreased, volcano-tectonic earthquakes have eased, and the rapid uplift and contraction of the summit that drew scientists’ attention in September has stopped.
OVSICORI laid out two possible explanations and said it is not yet possible to determine which is more likely. In the first, the volcano is adjusting after months of eruptions and after expelling part of the water in its crater lake. If that is the case, the recent signals would not necessarily lead to more eruptive activity.
In the second, the changes are precursors to new eruptions, possibly larger than any seen in recent months. The reason scientists are not treating the quiet spell as good news lies in how this volcano behaves. According to OVSICORI, eruptions here often follow a significant drop in low-frequency tremor. The observatory said a kind of seal can form inside the system, trapping gas until the pressure breaks through.
Put simply, a quieter volcano can be one that is holding pressure in rather than letting it out. OVSICORI also warned that an active volcano can produce eruptions, landslides and lahars suddenly, sometimes without warning signs that give enough time to react. The volcano remains at Level 3, or caution, on the observatory’s activity scale. On Oct. 1, the National Emergency Commission (CNE) raised its alert to yellow for five nearby areas: Dos Ríos and Aguas Claras in Upala, and Mayorga, Cañas Dulces and Curubandé in Liberia.
If you live near the volcano or are heading there, the rivers are the main hazard. The CNE has advised people to stay clear of river channels when volcanic material is coming down, not to drink or touch the water in those rivers, and not to let livestock drink it. Lahars can travel far from the crater and arrive with little notice.
If ash falls where you are, cover your nose and mouth. The national park’s crater trail remains closed until activity decreases, according to the Guanacaste Conservation Area. Other trails in the Las Pailas sector are not listed as closed, but tickets must be bought online in advance, and conditions can change quickly, so check with the park before you drive out.
Rincón de la Vieja has been one of the most active volcanoes since July, with plumes rising as high as 1,000 meters (3,280 feet) above the crater. OVSICORI continues to monitor its seismicity, gas output and ground deformation, and the coming weeks should show which of the two scenarios is playing out.
