The Volcán Rincón de la Vieja appears to be waking up after a 13-year nap.
The Observatorio Vulcanológico y Sismológico de Costa Rica reported small eruptions in the crater lake, evidence of thermal activity there and the death of fish in nearby rivers. The volcano threw out some material Sept. 16 that appears to have affected fish as much as 18 kilometers (more than 11 miles) downstream, the observatory said.
Scientists conducted a site visit from Tuesday to Thursday.
Parque Nacional Rincón de la Vieja is about 25 kilometers or about 15.5 miles northeast of Liberia in Guanacaste. The volcano is 1,916 meters high or about 6,286 feet. It is part of the mountain range that is the spine of the country.
The observatory said that the first reported activity from the volcano was in 1850. There was an explosive eruption in 1863 and more activity noted in 1912, 1922, 1940, 1955, 1963, 1965 and 1966.
In December 1966, the mountain put out black smoke and ash. Later researchers found chunks of rock the volcano had put out, the observatory said.
There was more ash and fine material expelled from 1969 to 1975 and 1983 to 1987. The most spectacular was Feb. 6, 1983, according to observatory records. Residents in Dos Río de Upala some 8 kilometers north of the volcano and residents a similar distance to the east heard the eruption and saw a large column of ash. The eruption produced an avalanche of mud and killed fish in the ríos Pénjamo and Pizote. There also were blocks or rocks thrown a kilometer from the crater, the observatory said.
There have been periodic eruptions since then through 1998, according to the observatory records.
The volcano is not as well known as Arenal, Poás, Turrialba or Irazú, in part because of its location. These four active volcanos are around the Central Valley.
The observatory said that what researchers saw near the summit suggests that there may be activity similar to the major events in the 1980s and 1990s with economic impact on the communities nearby.
They urged precautions, monitoring and that the communities be linked with a communication system in case of emergency.
Park officials have closed off the 13-kilometer (7.5 mile) hiking trail to the crater.
The researchers said there were two channels for runoff from the summit to nearby rivers. They said that the outpouring of mud and liquid from the volcano can reduce oxygen levels in streams and rivers, thereby killing fish.
There also is a temperature factor and toxicity from the sediment that is dumped into the waterways.
The crater lake is mainly a product of the heavy rains that fall on the Caribbean face of the mountain. The visitors said they could see convection in the lake caused by the heat of the volcano and small eruptions in the center.
There also are gas vents that are emitting vapor.
The mountain is tall enough that it shares two separate climates. On the west side, it experiences the dry climate of Guanacaste. To the northeast, the climate is similar to the Caribbean and the northern zone, according to the observatory.