Volcanologists say that the Volcán Turrialba is entering its third active phase and that the more recent displays are being augmented by heavy rains that are falling on the mountain.
An increase in activity in October has brought heightened official notice. The Comisión Nacional de Prevención de Riesgos y Atención de Emergencias said that emergency plans are being reviewed.
Monitoring stations can hear fluid moving around in the mountains, but no one has suggested that a major eruption is near. The internal activity has been on the rise since July, said a report from the Observatorio Vulcanológico y Sismológico de Costa Rica.
There was a small eruption about 6 a.m. Friday. Volcanologists reported that the eruption lifted smoke and ash 300 meters into the sky. There have been a flurry of small eruptions all last week.
The first eruptive cycle was from October to December 2014, said the Observatorio. The second was between March and July his year. The current activity is being aided by the infiltration of rain water, the report said.
The Observatorio said that sonic monitoring suggests that a small body of magma is making its way to the top of the volcano, and as it depressurizes it emits more gas and vapor.
Those most at risk are farmers who raise crops and animals of the skirts of the volcano, but they have had a lot of experience dealing with the mountain. The national park that surrounds the mountain has been closed for years. Police had erected checkpoints at entries to the area around the mountain. A lot of residents already have left.