Volcán Rincón de la Vieja cleared its throat Wednesday night, and scientists know that because the event registered for three minutes on seismic recordings.
The eruption began at 8:48 p.m. The monitoring station is about five kilometers away from the mountain.
There may have been an earlier eruption, said researchers at the Observatorio Vulcanológico y Sismológico de Costa Rica. There was a smaller seismic signal about 12:48 p.m., they said.
The researchers said that neighbors reported they did not hear or see anything, so the speculation is that all the material ejected fell back into the crater, which contains a lake just as does Volcán Poás.
The volcano was last active between September 2011 and January 2012. At that time, material was thrown from the volcano and polluted a local river.