Mud and ash spewed more than 1,500 feet into the air above Poás Volcano National Park.
Rangers said ash fell in various parts of the park.
The volcano regularly shows signs of gentle activity, according to geologists. But such a strong explosion had not been seen since 2006.
Experts said the activity was normal, but they will continue monitoring the volcano for any intensification.
Officials say that no visitors were threatened by the activity.
Photo: Poás Volcano National Park