The Turrialba volcano produced a 30-minute eruption early Monday afternoon, said the Red Sismológica Nacional at the Universidad de Costa Rica.
The day was overcast, but scientists could follow the eruption because the cameras at the volcano summit can record heat.
The eruption started at 1:08 p.m., and about 1:21 p.m. the volcano was throwing out flaming blocks of rock, said the Red. However, the expelled rock remained along the edge of the crater, the Red said. There also were some tremors in the earth, the Red added.
Officials have long awaited a major eruption from the mountain, and most of the residents have moved elsewhere. Those who have not received a shower of cinders Monday from the mountain. In communities such as El Central on the flank of the mountain, vehicles were covered with the fine, black grit.