Poas is a stratovolcano with several eroded calderas, cones, and two crater lakes at or near its summit. Poas has erupted at least 39 times since 1828 and is in state of nearly continuous mild activity. Most of the eruptions are phreatic , caused by the interaction of lava and the water in the crater lake.
Explosions vary in strength.
Note the steam rising from the active crater lake. Photograph copyrighted and provided by Steve O'Meara of Volcano Watch International. Boiling crater lake at Poas, 1986. Photograph by Mike Lyvers. In 1989, the crater lake at Poas gradually drained away.