The first eruption in the crater lake was at 5:57 p.m. Saturday, said the Observatorio Vulcanológico y Sismológico de Costa Rica. A second and smaller eruption took place at 11:20 a.m. Sunday.
In both cases, the Observatorio can be exact because the eruptions registered on a sensor that is located at the volcano.
The eruption did not send material outside the crater but did decorate the floor outside the lake with material in a distinctive design.
Such eruptions have been taking place since 2006 when the volcano re-entered an active phase, said the Observatorio.
The active crater of the volcano contains Laguna Caliente where the water is strongly acidic.
The activity is not considered dangerous and the volcano and its national park still are open to visitors.