They are helped in their investigation by statements from residents and photographs. The eruption also was detected by seismographs as if it were an earthquake.
The Observatorio Vulcanológico y Sismológico released photos Monday. The observatory also released photos from the air of the volcano crater taken on March 6 and again Saturday.
The normally blue lake in the crater has turned gray due to the injection of gas and expelled material, the observatory said.
The eruption was accompanied by what sounded like an explosion, according to residents in some of the nearby towns. The eruption appears to have thrown material from the lake bottom to the north flank of the crater, said the experts.
The trail to the crater has been closed by park rangers, but the national park that surrounds the mountain near Upala is open, official said.