The estimate came from geologists at the Instituto Costarricense de Electricidad who work with the Red Sismológico Nacional at the Universidad de Costa Rica.
The geologists, Waldo Taylor Castillo and Gerardo J. Soto, said in a report made available by the Red that an earthquake in July just 10 kilometers or about six miles from the volcano may have triggered the recent activity. They said that the infamous Limón earthquake in 1991 was believed to have triggered eruptions at the same volcano that year and the next.
The Observatorio Vulcanológico y Sismológico de Costa Rica at Universidad Nacional in Heredia released its report on the volcano late last week. That report noted that material ejected from the crater polluted streams and rivers and led to the death of many fish.
The volcano is right on the continental divide in northern Costa Rica not far northeast of Liberia.
The geologists said that activity began Aug. 15 when turbulence and bubbles were noted in the lake that fills the crater. That activity was also noted by guides.
The first eruption appears to have taken place Sept. 10. On Sept. 16 another geologist had visited the crater and heard an eruption a few hours later.
That event put material from the crater outside on the volcano slope.
The next day observers were able to see that the material eventually reached two nearby rivers and caused the death of fish.
Eruptions continued through Sept. 28, a day when the geologists said there was four times the previous activity.
Their estimates of the future are uncertain. They said they had to analyze their date of the activity in the mountain. But they thought that the activity would continue for at least several weeks more.
The national park in which the volcano is located is restricted now, and the local guides are not climbing to the top to show tourists the crater, the geologists said.
They said the video that gave them an idea of the height of the cloud was made by Walter Granada, a local guide.