"It was a brief eruption of ash for 30 minutes that was not preceded by a temblor inside the volcano," said Marta Lucia Calvache of Colombia's Volcanology Institute.
"But there is still a thin plume of ash leaving the crater, and we can't rule out the possibility of further eruptions."
The government this month ordered the preventive evacuation of thousands of people living in the shadow of the volcano amid signs of an imminent eruption.But many farmers are believed to have defied the order and stayed behind, fearful of losing their livelihoods by leaving crops unattended. Calvache
urged any families that remained in a wide area surrounding the volcano to leave immediately and seek medical treatment if they have trouble breathing.
"A lot of ash has fallen.