Twin blasts from the Calbuco volcano in southern Chile sent vast clouds of ash into the sky, covering the town with thick soot and raising concerns that the dust could contaminate water, cause respiratory illnesses and ground more flights.

The volcano erupted on Wednesday afternoon for the first time in more than four decades, spewing out a plume of ash more than six miles high. Emergency officials were taken by surprise and had only a few minutes to issue an alert.

Most of the 1,500 residents had evacuated after the initial eruption on Wednesday, with only about 30 people refusing to leave out of worry for their homes and animals.