About 8,000 people live in areas near the volcano where Pasto's mayor ordered an evacuation but "most of the city is not in danger," Fernando Gil, director of Colombia's Seismological Network, told The Associated Press by phone.
"It's still erupting," Gil said more than two hours after its initial eruption.
Gil estimated that the ash cloud reached five miles into the air.
He called it the most severe eruption of Galeras since the volcano reactivated in 1989.
"Depending on the wind direction it's going to spread ashes over the entire area."
"Most of (Galeras') eruptions are violent and short," he noted. He said Thursday's eruption had produced some lava flows that did not extend far from the volcano's crater.
A 1993 eruption of the volcano, near the border with Ecuador, killed nine people, including five scientists who had descended into the crater to sample gases.
In November 2005, the volcano spewed ash that fell about 30 miles away.
