(Adds new evacuations, LATAM Airlines' with normal operations, comments from head of geological service)
SANTIAGO, April 24 (Reuters) - Chilean volcano Calbuco, which erupted without warning on Wednesday, is still puffing out ash and smoke on Friday, prompting new evacuations and leading airlines to cancel flights to Argentine capital Buenos Aires, some 1,400 kilometers east.
Calbuco, considered one of the most dangerous along Chile's chain of around 2,000 volcanoes, erupted twice in 24 hours on Wednesday and Thursday, sending up a spectacular 17 kilometer-high (11 miles) cloud and coating nearby towns in a thick layer of grey ash.
Authorities have set up a 20 kilometer (12 mile) cordon around Calbuco, which is located in the scenic Los Lagos region, around 1,000 kilometers (621 miles) south of capital Santiago.
An increase in volcanic activity caused potentially deadly lahars, a mix of water and rock fragments that flow down a volcano's slopes and river valleys, prompting authorities to evacuate an additional 2,000 people.
"This is a complex (volcanic) process that could last for weeks," said Rodrigo Alvarez, head of Chile's mining and geological service.
Television images showed residents in a town close to Lake Chapo quickly driving down ash-laden streets away from the smoldering volcano. Some houses and schools located close to the volcano have collapsed under the weight of the ash.
The ash also presents a threat to air traffic, as particles in the atmosphere can cause problems for planes.
With winds blowing the ash cloud north-east into neighboring Argentina, Argentine air traffic was the worst affected by Friday.
Delta Air Lines, Air France and American Airlines suspended flights to the main international airport at Ezeiza, near capital city Buenos Aires.
"They were canceled as a preventive measure, for fear that after landing they might not be able to take off later," said a spokesman for Argentina's National Civil Aeronautic Administration. "We do not rule out the possibility that other airlines might take the same decision."
In the southern Argentine tourist city of Bariloche, flights were canceled for a second straight day, some roads were closed and children were kept home from school.
In Chile, the ash cloud had reached as far as port city Valparaiso, near Santiago. Delta and American Airlines cancelled flights to the capital. Chile's flagship carrier LATAM Airlines cancelled over 20 flights south on Wednesday and Thursday, but has since resumed normal operations.
The ash cloud was unlikely to spread much further under current conditions, said David Rothery, Professor of Planetary Geosciences at Britain's Open University.
"Even a lengthy sustained ash eruption would be very unlikely to produce enough ash to spread across the tropical zone," he said. It was not in line to be as disruptive as the eruption of Chile's Puyehue in 2011, which led to flights being cancelled as far away as Australia, he added.
Most of the mines in top copper exporter Chile are located in the north of the country, far from the volcano. Fears over contamination of salmon farms in the south, however, hit the shares of local producers on Friday.
Reporting by Rosalba O'Brien, Antonio de la Jara and Anthony Esposito in Santiago; Additional reporting by Nicolas Misculin in Buenos Aires; editing by Andrew Hay