(Adds evacuee comment, details from Argentina)

By Antonio de la Jara

PUERTO MONTT, Chile, May 9 (Reuters) - A towering plume of ash from an erupting volcano in Chile's remote Patagonia could rain down on the surrounding area and cause devastating damage, a volcano expert warned on Friday.

Luis Lara, a scientist with the government's geology and mining agency, said the column, which has soared 7.5 miles (12 km) into the air, was at a critical stage.

An abrupt descent would blanket vast areas with deadly hot gas, ash and molten rock, he said.

Authorities have evacuated thousands of people from the immediate vicinity of Chaiten volcano, 760 miles (1,220 km) south of the capital Santiago, and are forcing people within a 30-mile (50-km) radius to leave.

Chaiten began erupting eight days ago for the first time in thousands of years.

"We are at a critical point of this phase given the characteristics (of the eruption) have remained the same for several days," Lara said.

"The volcano is now at its limit and one possibility is that the column could collapse quickly, generating flows of ... material down its ravines," he said.

The column might descend gradually and do little damage. But in the worst-case scenario, the ash and fiery material would engulf the town of Chaiten, just 6 miles (10 km) from the volcano, and the areas around it.

Lara said the volcano could rumble on for years and suggested that the town, which is now deserted, be moved.

SOUTH ARGENTINA ALSO HIT HARD

The cloud has also caked towns on the Argentine side of the border with ash. Satellite images show a white stripe smeared across the southern part of the continent.

Ash that had drifted as far as Buenos Aires dissipated on Friday, and some airlines that had canceled flights overnight resumed service.

But towns in Argentine Patagonia were badly affected, with residents complaining of sore throats due to ash inhalation and being forced to pay exorbitant prices for bottled water because ground water had been contaminated.

Views of dramatic Andean peaks that serve as a natural border between the two countries were obscured by clouds of ash in the Argentine settlement of Trevelin, a popular tourist spot about 60 miles (100 km) from the volcano.

Shop owners put wet cloths and cardboard on the doorsteps as doormats to stop prospective customers from tracking ash into their premises. But they were losing the battle.

Some residents wore masks, but many did not.

"We keep cleaning, but still everything gets dirty at the same time. The dust and ash gets everywhere," said a hotel employee named Alejandra.

Back in Chile, many evacuees had no idea when they might be able to return to their homes and lives, and their frustration was mounting.

"We've been here so many days and no one tells us anything," said Iluminada Ide, who was evacuated to the southern Chilean town of Puerto Montt. "We can't go on like this." (Additional reporting by Jorge Otaloa in Trevelin, Karina Grazina in Buenos Aires) (Writing by Simon Gardner; Editing by Xavier Briand)

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