Puerto Varas (Chile) (AFP) - Southern Chile was on alert Thursday after the Calbuco volcano erupted for the first time in half a century, lighting up the night sky with spectacular bursts of volcanic lightning and lava, and forcing some 5,000 people to evacuate.

Authorities declared a state of emergency, sent in the army and evacuated a 21-kilometer (13-mile) radius around the volcano after the twin eruptions -- the first on Wednesday as night fell, the second early Thursday before sunrise.

The volcano, which last erupted 54 years ago, had not shown signs of increased activity.

The massive ash clouds belched into the sky forced airlines to cancel flights and drifted as far away as southern Argentina -- including the picturesque ski resort of Bariloche, which warned people to stay home to avoid inhaling dust particles.

There were no immediate reports of injuries. A 21-year-old mountain climber was initially reported missing, but later turned up alive.

The first eruption, which started around 6:00 pm (2100 GMT) Wednesday and lasted about 90 minutes, spewed a giant mushroom cloud of ash 10 kilometers into the sky, which turned hues of pink and yellow as the sun set over the area.

Seven hours later, Calbuco erupted again, this time shooting lava into the air and generating volcanic lightning, a phenomenon researchers believe is caused by the emission of large amounts of molten rock and ash charged with static electricity.

Bluish-white bolts of lightning cut through the red and orange plumes of lava in a stunning display of nature's power that played out against the backdrop of the night sky.

Chile's National Geology and Mines Service warned a third eruption could follow.

- 'Situation unpredictable' -

Volcanic ash blanketed the area -- nearly a meter (three feet) high in some spots, causing the roofs of some houses to cave in.

Road workers used heavy trucks to plow a path through the ash, as residents shoveled it off their rooftops.

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President Michelle Bachelet flew to the affected area along with several ministers.

"We don't know how the situation is going to evolve. It's pretty unpredictable," she said.

"The ash might damage crops, animal feed, bridges, roads, people's work routines, tourism and especially their health," she said, calling on residents to wear protective masks, which authorities were handing out in both Chile and Argentina.

Until minutes before the blast, volcano eruption monitoring systems had picked up nothing. In fact, volcano watchers in Chile had been watching another one, Villarrica, for a possible eruption.

But some experts said the event should not have been a surprise.

"The volcano wasn't at all dormant. It was active and alive," with regular emissions of gas and smoke, said Florent Brenguier, a seismologist at the University of Grenoble in France.

"A period of 50 years between eruptions is very short in the life of a volcano."

Some in the area reacted with panic. Others were fascinated.

"The explosion caused immediate hysteria among residents. We couldn't believe what we were seeing," said Marcia Claro, the owner of a cafeteria in Puerto Varas, a town of 38,000 people on the shores of Lake Llanquihue, at the foot of the volcano.

The mayor of nearby Puerto Montt, a tourist gateway to the popular Patagonia region, said residents were "very, very frightened," amid warnings that melting snow and ice could flood rivers.

Officials declared a red alert for both cities.

But another local, Alvaro Ascencio, said people were "more excited than afraid," flocking to the lake to take pictures of the volcano some 40 kilometers away.

- Turbulent geology -

The region, Los Lagos, is popular with tourists, with beautiful mountain landscapes dotted with volcanoes and lakes with beaches of black sand.

The 2,000-meter (6,500-foot) volcano, located some 1,400 kilometers south of the capital Santiago, had last erupted in 1961 and showed light activity in 1972, said the National Geology and Mines Service.

It is the second eruption in Chile since March 3, when the Villarrica volcano emitted a brief but fiery burst of ash and lava.

Chile has about 90 active volcanoes.

Geologist Alfredo Lahsen of the University of Chile told the long, narrow country, the world's largest copper producer, to look on the bright side of its turbulent geology.

"If we didn't have volcanoes, we wouldn't have the rich gold and copper mining activity that we have in Chile. You have to see the positives, too," he told radio station Cooperativa de Santiago.