By Anthony Esposito
SANTIAGO, April 22 (Reuters) - Volcano Calbuco in southern Chile erupted for the first time in more than five decades on Wednesday, sending a thick plume of ash and smoke nearly 20 kilometers into the sky.
Chile's Onemi emergency office declared a red alert following the sudden eruption at around 1800 local time (2100 GMT), which occurred about 1,000 kilometers (625 miles) south of Santiago, the capital, near the tourist town of Puerto Varas.
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As night fell, about 4,000 people had so far moved out of the area, an evacuation radius of 20 kilometers has been established and classes have been canceled in surrounding towns, authorities said.
President Michelle Bachelet is scheduled to travel to the affected area on Thursday.
There are no reports of deaths, missing persons or injuries, Interior Minister Rodrigo Penailillo said. He urged residents to evacuate and warned of possible lahars, a mix of water and rock fragments that flow down a volcano's slopes and river valleys.
Winds blowing northeast were pushing the column of ash and smoke toward Argentina.
"In this situation, with the eruption column so high, the main risk is that it collapses, falls due to gravity because of its own weight and causes a pyroclastic flow," Gabriel Orozco, a vulcanologist with Chile's geological and mining service, said on local TV.
A pyroclastic flow is a superheated current of gas and rock that can destroy nearly everything in its path and travel at speeds upwards of 200 to 300 kilometers per hour.
LATAM Airlines said it has canceled flights to and from neighboring Puerto Montt, the area's largest city, due to the presence of volcanic ash, which can potentially damage aircraft and make flying dangerous.
Trevor Moffat, who lives in Ensenada, some 10 km from the volcano, said he and his family fled when the volcano erupted. The last major eruption of the 2,003-meter-tall Volcano Calbuco was in 1961.
"It sounded like a big tractor trailer passing by the road, rattling and shaking, guttural rumbling. ... We left everything there, grabbed my kid, my dog, got in the car with my wife," Moffat said.
"All the neighbors were outside, a lot of young people crying. Armageddon type reaction," said Canada-born Moffat, who was driving to nearby Puerto Varas.
Television pictures showed a spectacular mushroom-shaped column billowing into the sky with occasional lighting bolts shooting through. The eruption was seen in towns at least 50 kilometers away.
"There are a lot of people out in the streets, many heading to the gas stations to fill up on gas," Derek Way, a resident of Puerto Varas, said.
"A friend told me to fill everything we have with water," Way added.
Chile, on the Pacific "Rim of Fire," has the second largest chain of volcanoes in the world after Indonesia, including around 500 that are potentially active.
In March, Volcano Villarrica, also in southern Chile, erupted, sending a plume of ash and lava high into the sky, but it quickly subsided. (Additional reporting by Rosalba O'Brien and Antonio de la Jara; Editing by James Dalgleish, Christian Plumb and Leslie Adler)
