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Pablo Sanhueza Gutierrez, Associated Press
Men remove ash from a building's roof, left by the Calbuco volcano, in Ensenada, Chile, Thursday, April 23, 2015. The volcano erupted Wednesday for the first time in more than 42 years, billowing a huge ash cloud over a sparsely populated, mountainous area in southern Chile, and is considered one of the top three most potentially dangerous among Chile's 90 active volcanoes.

PUERTO MONTT, Chile — Twin blasts from the Calbuco volcano in southern Chile sent vast clouds of ash into the sky, increasing concerns the dust could contaminate water, cause respiratory illnesses and ground more flights.

The volcano erupted Wednesday afternoon for the first time in more than four decades, spewing out a plume of ash more than 6 miles (10 kilometers) high. Emergency officials were taken by surprise and had only a few minutes to issue an alert.

Calbuco had another spectacular outburst early Thursday with lightning crackling through a dark sky turned reddish orange by the explosion.

As the ash cloud spread Wednesday, "people went into a state of panic," said Miguel Silva Diaz, an engineer who lives in Puerto Montt, a city about 14 miles (22 kilometers) from the volcano. "Then, at around 1 a.m., I heard a loud noise, as if somebody had detonated an atomic bomb."

No injuries were reported and the only person reported missing since the eruption was located Thursday.

Authorities evacuated 4,000 people as gas and ash continued to spew, and they closed access to the area around the volcano, which lies near the cities of Puerto Varas and Puerto Montt, some 620 miles (1,000 kilometers) south of Santiago.

"I was shocked. I had just arrived home when I looked through the window and saw the column of smoke rising up. We called our families, posted photos," said Daniel Palma, a psychologist who lives in Puerto Varas.

"We woke up today with a blanket of fog and it hasn't cleared. We have a layer of smoke above us," Palma said, adding that many were concerned about the possible effects of the ash on their health.

The Chilean national geology and mining service warned that people should prepare for a third and "even more aggressive eruption."

President Michelle Bachelet declared a state of emergency, saying the eruption of Calbuco was "more serious and unpredictable" than the one last month at the Villarica volcano, which also forced the evacuation of thousands.

"We don't have any problems with supplies, water or sewage up to now. That's not the problem," said Bachelet, who visited the area Thursday. "Our problem is a respiratory one, from inhaling all of this ash, and the fact that this ash could generate some sort of environmental contamination."

The short-term dangers related to the ash also include eye and skin infections as well as water contamination, said Bernardo Martorell, a physician and the head of the sanitary planning division at Chile's health ministry.

"That's why the people in the area need to evacuate," Martorell said.

Ash continued to fall Thursday in Puerto Montt and other nearby cities, said Patricio Vera, director of a local radio station. Varas said that after the initial eruption, hundreds of people rushed to buy gasoline, forcing stations to ration sales, while supermarkets closed early to avoid the risk of looting.

The 6,500 foot (2,000-meter) Calbuco last erupted in 1972 and is considered one of the top three most potentially dangerous among Chile's 90 active volcanos.

LATAM and other airline companies cancelled flights to and from Puerto Montt because airborne ash can severely damage jet engines.

In 2011, a volcano in the Caulle Cordon of southern Chile erupted violently, forcing the cancellation of hundreds of flights and the evacuation of more than 3,500 people. Thick, abrasive soot coated slopes in the ski resort city of San Carlos de Bariloche, over the border in Argentina.

By Thursday afternoon, much ash had made its way to Villa La Angostura, Argentina, a small town about 56 miles (90 kilometers) northeast of Calbuco. Cars and streets were coated with a thin layer of ash, but people were otherwise going about their business.

"We are praying that the volcanic activity will be as short as possible," said mayor Roberto Cacault.

Associated Press writer Luis Hidalgo reported this story in Puerto Montt and Luis Andres Henao reported from Santiago. AP writers Gonzalo Keogan in Villa La Angostura, Argentina, and Eva Vergara in Santiago contributed to this report.

Luis Andres Henao on Twitter: https://twitter.com/LuisAndresHenao