Firefighters were unable to fight the blaze because of dangers posed by the gas and said they would let it burn itself out.
"That's the safest way to deal with a fire like this," said Renee Preslar, a spokeswoman with the state Department of Emergency Management. It wasn't immediately known how long that would take.
The explosions occurred in the freezer section of the Cargill Meat Solutions plant, Preslar said. The fire involved an estimated 88,000 pounds of anhydrous ammonia, plus the plant had 100,000 pounds of nonflammable carbon dioxide, which is used in refrigeration systems, Preslar said.
Officials said dispatchers received the first emergency call about the fire at 12:58 p.m. Sunday.
Workers "were doing some welding on some fans," Logan County emergency manager Don Fairbanks said. "The welders had put their equipment up and turned around and there was a fire."
As the fire grew, officials said a series of small explosions rattled the 150,000-square-foot plant, which consisted of metal-framed buildings. By late Sunday afternoon, a hazardous materials team entered the smoldering plant to check the gauges on the anhydrous ammonia tanks. They all read empty, said Tonya Roberts, a spokeswoman for the emergency response effort,
"It either went up into the atmosphere or burned up," Roberts told reporters at a news conference. "The piping from the tanks runs all throughout the plant, so there are lots of places for it to leak or burn."
Booneville police officers said the evacuations began around 1 p.m. Residents of a nursing homes and patients at the city's hospital were among those chased from the area. State emergency officials said there was no need to open a shelter: the nursing home residents went to a nursing home in Greenwood and hospital patients went to another hospital in Waldron. Others who fled went to visit friends or relatives.
A witness said she heard the explosion while staying at a hotel near the plant. Meredith Voges, 22, of Connecticut, called the scene chaotic.
"The whole factory was ablaze with black smoke flying into the air, plumes of smoke," said Voges, in the area to shoot footage for a television program about a Booneville school principal.
At least one fire still burned Sunday night at the plant, which produces more than 2 million pounds of ground beef and steak a week. Cargill officials did not offer a damage estimate, but Logan County Judge Edgar Holt estimated the plant was worth more than $100 million before the fire.
"They just did a $40 million expansion and it's gone," Holt said.
Cargill Inc. has about 2,000 employees in Arkansas, according to a fact sheet on its Web site. Mark Klein, a spokesman for the Minneapolis-based company, said the plant is closed Sundays but about 20 contractors and a few other employees were at the site at the time of the fire.
Klein said the plant employs about 800 people, making it the largest employer in Booneville, a town of more than 4,000 people in western Arkansas.
Lori Hayes, a human resources manager for the plant, said corporate officials would come to the plant in the coming days to examine the damage. She said it was too early to say whether the plant would be rebuilt.
"We are asking all employees not to show up tomorrow," Hayes said.