Pahoa, Hawaii (CNN) -- Officials warned residents and airplanes to stay away from part of Hawaii's Big Island after a plume of ash from the Kilauea volcano rose 12,000 feet into the air.

Since the Kilauea volcano erupted May 3, it's been one nightmare after another for residents on the southeastern part of the Big Island.

The US Geological Survey issued a red alert Tuesday, which means a major eruption is imminent or underway and ash could affect air traffic.

The USGS' Michelle Coombs described the situation as " very hazardous for aviation" and said her team isn't quite sure what caused Tuesday's slightly more intense ash emissions.

Norwegian Cruise Line announced that Pride of America would skip a call at Hilo on Tuesday and Kona on Wednesday. Instead, the ship will spend an extra day on the island of Maui.

USGS officials have said a phreatic eruption could happen at a crater at the top of the Kilauea volcano. It could send ash plumes as far as 12 miles from the summit crater, the Hawaiian Volcano Observatory said.

These are steam-driven explosions that occur when water beneath the ground or on the surface is heated by magma, lava, hot rocks or new volcanic deposits, the USGS says. The intense heat may cause that water to boil and result in eruptions.

Gas seeping out of cracks

Besides the ash, which is not poisonous, residents have to worry about choking on sulfur dioxide.
Coombs said the gas is coming out of 21 fissures, or cracks in the ground, caused by the volcano.
Levels of the toxic gas are dangerous in some places, Hawaii County officials said.

"Severe conditions may exist such as choking and inability to breathe," the county's Civil Defense Agency said. "This is a serious situation that affects the entire exposed population."

Officials warned residents to leave the area and get medical attention if they're affected by the gas.

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