***drop banner when using***  Description:  Fissure 18 erupts from Kilauea near the town of Pahoa, Hawaii. Large chunks of rocks and magma fly with big explosions. Fissure 17 just opened yesterday afternoon and started a full eruption around 5am after multiple earthquakes overnight.  Credit:  Brandon Clement / LSM  Locations:  Pahoa, HI, USA    title: File uploaded by user duration: 00:00:00 site:  author:  published:  intervention: yes description:
Brandon Clement / LSM
***drop banner when using*** Description: Fissure 18 erupts from Kilauea near the town of Pahoa, Hawaii. Large chunks of rocks and magma fly with big explosions. Fissure 17 just opened yesterday afternoon and started a full eruption around 5am after multiple earthquakes overnight. Credit: Brandon Clement / LSM Locations: Pahoa, HI, USA title: File uploaded by user duration: 00:00:00 site: author: published: intervention: yes description:
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In this May 24, 2018 photo from the Federal Emergency Management Agency, lava erupts from a fissure in the Leilani Estates neighborhood near Pahoa on the island of Hawaii. Three lava flows from eruptions of Kilauea volcano are now flowing into the ocean off Hawaii's Big Island. Hawaii County officials say the third flow started pouring into the sea Thursday. Lava is spewing from a fissure in a rural community that's feeding the two other flows that are reaching the waves. (Grace Simoneau/FEMA via AP)
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PAHOA, HI - MAY 5: In this handout photo provided by the U.S. Geological Survey, lava errupts from a new fissure from Luana Street after the eruption of Hawaii's Kilauea volcano on May 5, 2018 in the Leilani Estates subdivision near Pahoa, Hawaii. The governor of Hawaii has declared a local state of emergency near the Mount Kilauea volcano after it erupted following a 5.0-magnitude earthquake, forcing the evacuation of nearly 1,700 residents. (Photo by U.S. Geological Survey via Getty Images)
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A plume of ash from the Kilauea volcano rose 12,000 feet into the air before dropping ash on sections of the Big Island of Hawaii, prompting officials Tuesday to warn residents to stay indoors and airplanes to stay away from the area.

“We’ve seen the waxing and waning (of volcanic activity over the past few days). It seems that the system up at the summit has been what we call somewhat open, relieving that pressure,” Michelle Coombs, a geologist who is the scientist-in-charge at the Alaska Volcano Observatory, told reporters in Hilo, Hawaii. “So, it intensified today but it wasn’t the big one.”

That doesn’t mean the “big one” won’t happen, Coombs said. The volcano could “plug up” as the magma level in the crater drops and the pressure below could build up, she said.

She said her team isn’t quite sure what caused Tuesday’s slightly more intense ash emissions.

US Geological Survey officials have said a phreatic eruption could happen at Halemaumau crater at the top of the Kilauea volcano. And it could send ash plumes as far as 12 miles from the summit crater, the Hawaiian Volcano Observatory said.

The USGS changed its aviation color code for the area to red.

“It’s very hazardous for aviation,” Coombs said.

Weather officials issued an ashfall advisory that ended around 6 p.m. local time.

According to the USGS, ash has fallen as far as 18 miles from the volcano.

Hazardous air

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.

Toxic sulfur dioxide seeps out of a street in Leilani Estates, Hawaii.
Sara Weisfeldt/CNN
Toxic sulfur dioxide seeps out of a street in Leilani Estates, Hawaii.

Since the Kilauea volcano erupted May 3, it’s been one nightmare after another for residents in the southeast part of the Big Island.

’It sounded like hammers in the dryer’

A fissure spews lava in Pahoa, Hawaii, after a massive eruption of the Kilauea volcano.
Mario Tama/Getty Images North America/Getty Images
A fissure spews lava in Pahoa, Hawaii, after a massive eruption of the Kilauea volcano.

Fissure 17 is the longest and has been shooting lava like a fountain and “sending spatter more than 100 feet into the air,” the Hawaiian Volcano Observatory said.

It is more than a mile long but its advance has slowed to 20 yards per hour, slightly more than a quarter-mile a day.

Coombs said the fissure had diminished in intensity, but officials said activity can go up just as quickly as it goes down.

Still, many of the fissures are emitting “quite a bit” of sulfur dioxide.

Two new fissures are small with not much lava coming from them, she said.

CNN’s Stephanie Elam said her heart started pounding as she approached one fissure.

“It sounded like hammers in the dryer,” she said. “The molten rock was such a deep vibrant orange that it looked technically altered. When the sulfur dioxide hit my lungs once, it took my breath away.”

The Hawaii State Department of Health is asking the general public to avoid any area with fissures because the gases emitted require special cartridge respirators.

Nina Bersamina heeded warnings to evacuate, saying residents are at the mercy of Pele, the Hawaiian goddess of fire.

“When you move to this volcano, or move to this island, you move into her house,” Bersamina said.

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More eruptions could follow

Scorching lava has already swallowed dozens of homes and vehicles. So far, at least 37 structures have been destroyed, and counting.

Lava engulfed houses in hard-hit Leilani Estates, Hawaii.
Sara Weisfeldt/CNN
Lava engulfed houses in hard-hit Leilani Estates, Hawaii.

In addition to the lava and toxic gases, now there are concerns about phreatic eruptions.

Those 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 US Geological Survey says. The intense heat may cause that water to boil and result in eruptions.

The lava lake in the crater has been dropping since May 2, which increases the chances for a phreatic explosion.

But it will be difficult to warn residents who may be in the path of such an eruption.

Janet Babb, a geologist from the Hawaii volcano observatory, said phreatic eruptions are “notoriously hard to forecast, and can occur with little or no warning.”

Scott McLean and Sara Weisfeldt reported from Pahoa, Hawaii. CNN’s Chris Boyette, Joe Sutton, Keith Allen and Chris Boyette contributed to this report.