Experts remained flummoxed Friday about when Hawaii's Kilauea volcano will calm down.

The Big Island volcano exploded at its summit Thursday, sending ash and rocks thousands of feet into the sky.

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Mike Guich holds a fresh piece of lava rock spit out of a fissure on Pohoiki Road, Friday, May 18, 2018, near Pahoa, Hawaii. (AP Photo/Marco Garcia)

Experts remained flummoxed Friday about when Hawaii's Kilauea volcano will calm down.

U.S. Air National Guardsmen stand near cracks on the road in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

Scientists said the eruption was the most powerful in recent days, though it probably lasted only a few minutes.

Lava shoots out of a fissure on Pohoiki Rd, Friday, May 18, 2018, near Pahoa, Hawaii. (AP Photo/Marco Garcia)

On Friday, Hawaii County officials warned residents that fast-moving lava was approaching an area east of those destroyed homes.

Lava shoot out of a fissure on Pohoiki Rd, Friday, May 18, 2018, near Pahoa, Hawaii. (AP Photo/Marco Garcia)

Earlier in the day, a new lava vent — the 22nd such fissure — was reported by county civil defense officials.

Lava crosses the road near Pohoiki Rd, Friday, May 18, 2018, near Pahoa, Hawaii. (AP Photo/Marco Garcia)

Much of the lava that has emerged so far may have been underground for decades, perhaps since a 1955 eruption.

Lava shoot out of a fissure on Pohoiki Rd, Friday, May 18, 2018, near Pahoa, Hawaii. (AP Photo/Marco Garcia)

Lava shoot out of a fissure on Pohoiki Rd, Friday, May 18, 2018, near Pahoa, Hawaii. (AP Photo/Marco Garcia)

U.S. Air National Guardsman Orlando Corpuz carries a gas mask as he walks over cracks on the road in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

"We have no way of knowing whether this is really the beginning or toward the end of this eruption," said Tom Shea, a volcanologist at the University of Hawaii. "We're kind of all right now in this world of uncertainty."

A military vehicles patrol in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

A military vehicles patrol in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

Edwin Montoya films as the lava shoots out of a fissure on Pohoiki Rd, Friday, May 18, 2018, near Pahoa, Hawaii. (AP Photo/Marco Garcia)

U.S. government scientists, however, are trying to pin down those signals "so we have a little better warning," said Wendy Stovall, a volcanologist with the observatory.

Wearing a gas mask, U.S. Air National Guardsman Orlando Corpuz uses an SO2 detector to monitor the air quality in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

Wearing a gas mask, U.S. Air National Guardsman Orlando Corpuz uses an SO2 detector to monitor the air quality in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

Toxic gases rise from cracks in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

The Federal Aviation Administration still has flight restrictions in place, but for a smaller area — up to 5,000 feet above sea level, and a 3-mile (5-kilometer) radius around the crater.

U.S. Air National Guardsman John Linzmeier looks at cracks as toxic gases rise near by in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

U.S. Air National Guardsman John Linzmeier looks at cracks as toxic gases rise near by in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

U.S. Air National Guardsmen, Orlando Corpuz, left, and John Linzmeier survey cracks on the road in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

Authorities have been measuring gases, including sulfur dioxide, rising in little puffs from open vents.

Abe Pedro watches lava shoot out of a fissure on Pohoiki Rd, Friday, May 18, 2018, near Pahoa, Hawii. (AP Photo/Marco Garcia)

Lindsey Magnani said Thursday that she and her family picked up some of the 2,000 masks distributed by county officials. She also closed all the windows at home to block air, which smelled heavily of sulfur.

Mike Guich holds a fresh piece of lava rock spit out of a fissure on Pohoiki Road, Friday, May 18, 2018, near Pahoa, Hawaii. (AP Photo/Marco Garcia)

Mike Guich holds a fresh piece of lava rock spit out of a fissure on Pohoiki Road, Friday, May 18, 2018, near Pahoa, Hawaii. (AP Photo/Marco Garcia)

Scientists said the eruption was the most powerful in recent days, though it probably lasted only a few minutes.

It came two weeks after the volcano began sending lava flows into neighborhoods 25 miles (40 kilometers) to the east of the summit, destroying 26 homes.

U.S. Air National Guardsmen stand near cracks on the road in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

U.S. Air National Guardsmen stand near cracks on the road in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

On Friday, Hawaii County officials warned residents that fast-moving lava was approaching an area east of those destroyed homes.

Hawaii County Civil Defense said police, firefighters and National Guard troops were securing the area and stopping people from entering.

About 40 homes in the area are now isolated.

Lava shoots out of a fissure on Pohoiki Rd, Friday, May 18, 2018, near Pahoa, Hawaii. (AP Photo/Marco Garcia)

Lava shoots out of a fissure on Pohoiki Rd, Friday, May 18, 2018, near Pahoa, Hawaii. (AP Photo/Marco Garcia)

Earlier in the day, a new lava vent — the 22nd such fissure — was reported by county civil defense officials.

Several open fissure vents are still producing lava splatter and flow in evacuated areas. Gas is also pouring from the vents, cloaking homes and trees in smoke.

The fresher, hotter magma will allow faster lava flows that can potentially cover more area, said Janet Babb, a geologist with the Hawaiian Volcano Observatory.

Lava shoot out of a fissure on Pohoiki Rd, Friday, May 18, 2018, near Pahoa, Hawaii. (AP Photo/Marco Garcia)

Lava shoot out of a fissure on Pohoiki Rd, Friday, May 18, 2018, near Pahoa, Hawaii. (AP Photo/Marco Garcia)

Much of the lava that has emerged so far may have been underground for decades, perhaps since a 1955 eruption.

Meanwhile, more explosive eruptions from the summit are anticipated.

Lava crosses the road near Pohoiki Rd, Friday, May 18, 2018, near Pahoa, Hawaii. (AP Photo/Marco Garcia)

Lava crosses the road near Pohoiki Rd, Friday, May 18, 2018, near Pahoa, Hawaii. (AP Photo/Marco Garcia)

Lava shoot out of a fissure on Pohoiki Rd, Friday, May 18, 2018, near Pahoa, Hawaii. (AP Photo/Marco Garcia)

Lava shoot out of a fissure on Pohoiki Rd, Friday, May 18, 2018, near Pahoa, Hawaii. (AP Photo/Marco Garcia)

"We have no way of knowing whether this is really the beginning or toward the end of this eruption," said Tom Shea, a volcanologist at the University of Hawaii. "We're kind of all right now in this world of uncertainty."

It's nearly impossible to determine when a volcano will stop erupting, "because the processes driving that fall below the surface and we can't see them." said volcanologist Janine Krippner of Concord University in West Virginia.

U.S. Air National Guardsman Orlando Corpuz carries a gas mask as he walks over cracks on the road in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

U.S. Air National Guardsman Orlando Corpuz carries a gas mask as he walks over cracks on the road in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

A military vehicles patrol in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

A military vehicles patrol in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

U.S. government scientists, however, are trying to pin down those signals "so we have a little better warning," said Wendy Stovall, a volcanologist with the observatory.

Thus far, Krippner noted, authorities have been able to forecast volcanic activity early enough to usher people to safety.

"If nobody warned people, and tourists were all over the place, that would have been devastating with those large rocks flying out — those are deadly," she said.

Edwin Montoya films as the lava shoots out of a fissure on Pohoiki Rd, Friday, May 18, 2018, near Pahoa, Hawaii. (AP Photo/Marco Garcia)

Edwin Montoya films as the lava shoots out of a fissure on Pohoiki Rd, Friday, May 18, 2018, near Pahoa, Hawaii. (AP Photo/Marco Garcia)

Wearing a gas mask, U.S. Air National Guardsman Orlando Corpuz uses an SO2 detector to monitor the air quality in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

Wearing a gas mask, U.S. Air National Guardsman Orlando Corpuz uses an SO2 detector to monitor the air quality in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

The Federal Aviation Administration still has flight restrictions in place, but for a smaller area — up to 5,000 feet above sea level, and a 3-mile (5-kilometer) radius around the crater.

The greatest ongoing hazard stems from the lava flows and the hot, toxic gases spewing from open fissure vents close to homes and critical infrastructure, said Charles Mandeville of the U.S. Geological Survey's volcano hazards program.

Toxic gases rise from cracks in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

Toxic gases rise from cracks in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

U.S. Air National Guardsman John Linzmeier looks at cracks as toxic gases rise near by in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

U.S. Air National Guardsman John Linzmeier looks at cracks as toxic gases rise near by in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

Authorities have been measuring gases, including sulfur dioxide, rising in little puffs from open vents.

For now, the ash seems to be more of a nuisance, rather than a severe threat. Some Big Island residents are covering their faces to keep from breathing small particles flying through the air, and some have found thin coatings of ash as winds blew the plume away from populated areas.

U.S. Air National Guardsmen, Orlando Corpuz, left, and John Linzmeier survey cracks on the road in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

U.S. Air National Guardsmen, Orlando Corpuz, left, and John Linzmeier survey cracks on the road in the Leilani Estates subdivision near Pahoa, Hawaii Friday, May 18, 2018. (AP Photo/Jae C. Hong)

Lindsey Magnani said Thursday that she and her family picked up some of the 2,000 masks distributed by county officials. She also closed all the windows at home to block air, which smelled heavily of sulfur.

Abe Pedro watches lava shoot out of a fissure on Pohoiki Rd, Friday, May 18, 2018, near Pahoa, Hawii. (AP Photo/Marco Garcia)

Abe Pedro watches lava shoot out of a fissure on Pohoiki Rd, Friday, May 18, 2018, near Pahoa, Hawii. (AP Photo/Marco Garcia)

Dr. Josh Green, a state senator who represents part of the Big Island, said the immediate health risk comes from ash particles in the air. Anyone with respiratory difficulties, such as asthma or emphysema, should limit exposure to the ash, he said.

"As far as me, I'm not sure whether it would do me any good to stay," said James Clapper, 70, an evacuated resident now sleeping in his truck. "The property out there has got huge, huge cracks. Is one of those cracks underneath the house? I don't know."