UPDATE: 8 P.M.

PAHOA >> Fast-moving lava crossed a road and isolated about 40 homes today in a rural subdivision below Kilauea volcano, forcing at least four people to be evacuated by county and National Guard helicopters.

Late this afternoon lava emerged from fissure 20 and traveled southeast where it crossed Pohoiki Road. Estimates from Hawaii County Fire Department aerial video at 6:30 p.m. indicate advance rate of 300-400 yards per hour; this rate may change with time.

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

The homes were isolated in the area east of Leilani Estates and Lanipuna Gardens — two neighborhoods where lava has destroyed 40 structures, including 26 homes, over the past two weeks.

Officials were assessing how many people were still in the newly threatened area. They were advising people to shelter in place and await further instructions.

Due to the volcanic activity, the following policies are in effect:

>> Pohoiki Road is closed.

>> Four people were safely evacuated by County and National Guard helicopters.

>> Residents from Isaac Hale Beach Park to Kalapana are advised to prepare for voluntary evacuation should Highway 137 become threatened.

>> For those evacuating, the Pahoa Community Center, Keaau Community Center, and Sure Foundation Church are open. Food will be provided and the shelters are pet friendly.

>> Highway 137 and 130 are open.

—

For more information on the hazards of volcanic ash and vog, go to volcanoes.usgs.gov and vog.ivhhn.org.

—

County officials have been encouraging residents in the district to prepare for potential evacuations.

Edwin Montoya, who lives with his daughter on her farm near the site where lava crossed the road and cut off access, said he was at the property earlier in the day to get valuables.

“I think I’m lucky because we went there this morning and we got all the batteries out, and all the solar panels out, about $4,000 worth of equipment,” he said. “They have to evacuate the people that are trapped up there right now in the same place that we were taking pictures this morning.”

He said no one was on his property, but his neighbor had someone on his land.

“I know that the farm right next to my farm . he’s got somebody there taking care of the premises, I know he’s trapped,” Montoya said.

Montoya said the fissure that poured lava across the road opened and grew quickly.

“It was just a little crack in the ground, with a little lava coming out,” he said. “Now it’s a big crater that opened up where the small little crack in the ground was.”

Experts are uncertain about when the volcano will calm down.

The Big Island volcano had an explosive eruption at its summit Thursday, sending ash and rocks thousands of feet into the sky. No one was injured and there were no reports of damaged property.

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.

A new lava vent — the 22nd such fissure — was reported Friday 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.

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 possible.

“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. 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.

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.

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

The area affected by lava and ash is small compared to the Big Island, which is about 4,000 square miles. Most of the Big Island and the rest of the state’s island chain is unaffected by the volcanic activity on Kilauea.

State and local officials have been reminding tourists that flights in and out of the Big Island and the rest of the of the state have not been impacted. Even on the Big Island, most tourist activities are still available and businesses are open.

6 P.M.

A fast-moving pahoehoe lava flow is threatening about 40 homes in a new area southwest of Leilani Estates, prompting authorities to use helicopters to check the area to determine if people need help.

The new flow crossed Pohoiki Road near Malama Ki Place, and police and fire personnel are working with Hawaii Army National Guard soldiers to secure the area and stop anyone from entering.

People in the area are asked to stay where they are and wait for further instructions, according to civil defense.

The new flow is about 3/4 of a mile east of Lanipuna Gardens along Pohoiki Roads, and the lava flow appears to be on a course that would direct it toward the Puna coastline north of Pohoiki.

5 P.M.

The Hawaiian Volcano Observatory said there have been no explosions and no earthquakes greater than magnitude 3.5 in the summit area in the past day. Background seismic levels have increased slowly over the course of the day.

Volcanic gas emissions at the summit remain high.

Fissure 17 is actively spattering and the flow is active but is not covering new ground. Fissures 18 and 20 are active, and the flow from fissure 18 has traveled approximately 0.6 miles in a southeast direction. An area 50 to 100 yards wide, parallel to and north of the line of fissures between Highway 130 and Lanipuna Gardens, is disrupted with many cracks. This long cracked area is currently being filled by pahoehoe lava flows from fissures 20 and 21.

Fissure 15 was active and produced a lava flow that crossed Pohoiki Road between Leilani and Hinalo Streets.

Volcanic gas emissions remain elevated throughout the area downwind of the fissures.

PREVIOUS COVERAGE

Hawaii County Civil Defense officials raised the number of structures destroyed by lava in the Leilani Estates subdivision to 40 today, with two more are being threatened.

It is unclear how many of the structures are actual houses since the data is based on aerial views. The latest estimate is an increase from earlier reports of 37 structures, including at least 26 homes, destroyed by lava.

Officials also said that a 22nd fissure opened in Leilani Estates overnight and that several fissures in the Lower Puna subdivision are spattering lava as the latest outbreak from Kilauea volcano’s 35-year ongoing eruption entered its third week today.

Fissure 17 is more active and providing pahoehoe (or more fluid) flows, Civil Defense officials said this morning. New fissures 21 and 22 are also producing pahoehoe. Fissures 15, 16 and 20 are active with steam coming out, but not flowing.

The tip of the actual lava intrusion is now between 45 and 80 yards below the surface underneath the depressed area, so there is now generally more fast-moving lava, officials said. It is located just north of the fissure area and south of the Puna Geothermal Venture power plant. A‘a is absorbing much of the newer pahoehoe loads, so it has not been advancing downslope, officials said.

Two GPS receivers in the Leilani Estates-Lanipuna Gardens subdivisions show an extension within the rift zone by 2.5 inches a day, so it is not moving fast, Civil Defense said.

In other Kilauea eruption developments:

>> Highway 130 remains open this morning, although there is steaming and cracks near it. Checkpoints along Highway 137, 132 and 130 are limited to resident access only.

>> Increasing sulfur dioxide levels led officials to advise Leilani Estates evacuees to be alert to possible decreased access to their properties. Evacuees can contact Civil Defense for the possibility of an escort if access is limited. Access is still being denied for residents of Lanipuna Gardens due to high levels of dangerous volcanic gases.

>> Pahoa Post Office has reopened.

>> The summit at Halemaumau Crater has been quiet since Thursday morning, but seismicity (or the frequency of earthquakes) is building slowly, officials said today. A radar image overnight of the summit caldera shows an enlargement of the summit eruptive vent on the floor of Halemaumau from about 12 acres on May 5 to about 34 acres on May 17.

>> Officials also said that free masks for ash protection will be distributed between 1 p.m. and 7 p.m. today in the Cooper Center in Volcano and the Ocean View Community Center. Masks do not protect against volcanic gases or vapors but will filter ash. The masks were distributed in Puna an Kau, starting Thursday, after a large, but short-lived, explosive eruption at Halemaumau Crater in Kilauea’s summit, which sent ash 30,000 feet in the atmosphere.

The Associated Press contributed to this report.