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

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

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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. Over the last few days, more cracks opened in the ground on Hawaii's Big Island, launching red hot lava and dangerous gas into the air. Other fissures, silent for days, have come back to life. Satellite images over the last two weeks show how much the volcano's main crater has expanded.

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 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.