Lava oozing out of cracks for two weeks in rural Hawaii neighborhoods took on new characteristics as fresher magma mixed with decades-old magma, sending a flow toward the ocean Saturday.

Since a first fissure opened in a community on May 3, lava was mostly spattering up and collecting at the edges of the cracks in the ground. Two neighborhoods with nearly 2,000 people were forced to evacuate as lava claimed 40 structures.

On Friday afternoon, the lava changed dramatically with one fissure ramping up and sending a flow across a road, destroying four more homes and isolating residents, some of whom had to be air-lifted to safety.

The change is attributed to new magma mixing with 1955-era magma in the ground, creating hotter and more fluid flows, scientists said.

"There's much more stuff coming out of the ground and it's going to produce flows that move further away," said Wendy Stovall, a U.S. Geological Survey volcanologist.

By Saturday morning, two of 22 fissures had merged, creating a wide flow advancing at rates of up to 300 yards (274 meters) per hour. Aerial footage from the USGS showed fast-moving lava advancing to the southeast. The flow was 1.5 miles (2.4 kilometers) from the ocean, scientists said.

In the background, the footage showed lava fountaining 328 feet (100 meters) high at one of the fissures. The fountains are created by vents closing, forcing magma to burst through a single outpoint, Stovall said.

If lava threatens main highways, more people will be told to prepare for voluntary evacuation.

A lava flow was less than a mile (kilometer) away from Highway 137 and would reach it in a matter of hours, officials warned Saturday afternoon. No one lives in its path and another highway remained open as an escape route, said Hawaii County spokeswoman Janet Snyder.

A handful of people were trapped when lava crossed a road Friday and some of them needed to be airlifted to safety.

"They shouldn't be in that area," said County Managing Director Wil Okabe. He wants people to heed evacuation warnings.

Edwin Montoya, who lives with his daughter on her farm near the site where lava crossed the road and cut off access, said the fissure 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 might calm down.

The Big Island volcano released a small explosion at its summit just before midnight Friday, sending an ash cloud 10,000 feet (3,048 meters) into the sky. The USGS's Hawaiian Volcano Observatory said eruptions that create even minor amounts of ashfall could occur at any time.

This follows the more explosive eruption Thursday, which emitted ash and rocks thousands of feet into the sky. No one was injured and there were no reports of damaged property.

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

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.

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."

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.

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 island and the rest of the Hawaiian chain is unaffected by the volcanic activity on Kilauea.

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

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Kelleher reported from Honolulu. Associated Press journalists Jae Hong and Marco Garcia in Pahoa, Sophia Yan and Audrey McAvoy in Honolulu, Becky Bohrer in Juneau, Alaska, Seth Borenstein in Washington, D.C., and Alina Hartounian in Phoenix contributed to this report.