Residents in parts of Hawaii are being warned that lava may cross a highway in the coming hours as a flow heads towards the ocean.
The lava had mostly been spattering up since a fissure opened earlier this month with nearly 2,000 people forced to evacuate as lava claimed 40 structures.
But the flow changed dramatically on Friday afternoon as one fissure ramped up, destroying four more homes and isolating residents, some of whom had to be airlifted 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 US Geological Survey (USGS) volcanologist.
By Saturday morning, two of 22 fissures had merged, creating a wide flow advancing at rates of up to 274 metres per hour.
Aerial footage from the USGS showed fast-moving lava advancing to the southeast. The flow was 2.4 kilometres from the ocean, scientists said.
If lava threatens main roads, more people will be told to prepare for voluntary evacuation.
A lava flow was less than a mile away from Highway 137 and would reach it in a matter of hours, officials warned on 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 on Friday and some of them needed to be airlifted to safety.
"They shouldn't be in that area," said County Managing Director Wil Okabe.
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 on Friday, sending an ash cloud 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 on Thursday, which emitted ash and rocks thousands of feet into the sky. No-one was injured and there were no reports of damaged property.
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."