A man was "hit with lava splatter" that can weigh as much as a refrigerator while sitting on their third-floor balcony in the first serious injury from the Hawaii volcano.

The development came as a stream of lava threatened to block a key Hawaii highway that serves as an escape route for coastal residents after fresh explosive eruptions from the Kilauea volcano.

Janet Snyder, a spokesperson for the Office of the Mayor, County of Hawaii, said: "A homeowner on Noni Farms Road who was sitting on a third-floor balcony got hit with lava spatter.

"It hit him on the shin and shattered everything there down on his leg," she said, adding that lava spatters "can weigh as
much as a refrigerator and even small pieces of spatter can kill."

As magma destroyed four more homes, molten rock from two huge cracks merged into a single stream, threatening to block escape routes.

It was expected to hit Highway 137 overnight if it kept up its rate and direction of flow, the County of Hawaii's Civil Defense Agency said.

A soldier of the Hawaii National Guard stands over a crack in a road in Leilani Estates during ongoing eruptions of the Kilauea Volcano in Hawaii (
Image:
REUTERS)
A massive fast moving lava flow consumes everything in its path, as the flames from the remnants of one home burns on the left, while it approaches another on the right (
Image:
REX/Shutterstock)
A fissure eruption near the intersection of Hinalo St and Pohoiki Road, consumes a home as eruptive activity continues on Kilauea's east rift zone, Pahoa, Hawaii (
Image:
REX/Shutterstock)
A lava flow is seen on a road in Pahoa (
Image:
REUTERS)
Molten rock from two huge cracks merged into a single stream, threatening to block escape routes (
Image:
REUTERS)
Journalists and soldiers of the Hawaii National Guard document road damage in Leilani Estates (
Image:
REUTERS)
Several fast moving rivers of lava threaten homes and property as they make their way to the sea (
Image:
REX/Shutterstock)

Authorities are trying to open up a road that was blocked by lava in 2014 to serve as an alternative escape route should
Highway 137 or another exit route, Highway 130, be blocked, Jessica Ferracane of the National Park Service told reporters.

The park service is working to bulldoze almost a mile of hardened lava out of the way on nearby Highway 11, which has
been impassable, she added.

The Hawaii National Guard has warned of mandatory evacuations if more roads become blocked.

Lava flows near a house on the outskirts of Pahoa (
Image:
REUTERS)
A man takes a photo of a lava fountain from a Kilauea volcano fissure on Hawaii's Big Island (
Image:
Getty Images North America)
Lava erupts on the outskirts of Pahoa (
Image:
REUTERS)
Wide-angle camera view captures the entire northern portion of the Overlook crater (
Image:
Barcroft Media)

For weeks, geologists have warned that hotter, fresher magma from Kilauea's summit would run underground and emerge some 25 miles east in the lower Puna district, where older, cooler lava has already destroyed 44 homes and other structures.

"Summit magma has arrived," U.S. Geological Survey scientist Wendy Stovall said on a conference call with reporters.

"There is much more stuff coming out of the ground and its going to produce flows that will move much further away."

A crack in the road is seen in Pahoa (
Image:
REUTERS)
A person is silhouetted against the light from lava in Pahoa (
Image:
REUTERS)
Lava erupts from a Kilauea volcano fissure near a home at dawn on Hawaii's Big Island (
Image:
Getty Images North America)
Lava from this fissure eruption isolated a community of 40 homes, and evacuations became mandatory (
Image:
REX/Shutterstock)
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Fountains of bright orange lava were seen spouting at least 20-feet high, and spewing rivers of molten rock on Saturday.

Carolyn Pearcheta, operational geologist at the Hawaii Volcano Authority, told reporters that hotter and more viscous
lava could be on the way, with fountains spurting as high as 600 feet, as seen in a 1955 eruption.

"We've seen the clearing out of the system," she said. "We call that the 'throat clearing' phase."

At the volcano's summit, another large explosive eruption occurred around midnight, sending up a nearly two-mile-high ash plume (10,000 feet), according to the Hawaiian Volcano Observatory.

Ash erupts from the Halemaumau Crater near the community of Volcano (
Image:
REUTERS)
People watch ash erupt from the Halemaumau Crater near the community of Volcano (
Image:
REUTERS)
Lava flows through forest on the outskirts of Pahoa during ongoing eruptions (
Image:
REUTERS)
Fountains of bright orange lava were seen spouting at least 20-feet high, and spewing rivers of molten rock on Saturday (
Image:
REUTERS)

There was another explosion around 4 p.m. (10 p.m. ET), according to a Reuters reporter.

Scientists expect a series of eruptions from Kilauea, one of the world's most active volcanoes, that could spread ash and
volcanic smog across the Big Island, the southernmost of the Hawaiian archipelago.

That could pose a hazard to aircraft if it blows into their routes at around 30,000 feet (9,144 meters).

An ash plume rising following a massive volcano eruption on Kilauea volcano (
Image:
AFP)
A bird rests on a wire as lava from a Kilauea volcano fissure erupts on Hawaii's Big Island (
Image:
Getty Images North America)
Lava from a Kilauea volcano fissure erupts on Hawaii's Big Island (
Image:
Getty Images North America)
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There had been no reported injuries or deaths since the eruptions began on May 3.

Around 2,000 residents of Leilani Estates and Laipuna Gardens housing areas near Pahoa, about 30 miles (48 km) south
of Hilo, were ordered to evacuate due to at least 22 volcanic cracks that have opened.

Many thousands more residents have voluntarily left their homes due to life-threatening levels of toxic sulfur dioxide gas
spewing from vents in the volcanic fissures.