PAHOA, Hawaii (Reuters) - A stream of lava blocked a Hawaii highway on Sunday that serves as an escape route for coastal residents, while the first known serious injury was reported from fresh explosive eruptions from the Kilauea volcano.

A homeowner on Noni Farms Road who was on a third-floor balcony had his leg shattered from his shin to his foot when hit by lava spatter, said Janet Snyder, a spokesperson for the Office of the Mayor, County of Hawaii.

She added that lava spatters “can weigh as much as a refrigerator and even small pieces of spatter can kill.” No other information was immediately available.

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

The erupting lava, which can reach a blistering 2,000 degrees Fahrenheit (1,093 degrees Celsius), crossed Highway 137 shortly before midnight local time (1000 GMT), Hawaii’s Civil Defense Agency said, and sent lava flowing into the ocean.

That prompted warnings of laze -- clouds of hydrochloric acid and steam embedded with fine glass particles formed when hot lava hits ocean water.

CLEARING A PATH

Authorities were trying Sunday to open up a road that was blocked by lava in 2014 to serve as an alternative escape route, 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.

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

But officials went house-to-house in the area to urge more residents to flee, Snyder said, though no head count of the new evacuation was available early Sunday.

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.”

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.”

Slideshow ( 13 images )

NEW EXPLOSIVE ERUPTION

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.

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).

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.