The unnamed man was hit by a “lava spatter” as he sat on the balcony of his property in the town of Pahoa on Saturday.
Authorities have warned that the chunks of molten rock can weigh as much as a refrigerator and that even small pieces can kill.
“A homeowner who was sitting on a third-floor balcony got hit with lava spatter,” said Janet Snyder, a spokesperson for the Office of the Mayor, County of Hawaii. “It hit him on the shin and shattered everything there down on his leg.”
Magma destroyed four homes on Saturday as molten rock from two fissures merged into a single steam, threatening to hit the nearby Highway 137 on Sunday, Hawaii's civil defence agency said.
Authorities are trying to clear around a mile of hardened lava from a road blocked by an eruption in 2014, in order to open up alternative escape route for coastal residents.
The Hawaii National Guard has warned mandatory evacuations would take place if more roads become blocked.
Several people were trapped when lava crossed a road on Friday, with some of the group needing to be airlifted to safety.
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.”
Kilauea, on Hawaii’s Big Island, began erupting in early May, causing damage to dozens of properties.
However, geologists now fear the situation is set to get worse for the island’s residents, with fresh magma predicted to emerge 25 miles away from the volcano’s summit in the lower Puna district.
“Summit magma has arrived,” US Geological Survey scientist Wendy Stovall said. “There is much more stuff coming out of the ground and it’s going to produce flows that will move much further away.”
Fountains of bright orange lava were seen blasting at least 20 feet into the air on Saturday, creating rivers of molten rock.
Experts are unsure as to when the volcano will calm down and more eruptions at the summit are possible. “We have no way of knowing whether this is really the beginning or towards the end of this eruption,” said Tom Shea, a volcanologist at the University of Hawaii. “We're kind of in this world of uncertainty.”
Additional reporting by AP
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