A projectile from the Kilauea volcano in Hawaii struck a boat carrying people watching lava from the two-month-old eruption, injuring 23, the fire department said.
It said 13 of the victims were hospitalised and the remainder were treated at a harbour when the boat engaged in a "lava tour" returned to the Big Island, also known as Hawaii.
Lava flowing into the Pacific is a spectacular sight, producing a foggy haze known in Hawaii as "laze".
One woman was in serious condition with a broken leg.
The total number of people on the boat was not immediately known, the Hawaii County Fire Department said.
In the early morning incident a "lava bomb" punctured the roof of the boat and damaged a railing, the department said. It gave no further details.
"It was an explosion, basically," said Janet Snyder, a spokeswoman for the county mayor, told the Tribune Herald newspaper. "It punctured a hole right through the roof of the boat," she said.
Ms Snyder described the boat as "covered with lava".
The eruption has destroyed hundreds of homes since it began on 3 May.
Kilauea is one of the world's most active volcanoes and one of five on the island.
A passenger who was on board the boat at the time, Will Bryan, captured the exact moment on video.
'Lava bomb' strikes boat in Hawaii (Courtesy Will Bryan) pic.twitter.com/8kwJhamRXs
— RTÉ News (@rtenews) July 17, 2018
