A volcanic explosion injured more than 20 people on a sightseeing boat that was watching lava flowing from Hawaii's active volcano into the ocean.

The boat was off the island's Eastern coast when the volcano sent what officials said was a basketball-sized "lava bomb" through the boat's roof and peppered it with volcanic debris.

The Hawaii County Civil Defence Agency said that of the 23 injured, four were taken by ambulances to the hospital, including a 20-year-old woman who was seriously injured after fracturing her femur bone.

Photos posted by Hawaii county officials showed a manhole cover-sized hole in the boat's metal roof. A woman who answered the phone at Lava Ocean Tours confirmed that it was the company's boat that suffered the explosion but declined to comment further. Video posted by reporters on social media showed a large explosion of lava into the air.

The tour boats typically depart from Hilo, the county seat for the island in the early morning for three hours tours that cost US$225-$250.

"Lava Ocean Tours Big Island lava boat tours are an exciting way to experience the molten hot lava entering the sea," the company says on its website. "See, Hear & Feel the heat from your front row seat onboard one of our world class catamarans."

According to Hawaii News Now, the boat had left with 49 passengers and three crew members on board and had been touring the area around Kapoho, an unincorporated section near the island's coastline where the lava explosions from ongoing eruptions at the Kilauea volcano have destroyed hundreds of homes and completely filled in a popular tide pool bay last month. But with timely evacuations and public attention, few have been injured seriously.

A video posted by Hawaii News Now reporter Mileka Lincoln taken from another lava tour boat captures a massive lava explosion in the water that she said affected the other boat.

"It's unclear how close Turpin's tour boat was to the Kapoho lava ocean entry when the explosion happened, but eyewitnesses report the boat appeared to be 'very close,' " she wrote, noting that the Coast Guard recently changed the mandatory safety perimeter required around ocean lava entries from 100 to 50m for the tour operators.

#LeilaniEstatesEruption #KilaueaVolcano LATEST: @HiloMedCenter confirms 13 patients came in with injuries from #lavabomb in #lavaexplosion; 12 treated & released for burns/abrasions; 1 airlifted to Oʻahu with serious pelvic/leg wound https://t.co/8EDWGQsk2Q @HawaiiNewsNow #HINews pic.twitter.com/5guiry4sJv

— Mileka Lincoln (@MilekaLincoln) July 17, 2018

State officials and police said they were investigating the incident. The US Geological Survey volcanologist Wendy Stovall told USA Today that lava bombs can result from the interaction between lava with the ocean, as water turns into steam and causes the lava to explode.

Government officials have also been warning about the dangers of laze, potentially dangerous mix of hydrochloric acid, steam and tiny glass particles in the air that results from the interaction of the lava and the water.

Shane Turpin, the captain of the boat in the incident, told AP that the group had been about 250m away from the lava.

"As we were exiting the zone, all of a sudden everything around us exploded," he said. "It was everywhere."

He said that the crew didn't pick up on any warning signs before the explosion.

The volcano's eruption on Hawaii has been an ongoing emergency on the island since May, sending thousands of residents from their homes, destroying hundreds of structures, and captivating the world through arresting images showcasing the earth's raw destructive power. The lava also created a small island metres off the coast this week.