Scientists from the U.S. Geological Survey Hawaiian Volcanoes Observatory said this morning’s explosion at the main ocean entry near Ahalanui was the typical result of an interaction between hot, molten lava and cool seawater.

The “littoral explosion” hurled hot lava rocks toward a boat, with “at least one being quite strong,” according to the USGS. The Hawaii County Fire Department said one of the explosions sent a “lava bomb” of molten lava on the metal roof of a Lava Ocean Tours boat and injured 23 passengers shortly after 6 a.m. today.

Four individuals were transported by ambulance to Hilo Medical Center, according to the state Department of Land and Natural Resources. The worst case was a 20-year-old woman who suffered a traumatic, leg injury.

What is unprecedented with this lava flow, said USGS geologist Janet Babb, is the high volume of lava generated from fissure 8, as well as the more shallow topography near Ahalanui warm ponds, the site of the ocean entry.

“Based on what’s going on near Ahalanui, the ocean is probably not all that deep in that area,” said Babb during this afternoon’s press conference by phone.

When molten, 2,000-degree lava makes contact with seawater, a steam explosion results, blasting fragments of molten lava, as well as solidified or semi-solidified lava fragments up in the air. Some of these pieces can be larger than a breadbox, said Babb.

For decades, including the ‘61g flow’ of 2016, Babb said the observatory has recommended a safe zone of 300 meters from the ocean entry, both landward and seaward. The U.S. Coast Guard also said in a press release that the safety zone currently stands at 300 meters, of 984 feet, from an ocean entry, with no exclusions.

The U.S. Coast Guard and state Division of Conservation and Resources Enforcement continue to investigate the incident.

Babb said the current volume of lava flowing out of fissure 8 is much higher, at about 50 to 100 cubic meters per second, compared to the ‘61g flow,’ where lava from Puu Oo pumped out about three to four cubic meters per second.

In addition, the Kamukona lava ocean entry had an off-shore topography that was very steep. The lava entering the ocean at Kamukona hit a steep slope, and was quickly carried down to deeper parts of the ocean, said Babb.

“Right now, conditions are ripe,” she said. “The amount of lava going into the ocean, the off-shore topography, are now producing these large, littoral explosions.”

During this morning’s overflight, USGS observed lava from fissure 8 continuing to erupt into the perched channel leading northeast from the vent. The channel was full, but not quite up to the rim. The southern margin of the flow was about a half-mile from Isaac Hale Park and the Pohoiki boat ramp.

Lava continued to ooze at several points along the flow along the ocean, currently about 3.7 miles wide.

Fissure 22 did not appear active this morning, but sounds were heard from it on Sunday night. A ‘collapse event’ with energy equal to a 5.4 magnitude earthquake occurred at Halemaumau Crater at 11:43 a.m. today, with no tsunami threat to Hawaii island.

Hawaii County Civil Defense warns the public that the ocean entry is a hazardous area because the interaction of lava with ocean creates flying debris from the explosive interaction, in addition to “laze,” a corrosive seawater plume plume laden with hydrochloric acid and fine, volcanic particles that drifts downwind and can irritate the skin, eyes and lungs.