A look at the science behind what caused Monday's lava explosion

KAPOHO, BIG ISLAND (HawaiiNewsNow) - What caused the explosion that rained down rocks and rubble onto a crowded tour boat on Monday?

The U.S. Geological Survey calls the event a littoral explosion.

"Littoral explosions occur when hot lava instantaneously converts sea water to steam. Results are dangerous if too close!!" the agency exclaimed in a tweet after Monday's incident, where molten shrapnel injured 23 people.

The agency says littoral explosions happen when molten lava and cool seawater mix, creating a steam explosion that can hurl rock fragments hundreds of meters away.

"When you see a littoral explosion, you can sometimes see the bright red, yellow-red incandescent molten material as well as a lot of black debris," Janet Babb, Geologist with the USGS said.

A video recorded by tour boat passenger Laci Mayton on Monday briefly shows a basketball-sized molten rock after it landed in the damaged vessel.

Read more: More than 20 injured after explosion hurls 'basketball-sized' lava bomb onto tour boat

The USGS says these types of fragments can fly as far as 300 meters. How far away the vessel was from the actual entry point during Monday's explosion is still being investigated by officials, but licensed tour boat operators are legally allowed to get as close as 50 meters to a lava ocean entry.

"Anytime lava enters the ocean, there is the potential for these explosions," Babb said.

But if lava has already been flowing into the ocean for nearly two months, what is difference during a littoral event?

Babb says there is an explosive interaction every time lava meets the ocean, but some are just too small to notice. Other times, when the conditions are right, these explosive interactions can be larger.

"I don't know that we have a complete handle on exactly what those conditions are," Babb said.

One thing is apparent at the moment: the conditions around Kapoho are right for larger explosions.

"The amount of lava, the offshore topography, and type of lava going in, it's all just right to where it's producing these larger littoral explosions," she said.

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