Will the Eruptions From Hawaii's Kilauea Volcano Turn Explosive? Get the Facts.

Scientists are warning of "ballistic rocks" hurled from the volcano's crater.

Volcanoes 101

Will the Eruptions From Hawaii's Kilauea Volcano Turn Explosive? Get the Facts.

Scientists are warning of "ballistic rocks" hurled from the volcano's crater.

Volcanoes 101

First, it was oozing lava streaming into residential areas on Hawaii's Big Island. Now, it looks as if Kilauea might be ready to explode.

Overnight, the U.S. Geological Survey issued a warning that the volcano could turn explosive, as falling lava levels inside the crater bring hot rock closer to cool groundwater. Already, the active peak has seen smaller scale bursts, as rockslides fall into the volcano's crater and create plumes of ash.

Until recently, the biggest threats from the volcano were this slow-moving lava inundation and inhaling the hazardous sulfur dioxide released from the molten rock. Since the volcano began actively erupting last week, more than a thousand people have been evacuated, and at least 26 homes have been destroyed. Dramatic video footage even showed lava engulfing a parked car.

But a large crater called Halema'uma'u sits near the center of Kilauea, and it's filled with a lake of bright red and yellow lava. Since late April, that lake has been receding into its magma column, the vertical passage that brings magma from Earth's interior up to the crater. Volcanologist Janine Krippner suspects the magma is either moving to a different storage area in the volcano or to the East Rift Zone, where fissures have been oozing lava, but the magma's movements can't be predicted with complete certainty. (Confused about the difference between magma and lava? Here are the facts.)

If the lava lake recedes far enough, it will move below the water table, allowing water to enter the crater and creating steam. More rocks falling into the crater could then form a cap over the steam, eventually building up so much pressure that the rock cap explodes.

“We are looking at the possibility of a steam-driven eruption at the summit, which is not as large as magmatic eruptions that we see at volcanoes like Mount St. Helens,” Krippner says. “The hazards are ballistic rocks around the summit area, and ash fall can occur in areas depending on wind strength and direction and the size of the eruption.”

Such dramatic action from Hawaii's famously gentle volcano is very rare. The peak has not seen this kind of major explosive eruption since 1924, when a steam-powered eruption caused more than 50 explosions over two and a half weeks. The blasts sent chunks of rock weighing as much as 14 tons hurling through the air.

Similarities between then and now have scientists concerned, but it's tricky to predict when and with how much force Kilauea might explode in this fashion.

“The magma is a constantly changing system ... we cannot see below the surface to actually look at what is going on,” says Krippner. “We see what is happening now, and we know a bit about what happened the last time ... so we can give warnings about what might happen next, but not with certainty.”