If Kilauea's lava lake falls below the water table, the results could be explosive

Here’s how that steam-powered boom could happen.

black lava steaming with an orange skylight in the middle, surrounded by forests
An aerial view of lava flows in Leilani Estates.U.S. Geological Survey

The Big Island of Hawaii's Kilauea volcano has a lot going on.

Down one of its sides (or flanks), plumes of gas steam out of cracks, known as fissures, directly under Leilani Estates, an orderly residential area. Houses, cars, and beloved mailboxes have quickly become engulfed in cooling lava flows as the ground trembles with seismic activity. Residents evacuated their homes, fleeing with pets and belongings in tow to nearby shelters. A geothermal plant in the area hastily removed stocks of flammable material as the heat of the earth crept closer to their facility than comfort allowed.

But that's only part of the story. At the summit of Kilauea, about 25 miles away from the fissures in Leilani Estates, the surface of a long-standing lava lake plunged deep into a vent, dropping 970 feet below the floor of the crater at the top. This swift and tunneling descent into the earth raises the potential for a different kind of eruptive event happening—one far more explosive than this volcano has seen in over 90 years.

The activity at Kilauea is driven by an underground reservoir of magma–molten rock hanging out in vents and chambers below the volcano. As the magma moves underground it pushes the earth around it, creating earthquakes and eruptions when it finally breaks through to the surface.

Geologists monitor the magma’s movement by tracking earthquake activity in the area and measuring changes in the slope of the volcano, which can shift as the magma inflates or deflates the overlying rock. But they’ve got another window too—that lava lake in the crater. Before the most recent rounds of eruptions started, the levels of the lava lake rose, spilling lava out on the floor. Now, they’ve dropped dramatically, as the magma underground found new outlets in the eruptions further downslope.

If the levels of the lava drop much further, they could head down below the water table where things could start to get interesting. Normally, the heat of the magma keeps water from intruding into the parts of the ground near the lava lake. But if the lava lake levels drop too far below the water table, then water in the suddenly non-magma adjacent areas would have an opportunity to inch closer into the cooling areas. If that happens, the water could interact with the lava, creating a ton of steam.

That steam alone is not hazardous, especially if the steam is forming near the surface of the lava. But that’s not the only factor at play here. The vent hosting the lava lake is fairly unstable, and it’s pretty common for rocks to fall into the lava, sending up plumes of volcanic ash (tiny rock particles). If enough rocks fall through, they could plug up the vent where the lava lake sits, barricading it below ground. That’s when the real action starts.

explosion
A "short-lived explosion" on May 9, 2018, caused by rocks falling into the lava lake. The lava lake is still above the water table at this point.U.S. Geological Survey

If that blockade gets erected while the lava is below the water table, all that steam could build up underground, increasing the pressure until suddenly....boom.

“It’s an unfortunate series of events that has to happen to form these large explosions” says Jessica Johnson, a volcanologist at the University of East Anglia, who spent two years studying Kilauea as a post-doctoral researcher.

Those rocks that bottled up the system will suddenly find themselves no match for the pressure built up below, and will rocket towards the sky, 12 ton champagne corks suddenly unstoppered. Like the top of a bottle of bubbly, they will arc upwards and then fall, but instead of burrowing into couch cushions or ricocheting off a ceiling fan, these boulders will slam into the surrounding earth, at a range of about one kilometer from the crater.

kilauea diagram
How an explosion at Kilauea's summit might occur.U.S. Geological Survey

“That explosion could be so energetic that everything in its path gets ripped apart. It could be rocks that had been previously fallen from the vent and fallen into, it could additionally rip more blocks away from the vent,” Johnson says.

Marble-sized rocks will fly further, and a tower of ash could zoom up into the sky.

In a press conference Thursday night Jessica Ferracane, a public relations specialist at Hawaii Volcanoes National Park, said that most of the park (which hosts both Kilauea and Mauna Loa) would be closed Friday “in anticipation of the possibility of an explosive steam event here at the summit of Kilauea.” At the time, they were not sure when they would re-open. “We are not expecting this to be a life-threatening event unless you’re in a closed area. So stay out of the park,” Farracane said.

The ash could fall over a larger area of the southern part of the island, depending, of course, on wind patterns and weather conditions. The USGS encourages people concerned about ash to look at some guidelines to better prepare for that event. And researchers are quick to caution, while there is a possibility that this might happen, there's no guaranteeing when, or even if it will.

While Kilauea's eruptions are generally more effusive—slowly spilling lava onto the surface, there is precedent for explosive events at the volcano. A 1924 eruption had an explosive phase in the crater. Luckily, in that case, no one was hurt, but an earlier explosive eruption, in 1790, killed about 80 people near the summit.

While researchers were present at the 1924 event, and geologists have intensely monitored the subsequent eruptions, there’s still plenty to be learned, says Johnson.

“In terms of the fissure eruptions down at Leilani Estates, the small earthquakes and ground deformation will give us information about where the magma is going, whether its being stored underground, what triggers it coming to the surface,” Johnson says.

At the summit, if an explosion happens like it did in 1924, there’s opportunity to apply new equipment and analysis to the event. “That's going to be a completely new data set for us and we’ll learn an awful lot about the way these explosions are triggered,” Johnson says.

Then, there are the earthquakes. Towards the start of this eruption, a 6.9 magnitude earthquake struck the area, the largest since a magnitude 7.7 hit in 1975. This new one occurred in an area where the weight of all the volcanic rock that makes up the Kilauea volcano pushes down on the underlying oceanic crust, and the boundary between the two is particularly fascinating for geologists.

“The interaction between the volcano and this boundary is a really important point of research as well. We want to learn about the evolution of this island, how it's going to grow, and how large earthquakes like this are triggered,” Johnson says.

There’s a lot still to learn about this eruption, but volcanologists like Johnson are acutely aware that this event, while fascinating, is still tragic. “As much as this is a very interesting episode, it's really unfortunate that its happened in the middle of a residential estate where people’s homes and livelihoods are being destroyed and there’s nothing they can do about it,” Johnson says.