A plume of ash rises from Kilauea Volcano on May 3, 2018.
A plume of ash rises from Kilauea Volcano on May 3, 2018. USGS/Reuters

Volcanic eruptions like this can have a long-term—and sometimes permanent—impact on the communities that live there.

An ordinary American neighborhood has been evacuated ... because of a volcano.

On Thursday evening, Hawaii County ordered roughly 1,500 people near Pahoa, Hawaii, to leave their homes. The cause: A new lava fissure opened on Kilauea, a massive volcano in the southeast of the state’s Big Island. Lava from the fissure has come within several hundred yards of homes, threatening two subdivisions in the area. The fissure is also releasing toxic amounts of sulfur dioxide, according to Hawaii News Now.

A ponderous lava flow, moving through trees: It’s not exactly the sudden explosion that many Americans imagine when they hear the words volcanic eruption. But for exactly that reason, “it’s the kind of eruption that makes volcanologists nervous,” says Erik Klemetti, a volcanologist at Denison University.

Right now, the U.S. Geological Survey is still trying to understand the new fissure. If the lava flow stabilizes, residents could return to unharmed homes in a week or two. But if the new fissure follows a pattern set by other fissures on Kilauea, then the evacuation could “last for a prolonged period of time,” says Klemetti.

And “because it tends to fall out of public view, it can have a long-term impact on the communities,” he says. It’s happened several times before.

Members of the National Guard stand outside the entrance to an evacuated subdivision on Friday. (Marco Garcia/AP)

Kilauea is “unlike a lot of volcanoes because it’s a shield volcano”—meaning it has long, sloping sides—“and because it’s huge,” Klemetti says. “The scale of it is hard to comprehend until you’re on the volcano and you realize you can drive 20 miles and still be on the volcano.”

Kilauea has also “been pretty much in eruption for the last 35 years,” he adds. There are long-simmering lava lakes within its crater, and every so often new flows appear within Volcanoes National Park. But the new fissure has appeared much farther down the face of the volcano, in an area where there hasn’t been an eruption since the 1950s. In that time, trees and wildlife in the area have largely recovered. And developers have constructed at least two subdivisions nearby, called Leilani Estates and Lanipuna Gardens. Authorities ordered residents to evacuate both of them on Thursday.

Even if the eruptions of the last century were destructive, they haven’t been sudden or violent. Mika McKinnon, a geophysicist and disaster researcher, says that Kilauea does not suddenly explode like Mount St. Helens did in 1980. This has to do with where and why the volcanoes formed on the Earth—and also with the chemistry of the Earth’s tectonic plates, those great, drifting chunks of rock that form the surface of the world.

There are two types of tectonic plates: oceanic and continental. Oceanic plates are denser than continental plates, and don’t contain very many granitelike rocks, such as quartz or silica. When an oceanic plate melts—which is what’s happening now at Kilauea—it tends to form lava that is very runny. This type of lava can get very hot, but it’s so liquid-like that any gas just bubbles out of it. “It creates more gentle eruptions and these beautiful, dome-shaped volcanoes,” says McKinnon.

Continental plates, on the other hand, are rich in silica. When a continental plate melts—which is what fuels the Pacific Ring of Fire’s volcanoes—it creates lava that is stickier and even slower moving. So gas, instead of escaping into the atmosphere, gets trapped inside this kind of lava. And when it escapes, “it’s going to be a much more violent or explosive eruption,” says McKinnon.

The main concern for the just-evacuated residents of Leilani Estates, in other words, are slow flows of runny, superhot lava. “Hawaiian volcanoes can be extremely deadly, but it’s a hazard you can walk away from,” McKinnon says. “That’s how you get these really close-up drone videos of them, or photos that show people in neighborhoods with lava in the background. Those people will still be able to escape.”

Since a week of small earthquake swarms preceded the new fissure, many residents had time to pack their cars with major possessions and valuables, McKinnon says.

But houses can’t be hauled away. Two months ago, Klemetti’s research brought him to the same area as the new lava fissure. He has driven through Leilani Estates. “It’s just these houses dispersed through the trees. It’s a standard neighborhood,” he says. “These are not big, fancy, expensive houses, just the houses of average Hawaiians on the whole.”

“There’s this perception that Hawaii is all luxury, but this part of Hawaii tends to be people who are of average means or below-average means,” he adds.

Lava moves through the Leilani Estates subdivision near the town of Pahoa on Hawaii’s Big Island. (USGS/AP)

Which is why he is so nervous. In the past few decades, at least two subdivisions have been destroyed by Kilauea’s lava flows. One of them, Royal Gardens, had to be completely abandoned. Some residents of Kalapana Gardens, another development, eventually rebuilt their houses on the black, fissured aftermath of a lava flow after an eruption in 2011.

These eruptions can have “a dramatic effect on people’s lives,” Klemetti says. “If you get displaced by a lava flow, you may get displaced permanently.”

For now, it’s too early to know what the new lava fissure will mean for Leilani Estates. “It might be at least months of lava flows. Or it might just end now,” Klemetti says. “There’s a likelihood that this is the establishment of a new lava-flow field and that it might be in action for a while.”

“There’s not a lot of words to capture a gradual, slow-developing disaster like this,” he tells me. “Even disaster doesn’t seem like the right word, because it implies something instantaneous or rapidly developing—and this isn’t. But it will still be quite a disaster for people whose homes are taken out.”

This post originally appeared on The Atlantic.

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