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.

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.”