The Slatest

When Will This Hawaiian Volcano Stop Erupting?

Plus: How fast is “fast-moving lava”?

Photo illustration: volcanic activity in Hawaii with a blue/white cutout. Photo illustration by Slate. Photo by Ronit Fahl/AFP/Getty Images.
Photo illustration by Slate. Photo by Ronit Fahl/AFP/Getty Images.

A “fast-moving lava flow” from Hawaii’s Kilauea volcano has destroyed at least 41 homes since the beginning of May. County officials made the rounds on Monday evening, advising local residents to flee the fast-moving front according to the Associated Press. Just how fast is “fast-moving” lava?

It’s surprisingly slow. Ten days ago, the U.S. Geological Survey reported that the Kilauea lava flow was advancing at rates of up to 300 yards per hour, or 0.17 miles per hour. Then on Memorial Day, USGS gave an update: “Vigorous fountaining resumed at Fissure 8, spawning a fast moving flow that moved north along Luana Street,” the agency said; by then its estimated speed had dropped to 0.01 mph.

At this “fast-moving” rate it would take most of a day for the Kilauea lava flow to advance the length of a city block. Any potential victims could stroll away from danger. (There has been just one reported injury so far, the result of a molten projectile. “Snapped my leg in half to where my ankle and foot and leg were a hinge,” the victim told a reporter. “It was super painful.”) As a practical matter, though, lava that advances at a few hundred yards per hour may still cause problems for emergency responders who must determine on the fly where the flow is headed and which communities must be evacuated.

The speed measurements at Kilauea refer only to the flow’s leading edge. That’s where the lava encounters vegetation and other sources of resistance (such as people’s homes), and slows down as a result. According to researchers on the scene, in the places where the flow has an established channel to the ocean—that is, where its path has been insulated by cooled lava on either side—it appears to be running at least 10 times faster than at the front.

The main determinants of lava’s speed are the rate at which it has been spewing from above, the degree to which it is being funneled through a lava tube or channel, the steepness of the slope, and the specific composition of its melted rocks. Kilauea lava tends to move pretty quickly, in relative terms. In the academic literature, flow speeds in the ballpark of 0.10 mph will be described as “fast-moving” or even “very fast moving.” Other, slower volcanoes have flows that peak at .001 mph or less.

The fastest-known lava flow occurred at Nyiragongo volcano in the Democratic Republic of Congo in 1977. In that eruption, very hot, very runny lava erupted all at once from 12 miles’ worth of fissures. The resulting flow was estimated to have an average speed of 19 mph, with some witnesses reporting seeing flows move at twice that rate.

Bonus Explainer: When will this damn volcano stop erupting? The Explainer addressed a similar question 12 years ago, after the eruption of Peru’s Ubinas volcano. That one ended up lasting three months, though there was no way to know it at the time. About half of recorded eruptions last less than two months, and one-tenth end within a day. But some eruptions drag on for years or decades. As that column noted, Kilauea has been spewing lava pretty much nonstop since 1983. That fact hasn’t changed in the last few weeks. The difference is that we’re now seeing major lava flow from new locations on the volcano where lives and property are at risk.

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Explainer thanks Brett Carr of the Lamont-Doherty Earth Observatory.