From thundering pyroclastic flows to scorching rivers of lava, volcanoes are famed for their furious fireworks. They can, however, be just as awe-inspiring during their quieter moments—like when a simmering peak blows a lazy smoke ring into the sky.
Such ephemeral apparitions have been spotted flitting above multiple volcanoes across the world, but it’s been unclear how they are made. Although they can be hundreds of feet across, the rings are short-lived and unpredictable, making them difficult to study. That’s why scientists led by Fabio Pulvirenti, a senior fellow at NASA’s Jet Propulsion Laboratory, decided to do the next best thing and simulate them on a computer.
The team’s virtual volcano, presented at a gathering of the Asia Oceania Geosciences Society in Singapore earlier this month, shows that “what happens in the mouth of a smoker happens more or less exactly within a volcano,” says study coauthor Carmelo Ferlito, a volcanologist at the University of Catania in Italy.
Smoke and mirrors
Despite the name, these smoke rings aren’t made of smoke, says Boris Behncke, a volcanologist at Italy’s National Institute of Geophysics and Volcanology who was not involved with the research. Instead, the volcanic variants are largely made of condensed gases, predominantly water vapour, escaping from the magma and propelled from the volcano’s vent.
Strong winds can prevent rings from forming or lasting for more than a brief moment. Otherwise, rings forged from a decent supply of volcanic vapour—which is warmer and less dense than the surrounding air—drift toward the sky and start expanding. Eventually, when they shed most of their vapour, the rings vanish from sight.
But only some volcanoes seem able to make smoke rings, and only at certain times. Back when Pulvirenti first learned about them in 2013, he found that the scientific literature offered no substantive answers. So, when his curiosity became too much to bear, he recruited some colleagues and took a crack at solving the mystery.
The team perused plenty of observations of volcanic smoke rings, seeing how high they rose, how fast they moved, how quickly they cooled, how much their compositions varied, and how frequently they contained ash. The scientists also read up on how magmatic gases migrated through and escaped from volcanic conduits, and they dove into the complex physics of vortex formation in fluids, gleaned from laboratory experiments.
A series of infrared images shows a smoke ring forming and dissipating above the Yasur volcano.
Although volcanic smoke rings can be hundreds of feet across, they are short-lived and unpredictable, making them difficult to study.
IMAGE BY BEN SIMONS
They then plugged all their findings into a computer model. By fiddling with the build-up of pressure within the conduit, as well as the geometry of the virtual volcano’s vent, the team worked out just what it took to make smoke rings.
As magma rises through the conduit, the surrounding pressure drops, allowing dissolved gases to emerge as bubbles. If the magma is not too viscous, bubbles can merge into singular pockets of pressurised gas. When they approach the vent, these gas pockets can violently depressurise and explode, propelling hot vapour upward, sometimes at near-supersonic speeds.
Here, volcanoes are comparable to simple smoke cannon toys, which eject fog through a narrow circular opening. In both cases, there needs to be a substantial amount of vapour collected and then expelled quickly to form a decent ring.
In the simulated volcano, vapour being ejected from the vent interacted with the rocky sides, causing the ball of gas to roll up around the edges. Slow-motion videos show the exact same thing happening with smoke cannon toys, Pulvirenti says. Then, as the rolled-up vapour ring meets the cold atmosphere, it chills, decelerates, condenses, and becomes visible, a bit like the contrails of airplanes.
Crucially, to make rings, a volcanic vent must be fairly circular, and the sides of the vents have to be the same height. If the vent is too irregularly shaped or broken up, the ring may be severely warped, unstable, or won’t form at all.
Fellowship of the rings
As a doctoral student of volcanology at the University of Auckland, Benjamin Simons has seen smoke rings at several persistently active volcanoes, including Mount Yasur in Vanuatu. The majority of the rings he saw escaped from skylights, roughly circular natural openings perched above the level of volcanic vents that are open to the “beautiful night glow of magma” below.
When small puffs of volcanic gas were forced through these narrow openings, smoke rings appeared. They rose ponderously, he says, rarely having the power to leave the summit crater before fading away. The results of the new computer model match with Simons’ own observations; the more circular the opening, “the more likely it is to produce a smoke ring,” he says.
Although this work is yet to be peer reviewed, it appears to explain why smoke rings aren’t seen at every volcano all the time, since the rings require such precise conditions to form. (Find out where the world’s largest volcano sits.)
Even when these conditions are met, however, smoke rings don’t always appear, suggesting that there is more to the gassy pandemonium within volcanoes than we currently know. Fortunately, the team is still working on it, hoping to dig up more answers.
The research isn’t just about scratching a scientific itch. Volcanic smoke rings form in the same rocky pathways that shuttle magma to the surface during eruptions, so it’s possible that studying them can help geologists better understand what’s going on inside the steamy throats of volcanoes.
And while this work may not be tackling the most pressing problem in volcanology, Ferlito says, it’s satisfying to try solving a mystery that is both entertaining and beautiful to behold.