“The rings look pretty much like the ‘smoke rings’ produced by an able smoker,” Boris Behncke, a volcanologist at the Italian National Institute of Geophysics and Volcanology’s Etna observatory, said in an email Monday.

He explained that the rings were formed by magma — molten rock with gas — bubbling beneath a cylindrical vent that opened into a crater that formed on Etna’s surface last week.

“Imagine a very narrow, cylindrical conduit, within which, at a certain depth, there is magma,” Behncke said. “Every so often, a bubble forms at the surface of the magma, bursts, and sends a slug of gas at high speed through that conduit.”

As pulses of gas squeeze up the narrow, circular vent, they take on its ring-like shape — forming the puffs. “You won’t get rings from a more irregular shape of the vent,” he said.

The gas rings are composed of about 80 percent water vapor, with the remaining 20 percent mostly made of sulfur dioxide and carbon dioxide, he said. It is the water vapor that gives the rings their white hue, making them resemble smoke rings.

Mount Etna first erupted about 500,000 years ago. Since then, it has frequently dazzled tourists and Sicilians alike with its lava flows — including a molten fountain that reached heights of close to a mile in 2015.

Its volcanic activity has also caused serious disruption and damage, emitting hazardous ash clouds that forced the nearby airport to close and encased villages in ash. In 1979, a fluke crater explosion killed nine tourists — although deaths are uncommon. In a 17th-century explosion, Etna buried the town of Nicolosi, destroyed much of the city of Catania and fired molten rock into the sea 18 miles away.

According to Behncke, the rings are not particularly rare phenomena. Etna is “the most prolific volcano on this planet in terms of vapor rings,” he said.