Italy's Mount Etna volcano has released volcanic vortex rings, a rare phenomenon caused by a constant release of vapours and gases.
Australian National University volcanologist Ana Casas Ramos said the rings, which have been rising since Wednesday, were often wrongly described as smoke.
"It is condensed water vapour," she said.
"On its own it is colourless but once it reaches the condensation level then it becomes whitish — and that's what we are seeing.
"The reason why you have this water vapour coming up and condensing so quickly and then forming these rings is because of the temperature difference.
"The water vapour is coming out very hot and then once it reaches shallow levels, like atmospheric levels, it then encounters cold air and that's when you get this condensation."
There were no reports of disruptions in the villages surrounding the volcano or at Catania airport, which often closes in the case of major eruptions.
Dr Casas Ramos said the phenomenon was more common at Mount Etna than at other volcanoes, and the shape was on account of the geometry of the conduit.
"It's a very active volcano. It has a very defined geometry internally and it's a phenomenon that can be explained by physics," she said.
"It's a very open-system volcano.
"It tends to form very large bubbles."
"The shallower they get, the bigger they get, and so it forms this circular bubble towards the surface then you have only the edges of the vents that are allowing these rings to form."
Mount Etna is one of Europe's most active volcanoes. However, Dr Casas Ramos said the likelihood of pressure building and causing "explosive eruptions" was reduced because it was constantly degassing.
"If it does culminate in an eruption, let's say in the coming months or weeks, it'll be just a couple of puffs, a bit of ash, maybe a bit of magma but nothing violent," she said.
The phenomenon has led locals to refer to the volcano as "Lady of the Rings".
Dr Casas Ramos said while she understood the appeal of visiting Mount Etna at this time she would urge those visiting to be cautious.
"It's such an unusual shape and it tells you that the volcano is doing something very distinctive," she said.
"Yes, it's beautiful, but it's telling me that something is changing and in that sense it would be very smart to have caution."
ABC/wires
