Etna is the tallest and most active volcano in Europe, situated where the European and African geological plates are colliding.

Such rings have never been seen in such detail before

Dr Jug Alean and Dr Marco Fulle have been investigating Etna's growing level of activity and in February they saw the ejection of several spectacular hoops from the Bocca Nuova region of the mountain.

"This wonderful specimen gently drifted overhead and past the Sun which was tinted orange by aerosols in the smoke," Dr Alean told BBC News Online.

It is difficult to gauge the size of what the scientists are calling "steam rings". They drift across the blue sky with no points of reference. However, the volcanologists estimate the hoops to be about 200m across and up to 1000m above the ground.

Stable shape

Smoke rings have been seen at volcanoes before but never in such detail. This time, there was hardly any ash on Etna and the gas billowing from vents had a high steam content. It is for this reason that Drs Alean and Fulle are using the term "steam rings".

The rings would last up to 10 minintes