It’s unusual – but not entirely unheard of – to see volcanoes blowing smoke rings during inactive periods. But given their unpredictability, scientists had not studied this phenomenon in much depth. In a recent presentation, though, a group unveiled results from numerical studies of volcanic vortex rings. They found that the decreasing pressure on rising magma allows dissolved gases to emerge as bubbles. If the magma has the right viscosity, those bubbles can merge into one big pocket that depressurizes explosively in the vent. As the hot gases burst upward, the walls of the vent cause them to curl up into a vortex ring, provided the vent is fairly circular and uniform. That sends the roiling vortex up into the atmosphere, where it cools, condenses, and becomes visible.
The need for a circular vent matches observations of volcanic vortex rings in nature, like the infrared image shown above. Volcano watchers find that vortex rings only form from some vents, and the more circular the vent, the more likely it can produce vortex rings. (Image credit: B. Simons; research credit: F. Pulvirenti et al.; via Nat Geo; submitted by Kam-Yung Soh)