Volcán Reventador's unusual umbrella

Photographs of a volcanic column in a recent eruption of Reventador show a prominently scalloped umbrella that is unlike any umbrella previously documented on a volcanic column. We propose that the scallops in this umbrella are the result of a turbulent Reyleigh-Taylor (RT) instability, a type of fluid instability with no precedents in volcanology. Negative buoyancy drives this instability, and we ascribe the the fact that the Reventador column fed on a cool co-ignimbrite cloud. From the wavelength of the scallops, we estimate a value for the eddy viscosity of the umbrella, V_d ≈ 4,000 m²/s, the first such value to be inferred directly from an observation in the field. Collapse of the umbrella back to the ground could result in a previously unrecognized hazardous flow. We hope this work will elicit new reports on scalloped umbrellas and further study of the characteristics and evolution of such umbrellas.