A Collapsing Umbrella

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Science  24 Mar 2006:
Vol. 311, Issue 5768, pp. 1675
DOI: 10.1126/science.311.5768.1675b

Observations of volcanic plumes have provided fundamental insight into volcanic processes, one notable instance being Pliny the Younger's descriptions of Vesuvius in 79 AD. Large eruptions, like nuclear explosions, often form an umbrella-shaped plume. The top of the umbrella forms when hot gases and particles in a central eruption column reach neutral bouyancy and mix with cold dense air that is being driven upward; this process helps to stabilize the umbrella, allowing ash to fall gradually. Most such plumes have a cauliflower-shaped outer surface.

Chakraborty et al. describe a more ordered umbrella that formed during the November 2002 eruption of Reventador in Ecuador. In this instance, the edge of the umbrella formed large regular undulations approximately every 0.7 km, producing a shape similar to the edge of a scallop. The authors ascribe this phenomenon to an instability that occurs when the outer rim of the umbrella becomes too dense to be neutrally buoyant, a plausible result of this relatively cool eruption. Such a loss of buoyancy could lead to collapse of the umbrella, which would produce another type of volcanic flow. — BH

Geophys. Res. Lett. 33, L05313 (2006).

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