Mount Redoubt Eruption: The Umbrella from the Volcano

The umbrella metaphor extends in two distinct directions. First, is the "security" aspect--going to its function of sheltering users from rain or sun. Second, is the shape; this may have two aspects--the ribbed underbelly of an umbrella or the view from above. The latter is in the news as Alaska's Mount Redoubt eruption awakens public discussion of volcanoes. According to an NPR story, a "volcanic plume is essentially a column of hot gases and dust, topped with a horizontal 'umbrella.'"

While there is no example of this eruption's plume, here is a picture from the last eruption, in 1990. NPR, citing a recent article in Nature, points out that "inside that umbrella-shaped haze of gas and dust, a lightning-covered cyclone rages, spitting out waterspouts and dust devils."

More interesting is that the metaphor of an umbrella must be of more recent origin. The NPR story points out that "[v]olcano watchers described the general shape of these plumes as early as the eruption of Mount Vesuvius in A.D. 79." and notes a reference to this bizarre phenomenon in an obscure letter by Capt. S. Tillard, published in 1812, who was navigating the seas around the Azores when a volcanic vent erupted and he refers to the column of smoke as rotating on the water "like a horizontal wheel." So, if the umbrella now a universal metaphor for a shape, did it replace some other generally accepted concept or become its own permanent thing?

For an animation of an eruption, including depiction of the umbrella region, check out the San Diego State University's College of Science website. "The column thus spreads out in the umbrella region. The bottom of umbrella region is where densities of the plume and the surrounding air are equal. Continued upward mobility towards the top of the umbrella region is controlled by momentum. The umbrella region is often asymmetric due to the effect of high atmospheric winds in the stratosphere." This raises an issue of whether this metaphor derives from the shape or from the opening up of an umbrella.