Erupted grain sizes key to determining eruption column height

Photograph: CHRIS WEBBER (AP)

Despite the social and economic consequences caused by the 2010 Eyjafjallajokull eruption in Iceland was relatively small, emitting only 0.24 cubic kilometers of material, enough ash to fill 96 thousand Olympic swimming pools.  In comparison, the 1980 Mt. St. Helens eruption produced four times as much ash and an ash plume twice the size as Eyjafjallajokull’s. While no human lives were lost as a result of the eruption of Eyjafjallajokull, its economic impact was outsized: the eruption of Mt. St. Helens cost US$2.74 billion (by todays estimates) while Eyjafjallajokull cost the global economy a whopping US$5.13 billion.

So the big question is how did an eruption a quarter of the size of Mt. St. Helens cause so much economic destruction?

While there are many factors that contribute to effects of a volcanic eruption (e.g., volcano location, location relative to major aviation routes), the presence of a strong ambient wind had a major effect on Eyjafjallajokull.  The strength of the surrounding winds was much greater than the strength of the eruption column, causing it to bend.   During the Mt. St. Helens eruption, weaker ambient winds relative to a stronger eruption column allowed for the ash to rise without bending – it only began to sheer when the ash formed an umbrella cloud similar to the clouds from nuclear explosions. 

The jet stream blew the ash from Eyjafjallajokull over Europe, grounding hundreds of thousands of flights due to unsafe levels of ash in the air that could clog jet engines resulting in engine failure. To predict the ash concentration, scientists use height of the plume and rate of eruption to determine how the ash is transported.   The current model for determining plume height of bent ash columns requires the ambient wind velocity, velocity of the ash cloud, and the rate that air mixes with the plume.

A recent study published in the Journal of Geophysical Research: Solid Earth has modified the traditional model for bent ash clouds by incorporating the influence of ash size on the plume.  When grains are greater than a few millimeters, it can fall out of the ash cloud before it cools, resulting in the overall temperature of the ash cloud decreasing.  Since heat affects how high the plume rises, this loss of heat due to fallout plays an important role in determining the height of the plume.

These scientists tested their model against the 2010 Eyjafjallajokull eruption.  When the effects of grain size were included in the model, it more accurately predicted the plume height.  They also found that detailed local meteorological data was critical to model success.  It superseded the effects of tephra – without the atmospheric data, the simulated plume underestimated the height of Eyjafjallajokull.

It is crucial to develop these models for bent plumes because bent ash clouds are more often produced by smaller eruptions, which are more frequent than large, catastrophic ones.  By developing models to study bent eruption clouds, we can better predict where ash will spread during an eruption and more strictly constrain ‘no-fly’ zones, limiting their economic impact.

Original Article: Woodhouse, M. J., Hogg, A. J., Phillips, J. C., & Sparks, R. S. J. (2013). Interaction between volcanic plumes and wind during the 2010 Eyjafjallajökull eruption, Iceland. Journal of Geophysical Research: Solid Earth.

Synopsis by Meghan Fisher