Abrasive skies
On April 14, 2010, the Eyjafjallajökull volcano entered a new, explosive phase. An eruption had been underway since March 20, but this second stage occurred directly beneath the volcano's summit ice cap. The interaction between 1,200°C magma and glacial meltwater was immediate and violent. This phreatomagmatic event flash-chilled the molten rock, shattering it into exceptionally fine, sharp-edged particles of glassy ash.
The resulting plume of steam and ash shot 9 kilometers (30,000 feet) into the atmosphere, directly into the high-velocity jet stream. This eruption, rated a 4 on the Volcanic Explosivity Index (VEI), ejected an estimated 250 million cubic meters of tephra. The ash particles, many smaller than the width of a human hair, were composed largely of abrasive silicate glass.
For jet aircraft, this cloud was a major threat. Modern jet engines operate at temperatures around 1,400°C, well above the melting point of volcanic ash, which is about 1,100°C. When ingested into an engine, the microscopic glass shards abrade internal components. They then melt in the combustion chamber and fuse onto the cooler turbine vanes at the rear of the engine, solidifying back into glass. This buildup blocks airflow and can cause the engine to stall and flame out. The airspace closure over Europe, which lasted for an initial six days, was the largest disruption to air travel since World War II.
Modeling the plume
Predicting the path of this airborne hazard fell to the Volcanic Ash Advisory Centers (VAACs), a network of nine global centers established by the International Civil Aviation Organization. The London VAAC, operated by the UK's Met Office, took the lead for the Eyjafjallajökull event. The VAACs use volcanic ash transport and dispersion (VATD) models, such as the HYSPLIT model, to forecast the movement of ash clouds.
These computer models are complex, combining meteorological data—wind speed and direction at various altitudes—with volcanological inputs. The main challenge is getting accurate data for the eruption source term. This includes the height of the ash column, the mass eruption rate, and the size distribution of the ash particles. These parameters are difficult to measure in real-time during an explosive eruption, especially when steam and weather clouds obscure satellite views.
The initial models in 2010 were based on a conservative "avoid all ash" policy. A lack of precise data on ash concentration within the cloud meant that vast areas of airspace were closed as a precaution. The event exposed the limitations of existing models and spurred research into better characterizing eruption plumes and determining safe concentration thresholds for aircraft. Subsequent improvements in modeling and coordination mean that a similar eruption today would likely cause about half the number of flight cancellations.