Volcanic ash

Drifting clouds of fine silica-rich particles pose severe danger to all forms of aviation.

By Karsten Shein
Comm-Inst, Climate Scientist

Iceland’s Eyjafjalla volcano erupted in Apr 2010. Despite the relatively small scale of the eruption, the ash plume forced the shutdown of a great deal of European airspace for several days.

When Eyjafjalla, the Icelandic volcano beneath the glacier Eyjafjallajökull, erupted in mid-April, the impact on aviation was as dramatic as it was sudden. Not since WWII had nearly all civilian aviation activity over western Europe come to a halt.

Even though the eruption and accompanying ash cloud were relatively minor in terms of volcanic activity, its geography in relation to both the prevailing global wind patterns and major transatlantic flight corridors ensured that flight operations would be suspended over some of the most heavily used airspace on Earth.

In fact, for 6 consecutive days, the airspace over Ireland, Great Britain and several continental countries was completely closed to all but emergency operations. In the aftermath, early estimates placed the cost of the ash plume to aviation somewhere around $2 billion, and that figure is only direct revenue loss by commercial carriers—it does not include additional billions lost by business aviation operations and revenue lost by the countless thousands of people and companies that depend on aerospace to get goods and services where they need to be in today’s just-in-time economy.

The decisions to close sovereign airspace were universally explained as necessary to ensure passenger safety. Nearly all cited the 1982 case of a British Airways Boeing 747-200 that penetrated the ash cloud of Mount Galunggung, about 110 miles southeast of CGK (Intl, Jakarta, Indonesia).

At an altitude of 37,000 ft, the aircraft lost power to all 4 engines, and the crew was only able to restart them at 11,500 ft. A Singapore Airlines 747 lost 3 engines in the same ash plume 3 weeks later. Still others cited the similar 1989 incident of a KLM 747-400 approaching ANC (Anchorage AK) which lost power to all 4 engines after encountering an ash plume from Mount Redoubt, which had erupted the day before.

However, unlike the British Airways crew, which did not see the plume, the KLM crew was advised of the ash plume and noted it, but was unable to avoid its effects. However, despite these historical encounters, with Europe’s flight corridors reopened, commercial carriers are now questioning the need to close the airspace in the first place.

They assert that the decision should be left to individual operators to determine if it is safe to fly through an ash plume. Where once these same carriers balked at assisting the Intl Airways Volcano Watch Operations Group (a committee of ICAO) in setting minimum safe ash concentration levels, out of fear of establishing liability thresholds, they are now calling for these levels to try to limit interruptions of operation as they just experienced.

Regardless of acceptable ash limits or any debate of risk versus revenue, volcanic ash is something most pilots will seldom, if ever, find themselves having to deal with. It is for exactly that reason that we do not generally learn much about volcanic ash in flight school, or bother to educate ourselves about its behavior and dangers thereafter.

But anyone who flies in the vicinity of active volcanic zones—and especially those who pilot the current generation of globehopping business jets—should be very familiar with the fundamentals of volcanic ash.

Volcano basics

Ash from the 1980 eruption of Mount St Helens in Washington state. Note how many of these particles resemble glass, due to their high silica content. Such particles are easily ingested into engines and pitot tubes as well as acting as an efficient abrasive.

According to the Smithsonian Institution, there are currently about 1500 active volcanoes around the world. This means that these volcanoes have erupted in the past 10,000 years. However, about 600 of these have erupted in recorded history and about 60 will erupt in any given year. In fact, at any given moment, there are about 20 volcanoes actively erupting.

And these numbers don’t even include the many undersea volcanoes that will likely never see sunlight. Fortunately, most terrestrial eruptions are not the massive, explosive type we imagine when we think about volcanic eruptions.

For example, Stromboli in Italy has been “erupting” small puffs of ash continuously for about 1000 years, and Mauna Loa in Hawaii erupts primarily by oozing lava across the landscape. However, for aviation it is the occasional explosive eruption, such as we saw with Mount St Helens and Mount Pinatubo, with which we are most concerned.

Every continent on earth has at least 1 volcano capable of erupting explosively. Explosive volcanoes come in all shapes and sizes. Some are small hills, known as cinder cones, that have spewed a few small rocks into the air at some point in their lives.

Others are super volcanoes that may span hundreds of miles, such as the volcano containing the Yellowstone Basin in the western US. The basin is actually the caldera—a depression resulting from a collapsed magma chamber—of a massive volcano that last erupted about 640,000 years ago with a force roughly 2500 times that of Mount St Helens.

Although a volcano may form anywhere a weak spot in the Earth’s crust allows magma to seep toward the surface, the high pressures and stresses associated with certain parts of the crust make those regions conducive to Earth’s more active volcanoes.

These regions are generally the boundaries between the large tectonic plates that make up Earth’s crust. For example, Eyjafjalla is part of the Iceland volcano, which rises up from the mid-Atlantic ridge. The ridge is the boundary between the North Atlantic and Eurasian plates, which are moving apart from one another, creating a rift beneath the Atlantic Ocean where subsurface magma can escape.

However, the majority of volcanoes occur where 2 plates are colliding and 1 is forced beneath the other—at a boundary known as a subduction zone. In fact, about 75% of the world’s active volcanoes occur around the so-called Ring of Fire.

This ring represents the boundaries of the plates that comprise the Pacific Ocean Basin. These boundaries have been very active historically and contain the stratovolcanoes of western North America, South America and the Indo-Pacific.

The subduction of the plates creates tremendous pressure that forces the magma through these weak spots, and over time the cooled lava forms the volcanic mountains that we see. However, the same magma that created the volcanoes also poses a grave threat.

The magma in these zones tends to be high in silica, which makes the magma very viscous—it cannot flow easily through the volcanic vents. As a result, the vents may become blocked, allowing pressure to build up due to the rising magma.

Eventually, if the pressure is not released elsewhere, perhaps by an earthquake or a minor eruption in a neighboring volcano, the volcano can erupt explosively. When a volcano erupts explosively, it can do so with the energy of dozens of nuclear bombs, and launch millions of tons of rock high into the air.


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