By Jan Zalasiewicz
Volcanoes can take one by surprise. That was the case with Mount St. Helens, that famously erupted sideways rather than upwards, and it was certainly so, two millennia back, when sleeping Vesuvius awoke to bury Pompeii and many of its citizens. Eyjafjallajokull may not have been quite so dramatic, but its effects, in tearing a large hole in our complex and delicate network of global airline communication, certainly rippled around the world.
To a geologist, the presence of a volcano on Iceland isn’t at all surprising. After all, Iceland is literally, and continuously, splitting apart, as this island sits exactly on the Mid-Atlantic Ridge. That mighty planet-sized fracture is continuously oozing magma, as the Americas pull ever farther apart – by a couple of centimeters a year, maintained for over a hundred million years – from Africa and Europe.
What raised a few eyebrows, though (mine, for sure) was the sheer filthiness of the eruption, and the amount of ash that it hurled high into the atmosphere, to the alarm of airline companies just about everywhere. For volcanoes that sit astride mid-ocean ridges are by nature generally placid by nature. For sure, they produce what seem like spectacular firework displays for the TV cameras, and flowing lava can, here and there, play merry hell with real estate values.
This is nothing, though, compared to the paroxysmal eruptions – Krakatoa, Pinatubo and the like – that occur in those parts of the world where tectonic plates are colliding. The violence of such cataclysms can destroy a whole country (and even Krakatoa was small compared to the great eruptions of the deep geological past). So why was Eyjafjallajokull trying to behave like one of the bad boys?
One answer is ice. Lying far north, Iceland is a land not just of volcanoes but of glaciers too – one of which lies on top of Eyjafjallajokull. As the uprushing magma came into contact with this, the ice flashed into steam, the expansion of which added quite a bit of oomph to the eruption. The lava, in turn, rapidly chilled by the ice, solidified quickly as it emerged, the thermal stresses shattering it into countless tiny fragments. This produced lots of ash, to be carried high into the atmosphere in the steam-driven (turbo-charged, if you like) eruption plume. It’s a bit (only a bit, mind) like putting a lot of wet wood and leaves on to a bonfire. This was one smoky volcano, and it seriously annoyed the neighbours.
Volcanic ash, of course, is feared by airline pilots, and justifiably so. One of the scariest experiences in all of flight history took place in 1982 when a British Airways Boeing 747, carrying 263 people, flew into an ash cloud from an erupting Indonesian volcano, Mount Galunggung. Ash particles entered the jet engines, melted against the hot metal, and, in effect, clogged them with reconstituted magma. All four engines failed, and the airplane, now completely without power, began to plunge towards the Indian Ocean.
The pilots kept their nerve, and prepared to ditch into the sea, while at the same time trying to restart the engines. The attempts failed until, when just a few thousand meters above the sea, the engines – amazingly – coughed back into life. They were able to fly to Jakarta, and landed safely (though not without difficulty, as the windscreen was almost opaque through being sandblasted by the sharp ash particles).
They had been saved by the same phenomenon that made Eyjafjallajokull such a disruptive volcano: thermal shock. As the stricken airplane descended, the cold air rushing through the lifeless engines chilled the molten ash, freezing it into solid volcanic glass. The chilling was fast enough for thermal stresses to shatter this glass, causing enough of it to break off to allow the engines to re-start. It was a lucky squeak.
That