16 April 2010
Today’s volcano word is tephra, another term that’s directly related to the Eyjafjallajökull- Fimmvörduháls eruptions going on in Iceland at the moment. Tephra (“teff-rah”) refers to any fragmented material thrown from a volcanic vent during an explosive eruption. It comes in different sizes, all of which have their own names (just to make things even more difficult!)
Bombs or blocks are large rocks – 64 mm and greater in diameter (cobble to boulder sized; see the photo at left, which is an example of a really big bomb on a scoria cone on Mount Etna). Lapilli are smaller, from 2 mm to 64 mm (the size of the material underneath the bomb at left). Ash is any material smaller than 2 mm, and is one of the main constituents of a volcanic eruption column, such as the one that’s disrupting air traffic over northern Europe at the moment. Volcanic ash is composed of fragmented glass, rock, and phenocrysts (crystals), unlike the ash you get from fires (which is mostly carbonized organic material).
The other things that make up an eruption column are typically gases (including water vapor), ambient air that’s been entrained and heated, and some lapilli and bomb-sized particles. Some recent news reports have been saying things like “ash and smoke” to describe the Eyjafjallajökull-Fimmvörduháls eruption column, which is incorrect. There is no “smoke” in an eruption column, at least in the sense that most people think of it (as a byproduct of burning materials). The column appears to be smoky, but only because of the presence of the ash, which is generally some shade of gray or black.* (The photo below, from a February 2010 eruption of the Caliente dome at Santiaguito, is quite gray to begin with, but I can guarantee that it’s not because something in the vent is burning.)
Tephra is a major hazard associated with volcanoes. Bombs tend to be more of a problem in the vicinity of a volcano, but as many people in northern Europe are finding out, smaller particles like lapilli and ash can travel much higher and farther. Ash from a powerful eruption can reach the upper atmosphere, far higher than airplanes can fly; and because glass makes up a good portion of those ash particles, any plane that does fly through an ash cloud risks sucking glassy particles into its engines, where the glass can melt and re-solidify. This is bad – it could mean total engine failure, which is what happened to a flight over Alaska in 1989. No sane pilot is going to fly a plane into that.
*Okay, maybe some lichen is getting toasted, but that still doesn’t mean you can call ash “smoke”.