For a University open day for gifted and talented high school kids the other week we once again brought out the liquid nitrogen volcano (see previous post here). As the usual volcanologists in charge are off on some fieldwork jolly, looking at real volcanos/lying about in the sun in some tropical paradise (not jealous at all), I was put in charge.
As this was my first time operating the volcano we decided to have a run-through a few days beforehand which resulted in a few interesting observations.
First we discovered that if you don’t screw on the lid quite right the pressure can’t build up and the liquid nitrogen gently boils off releasing a cloud of gas with minor bubbling. This rather nicely models a non-explosive eruption where the lava degasses easily (due to its low viscosity), for example the basaltic eruptions on Hawaii which produce some spectacular fire fountaining as the lava degasses but usually only erupt with quite gentle effusive lava flows. We recreated this for the students on the open day by putting a small hole in the bottle top to allow for gas escape, the lack of drama of this ‘eruption’ was to the great amusement of the large crowd of builders who’d gathered expecting (from the way we still wore facemasks and ran away from the barrel) an impressive explosion.
Fire fountaining as lava degasses at the Fimmvorduhals volcano eruption in 2010, from http://www.youtube.com/watch?v=1bZ9IJjtuCo
Secondly, we also discovered the dangers of liquid-nitrogen-powered-oranges. My supervisor had come up with the bright idea of adding large oranges in with the plastic balls to represent pyroclastic bombs of varying sizes and densities. Of course it was expected that these denser projectiles would land close to the vent and 4 out of 5 of them did. However, (as shown in the detailed technical diagram below) one rogue orange managed to travel in a very high arc, taking a good 3 seconds to clear the 20m ‘exclusion zone’ between the volcano and where we were standing and flatten itself at high velocity into my mate’s (who’d come down to watch) forehead! Needless to say he didn’t find it as funny as we did and threw a proper little paddy – which just added to the amusement. A similar thing happened when we repeated the experiment for the kids but we’d learnt our lesson and added an extra 10m to the exclusion zone.
|Completely scientifically accurate diagram of flight path of orange volcanic bomb, unfortunately there is no idea on the launch velocity so cannot work out how high it went.|
This really emphasised just how much uncertainty there is in predicting the hazards from a volcanic eruption as there are so many unknowns and shows that plans and hazard maps definitely must err on the side of caution! In real highly explosive eruptions the bombs can be over 6m in diameter and even these have been known to travel over 500m from the vent, with smaller ones in the region of 0.5-1m often being found more than a kilometre from the vent! (Here’s an amusing story I found about a different danger of volcanic bombs in an airport)
Explosive eruption emitting large volcanic bombs from Sakurajima volcano, Japan, video from http://www.youtube.com/watch?v=Jav4-GLXecQ
Here's a close up of a volcanic bomb I found in Iceland on a fieldtrip a few years ago, my size 9 boot for scale