It is interesting to note that not long after we had revealed Aso as #4 in our NDVP series, activity seemed to intensify and cause expectations of a large eruption, possibly even the “Big One”, within the near future. Such excitement is only natural but is Aso really on the verge of a major eruption?
There is a philosophical thought experiment to raise our awareness about the connection between our observations and perceptions of reality – “If a tree falls in a forest and no one is around to hear it, does it make a sound?”. Leaving philosophy behind, we can use that as the starting point of another observation: One day you hear a sound from the forest that you have never heard before and when you turn around to investigate, you find that a tree has just fallen. From that day on whenever you hear a loud noise from the forest, you conclude that another tree has fallen. It is similar with Aso.
After Sakurajima, Aso is the most active of all the volcanoes of Japan. Periods of unrest are not uncommon and are associated with the intrusion of minor amounts of juvenile magma into the inferred sill structure at ~15.5 km depth. Such an intrusion or series of intrusions can lead to heightened activity over a longer period of time such as happened in 2006-7. Analysis of recent ash emissions at Kyoto University show that only a very small portion can be attributed to fresh magma and that it is almost exclusively older rock that has been pulverised by the explosions. They are thus phreatic in nature.
So what is happening? Well, with the infusion of juvenile magma at depth, energy in the form of heat enters the system and even if the magma itself remains at depth, heat rises in the system together with volcanic gasses (read “superheated steam”, e.g. water at very high temperature and pressure) and spreads upwards through pre-existing cracks and fissures. As the “superheated steam” approaches the surface, it will reach a critical point where it instantly flashes into steam proper, increasing its volume by a factor of hundreds of times. It is exactly as if a large charge of dynamite was set off. The immediately surrounding rock is pulverised and cracks form through which the mix of steam and pulverised rock is ejected. This is what we are seeing on the webcams.
Over time, these explosions will weaken and erode away the lid of rock enclosing the upper magma reservoir which is thought to lie between about 4 – 10 km depth, contain around 100 km3 of magma which contains at least 10% melt. At present, this is much too low – it needs to approach 40% before becoming “critical” – to be a cause for immediate concern. For Aso to have a major eruption, a major infusion of fresh magma on the order of at least 100 times greater than those observed over the past decades, is required. At present, there are no signs to indicate that this about to happen.
However, one should never discount the possibility of a major eruption as being “impossible”. This is clearly illustrated by the 1886 eruption of Mt Tarawera where several cubic kilometres of basaltic magma apparently traversed the entire thickness of the crust in a matter of a day and a half. The c. 2.1 kY BP basaltic VEI 6 ignimbrite eruption of Masaya, Nicuaragua, is another example of what a nasty substance water can be in the wrong location. The reason Aso made it to number four in the NDVP list is the huge serpentinised (water-rich) wedge sitting just below the crust and that such a wedge can result in very large quantities of the water-saturated magmas required for a very large eruption within a very short lead-up time. Let me finish by stressing the point that is not what we are seeing at the moment but beautiful examples of phreatic explosions.
Aso Webcam: http://www3.nhk.or.jp/news/realtime-1/
Alternate hosting: http://wwitv.com/tv_channels/b6808-Aso-Vulcano.htm
Kyoto University Aso monitoring page: http://w3.vgs.kyoto-u.ac.jp/SUN/index.html