Japanese volcano Mount Ontake erupted suddenly over the weekend, when the popular recreational site was crowded with hikers, resulting in significant casualties.
Ontake’s last major eruption was in 1979. There have been no fatalities from Japanese volcanoes since 1991.
NZ focused media coverage of the eruption includes:
The Science Media Centre has gathered the following expert reaction to news of the eruption.
Prof Shane Cronin, Earth Sciences, Institute of Agriculture and Environment, Massey University, comments:
“The monitoring of Japanese volcanoes is excellent. There was some two-weeks of slight unrest before the event, but at a frequently active volcano such as Ontake, there is commonly seismicity associated with hydrothermal systems. The eruption was a true ‘blue-sky’ event in terms of not being preceded by major unrest.
“We are familiar with similar types of eruption at NZ volcanoes, such as 2007 Ruapehu and 2012 Te Maari/Tongariro. The suddenness of these eruptions is a real challenge for any monitoring technology. We have been very lucky in past situations in NZ to avoid such calamitous situations, although such blue sky eruptions remain a real possibility at both Ruapehu and Tongariro.”
What warning signs would scientists have expected to see before an eruption of this kind?
“Typically, an increase in seismicity – mainly small earthquakes associated with heating up of hydrothermal systems. There may also be changes in hot spring temperatures, gas release, fumaroles and possibly ground surface changes in the vent area (bulging, cracking and similar).
“In the case of Tongariro 2012, there were around 3 weeks of preceding earthquakes, although their number had decreased by the time of the August 6 sudden eruption. The November 21 eruption was preceded by no seismicity. The challenge, even once earthquakes or other changes are detected, is to pinpoint the exact time of an eruption. Such volcanoes present often with signs of unrest, many of these periods of unrest, however don’t necessarily lead to an eruption.”
What were the main hazards faced by people on the mountain at the time?
“Hazards in this case were fast-moving currents of ash, gas and steam… these would kill by asphyxiation, physical impact of large rocks, and possibly burning. These are similar to the energetic ash currents generated near to Te Maari crater in 2012 at Tongariro. Other dangers are ballistic blocks – which are large rocks (up to metres in diameter) being thrown out like mortar shells from the vent site.”
Are there any parallels that can be drawn with New Zealand volcanoes and recreational use of active volcanoes more generally?
“There are direct parallels with Tongariro, as mentioned above, and with Ruapehu. There are many volcanoes around the world with high-level heat flows in the upper summit areas – many in temperate or tropical climates have hydrothermal systems that store heat and super-heated water. Often in such cases, subtle changes in the heat or gas flow from below can lead to the hydrothermal system becoming destabilised. If a cap or sealing plug develops in such systems, water can be super-heated, like within a pressure cooker, once the pressure is too great for the rock cap to hold in – sudden release generates eruptions with very little if any warning. At Tongariro, a small landslide triggered the breaking of such a seal on the hydrothermal system and triggered the August 6 eruption.”
What advice can be offered to people who find themselves up close to an erupting volcano that might increase their likelihood of survival?
“Be aware of all hazards when going into an area. If there is an eruption – make your way out of the area as quickly as possible obviously, but take stock of which particular areas are mostly being affected. Many ground-hugging currents will follow a valley pathway – hence avoid the valleys, even those well downstream of an affected area. Note the wind direction and try to evacuate up-wind.”
Mike Rosenberg, GeoNet Duty Officer, comments:
“While we are interested in learning more about Mount Ontake’s activity from a scientific perspective, more importantly, we send our deepest sympathy to the families, rescuers and scientists during this very difficult time. The eruption of Friday in Japan was a tragic example of what can happen on any cone volcano.”
“Often, volcanoes let us know about ahead of time about eruptions by producing a swarm of small, shallow earthquakes that have unique energy signatures which can be identified. It is rare that a cone volcano will erupt without giving us a heads up first, but it has happened in the past decade in New Zealand.”
A GeoNet blog post with further information is available on their website.
Our colleagues from the UK SMC gathered the following comment:
Prof David Rothery, Open University, said:
“This eruption of Ontake was not especially powerful, but distributed several cm of ash across areas close to the summit. It seems similar to its 1979 eruption, which is the only other eruption of this volcano in historic times. I am surprised that it erupted without warning, because so far as I know there are seismometers on Ontake that are supposed to detect signs of internal magma movement. ‘Sneak’ eruptions lacking in pre-cursor signs are not unknown, but analysis of the records may reveal data that, with hindsight, should have been read as a warning sign. This is how we learn how to do it better next time.
“Ontake volcano sits above a site where the floor of the Pacific ocean and the Phillippine Sea is being pushed below (subducted below) Japan. This leads to melting at depth, which produces the magma. Gases, such as water vapour, excaping from the subducted material expand as magma rises, making the eruptions explosive.”