Volcanic eruption in Japan - experts respond
Japanese volcano Mount Ontake erupted suddenly over the weekend, when the popular recreational site was crowded with
hikers, resulting in significant casualties.
Rescue efforts are ongoing and the total loss of life has yet to be confirmed. Latest reports indicate over 30 deaths,
with dozens more injured in the eruption.
Ontake's last major eruption was in 1979. There have been no fatalities from Japanese volcanoes since 1991.
The Science Media Centre has gathered the following expert reaction to the news. Feel free to use these quotes in your
stories.
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, 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 6eruption."
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."
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."