Tuesday, January 23, 2018

Unpredictable explosions

Some volcanic eruptions are less foreseeable than others

In today's tragedy at the ski-resort under the Kusatsu-Shirane volcano, one soldier was killed by flying rocks after a volcanic explosion. Several more people were hurt. The accident recalls the much greater disaster on Ontake three years ago, when 63 people died.

Eruption at Kusatsu-Shirane in December 1982
Photo: Japan Meteorological Agency
Both mountains belong to Fukada Kyūya’s One Hundred Mountains of Japan. Reading his write-ups, one would never guess that climbing these peaks might be a hazard to life and limb. That’s not because the Hyakumeizan author was negligent in his research, but rather the effect of timing. When Fukuda wrote about Ontake, in the late 1950s or early 1960s, the volcano had never erupted during historical times. Its first recorded eruption was in October 1979, eight years after Fukada’s death. Until then, the mountain was thought to be inactive.

Fissure eruption at Kusatsu-Shirane in 1942
Photo: Japan Meteorological Agency
Kusatsu-Shirane too appears quite innocuous in Fukada’s account. In the relevant Hyakumeizan chapter, he notes that the mountain was ascended by the Confucian scholar Asaka Gonsai in the summer of the ninth year of Tempō (1838). Today, Fukada adds, “one often meets old women and frolicsome children on this path.”

Yet Gonsai’s description, as quoted by Fukada, leaves no doubt as to Kusatsu-Shirane’s volcanic temperament: “All the peaks are scorched partly red, partly black, by the sulfurous vapors. Bare bones, stripped of flesh, not a tree or blade of grass, an exceedingly strange and ghastly scene.”

"An exceedingly strange and ghastly scene"
A pre-war postcard view of Kusatsu-Shirane

Indeed, it appears that Kusatsu-Shirane has erupted much more frequently than Ontake. About 14 eruptive episodes have been recorded since 1805. Taking a simple average, therefore, you'd expect such explosions to take place slightly more often than once every two decades.

In reality, though, the volcano erupts at quite irregular intervals, and it so happened that Fukada wrote it up roughly in the middle of a lull that lasted more than thirty years. So it’s fair to conclude that Kusatsu-Shirane’s eruptive potential could not be in the forefront of his mind.

Eruption on Kusatsu-Shirane (from a pre-war postcard)
On both Ontake and Kusatsu-Shirane, all historical eruptions have been phreatic – that is, caused by water coming into contact with hot rocks rather than by rising magma. Unfortunately, steam explosions are harder to predict than eruptions caused by upwelling magma, as they are less likely to give out clear seismic signals in advance.

“Eruptions can occur without warning, so stay alert to what is happening in and around the crater,” warned the Japanese authorities last year in a pamphlet distributed to hiking and climbing organisations. That advice remains as valid as ever. At the same time, it should be recognised that a volcano may give no usable warning.


David Lowe said...

Interesting, so Fukuda actually omitted candidate mountains based on their eruption history or eruptive potential? On the one hand it makes sense as it’s a bit hard to write about a mountain when you can’t even safely make it to the top, though at the same time can’t help feel hard luck for those volcanoes active during his writing.

Project Hyakumeizan said...

David, thanks for reading and for raising an interesting point - did Fukada leave a volcano or two out of his list because they were active while he was writing? Well, he did make it rule not to write about mountains that he hadn't personally climbed. But as just under half his "Hyakumeizan" list consists of volcanoes, most of them still active in some way, it appears that recent activity didn't put him off - eg in the write-up of Tokachi-dake, there is mention of a big disaster in 1926 that devastated a village, and a more recent eruption in 1962 that resulted in the evacuation of the local onsen. In the Akan-dake chapter, he says that Me-Akan had just started erupting when he visited in 1959, so he climbed O-Akan, a lower peak, instead. But, up there, he had a near-encounter with a bear. This didn't stop him adding the twin Akan peaks to his list, though ....