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 have been 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.