Mt Fuji is a true "yokozuna" among island-arc volcanoes
But how high is Mt Fuji really? Not in the arbitrary coinage of feet or metres or shaku, but compared to its peers. Can it be considered a true yokozuna among volcanoes when set beside such hulks as Chimborazo (6,268 metres), whose summit is further from the centre of the Earth than Everest's? Or Ojos del Salado (6,893 metres) on the Chilean-Argentina border, which may be – nobody is quite certain – the world’s tallest volcano?
The comparison might be unfair. While the volcanic giants of the Americas stand on the solid footing of a continental craton, Fuji has to make do with the thin crust of an island arc - where it sustains itself by little more than "hinkaku", the ‘gracefulness’ that Fukada Kyuya attributes to famous mountains.
In fact, it’s hard to find island arc volcanoes that rise much higher than Fuji. Tambora, an Indonesian heavyweight, once soared above 4,000 metres, it is alleged. Alas, it met its Waterloo in 1815, when it blew apart in a massive eruption. The stump is now a thousand metres shorter than Japan’s top mountain.
Then there’s Klyuchevskaya Sopka (4,750 metres), the tallest mountain on Kamchatka and the largest active volcano in the Northern Hemisphere. But – hold it – Kamchatka is not quite pukka as an island arc, being a peninsula joined to the Siberian mainland. Like a sekiwake implicated in a sumo match-fixing scandal, Klyuchevskaya must be disbarred on suspicion of soliciting illicit help from the Russian continental Underworld.
When it comes to its peers on true island arcs, Mt Fuji more than holds its own. It overtops the highest volcanoes in New Zealand and the Philippines by more than a thousand metres. And it stands almost crater-to-crater with Indonesia's tallest volcano, Sumatra's Mt Kerinci (3,800 metres). That close coincidence of heights set Project Hyakumeizan wondering. Is it just chance that the greatest volcanoes of Indonesia and Japan top out at roughly the same altitude? And how high can a volcano grow anyway?
It turns out that few savants have considered this question. One who did was Peter Vogt of the US Naval Oceanographic Office in Washington. Looking at oceanic island volcanoes, such as Hawaii’s, he concluded that their heights were related to the thickness of the crustal plate on which they stood. The thicker the plate, the higher the volcano. Beyond a certain limiting height, the lava no longer has enough pressure to keep erupting from the volcano's summit.
Vogt ended his his 1974 paper with an intriguing speculation about the huge volcanoes discovered on Mars – Olympus Mons (火星富士: the Mt Fuji of Mars), the tallest, is almost three times as high as Everest, even though Mars is a much smaller planet. Such huge mountains, Vogt suggested, would need to be supported by a crust that is two or three times thicker than the Earth's.
If Vogt's hypothesis holds for island-arc volcanoes, it may be that Mt Fuji has already grown as tall as it ever will. After all, it is already more than 700 metres higher than Ontake, the next-loftiest Japanese volcano. Fuji’s most recent activity, from a side-vent in 1707, added nothing to its stature. We’ll have to wait for the next eruption to see if Japan’s top mountain intends to make another bid for that coveted 4,000-metre brevet.
Volcano height and plate thickness. Peter R. Vogt, US Naval Oceanographic Office, 1974
Image of Kasei-Fuji (火星富士) by courtesy of Jet Propulsion Laboratory