Mount Fuji: its height, the crater and why the cone is so symmetrical
3,776 metres, a crater at the top and an eruption from the side in 1707. Why Mount Fuji's cone is so even, what happened the last time it was active, and why it became a cultural World Heritage Site.
Mount Fuji is 3,776 metres high, measured at Kengamine on the crater rim, and the highest mountain in Japan. It is a volcano that last erupted in 1707, and the shape everyone recognises comes from a mountain built up layer by layer from the same point.
It stands about 100 kilometres southwest of Tokyo and can be seen from the city on a clear day. Here is what makes the cone so even, what happened in 1707, and why the mountain became a World Heritage Site for its culture rather than its nature.
A volcano built on two older ones
Fuji is a stratovolcano, built from alternating layers of lava and ash laid down by repeated eruptions. What you see today is the youngest of three volcanoes in the same place. Beneath it lies the oldest, Komitake, and then Ko-Fuji, Old Fuji, which was active around 100,000 years ago. The present cone, Shin-Fuji, has been built over roughly the last 10,000 years and covers the older ones.
That is the reason for the symmetry. When eruptions come out of more or less the same crater over a long period, material is spread evenly in every direction and the slopes end up with the same gradient all round. A young cone has also had less time to erode than an old mountain, where water and ice have had time to cut valleys into its sides.
The crater at the top
At the summit is a crater about 780 metres across and 240 metres deep. Its rim carries several high points, and the highest of them, Kengamine, is where the 3,776 metres is measured. From a distance the crater is invisible. Only from above, or in relief, does it become clear that the top is not a point but a ring.
1707: the eruption from the side
Fuji's most recent eruption began on 16 December 1707, according to the Japan Meteorological Agency. It did not come from the summit crater but from the southeast flank, where a new crater opened. Earthquakes had shaken the area in the weeks before. Ash fell on Edo, today's Tokyo, the same day, and lay five centimetres thick in Kawasaki. The eruption died down towards the end of the month.
The crater from 1707, the Hōei crater, is still there as a clear notch in the otherwise even cone. It is one of the few things that breaks the symmetry, and it shows up well in relief when the light comes from the side.
World Heritage for its culture
Fuji became a UNESCO World Heritage Site in 2013, but as a cultural site. The citation is about the mountain as a sacred place, a pilgrimage destination and a subject of Japanese art, not about its geology. It says something about how long the mountain has been a symbol, long before anyone measured it.
The whole cone on one piece
Fuji is an unusual subject, because the mountain is so big and so alone. The window on the Mount Fuji piece is 34 kilometres wide, more than twice that of most Alpine peaks, so that the whole cone fits from its foot to the crater. Up close you can see the summit crater and the Hōei crater on the southeast flank. From across the room you see what everyone recognises: the even cone.
There are more volcanoes in the range: Etna on Sicily and Teide on Tenerife. The full selection is among our 3D topographic maps.
