Mont Blanc 'grows' almost two metres to 4,807.47 m, but not because of the rock

The roof of western Europe has “grown”. The new official survey puts Mont Blanc’s summit at 4,807.47 metres, according to France’s National Geographic Institute (IGN): 1.88 metres higher than in 2023. But don’t be fooled: the mountain isn’t rising. What changes is the snow and ice on top.

A summit on the move

Mont Blanc’s top isn’t rock but a cap of snow and ice. The rock lies beneath it at about 4,792 metres, some 15 metres lower. The cap grows or shrinks with each season’s snowfall and wind, and 2023 had been much drier at the summit.

That’s why the height is measured every two years, and each time the figure is different.

The summit is “constantly shifting” and no single measurement means much on its own, Jean des Garets, chief surveyor for the Haute-Savoie department, told Le Monde.

How it was measured

The team from France’s Order of Chartered Surveyors:

1. Acclimatised by climbing the Bishorn (4,153 m).
2. Took measurements at several points around the mountain over several days.
3. Climbed Mont Blanc by the normal route on 15 and 16 September.
4. On top, used a GNSS pole, high-precision satellite positioning, to pin down the highest point in real time.

The big novelty: for the first time a gravimeter was placed on the summit. It read 9.7944689 m/s², compared with 9.81 m/s² at sea level in Marseille. Repeating this measurement in future will show whether it’s the ice mass that changes or the rock itself.

So what height should I use?

Experts advise guides to wait before updating their figures: next time it could drop. What doesn’t change is the rock, about 15 metres below the snow.

Mountains changing height isn’t unusual: the Himalaya really is rising because the Indian plate keeps pushing, as we explain in did you know the Himalaya is still growing?. And if you dream of walking around Mont Blanc, here’s our guide to the Tour du Mont Blanc.