The mountaintop that wandered: Rosablanche’s riddle

The eventful history of Rosablanche (today 3336 m above sea level) began in 1888. At that time, state surveyor Max Rosenmund made the summit a fixed point in the Swiss triangulation network. This network consisted of highly visible points spread across the country, often marked with a signal. Well-known signals are the ‘pyramids’, such as those found on the Napf, the Gurten or the Chasseral.

The position and height of the individual fixed points were determined using angle measurements and triangular calculations. This work was of great importance because the triangulation network created an accurate basis for the maps of Switzerland.

It took more than a quarter of a century for the triangle point on the Rosablanche to become a ‘real problem child’, as engineer Johann Ganz put it in 1916.

In September 1914 a team left Topography of the country (Today: swisstopo) to the top to take angle measurements. The measurement results showed that something special had happened on the Rosablanche. According to Hans Zölly (1880–1950), primarily responsible for land triangulation at the time, ‘unpleasant surprises occurred’: all triangulation calculations related to the Rosablanche did not work.

Initially it was suspected that measurement and calculation errors were the cause, but the double calculations carried out for verification confirmed Zölly’s observation: the triangle point on the Rosablanche had shifted approximately 3.5 meters between 1895 and 1914. The engineer drew a dramatic conclusion: “We were standing […] from the fact that one of our main trigonometric fixed points was not a fixed point.

As a result, the state topography did everything it could to track whether and how the fixed point moved. Between 1915 and 1921, engineers and their survey assistants climbed Rosablanche almost every summer. You could literally see the movement of the top:

Calculations from 1921 ultimately showed that the fixed point was more than 21 meters lower than at the first measurement in 1891. The top, which was once clearly separated from the rest of the mountain, had sunk so far that it was increasingly part of the mountain. became. ridge.

Initially, the country’s topographical engineers assumed that an earthquake had caused the Rosablanche to move. However, because no other triangulation point in the region had undergone a similar migration, this assumption quickly seemed unlikely. Porous rocks or tectonic shifts can also be ruled out as causes.

The state surveyors were in the dark for years until around 1920 they became aware of the geology professor Émile Argand (1879–1940) from Neuchâtel. Argand was an expert on the Valais Alps and had also studied the mountain closely. There was a certain irony of fate: unbeknownst to each other, the engineers and the geologist had sighted the Rosablanche at the same time.

Émile Argand had already discovered in 1916 that melting glaciers had caused the rock masses to slide. Directly below the peak of the mountain was the Glacier de Prafleuri. The ice stabilized the summit area not only from the outside, but also underground: the glacier had undermined and supported the summit of Rosablanche with its massive ice masses for thousands of years. But around the turn of the century, the Glacier de Prafleuri began to melt rapidly. As a result, the summit lost the icy foundation on which it stood – the summit movement began.

Argand’s findings also had implications for the Swiss national survey: the unstable summit had no longer served as the location of a triangulation point. The “central, beautiful peak” La Ruinette took its place.

The case of Rosablanche is an example of how Switzerland’s mountains have come under increasing scrutiny since the 19th century. National surveys, glacier research and mountaineering helped to document and detect changes in the Alpine region at an early stage. This trend continues to this day: high-precision elevation models, the GLAMOS glacier monitoring network and the PERMOS permafrost monitoring network, to name just a few examples, now document changes in mountain areas in great detail.

The demise of the Rosablanche in the 1910s also drew early attention to the relationship between ice and rock. The fact that glaciers and permafrost play an essential role in literally holding the Alps together is becoming increasingly clear as the climate continues to warm.

The most recent example is the huge landslide on the Ruhehorn in the Swiss-Austrian border area. One million cubic meters of rock fell there on June 11, 2023, making the top 19 meters shorter. The cause: the thawing of the permafrost and the melting of the Ruhehornferner. This glacier had supported the western flank of the Ruhehorn until it retreated and – as the Glacier de Prafleuri once did – rocked the mountain.