Stage 17 | Saint Gaudens - Peyragudes / A tour in the earth's molten crust

This mountain stage brings the riders close to the center of the Pyrenees.

Young rocks in the foothills of the Pyrenees

In an east-west direction we call it the Central Pyrenees, in a north-south direction it is the Axial Zone. It is also the area with the highest peaks, above 3000 m. The riders go westwards from Saint-Gaudens along the border between the Pyrenean foothills (~100 million year old rocks) in the south and the younger products of erosion of the High Pyrenees, which are still being deposited to the north. After about 40 km they turn southwards and start riding through ever older rocks.

Into the older and hotter rocks of the Pyrenees

After Arreau, the riders take a detour to the west, following a half circle that brings them close to the Néouvielle granite (granodiorite), a very large 300 million years’ old magma chamber. The stage ends with several climbs through the even older sediments of the Axial Zone to Peyragudes. This part of the Pyrenees shows spectacular evidence for two mountain-building cycles: the Hercynian mountains around 300 million years ago, when the supercontinent Pangea was formed, and the recent Alpine cycle that is best seen in the Gavarnie area. During the Hercynian cycle old sediments, derived mainly from the erosion of the southern continent Gondwana, were brought to 10-20 km depth where they were heated to temperatures between 300 and 700 °C. The rocks changed appearance (metamorphism) and various minerals were formed that can be seen with the naked eye, for example andalusite, staurolite, and sillimanite.

Melting the deep domains of the Pyrenees

At the highest temperatures, the rocks underwent a special process that we call “partial melting”. It has been known since at least the bronze age that rocks do not melt completely. Instead, some metals and minerals melt at lower temperatures than others. For example, tin and lead melt at much lower temperatures than copper. Apart from these metals (or metal sulfides), most minerals are silicates. Some combinations of silicate minerals melt at 650-700 °C. When the melt escapes, it can collect upwards and form large granite (or granodiorite) bodies that we see all over the Pyrenees. Minerals with higher melting temperatures stay behind in the source rock, which we call restite. In many places, some melt also stays behind, and the rock then shows layers or patches of restite and melt. This rock type we call a migmatite. Some of the best areas to see this are in Gavarnie and some valleys south of the finish line. And in nicely polished kitchen worktops!

I am a geologist who specializes in high-temperature processes in the deep continental crust. My motto in research and teaching is that knowing the history of our planet will help making reasonable predictions about our future. Main professional passions are fieldwork, microscopy of rocks, and teaching. Check the Geo-TdF-team-2022.

Leo Kriegsman