Stage 11 | Albertville - Col du Granon / A ride through a former subduction zone

A steep climb in the Alps, or deep dive beneath Earth's crust?

Today’s ride is both! Journey with the riders as they traverse what used to be the edge of Europe, pass over a collisional zone where fragments of oceanic crust are caught up in the wreckage, and then enter a terrane of rocks that were dragged deep into Earth’s interior, 45 million years ago. However, over the years, they’ve made their way back up to the surface. Rocks like these ones comprise large parts of the incredible topography of the Alps. To see these deeply buried rocks, the peloton will battle a series of rolling hills that caps out at ~2600 m at Souvenir Henri Desgrange. Marking roughly two-thirds through today’s trek, this peak is situated at the boundary between French provinces Savoie and Hautes-Alpes and is aptly deemed the heart of the French Alps. Today’s climb is a small price to pay to reach the rare rocks that formed 70 km beneath the Earth’s surface!

The Earth is a pressure cooker, and rocks are the ingredients

As the riders close the first ~45 km of today’s 149 km, the peloton approaches the foothills of the Alps. If the weather allows, keep an eye out for rocks with sparkling flecks that shine in the sunlight. These sparkling minerals are sheet-like crystals of mica which testify to the rocks’ high-pressure history. When rocks are buried deep in the Earth due to tectonic forces, the chemical constituents reorganize and mineral structures change to equilibrate with the higher pressures and temperatures during a process called metamorphism. When cold, dense oceanic crust plunges beneath less-dense continental rocks, high-pressure metamorphism occurs in a subduction zone environment. Subduction zones bring surface rocks 1000s of km down into the mantle, but sometimes subducted rocks come back to the surface, even from depths of more than 100 km, where they can make diamonds! Even though the shiny minerals on today’s route are not diamonds, metamorphic petrologists think they’re even more special. This is because each metamorphic mineral is a direct witness of the depths, temperatures, and chemical environments in which they formed. By measuring the chemistry and structure of metamorphic minerals, geologists can reconstruct tectonic conditions of the past. Therefore, we know that there was a subduction zone that made these rocks and it was plunging towards the south about 45 million years ago. As the riders crest their climbs at Col du Telegraphe and then at Souvenir Henri Desgrange, they are traversing deeper and deeper into the ancient subduction zone that consumed a small ocean that separated Europe from the microcontinent called Brianconnais.

A subduction zone today

If these rocks in the Alps formed in a subduction zone 45 million years ago, why do we care about them at all? As with any geoscience field, looking at rocks from ancient systems teaches us about processes that occur today. Subduction zones make up over 55,000 km of the Earth’s plate tectonic boundaries, for example along the western edge of South America, up around the Pacific ring from the Aleutians to Japan and Sumatra. Subduction zones continuously recycle oceanic crust back into Earth’s interior, generate the largest and most destructive earthquakes on the planet (Magnitude 8 or 9 quake? That was a subduction zone!) and host the most explosive volcanic eruptions (like the recent one in Tonga). Studying subduction zones in the rock record can give us clearer insights into why earthquakes occur, what triggers volcanic eruptions, and how often these destructive events occur. Although these events are some of the most devastating natural hazards to our human perceptions, they are barely blinks of an eye in geologic time. Nevertheless, they show us that our Earth is a living, breathing system and its pulse persists over the eons. And the temporary end product of all this raw tectonic power makes for spectacular bike races!

My goal is to figure out how subduction zones get started, and after they start, how they change through time. I combine structural geology in the field and mineral and isotope chemistry in the lab (metamorphic petrology and geochronology) to reconstruct plate tectonic histories from ancient subduction zones. I’ve studied rocks from Greece, Oman, and Quebec, and I'm adding to the list! Check the GeoTdF-team...

Alissa Kotowski