The Tour de France Femmes starts today in the centre of Clermont Ferrand, home to a volcanic cathedral. The area around Clermont Ferrand is well-known by geologists for the Chaine des Puys (see stage 9). To remind you: it’s a 32 km long chain of 80 volcanoes. The Puy de Dôme is the most famous and prominent one.
As the helicopters circle around the starting point, you will certainly get a good view of the chain. However, your attention will soon also be drawn to the great black Romanesque-Gothic cathedral, Notre Dame de l’Assomption. It towers over the city. The enormous, dark coloured cathedral from the thirteenth century with its prominent spires and delicate architectural details is an iconic sight. This is not at least because of the special type of rocks that are used for its construction. It’s a volcanic cathedral!
The rocks of the cathedral
The complete walls and spires of the cathedral are constructed from blocks of black solidified lava, called Pierre de Volvic. Due to its hardness and homogenous texture, containing numerous small irregularly shaped bubbles (vacuoles), this basaltic rock is both a strong and lightweight material that is easy to sculpture. The master builders were lucky to have this excellent building material in their vicinity. It allowed them to design such a huge cathedral with slender and high towers and a lot of little details.
In the era our volcanic cathedral was built (from 1248) it was of course impossible to transport heavy materials overland over longer distances. The builders had to rely on local resources. Pierre de Volvic is quarried about 20 km northwest of Clermont Ferrand. The riders reach the village Volvic, mainly known in France for its mineral water source, near the end of today’s ride. The rocks are quarried from a lava flow of the northernmost volcano of the Chaine des Puys, the Puy de la Nugère (west of Volvic).
The volcanic origin of the building material
Our volcanic cathedral is built from – eh yes – volcanic building material. The Puy de la Nugère is an extinct stratovolcano. It had a complex history with several eruptions that resulted in the formation of a cylindrical cone with a layered structure. Today the volcano is covered by a dense forest but its structure with its 82 metres wide main crater is still visible in the landscape.
The lava from which the building stones of the cathedral originated was released during one of the most recent eruptions 10’000 years ago. It had a very high gas content (water, carbondioxide and sulfurdioxide) and the eruption was thus highly explosive. A small part of the gas was trapped in the lava. However, the major part was released into the atmosphere in a dense cloud, together with aerosol particles, droplets and ash. A prominent example of a similar volcano that is still active is the Eyjafjallajökull in Iceland. You might remember that it released such a big ash cloud during a recent eruption in 2010 that air travel throughout the world was disrupted for several days. Another example is he Mount Tambora. This eruption caused a global climate anomaly. That resulted in an agricultural disaster and famine in central and southern Europe and northeast America.
Pioneer: Katia Krafft
To experience the intensity of volcanic eruptions, you should watch the following trailer from a 2022 film about the lives and careers of pioneering volcanologist Katia Krafft and her husband Maurice.
The couple documented and provided photographic and film material of almost all volcanic eruptions between 1975 and 1992. They also wrote several books about active volcanoes around the globe. Besides that, they educated the population living in the vicinity of the volcanoes about the potential hazards. Katia Krafft contributions are remarkable. She was among the few volcanologists who ventured to the very edge of the craters to measure gas emissions and to collect mineral samples.
Building the future
There are several other rock types with a volcanic origin but with different properties, like granite or tuff, that are used in construction for variable purposes. Nowadays, only a neglectable part of new buildings are constructed using natural stone. Concrete is by far the most popular building material. However, the global production of concrete is on itself responsible for 8-10% of the global carbon emissions. Could local natural building stones be a CO2 neutral alternative for today’s building industry?