|
195 km |
Hills |
Volcanism of the Massif Central |
Stage 14 will lead the peloton through the eastern part of the old Massif-Central crystalline massif. But not all rocks are old: parts of today’s stage will come through volcanoes that are young (for a geologist)! The rocks of the Massif Central crystalline massif were formed during the collision of the Armorica and Laurasia continents during the Hercynian mountain building event between 335 and 300 million years ago. These crystalline rocks, many of granitic composition, were formed by cooling of 
large magma chambers related subduction of tectonic plates and the collision of continents. The granites used to be buried deep within the mountain range that was as high as the Himalayas today. These rocks are now exposed, because of ~300 million years of erosion. The start and the end of stage 14 will lead over the root of this old mountain belt (see Figure).
Young volcanism
After nearly 300 million years of erosion the area experienced a new phase of volcanism. Scientists do not entirely understand why this volcanism occurred: some think that it was associated to stretching of Central France (also making the Rhone rift), others think there was a plume of hot mantle material rising up below the Massif Central, like below for the Canary Islands making last year’s La Palma eruption. In any case, the rocks of the in the mantle below the crust were partially molten, and the molten rock rose to the surface as magma. The magmatic activity on the surface of the Massif Central started in the early Miocene at around 23 million years ago and continued until very recently with the youngest volcano erupting around 1000 BC. There are more than 400 volcanoes in seven main volcanic provinces of which the Chaîne de ‘Puys’ is most famous with its “Puy de Dôme” followed by the Mont-Dore, Cézallier, Cantal, Aubrac, Devez and Velay Oriental (see Figure). The Puy de Dôme is a Hors Categorie climb, where the likes of Coppi, Bahamontes, Gimondi, van Impe, and Zoetemelk won stages.
Volcanoes in the landscape
The volcanism throughout the Mio-, Plio and Pleistocene was highly variable in character varying from highly explosive to effusive (calm) resulting in a very different style of volcanoes 
now visible in the landscape. Explosive volcanism is associated with magma that is viscous (i.e., it is too syrupy to flow very well). These viscous magmas form steeper volcanoes built of dome structures, or when the magma interacts with water, enormous volcanic explosions can form deep round craters in the ground called calderas (like in Tonga last winter). Effusive eruptions are associated with more fluid magma resulting in less steep volcanoes built of lava flows and spatter (scoria) cones (like in La Palma and Iceland). The volcanic areas of the Massif Central are thus composed of large Etna style
volcanoes (the Cantal volcano was at least twice the size of Etna), volcanic plateaus, volcanic domes, small cones structures, but also explosive ‘maar’ calderas. Halfway stage 14, the peloton passes through Puy en Velay which is built on an old volcano. Two eroded volcanic necks, remnants of the volcano magma pipes, stand out in the landscape to the west of the D103: the ‘St Michel de Aiguille’ with its Chapel on top and the “Rocher Corneille’ with the ‘Notre-Dame de Puy’ Cathedral. Will today’s stage yield an explosive (Mathieu van der Poel, Wout van Aert) or an effusive (Thomas de Gendt, Nance Peters) winner?
I am a deep Earth geochemist using the composition of volcanic rocks to understand large-scale tectonic processes that control deep elemental cycles. I am specalised in analysing compositions of very small (<1mg) samples using advanced mass-spectrometry techniques. I analyse magmatic minerals, but also dust in ice-cores, precious archaeological artefacts and/or famous paintings. Check the Geo-TdF-team-2022.
Janne Koornneef