De la pierre au papier, une carte innovante du sous-sol de Paris et ses environs

From stone to paper, an innovative underground map of Paris and its surroundings

Educational geological map of Paris and its surroundings. BRGM
Frédéric Simien , BRGM and Nicolas Charles , BRGM

The emblematic zinc roofs of Paris, the white tiles of the metro, the iron of the Eiffel Tower, the stones of Notre-Dame and Haussmann buildings are some elements of an unknown mineral world which nevertheless reigns supreme over our capital and its surroundings.

Despite this discreet omnipresence, we rarely make the connection with the basement. However, in addition to these famous examples, many minerals are used in our daily lives (health, transport, construction, energy). Although recycling is developing more and more, the extraction of raw materials remains an industry that is still active in the Paris Basin.

In order to make it easy to visualize where our resources come from, the location of groundwater and the sectors that may present natural risks, we have developed a new geological map for educational purposes .

Mineral Resources Without Borders

Mineral resources have no boundaries and are not randomly distributed.

They are the result of a long geological history over several million years, a duration that is often difficult to grasp for most of us. Coming and going of the sea, sedimentation and erosion, extinctions and appearances of species, climate change...

The quarries around Paris are thus so many windows on the subsoil, which is difficult to observe in this plain region. Among other things, they have made it possible to develop the study of Earth sciences, such as paleontology (which studies fossils and therefore the biodiversity of the past) or stratigraphy (which studies the spatial and temporal arrangement of rock strata).

Paris: world cradle of geology

Lavoisier, Lamarck, Cuvier and so many others studied the Lutetian limestone, the beautiful stone of Notre-Dame de Paris (among others). This rock was formed around 40 million years ago, in what was then a warm, tropical sea!

It was extracted from the side of the Sainte-Geneviève mountain, at Montsouris, at the Trocadero, etc. To restore Notre-Dame Cathedral, it was necessary to find stones with very similar characteristics to replace those damaged during the fire.

Great scholars have also studied the quarries of the reliefs located to the north and east of the capital. A completely different resource was sought there: gypsum, extracted to make plaster in Montmartre, Belleville, Ménilmontant and the Buttes-Chaumont. Gypsum which is still exploited north of Paris.

It is interesting to note that former gypsum and limestone quarries are now green spaces and constitute havens of biodiversity. This is for example the case of the Montsouris park, the Arènes de Lutèce, the Trocadéro gardens, the Buttes-Chaumont park, etc.

Useful mineral resources

The extraction of mineral resources has always met the needs of our societies. This is particularly the case in the digital era and the energy transition, where the dematerialized world and new modes of mobility are major consumers of mineral materials. Current demand is therefore particularly high.

Take the example of concrete. To produce 1 m 3 , you need 1,200 kg of aggregates, 700 kg of sand, 300 kg of cement, 150 liters of water… and energy. To better integrate the environmental impact, it is also necessary to consider that the manufacture of a ton of concrete rejects 500 kg of CO 2 , and this without counting the transport of materials upstream (from the quarry) and once the product is finished.

Sand is thus the 3ᵉ resource consumed in the world after air and water.

As for gypsum, used to make plaster and cement, it is classified as wealth of national importance and European interest. A resource that is all the more strategic in that a very large part of the resources is difficult to reach due to rampant urbanization.

The gypsum quarry of Montmorency (Val d'Oise) is one of the largest in France (Société du Grand Paris).

Its consumption in France is around 100 kg per year and per inhabitant . With a production of 3 to 5 million tonnes per year, France is one of the main European producers, and two thirds are extracted in Île-de-France.

Water, a vital and… often underground resource

It's not just rock in the basement. There is also water!

In France, an inhabitant consumes an average of 150 liters of water per day . The daily needs for the Ile-de-France population are thus 1.8 million m 3 , or more than 500 Olympic swimming pools.

Much of the water we consume comes from groundwater. Knowledge of the subsoil therefore plays a major role in characterizing this vital resource.

In Île-de-France, there are several superimposed water tables, mainly in limestone and sandy soils. These aquifers are isolated by two major clay screens, often marked on the surface by the presence of springs from which the water has been captured and transported to the heart of the capital.

Thus, aqueducts have been built over the centuries to supply Paris, and have left traces in the landscape (you can also follow them on our map) or in place names (Arcueil, for example, takes its name from the arches of the 'aqueduct).

Interestingly, the compositions of mineral salts contained in the water vary according to the nature of the surrounding rocks. Between the north and the south of Paris, we do not find the same levels of sulphates, carbonates, calcium, magnesium and sodium. Because if the plateau of Longboyau (to the south) is made up of limestone from Champigny, it is gypsum which overcomes the clay level of the plateau of Romainville (to the north).

Developments and risks

Knowledge and control of the subsoil represent major challenges for the establishment of constructions and underground networks.

It is thus necessary to avoid the old quarries, the zones of dissolution of the gypsum, to pay attention to the landslides and to the sectors where the underground water tables are likely to go up, or to anticipate the phenomenon of shrinkage-swelling of the clays which can cause major disorders on the surface buildings.

As a result, the realization of monuments has sometimes represented technical challenges. Thus, the Sacré-Coeur basilica required the installation of 83 wells 33 m deep, subsequently filled with concrete. They are materialized on the cut appearing at the bottom of the card.

Cartographic renewal for the City of Light

As it is often difficult to represent the subsoil, and as reading geological maps requires advanced knowledge to understand them, we wanted to propose new documents.

View of the educational map of Paris and its surroundings. BRGM , Provided by the author

The interest of this new geological map of Paris is to make information on the subsoil accessible to a wide public in order to better understand the issues, the organization of the rocks and the different stages of their formation, their environment of formation, their deformation, or their alteration which have marked a long geological history.

Compared to a traditional geological map, it is easier to perceive the nature of the rocks, the location of the underground water tables and the sectors that may present natural risks (cavities, erosion, landslides, etc.).

All these elements make it possible to better understand the environment and the organization of the Parisian underground, so present in daily life without the inhabitants always being aware of it. Starting with the card paper! (Yes, it also contains minerals, such as kaolin).

The National Geology Days will take place from May 20 to 22, 2022. Find all the appointments here . The Conversation

Frédéric Simien , Head of Publishing, BRGM and Nicolas Charles , Geologist, PhD, BRGM

This article is republished from The Conversation under a Creative Commons license. Read the original article .

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