Natural and volcanic site at Neyrac-les-Bains, Meyras in Ardèche

Opening : All year round.

Located on the thermal site of Neyrac-les-Bains, the Maar Doris is an explosion crater. This type of maar is created when a very hot magma (lava) 1200C° meets the water contained in the subsoil several hundred meters deep.

Part of the Maar Doris can be seen from the spa town of Neyrac-les-Bains or from the heights of Via Domitius, on the heights of Meyras.

To the trained eye, the Maar Doris corresponds exactly to the site of the spa. It acts as a natural sink, trapping nearby and adjacent waters, which is why the Neyrac-les-Bains spa was built in this strategic location. The carbon dioxide emitted in this natural trap adds to the special character of Neyrac-les-Bains a mofette (a place where carbon dioxide is released into the air at the surface), already known to the Romans. The maar structure is easy to spot from downstream of the car park at the foot of Neyrac-Ciné. It is even easier to see from the small ridge road linking Meyras to Thueyts.

How is a Maar created? When molten magma meets water, it produces water vapour, which, unable to escape, leads to an increase in pressure and thus a 1st explosion, then a second and so on until it reaches the earth’s crust. The rocky debris is then ejected several kilometres into the air. After the explosion, the walls collapse and partially fill in the crater. Several thousand years later, there are 2 possible scenarios: either the crater fills with water and becomes a lake (Lac d’Issarlès), or the water contained in the subsoil mixes with the earth and the result is a marshy depression also known as a narcre (case of the Maar Doris). The site of the spa, although known for a long time, has only very recently been interpreted as a maar crater. Moreover, it is not listed on the geological map published in the 1980s. It is a sub-circular device, the internal surface area of which is exactly that of the flat area at the heart of the station. The characteristic phreatomagmatic explosion products can be seen both by the roadside (just before entering the resort) and behind the spa. They consist almost exclusively of more or less pulverised basement fragments (gneiss, granite).