Located at Neyrac spa, this is an explosion crater. Volcanoes of this type happen when very hot magma at 1,200C° (lava) meets water several hundred meters below the earth.
This encounter will cause the water vapor to be trapped, whom can’t escape and will cause an increase in pressure and therefore a first explosion, then a second and so on until reaching the earth’s crust. The rocky debris is then ejected several kilometers above sea level. After the explosion, the walls collapse and partially collapse the crater. Several thousand years later, we obtain two cases: either the crater fills with water and becomes a lake (such as the Issarlès lake), or the water contained in the basement mixes with the earth and there will be a marshy depression also called narcre (case of Maar Doris).
The site of the spa, although known for a long time, has only very recently been interpreted as a crater of maar. Moreover it is not listed in the geological map published in the 80s. It is a device of sub-circular shape, whose internal surface is rigorously that of the flat located in the heart of the station. The structure of the maar is perfectly identifiable from downstream of the parking located at the foot of “Neyrac-Ciné”. It stands out even better from the small ridge road connecting Meyras to Thueyts. The phreatomagmatic explosion products, quite characteristic, are observable both along the road (just before entering the station) and behind the spa. They consist almost exclusively of fragments of the base more or less pulverized (gneiss, granite)
All year round.