Aveyron is a well-known "département" in regards to its minerals' wealth and its very diversified landscapes.
One of its geological curiosity is the "burning mountain" of Cransac. The small town of Cransac is located at the end of the Decazeville coalfield, 9 km from it, in the heartland of a 300-hectare forest of locust trees, Black locusts (Robinia pseudoacacia, the real Latin name being Acacia fabaceae mimosoideae), the largest forest of this type in Europe. To note, donuts made with its flowers are excellent.
An important amount of sulfur is contained in Cransac's coal. When the mine was in operation from 1880 to 1962, it got nicknamed the "fire mine", as spontaneous bursts of fires were to be frequent. Humid air finds its way through the rocks; a chemical reaction follows with sulfides' oxidation and the coal and coal shale would enter in combustion.
This is a natural phenomenon. Cransac has been hence known for its hot gases found along natural ovens and moreover built a reputation around its thermal waters.
The gases are coming to air at multiple points around the Montet Hill, at a temperature of 120 C (248 F) to 180 C (356 F). They escape through cracks of the surrounding ground. As it has been on-going for so many years, one can see chimneys made of sulfur and alunite (hydrated aluminium potassium sulfate mineral). The Holed Rock, one of the largest of the blocks found around is quite interesting as a natural phenomenon. These coloured rocks have been transformed by temperature and pressure. When the shale is cooked it would become argilite—a fine-grained sedimentary rock such as indurated clay particles—then porcelanite—a hard, dense rock similar to an unglazed porcelain.
According to some historians, thermalism at Cransac would have started with the Romans. 1646, was the first time Claude Desbruyères, a medical doctor, mentioned in writing the area where huts had been erected along the fuming cracks. He subsequently hailed these steam rooms as being the best in France.
In 1789, at the start of the French Revolution, the utmost recognition came: the Cransac thermal waters of national and public interest. Cransac's springs found their climax under the reign of the Emperor Napoléon III. However, original springs have been disturbed by the coal mining starting in 1859. Hence thermalism became more of a curiosity as the industrial revolution was on-going and coal extraction was a priority.
Only the steam rooms were still in existence at the start of the 20th century. (See the postcard above from this era)
In 1961, as coal mining became not profitable anymore, mines got shut down. Some mine shafts got even blasted down.
Slowly, the town adjusted itself back into thermalism.
These thermalism springs are heavily charged with minerals. Rain waters find their way through these burnt and hot lands before being stopped underground, staying there at 24 C to 28 C (75.2 F to 82.4 F). These waters finally find their way out bearing fair amounts of calcium, iron, manganese and magnesium.
Nowadays, gases from down are brought back to surface via pipes. Patients with with osteoarthritis and other conditions such as rheumatic diseases are treated all year long.