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Earth of fire

Actualité volcanique, Articles de fond sur étude de volcan, tectonique, récits et photos de voyage

Publié le par Bernard Duyck
Publié dans : #"Readers' Words"

In the heart of the Massif Central, the volcanic massif of Cantal is the largest of the French stratovolcanoes, and one of the most important in Europe: with an area of ​​nearly 2,500 km², it has the shape of a low cone , 60 km. N-S for 70 km. E-O, at its base.

Its current maximum altitude at Plomb du Cantal culminates at 1,855 m. Projections allocate to the Cantal volcano an altitude of more than 3,000 meters before its major collapses.

It still has many spaces above 1,000 m. altitude, which gives it a strong mountain character.

Simplified geological section of the Cantal stratovolcano - Doc. Geological excursion in Cantal - AVG - one click to enlarge

Simplified geological section of the Cantal stratovolcano - Doc. Geological excursion in Cantal - AVG - one click to enlarge

The Cantal volcanic massif - Simplified geological map / BRGM- 1- Supracantal basalts 2- Phonolites 3- Debris flow deposits 4- Debris avalanche deposits 5- Hercynian basement 6- Trachyandesitic complex and post-avalanche trachyandesites 7- Basalts infracantaliens 8- oligo-miocene sediments

The Cantal volcanic massif - Simplified geological map / BRGM- 1- Supracantal basalts 2- Phonolites 3- Debris flow deposits 4- Debris avalanche deposits 5- Hercynian basement 6- Trachyandesitic complex and post-avalanche trachyandesites 7- Basalts infracantaliens 8- oligo-miocene sediments

The history of Cantal extends from the Miocene to the Pliocene, between -11 Ma and -3 Ma.

Over 8 million years, the eruptive activity has alternated effusive and explosive phases; eruptive materials – basaltic flows, explosion and collapse breccias, Strombolian projections and domes – piled up to form the Cantal massif, which was reshaped by glacial erosion in the Quaternary.

Saint-Flour - Infracantal basalt flow, in the lower town - - photo © J-M M. - click to enlarge

Saint-Flour - Infracantal basalt flow, in the lower town - - photo © J-M M. - click to enlarge

The town of Saint-Flour is built on a basaltic flow set up 8.8 Ma ago, prior to the construction of the Cantal stratovolcano itself (infracantal basalts) and therefore to the flows that covered the planèze of Saint-Flour ( supracantal basalts).

The Saint-Flour basaltic flow consists of an olivine basalt, rich in feldspars.

Saint-Flour - Infracantal basalt flow, in the lower town - photo © J-M M. - one click to enlarge

Saint-Flour - Infracantal basalt flow, in the lower town - photo © J-M M. - one click to enlarge

Saint-Flour - olivine basalt - photo © J-M M.

Saint-Flour - olivine basalt - photo © J-M M.

A single basaltic flow in 2 large parts, differentiated by cooling according to 2 geothermal gradients. which perpendicular to the surfaces of the flow and directed towards the heart of the flow…

The basaltic organs show a colonnade of fairly regular large prisms, surmounted by a very thick entablature of small confused prisms.

(Geological excursion in Cantal, by Gaston Godard.)

Saint-Flour - Infracantal basalt flow, in the lower town - photo © J-M M. - click to enlarge

Saint-Flour - Infracantal basalt flow, in the lower town - photo © J-M M. - click to enlarge

Coltines is located on the Planèze de Saint Flour, a basaltic plateau with a slight slope and approximately triangular in shape, bounded by the valleys of Lagnon, Alagnon and Epi(e).

The Planèze de Saint-Flour is made up of a supra-cantalian basalt rich in olivine.

The planèzes of Cantal - Doc. CPIE of Haute-Auvergne

The planèzes of Cantal - Doc. CPIE of Haute-Auvergne

Coltines – The Planèze de Saint-Flour - a supracantalian basaltic plateau - photos © J-M M. - one click to enlarge
Coltines – The Planèze de Saint-Flour - a supracantalian basaltic plateau - photos © J-M M. - one click to enlarge

Coltines – The Planèze de Saint-Flour - a supracantalian basaltic plateau - photos © J-M M. - one click to enlarge

Sources:
- Guide to the volcanoes of Europe and the Canaries / Cantal - M. Krafft and Larouzière, ed. Delachaux & Niestle
- Debris avalanche deposits in Cantal (France): witnesses
of the construction of the largest European stratovolcano of Miocene age - by Nehlig Pierre, Dardon Arnaud, Fréour Gwenael, Huguet David, Leyrit Hervé. In: Geomorphology: relief, process, environment. April-June, vol. 7, no.2. p.p. 107119.
- The volcanism of Cantal / BRGM
- Geological outing in Cantal 24 - 25 and 26 May 2015, under the direction of Gaston GODARD, Lecturer at the University of Paris-Diderot
- Geological excursion in Cantal, by Gaston Godard.

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