The cycling Tour de France has just passed the passes of the Pyrenees, and it's time to talk about the regional volcano, the Pic du Midi d'Ossau.
This proud Béarn volcano, close to the border with Spain, remains an exceptional case in the Pyrenees.
The Pic du Midi d'Ossau, from Lake Gentau - Photo Capbourrut / Personal work, CC BY-SA 4.0, commons.wikimedia.org
The region is rich in 400 million years of geological history, from the Devonian series to the glacial reliefs which give their forms to the current valleys:
* from -410 to -360 Ma: it is occupied by a tropical sea of the southern hemisphere. In particular limestones are formed there (Pic Castérau, Pène Peyreget)
* from -360 to -290 Ma: this is the period of collision and the formation of a mega-continent, "Pangea"; the world-wide Hercynian mountain range is forming. The previously formed sedimentary series are folded (fold of the Pic Castérau, visible when we go around the lakes)
The Pic du Midi d'Ossau, south face - Photo Capbourrut / Personal work, CC BY-SA 4.0, commons.wikimedia.org
* from -300 to -250 Ma: the Hercynian range undergoes erosion. The volcanism of Ossau and Anayet takes place in a phase of extension which follows the Hercynian compression.
During the Permian, the formation of grabens and the opening of fractures causes the rise of the deep magma from the upper mantle, thus forming a magmatic chamber which flows out on the surface and constitutes the Ossau volcano. The volcanism of Ossau takes place in Autunian in two episodes dated at 278 ± 5 and 272 ± 3 Ma.
Formed of andesite, the volcano sees prolonged emissions of volcanic lava gradually emptying its magmatic chamber. During an eruption - certainly very violent - the roof of the volcano collapses in the magma chamber, constituting the caldera. The volcanic activity then resumes through marginal cracks at the level of this caldera, which form the annular wall made up of dacite and rhyolite. The body of the Pic du Midi d'Ossau then forms at the level of this ring. The volcanic activity of Ossau ceased in the Permo-Triassic, about 250 million years ago.
The Permian continental deposits bear witness to a hot and arid continent.
The Ossau caldera before the formation of the Pyrenees and its deformation during the establishment of the mountain range - Doc. Capbourrut / Personal work, CC BY-SA 4.0, commons.wikimedia.org
* from -250 to -100Ma: the Iberian plate on which we are in Ossau probably remained emerged to be finally invaded by the sea. Massive limestones settle there.
* from -60 Ma: the collision of the Iberian and European plates leads to the formation of the present-day Pyrenees. The Ossau caldera is dislocated in the overlaps caused by compression.
Several intrusive arches can still be identified: the Moundelhs arch, the Peyreget arch, and the Ayous arch. The Pic du Midi d'Ossau is part of the intrusive arc of Moundelhs, the peak forms a laccolite, that is to say a very significant thickening of the annular vein. During the formation of the Pyrenees, this laccolite is tilted and then lifted to a height much higher than the rest of the old caldera. The Moundelhs arc overlaps that of Peyreget at the level of the Peyreget pass, a central area of about 200 ha is emerging between these two arches, at the level of the Embarradère and Moundelhs cirque. This space represents the heart of the caldera, the crater of the Ossau volcano
* from -2.6 Ma, period during which homo habilis fished its first tools, to -12000 years: glaciations model the Pyrenean landscapes.
Below is a summary in pictures of the geological history of the Pyrenees, provided by the Parc Nationla des Pyrénées. Good reading.
- BIXEL F. (1984) - The Stephano-Permian volcanism of the Pyrenees. Thesis. Toulouse.
- Information saga - Birth, life and death of a volcano: the paleovolcano at Pic du Midi in Ossau Dominique Rossier, member of SAGA.
- L'Ossau, from Bious Artigues to the lakes of Ayous 2019 UTLA - geology course in the field in partnership with GéolVal
- Pyrenees National Park.
The Ossau volcano - Doc. Pyrenees National Park