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Earth of fire

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

"readers' words"

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

Leaving Auvergne, on the road back to Belgium passing between Issoire and Besse, the visit of a troglodyte fortress dug in the heart of the volcanic tuff allows a beautiful discovery: the caves of Jonas.

 

The Jonas fissural volcano, with its aligned cinder cones, and the nearby volcano, Saint-Pierre peak, have mixed pyroclasts with the lava to form a red tuff soft enough to be dug down to a depth of 5 meters.

The cliff is 500 meters long and 100 meters high; it is the result of volcanic activity that took place between -18 and -3Ma.

The fissural Jonas volcano, with its aligned cinder cones and Saint-Pierre peak - photo © J-M M. - one click to enlarge
The fissural Jonas volcano, with its aligned cinder cones and Saint-Pierre peak - photo © J-M M. - one click to enlarge

The fissural Jonas volcano, with its aligned cinder cones and Saint-Pierre peak - photo © J-M M. - one click to enlarge

Cliff and troglodyte site of Jonas - photo © J-M M. - one click to enlarge

Cliff and troglodyte site of Jonas - photo © J-M M. - one click to enlarge

The cliff was first dug in the 4th century BC by the Celts, who established a sanctuary there. The Romans took over the place, then Christian monks made an oratory there.

The caves served as a refuge during the Barbarian invasions, between 850 and 915.

At the beginning of the 12th century, the knight Anet Dalmas de Jaunac, owner of the land, had a stronghold built there, capable of resisting invaders and controlling the valley.

From generation to generation, the site will prosper to pass in 1683 to the Montal-Nozières family.

The caves were classified as Historic Monuments in 1886.

Jonas troglodyte site - banal oven - photo © J-M M. - one click to enlarge

Jonas troglodyte site - banal oven - photo © J-M M. - one click to enlarge

Troglodyte site of Jonas - In the 10th century, an oratory became a chapel, now called Saint-Laurent church. - photo © J-M M. - one click to enlarge

Troglodyte site of Jonas - In the 10th century, an oratory became a chapel, now called Saint-Laurent church. - photo © J-M M. - one click to enlarge

Jonas' frescoes are the oldest known in Auvergne. (9°-11° century) They represent the episodes of the end of Christ's life. In a cul-de-four, a Virgin in majesty with long hands and slender fingers, seated on a cathedra, holding the child in her lap - photo © J-M M. - one click to enlarge

Jonas' frescoes are the oldest known in Auvergne. (9°-11° century) They represent the episodes of the end of Christ's life. In a cul-de-four, a Virgin in majesty with long hands and slender fingers, seated on a cathedra, holding the child in her lap - photo © J-M M. - one click to enlarge

The frescoes of Jonas date from the 9th to the 11th century. They represent episodes from the end of Christ's life.

Only a few pigments were used: red ocher, yellow ochre, black and white (lime).

 

Sources:
- Sacred places – Canalblog – Saint-Pierre-Colamine – Jonas caves
- Auvergne Center France – the troglodyte site of Jonas
- Wikipedia - Caves of Jonah

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Publié le par Bernard Duyck
Publié dans : #"Readers' Words"
Cantal Massif - Le Puy Mary - photos © J-M M.

Cantal Massif - Le Puy Mary - photos © J-M M.

Puy Mary consists of the remains of a cumulo-dome dated 7.2 Ma (M.Krafft), largely eroded by glaciers.

It consists of benmoreite, a viscous lava rich in phenocrysts of plagioclase, sanidine, and brown hornbland, embedded in a paste of microcrystals of sanidine, biotite and tridymite. Veins of basalt or phonolite lard it.

From the summit, at 1,783 m., the entire summit part of the Cantal massif can be discovered.

Cantal Massif - panorama of the summit in autumnal colors - photos © J-M M. - one click to enlarge

Cantal Massif - panorama of the summit in autumnal colors - photos © J-M M. - one click to enlarge

On the way back, Salers, one of the most beautiful villages in France, can be discovered along the cobbled streets.

This city, fortified during the Hundred Years War, preserves ancestral houses in dark stones, with turrets and slates, architectural characteristics of the region, which are completed by a remnant of a rampart and its belfry.

The statue of Tyssandier d'Escous, renovator of the Salers cattle breed, with a dark mahogany robe and light lyre-shaped horns, enhances part of the main square.

Salers - the door of the belfry - photos © J-M M.

Salers - the door of the belfry - photos © J-M M.

Salers - Place Tyssandier d'Escous - photos © J-M M. - one click to enlarge

Salers - Place Tyssandier d'Escous - photos © J-M M. - one click to enlarge

Salers - It is good to walk there and have a tasting on the terrace - photos © J-M M. - one click to enlarge

Salers - It is good to walk there and have a tasting on the terrace - 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
- Salers - an architectural and historical heritage.

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

In the heart of Cantal, the Cère valley has been deepened into umbilicus, separated by locks of hard breaches. These locks have been cut by the rivers in gorges, sometimes in waterfalls.

 

The Faillitoux waterfall, in the Thiézac region (Aurillac), crosses a flow of ankaramite (*) prism vertically. The prisms, 0.5 m in section at the base, rise to the full height of the cliff.

This flow was dated at - 9.5 +/- 0.5 Ma (K/Ar).

The waterfall is fed by the Lasmolineries stream which rises on the southern slope of the Élancèze and flows into the Cère.

Thiézac - the Faillitoux waterfall and the prism ankaramite flow - photo © J-M M. - one click to enlarge

Thiézac - the Faillitoux waterfall and the prism ankaramite flow - photo © J-M M. - one click to enlarge

Thiézac - the Faillitoux waterfall which drops 41 m. on the prism columns of ankaramite - photo © J-M M.

Thiézac - the Faillitoux waterfall which drops 41 m. on the prism columns of ankaramite - photo © J-M M.

(*) Ankaramite is a volcanic rock close to basalts and basanites. It is a dark rock (melanocratic), with a porphyritic microlithic structure. The phenocrysts include black automorphic olivines and clinopyroxenes (= augite) which can reach 2 cm in size. The matrix contains microlites of the same nature to which are added plagioclases associated with traces of titanomagnetite and biotite.

It belongs to a flow interbedded in trachyandesitic breccias and topped by trachyandesites and supracantal basalts.

The extension of the glaciers in Cantal - in orange dotted line, the extension limit of the Cantal glaciation during the Würm glacial maximum (-20,000 years). - Doc. The volcanism of Cantal / AVG

The extension of the glaciers in Cantal - in orange dotted line, the extension limit of the Cantal glaciation during the Würm glacial maximum (-20,000 years). - Doc. The volcanism of Cantal / AVG

Le Chaos de Casteltinet, on the left bank of the Cère:

The landslides of large volumes of breccias that occurred during the disappearance of a glacier 12,000 years ago, as a result of decompression phenomena on the edges of the glacial valley and water infiltration from the liberated plateaus first, forming listric faults (**) which destabilized the cliff.

The upper part of the cliff which overlooks this chaos is made up of prismatic columns corresponding to an old trachyandesitic flow which overcomes mud and debris flows (piling of lahars with carbonized wood) interstratified with lava flows, themselves overcoming sandstone formations.

The breach, thus detached, takes on the phantasmagorical shapes of needles, of pyramids. The "lion's door" is an arch made of these debris and which evokes this animal.

(**) : A listric fault is a normal fault of great spatial extension which is concave at depth, involving a deep shear level.

Le Chaos de Casteltinet - the "lion's door" - photo © J-M M. - one click to enlarge

Le Chaos de Casteltinet - the "lion's door" - photo © J-M M. - one click to enlarge

Le Chaos de Casteltinet - the "lion's door" - photo © J-M M.

Le Chaos de Casteltinet - the "lion's door" - photo © J-M M.

A quick look at the passage to Puy Griou.

It is a phonolite dome located in the center of the remains of the Cantal stratovolcano. It would have been in place 6 Ma ago, after the paroxysmal eruptive phases of Cantal (between 8.5 and 6.5 Ma).

The dome was remodeled under the action of freezing and thawing which cut it into slates piled up in a scree sleeve.

Puy Griou - photos © J-M M. - one click to enlarge
Puy Griou - photos © J-M M. - one click to enlarge

Puy Griou - photos © J-M M. - one click to enlarge

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Publié le par Bernard Duyck
Publié dans : #"Readers' Words"
The three necks aligned - from left to right: Bredons and its church, Bonnevie, and Chastel-sur-Murat. - photo © J-M M. - one click to enlarge

The three necks aligned - from left to right: Bredons and its church, Bonnevie, and Chastel-sur-Murat. - photo © J-M M. - one click to enlarge

Murat is a very old city, cited in the texts of the evangelization of Auvergne from 270.

Human traces have been discovered on the rock of Chastel-sur-Murat, dating from the Middle Neolithic.

In the Celtic language, Murat could mean "steep rock ".

In the Middle Ages, the fortified town nestled under the rock of Bonnevie, the summit of which was occupied by a castle, demolished by Richelieu in 1633.

Sheltered by its walls, this city was very active: fairs and markets, crafts, liberal professions, inns, etc... During its rich historical past, Murat was fortified on many occasions: there are no less of three successive enclosures.

In 1944, the city was one of the highest places of the Cantal Resistance. Today, this small industrial center is a highlight of Cantal tourism.

To see the old houses of the XVIIth century, with arcades in handle of basket on column, nicely covered with lauzes and typical of the constructions of the city.

Murat - its volcanic stone houses and slate roofs lean against the neck of Bonnevie with its prismatic summit - photo © J-M M. - one click to enlarge

Murat - its volcanic stone houses and slate roofs lean against the neck of Bonnevie with its prismatic summit - photo © J-M M. - one click to enlarge

The city is framed by three basaltic peaks:
- the rock of Bonnevie, in the North, carries a virgin. The basalt organs are the finest in Europe,
- the rock of Bredons, to the south-east, whose 11th and 12th century church was built with very beautiful freestone,
- the rock of Chastel, in the North-West.

These rocks, born from the lava, have been transformed into necks thanks to erosion.

Murat - Bonnevie's neck - photo © J-M M. - one click to enlarge
Murat - Bonnevie's neck - photo © J-M M. - one click to enlarge

Murat - Bonnevie's neck - photo © J-M M. - one click to enlarge

The 3 necks, composed of basanite, line up on the same eruptive fissure.

These three bare necks correspond to fillings of diatremes, as shown by the breccia sheaths surrounding their base. They are therefore the result of explosive phreatomagmatic volcanism which pulverized the basement and led to the formation of maars, which were then filled with magma. (Geological output – G.Godard / in source)

Murat - Neck de Bredons - prismation in sheaves under the summit - photo © J-M M. - one click to enlarge
Murat - Neck de Bredons - prismation in sheaves under the summit - photo © J-M M. - one click to enlarge

Murat - Neck de Bredons - prismation in sheaves under the summit - photo © J-M M. - one click to enlarge

The Neck of Bredons carries the priory church of Saint-Pierre and Saint-Paul de Bredons (built in the 11th century then fortified in the 14th-15th centuries).

It has prismations arranged in sheaves clearly visible from the side of the road, oriented horizontally just above it, then almost vertically below the summit.

Chastel-sur-Murat - the finely prism neck - photo © J-M M. - one click to enlarge

Chastel-sur-Murat - the finely prism neck - photo © J-M M. - one click to enlarge

Chastel-sur-Murat - scree at the foot of the neck - photo © J-M M. - one click to enlarge

Chastel-sur-Murat - scree at the foot of the neck - photo © J-M M. - one click to enlarge

Chastel-sur-Murat - Saint Antoine Chapel (12th and 16th centuries) - photo © J-M M. - one click to enlarge

Chastel-sur-Murat - Saint Antoine Chapel (12th and 16th centuries) - photo © J-M M. - one click to enlarge

The necks of Bredons, Bonnevie and Chastel-sur-Murat are chronologically linked to the eruptions of the supracantalian planezes.

The Bredons nepheline basanite, highly deficient in silica, is - 4.3 +/- 0.1 Ma old (K/Ar dating). It is one of the most recent in Cantal.

 

Source :

Geological outing in Cantal 24 - 25 and 26 May 2015, under the direction of Gaston Godard, Lecturer at Paris-Diderot University

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

Near Issoire, the village of Usson is built on an old basalt peak, which culminates at 639 meters.

This surprising geological formation is linked to a phreatomagmatic eruption, resulting from the contact of lava with water. The region was then a vast lacustrine zone, in which the magma made its way.

This volcanism began 25 Ma ago, and is older than the volcanic events that gave rise to the volcanoes of Cantal, Mont-Dore or those of the chaîne des Puys .

The volcanic organs of Usson - photos © J-M M. - one click to enlarge
The volcanic organs of Usson - photos © J-M M. - one click to enlarge

The volcanic organs of Usson - photos © J-M M. - one click to enlarge

The organs of Usson - "Prisms for building" - explanatory panel - photo © J-M M. - one click to enlarge

The organs of Usson - "Prisms for building" - explanatory panel - photo © J-M M. - one click to enlarge

The organs, located south of the village, are the remains of the deep part, the chimney of an old volcano which has now disappeared under the effect of erosion. Usson's vertical organs are cooled from above, not from the sides of the chimney. The regularity of the prisms is also associated with a rather slow cooling.

 

The village, classified among the most beautiful villages in France, was built thanks to the hexagonal prisms, and the arkose, visible in the walls of the church of St Maurice.

The Castle of Usson, demolished on the order of Cardinal de Richelieu, had previously been, from 1585 to 1605, the place of house arrest of Marguerite de Valois (1553-1615), nicknamed by Alexandre Dumas Queen Margot, first wife (1572-1599) of King Henry IV of France.

The volcanic organs of Usson - photo © J-M M. - one click to enlarge

The volcanic organs of Usson - photo © J-M M. - one click to enlarge

The Saint Maurice church on the hill of Usson - photo © J-M M.

The Saint Maurice church on the hill of Usson - photo © J-M M.

"Prisms for building" - a basalt wall - photo © J-M M. - one click to enlarge

"Prisms for building" - a basalt wall - photo © J-M M. - one click to enlarge

The Château du Sailhant, in the commune of Andelat, was built in the 13th century on a triangular basalt spur, about 20 to 25 meters high.

It is also known for the presence of the Sailhant (or Babory) waterfall nearby.

 The Babory stream flows from a height of about twenty meters at this point into a chasm whose depth is estimated between 6 and 7 meters.

Sailhant castle - photo © J-M M. - one click to enlarge

Sailhant castle - photo © J-M M. - one click to enlarge

Sailhant or Babory waterfall - photos © J-M M. - one click to enlarge
Sailhant or Babory waterfall - photos © J-M M. - one click to enlarge

Sailhant or Babory waterfall - photos © J-M M. - one click to enlarge

Sources:
- Futura - The Exploratorium: the organs of Usson in the land of volcanoes.
- Sailhant castle - wikipedia

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

Before definitively stopping writing my blog, for various personal and technical reasons, I am sharing for a few days the photos of my Belgian volcanophile friends, all recent, from their visit to the Cantalien Massif (France), via La Limagne .

 

Some sites of La Limagne, to whet your appetite!

The cliffs of Jussat and the stromatolites:

Limagne is the region of France richest in fossil stromatolites, of ages corresponding to the Upper Oligocene / Lower Miocene.

The Jussat deposit presents a cliff made up alternately of sand with quartz levels, and others of marl and limestone, with cauliflower balls of stromatolites, created by colonies of cyanobacteria, overhanging a less resistant layer. to erosion.

Jussat cliff - overhanging, its ball-shaped stromatolites - photo © J-M M. - one click to enlarge
Jussat cliff - overhanging, its ball-shaped stromatolites - photo © J-M M. - one click to enlarge

Jussat cliff - overhanging, its ball-shaped stromatolites - photo © J-M M. - one click to enlarge

The necks of Buron and Saint-Babel:

They were formed around 20 million years ago, when a rise of magma through sedimentary rocks dating from the Tertiary period formed a lava lake. This lake then cooled and solidified, then was revealed by the erosion of the sedimentary rocks that surround it, very loose and friable.

The neck of Saint-Babel overlooks an area rich in peperites, which result from the encounter between magma and water (underground or surface), during a phreatomagmatic eruption.

Neck de Buron - photo © J-M M. - one click to enlarge

Neck de Buron - photo © J-M M. - one click to enlarge

Neck of Saint-Babel - photo © J-M M.  - one click to enlarge

Neck of Saint-Babel - photo © J-M M. - one click to enlarge

Neck of Saint-Babel - photo © J-M M.  - one click to enlarge

Neck of Saint-Babel - photo © J-M M. - one click to enlarge

Neck of Saint-Babel - photo © J-M M.  - one click to enlarge

Neck of Saint-Babel - photo © J-M M. - one click to enlarge

Neck of Saint-Babel - detail - photo © J-M M.  - one click to enlarge

Neck of Saint-Babel - detail - photo © J-M M. - one click to enlarge

Sources:

- Guide to the volcanoes of Europe and the Canary Islands / La Limagne - M. Krafft and Larouzière, ed. Delachaux & Niestle.

- Planet Terre: Auvergne, a museum of stromatolites / The cliffs of Jussat (Chanonat, Puy de Dôme).

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