Geoconservation Research
Volcanism & Geoconservation
2023, Volume 6 / Issue 1 / pages(207-232)
Original Article
Jean-Paul Raynal1, Emmanuelle Defive2, Nicolas Klee3,*, Roxane Buso4, Didier Laporte4
1 Université de Bordeaux, CNRS, UMR 5199 PACEA, Pessac, France
2 Université Clermont Auvergne, CNRS, Université de Limoges, GEOLAB - F-63000 Clermont-Ferrand, France
3 Parc naturel régional des Monts d'Ardèche, Géoparc mondial UNESCO, 07 380 Jaujac, France
4 Université Clermont Auvergne, CNRS, IRD, Laboratoire Magmas et Volcans, OPGC, 63000 Clermont- Ferrand, France
Abstract
Nicolas Klee Parc naturel régional des Monts d’
Ardèch, France Email : [email protected]
The Regional Natural Park of the Monts d’Ardèche, located in south-eastern France, became the Monts d’Ardèche UNESCO Global Geopark in September 2014. This territory possesses significant volcanic features dating from the Miocene to the late Pleistocene. The UNESCO Global Geopark label helped to formalize a long-standing partnership with the University of Clermont Auvergne which includes support for scientific research, establishing conservation and protection priorities, establishing geosites for the public and involvement of local people and communities in geotouristic initiatives. Here we focus on some peculiar geosites that allow us to question magmatic processes, eruptive dynamics, morphological evolution of landscapes, the chronology of eruptions, relationships between humans and volcanoes during the Pleistocene, and highlight the delicate alliance between economic pressures, heritage conservation, and scientific tourism.
Keywords:UNESCO Global Geopark, Ardèche, Haute-Loire, Velay oriental, Bas-Vivarais, Volcanism, Magmas,Cchronology
© Author(s) 2023, this article is published with open access at https://gcr.isfahan.iau.ir/
This work is licensed under a Creative Commons Attribution 2.0 Generic License.
e-ISSN: 2588-7343 p-ISSN: 2645-4661
Geoconservation Research
How to cite: Raynal JP, Defive E, Klee N, Buso R & Laporte D (2023). A tale of old and young volcanoes in Monts d’Ardèche UNESCO Global Geopark (South-Eastern France). Geoconservation Research. 6(1): 207-232. doi: 10.30486/
gcr.2023.1983724.1136
First Publish Date:2023-07-04
Accepted:2023-06-28
Received:2023-04-12
DOI:10.30486/GCR.2023.1983724.1136
Article information
The Regional Natural Park of the Monts d’Ar- dèche, located in south-eastern France on the east- ern range of Massif Central, obtained the status of UNESCO Global Geopark in September 2014. At this point, it became integrated with the network
of 177 UNESCO Global Geoparks throughout the world. It is the fifth labeled geopark in France out of the seven in this country and the only one with a clear volcanic heritage (39 of the 61 geo- sites; Fig. 1). The UNESCO Global Geopark label promotes territories with international geological
significance, and which combine protection, ed- ucation, geotourism and sustainable development by involving local communities and various ac- tors. Amongst its many commitments, the Monts d’Ardèche UNESCO Global Geopark is strength- ening its involvement in scientific research and outreach to better understand and promote its young and old volcanic heritage. The Geopark addresses several scientific questions, for ex- ample, magmatic processes, eruptive dynamics, morphological evolution of landscapes, and re- lationships between humans and volcanoes. We develop here three significant examples of geo-
.sites which fully illustrate these different topics Monts d’Ardèche UNESCO Global Geopark has been associated with volcanism for 10 Ma (million years). The volcanic activity is diverse, providing many different landscapes including strombolian craters, phonolite extrusions, maars, dykes, necks, and basaltic flows, all of which have shaped the landscape we observe today (Klee 2022). Three major volcanic episodes have been identified in Monts d’Ardèche and these created different vol- canic provinces.
The first episode created the Eastern Velay volca- nic province. It began (10-8 Ma) in Mont Mézenc- Mont Gerbier with the building of a vast plateau made of piled basaltic to trachy-andesitic lava flows. During the second episode (8-6 Ma), tra- chytic to phonolitic magmas created intrusions and extrusions (domes and peaks locally called “Sucs”). These forms and the later intense erosion have shaped a unique and scenic landscape, includ- ing one of the largest phonolitic massifs in Europe. The best-known places are Mont Gerbier de Jonc, easily identifiable with its typical sugarloaf shape which hosts the source of the Loire River, and Mont Mézenc, which reaches an altitude of 1753 m (Fig. 2).
At the same time (8-6 Ma), another volcanic prov- ince formed, the nearby massif of Coiron, which is characterized by more fluid basaltic lavas fos- silizing the paleo-course of the Ardèche River which ran towards the Rhône Valley. The land- scape exhibits various features here including in- truded dykes, volcanic necks, and lava flow stacks (Fig. 3).
The last major period began some 200,000 years ago and created the Bas-Vivarais volcanic prov- ince (named also the “Ardèche Young Volcanoes”) (Fig. 4). These eruptions have generated almost perfect strombolian volcanoes, still easily visible in the landscape despite the vegetation cover (Fig. 5 A). Most of them are relatively young, aged be- tween 40,000 and 12,000 years ago depending on the dating methods used. The oldest ones were contemporary with Neandertal occupation, then with Homo sapiens prehistoric settlements in Ar- dèche, notably marked by the Chauvet cave mural drawings that are known to be part of the oldest in the world (36,000 years) and certainly one of the first chef-d’œuvres (Pastre et al. 1994; Sanzelle et al. 2000; Raynal & Defive 2019 and references herein). While cooling down, lava flows originat- ing from these volcanoes created basalt columns showing a remarkable prismatic shape (columnar jointing; Fig. 5B), well described by the pioneers of volcanology: Faujas de Saint-Fond (1778), Gi- raud-Soulavie (1780) and Poulett-Scrope (1827). The longest lava flow is about 21 km long, starting at the Ray-Pic waterfall and ending at Pont-de-La-
baume. Among the young volcanoes, there are a number of maars, created by intense and violent phreatomagmatic activity. The largest one in Monts d’Ardèche is the Vestide du Pal, with a diameter of
1.7 km and a circumference of over 5 km, which is one of the largest maars in continental Europe.
The “basaltic and prehistoric cliff of Longetraye”
is the 55th geosite of the Monts d’Ardèche UN- ESCO Global Geopark (Fig. 1). It combines the highest prehistoric site in the Massif Central (1210 m) with an exceptional geological situation that allows us to observe the Miocene morphological context, the initial volcanism of the Mézenc pla- teau, and the impact of Pleistocene climatic vari- ations on the landscape (Fig. 6). It thus makes it possible to question current climate change and the economic future of this mid-mountain area.It is located on the south-western edge of the Mézenc massif, in the heart of the Eastern Velay volcanic province, where basaltic to trachy-andesitic lava
flows, emitted between 10 and 8 Ma, formed a vast plateau above the granitic basement (Fig. 2). Be- tween 8 and 6 Ma, the latter was pierced by trachyt- ic to phonolitic lavas (domes, dome-flows and lava flows) which form all the visible summits. This ensemble is extended to the south and southeast by plateaus gently sloping towards the northwest (1300 m to 1250 m approximately; Defive 1996
; Defive et al. 2011). At Longetraye, two basaltic
flows with regular columnar jointing and thick en- tablature are stacked. The upper one was dated by the Potassium/Argon (K/Ar) method at 8.81 ± 0.13 Ma (Defive et al. 2023; Raynal et al. 2023) and has been quarried since the 1970s. These lavas covered a poorly differentiated topography, in the hollows of which were present clayey-sandy lacustrine de- posits, themselves covered by coarse alluviums with cherts and quartz pebbles which were depos-
ited under warm climates in a very wide, shallow, cradle-shaped valley, the axis of which lies slightly further west of the site.
At the time the Longetraye lava flows were emit- ted, the Orcival Valley did not exist, and we must imagine then a vast plateau on which the water- courses settled and then became entrenched, down- cutting the lava flows and the granitic basement. This happened under the hot climatic conditions of the end of the Miocene, then during the final- ly cooling climates of the Pliocene, none of these conditions were favourable to the development of narrow valleys. This gave birth to the slopes that frame the Orcival Valley today.
The last phases of their evolution took place in the cold atmosphere of the Quaternary glacial times, which lasted several tens of thousand years and were separated by shorter, more temperate inter- glacials, such as the Holocene in which we live. During the last of these glacial episodes, all the massifs of the western Massif Central were gla- ciated (Defive et al. 2019; Ancrenaz et al. 2022; Ancrenaz 2023) but most authors consider this sector of the Mézenc massif evolved under peri- glacial but not glacial conditions, i.e. in very cold and more or less dry environments, marked by the great efficiency of all the erosion processes linked to frost and snow, but without the presence of gla- ciers. It was in this context that the Longetraye
rock shelters were finally hollowed and that the valley-side of the Orcival below evolved, giving rise to the shape that characterizes it today (Defive et al. 2015; Fig. 7). This slope is broken by a suc- cession of flats and scarps linked to the past creep of thick slope formations rich in basaltic boulders resulting from the dismantling of the summit cor- nice, by gelifraction and gravity. In these cold con- texts, the sandy-clay deposits present at the contact between the base and the lava flows have favored creep dynamics. Entire portions of the lava flow’s entablature were gradually detached from the cor- nice and line the slope on which they were dragged after having slowly subsided in the middle of the debris produced by gelifraction.
Forming a cornice above the Orcival valley, the rim of the upper flow is marked by a line of rock shelters developed at the expense of the columnar jointing, under the overhang of the entablature. The formation of shelters at the foot of a basaltic flow with false-prisms entablature is generally ex- plained by the differential resistance to the frost of the different parts of the lava mass, which is responsible for a more rapid retreat of the colum- nar jointing (Kieffer & Raynal 2001; Raynal & Kieffer 2003). The most important cavity in this line of shelters, Longetraye Cave, seems to have developed at the expense of a voluminous breccia package whose covering by the lava flow locally
thwarted the formation of the columnar jointing, the prisms reappearing and then lengthening on either side of this accident. Configurations of the same type, clearly visible on the working faces of the neighboring quarry, give a clear image of the situation before the hollowing out of the cavity.
At the foot of the line of rock shelters and the “cave”, in a deposit of more than 2 m, several ar- chaeological levels of the Late Upper Palaeolithic, Mesolithic, Neolithic and Iron ages, were revealed by several excavations. The archaeo-sequence of the Longetraye site attests to a repeated presence at high altitudes since the end of the Ice Age and to the circulation of materials and/or humans with- in extended spaces, which are essential elements for understanding human activities in the terminal Pleistocene and Holocene in this high-altitude area at the junction of the Mediterranean and Atlantic domains. It is a key complement to the information delivered by the sites in the upper Loire and Allier valleys, Le Puy basin and the Vivarais region.
The severe climatic conditions mentioned by suc- cessive excavators from last century have perhaps been mitigated today, but the annual snow cover (Fig. 8) still causes significant percolation in the mass of the archaeological deposit, which is well protected by suitable backfill, and the route adopt- ed for the visit of this part of the geosite does not affect it in any way. The same cannot be said of the
Miocene basalt flow, which has been vigorously eaten away over the last five decades by a litho- phagous quarry, which nevertheless offers to the visitor some remarkable views of its internal archi-
tecture. The delicate connection between econom- ics, heritage conservation, and scientific tourism is thus achieved here in relative harmony.
This geosite has many facets and a virtual exten- sion that goes far beyond the perimeter of its for- mal presentation with the help of various adapted furnishings (Fig. 9). These allow us to address the geological history of the Vivaro-Vellave plateau, repeated changes in the past climate and its future, digging and shaping of the slopes of the Orcival Valley and, finally, the dynamics of human cir- culation and settlements at altitude since the end of the Ice Age. In this way, hunting and gathering, pastoralism and agriculture provide a rich conser- vatory from flaked stone to polished stone, ceram- ics and metals; from the open spaces of prehistory to historical appropriations of all kinds. Therefore we see many entries into the various “eohistorical” registers, suggested by the graphic and textual in- formation brought to our attention on this atypical geosite.
The commune of Borée is located in the heart of the Hautes Boutières graben, in the Eyrieux basin, a right bank tributary of the Rhône. It occupies a remarkable place within the Monts d’Ardèche
UNESCO World Geopark, with six geosites, all related to volcanism, i.e. about 20 % of the volca- nic geosites and nearly 10 % of the geosites of the Geopark (Fig. 1). Borée territory also concentrates a quarter of the Geopark sites included in the Na- tional Inventory of Geological Heritage.
The geological and geomorphological history can be approached in great temporal depth. The spec- trum of landscape forms ranges from the flattened and low-lying topography, punctuated by lacus- trine and fluvial deposits covered by the lava flows of the Eastern Velay at the end of the Miocene (10-8 Ma), to the current anthropized landscapes. Local manifestations of recent volcanism of the Bas-Vivarais (here dated between 229,000 and 147,000 years, taking into account the margins of uncertainties), are contemporary with the gla- cial-interglacial alternations of the Middle and Up- per Pleistocene (Fig. 10).
At the western extremity of the commune and on the southern side of Mont Mézenc, the Cirque des Boutières geosite is popular for its striking topo- graphical contrast and the sumptuous scenery of-
fered to the Alpes at the Croix de Boutières, from Mont Blanc to Mont Ventoux. The cirque itself, at the Saliouse headwaters, has been sculpted by millions of years of erosion which has torn open the heart of an ancient polygenic volcano (Fig. 11). The latter appears to be made up of lava flows, sco- riaceous projections and hyaloclastic breccias with palagonite which characterize Surtseyan hydro-
magmatic activity. All are cut by mugearitic dykes and a phonolite intrusion. Here, as at the Saint-Clé- ment geosite (a plateau at the north-eastern limit of the Hautes Boutières where 13 lava flows evolv- ing from basanite to trachyte were superimposed between 10 and 8 Ma) we observe a progressive differentiation of the magma within an intracrustal reservoir (Berger et al. 1976).
(7) (photos and CAD E. Defive).
The other striking features of the relief, whether you look eastwards from the Croix de Boutières or westwards from the Borée Virgin or the Molines quarry, are the numerous “sucs” (trachytic to pho- nolitic lavas) which create a rugged topography and give the landscape a monumental character.
They correspond to the “pasty” lava flows which, between 8 and 6 Ma, made their way through the Eastern Velay basaltic plateaus previously estab- lished (10-8 Ma). In this heart of the massif, which is the richest in Europe in terms of phonolitic oc- currences, the commune of Borée concentrates the
greatest number of sites. Most of them are lavas that protruded and did not reach the surface (Touron, Chabrières, Gouleïou, Roches de Borée, Rocher des Pradoux, Pialoux, Combeyre). Only the Suc de Sara (Fig. 12), a remarkable annular structure close to that of the Rocher des Pradoux (Fig. 12), seems to have just poked its nose above the basement, while 500 m lower down, the vein, deeply exposed by erosion, presents a micrograined structure typical of a hypovolcanic facies (Mergoil 1968). Similar examples are found in the western part of Eastern Velay, where most of the phonolitic lavas which reached the surface formed the peaks that currently dominate the plateau (extrusion of Mont Gerbier de Jonc; domes of Mont Signon, Suc de Montfol or Bachat; dome-flows of Mont d’Alam- bre or Rocher Tourte; lava flows of Les Roches, La Lauzière or Les Coux), and the Mézenc which is a composite dome and a geosite of the commune of Borée (Fig. 12). Being the highest point of the plateau (1753 m) and the 4th highest peak in the Massif Central, it straddles the divide between the Atlantic and Mediterranean watersheds. A certain number of remarkable petrographic characteristics come in addition to the interest in the structures and reliefs of these multiple “sucs”.
Two groups can be distinguished within the Eastern Velay phonolites, according to the order of crystal- lization of the minerals. The lava from Rocher des
Pradoux represents the group of miaskitic phono- lites with a normal order of crystallization; the tin- gaïte from the Suc de Sara represents the group of agpaïtic phonolites with an inverse order of crys- tallization.
The remarkable Suc de Sara also offers the pos- sibility to observe the contact modalities between the ascending lava and the surrounding basement and the consequences for its texture. Various clues prove that the lava, which usually rises tempera- tures around 800– 900°C, was at the limit of consol- idation: at the external contact with the granite, the phonolite forms rocky blades linked to the shear- ing that affected the almost solidified lava during its ascent; these blades are absent in the heart of the vein. In this same edge zone, the arrangement of crystals in the rock, where dark veins of ægyrin appear, also testifies to the shear stress, which is absent in the heart of the vein where the ægyrin crystals are homogeneously distributed (Fig. 13). At the inner edge of the vein, the appearance of the rock shows a more rapid cooling, interpreted as being linked to the maintenance of a void between the very pasty lava and its granitic casing. Finally, at the edges of the vein, large quantities of gases and fluids of varying degrees of heat and aggres- sion must have circulated before, during and after the lava was emplaced, and these have profoundly transformed the surrounding granite.
The Suc de Chabrières, another geosite in the commune of Borée, opens up other petrographic and magmatic questions which make it a unicum in the Eastern Velay volcanic province (Fig. 14). Aged 8.2 ± 0.2 Ma, it presents a good example of “ball phonolite”, a classic facies for this type of rock in which spheric figures develop (ocelles or spherulites). Under the effect of erosion, they take on the appearance of large lumps, the famous “balls”, which eventually break off, contributing to the degradation of the rock. These structures seem to develop preferentially at the edge of the intru- sion in connection with the fluids and gases that circulated there at the time and in the late phases of the setting and solidification of the lava (Machin 2008). They also appear to be associated with fine tubular conduits, perhaps linked to these circu-
lations. More original is the trachytic explosion breccia with ignimbrite facies which is spacially associated with the intrusion but is more recent and without a genetic relationship with it. It can be observed over a thickness of nearly 200 m and within a radius of 1,000 m at the foot of the “suc” and in the neighboring Valley of the Saliouse. It is thought to be the result of the sudden emptying of a chamber containing magma infused with car- bonatite fluids (Hodges 1991; see also in Ray-Pic chapter infra). An uncharred sequoia trunk was discovered there, testifying to a setting tempera- ture below 200°C during a very violent eruption that must have erased the surrounding plant cover (Naud 1998).
The recent volcanism of the Bas-Vivarais is also represented in the commune of Borée by the geo-
site of the Echamps-Molines maar (Fig. 15). Its age is now estimated between 219,000 and 156,000 yr (Nomade et al. 2014; Sasco 2015; Sasco et al. 2017). At that time, the valleys were already dug almost to their present level. The volcano was es- tablished on a ridge bearing only remnants of the
old Eastern Velay volcanism (10-6.5 Ma), between the Eysse and Saliouse valleys and at the head of the small Azette valley. The activity began with a phreatomagmatic phase, forming the deposits visible in the Molines quarry where all the typi- cal sedimentological features of this eruptive dy-
namism can be observed. A gradual transition to a strombolian phase is recorded, during which several cones were built around the maar. A ba- saltic lava, rich in xenocrysts and xenoliths from the basement and the mantle (Berger & Prinzhofer 1977), was emitted and filled up the crater form- ing the Échamps plain, an unusual topographical plane perched above deep valleys. From there, the basalt overflowed at two points and then followed the Eysse Valley as far as Saint-Martial and the Azette Valley as far as the Pont des Lièvres, 8 km downstream at the confluence with the Saliouse valley. In both valleys, the arrival of the lava flow was preceded by syneruptive lahars reworking the freshly emitted phreatomagmatic products, and covering the coarse alluvium of the talwegs. The lava flows preserved them from total erosion. These spectacular outcrops are regularly integrat-
ed into scientific excursions and training courses organized by French and foreign universities and leading scientific graduate schools.
The rich volcanic heritage of the commune of Borée is expressed first and foremost through its relief. The monumental character of the landscape, which has already been highlighted, is of course due to the geological structures created by internal geodynamics, but also to the erosion which, over the last 8 Ma, has progressively loosened these structures as the valleys were excavated and fa- vored the dismantling of the rocky masses by geli- fraction during the cold periods of the Quaternary (Figs. 10, 16). This dismantling was particularly active because the fluidal structure of the phono- lites renders them particularly sensitive to frost action at all scales. The imposing scree slabs thus
produced which surround the “sucs” make them even more visible in the landscape. The periglacial context which ruled these dynamics did not prevent the development of large firms or small glaciers in the most favorable cirques that border the western edge of the Hautes Boutières. This was the case at the Cirque des Boutières already mentioned and at the north-eastern extremity of the Mézenc, at the Cirque de Médille and in the Antraygues Valley (a tributary of the Saliouse), where a black glacier that was not very active built a frontal moraine, still vis- ible in the landscape (Valadas 1984; Veyret 1980, 1981; Neboit & Veyret 1976; Defive et al. 2019).
The variety of environments and landscapes that currently characterize the commune of Borée is the result of this long history and of the different ways in which peoples have developed their habitations (Fig. 17). Numerous signs of the complex web of interactions between societies and environments which, for thousands of years, have shaped the en- vironments and landscapes and include a variety of soils, parcels of land, specificities of the vernacular architecture using the petrographic spectrum of lo- cal rocks, the Échamps dolmen made of large slabs of phonolite, the management of water supplied in abundance by the Molines spring draining the imposing natural reservoir of the Échamps maar
diatrem, and finally the development of the paths to the summit of the Mézenc to channel the flow of tourists to reduce the degradation of the plant cover and sensitive soils. All these fully justify the advent of geo-patrimonial values.
The Ray-Pic is one of the most visited volcanic geosites in the Monts d’Ardèche UNESCO Global Geopark (around 35,000 visitors per year accord- ing to the counter installed at the site, but with significant inter-annual variability). It is located at Péreyres in Ardèche, less than 2 km as the crow
flies southwest of Lachamp-Raphaël village, at the headwaters of the Bourges (Figs. 1, 4). Here, at the transition between the high valleys and the first gorges down-cut nearly 400 m below the summit planes, the valley had already been dug to its pres- ent level 34,000 years ago (34 ± 8 ka by 39Ar/40Ar in Nomade et al. 2014; Sasco 2015; Sasco et al. 2017).
Linked to the volcanism of the Bas-Vivarais, better known to the public as the “Ardèche Young Vol- canoes”, this volcano was mentioned in the 18th century by the first scientific visitors who explored this region and recognized the volcanic nature of
peculiar edifices and associated rocks (Faujas de Saint-Fond 1778). Two phases of activity succeed- ed one another during the opening of this crater, as for many other volcanoes in the Bas-Vivarais. The activity began with a violent phreatomagmatic phase, which resulted in typical bedded deposits more or less loaded with pebbles and blocks, es- sentially from crystalline rocks. Then the eruptive dynamism changed quite abruptly to a strombolian
phase (Fig. 18). In addition to the scoriaceous ac- cumulations, a lava lake formed, and a lava flow poured into the Bourges valley. The subsequent re-entrenchment by the river allows the obser- vation of the intra-crateric contact between the basalt and the surrounding basement and of the spectacular prismatic sheaf-like structure of the lava mass, about 50 m thick and cascaded by the Bourges (Fig. 19).
The Ray-Pic flow cannot be separated from the geosite sensu stricto. The 21 km it covered in the Bourges Valley and then in the Fontolière Valley as far as Pont-de-Labeaume, at the confluence with the Ardèche, make it one of the longest flows in the Massif Central. This testifies to both the huge volume of lava emitted and its great fluidity, to the point of having left no traces of its passage between the crater and Péreyres (Berger 2007). Further on, it is preserved in places perched on the left or on the right bank or still occupying the Bourges tal- weg and causing the development of beautiful wa- terfalls, as at Chastagnas bridge between Péreyres and Burzet. It thus appears that in a certain number of places, the work of re-incision after the outpour- ing of the flow was not successful in retrieving the talweg of 34,000 years ago. Because of its length, the Ray-Pic lava flow has also allowed, since the pioneer researches, the establishment of a relative chronology concerning several of the Bas-Vivarais volcanoes whose lava flows met at various con- fluence points. At the Bourges-Fontolière conflu- ence, the Ray-Pic lava flow is thus covered by the Gravenne de Montpezat lava flow. The absence of paleosol and erosion marks at the contact between the two lava flows suggests that the Gravenne de Montpezat erupted shortly after the Ray-Pic. Fur- ther downstream, at the confluence between Fon- tolière and Ardèche, the Souilhol flow overlaps the extremity of the Ray-Pic lava flow, there with clear evidence of a time lag marked by an intermediate phase of erosion, successive fluvial deposition and then pedogenesis (Berger 2007, p. 105).
Along with the other volcanoes of the Bas-Vivara- is, the Ray-Pic geosite finally allows us to address questions of magmatology, which are the subject of the most recent works on this exceptional volcanic province (Buso 2019; Schiavi et al. 2020; Buso et al. 2022; Buso 2023). These works aim to answer two fundamental questions: where and how did the magmas form? How long did it take for magma to reach the surface? To answer the first question,
we studied the tiny glassy inclusions contained in the olivine crystals of volcanic products (Fig. 20). These inclusions, often less than 50 µm in size, are small droplets of magma that were trapped during the growth of the olivine crystals and that recorded the early stages of the host magma’s history. Their systematic characterization by different chemical analysis techniques and high pressure-high tem- perature experiments has allowed the composition and formation conditions of the magmas at depth to be specified. The Bas-Vivarais magmas have basanitic compositions and were formed at a depth of more than 70-80 km and temperatures close to 1300 °C. Their most remarkable feature is a very high carbon dioxide content, with values up to 4-5 wt % CO2, which is a world record for this type of volcanism (Buso et al. 2022; Buso 2023). It is probable the percolation of carbonatitic fluids of deep origin in the source mantle of the Bas-Vivara- is magmas that explains their CO2 richness.
Moreover, it is in the strombolian projections of Ray-Pic, whose lava is reputed to be rich in mantle enclaves, that we have found the largest peridot- ite enclave known in the Massif Central (approxi- mately 50 x 32 x 26 cm, i.e. an estimated mass of 120 kg; Fig. 21). How could such a large enclave, denser than the containing magma, reach the sur- face? Calculations made considering the character- istics of the Bas-Vivarais lavas (composition, dis- solved water content, viscosity) give a fall velocity of the enclave in the magma of more than 50 km/h (for a magma temperature of 1275°C), and there- fore an even higher magma ascent velocity for the enclave to reach the surface. Apart from this huge enclave, peridotite enclaves rarely exceed 20 kg and correspond to an average ascent velocity of the Bas-Vivarais lavas of around 10 km/h (Buso 2019). These high ascent rates are presumably re- lated to the exceptional content of dissolved CO2 in the magmas. This information takes on another dimension in terms of volcanic risk management. It turns out that such an ascent velocity, with mag-
Figure 20. Optical microphotography showing a glassy inclusion (brown) containing a dark CO2 bubble in the core of an olivine phenocryst in a basanite from the Bas-Vivarais volcanic province. The olivine phenocryst is embedded in a volcanic matrix (a mixture of microlites and interstitial glass) visible in the upper left and lower right corners (from Buso 2023).
ma source regions located at a little more than 70- 80 km depth, would result in their arrival at the surface in less than ten hours, leaving very little time for the detection of possible precursor signals of an eruption.
The Ray-Pic geosite is, therefore, a very good ex- ample of the links that can be established and em- phasized between scientific values, heritage values and societal utility. Its high level of use requires adapted facilities and management that also con- siders the preservation of the environment and the risks inherent in rocky rugged topographies and torrential rivers.
This raises the delicate question of how to rec- oncile the preservation of the “natural” character of the sites, freedom of movement and controlled (managed, channeled) use for the benefit of knowl- edge, environmental education, and the local econ- omy (tourism).
When the Monts d’Ardèche Regional Nature Park embarked on the process of obtaining the UNE- SCO Global Geopark label in 2012-2013, it was primarily for tourism and heritage reasons. Sus- tainable tourism is of course a major pillar of the dynamics of a Geopark. Discovery of the land- scape and geological heritage cannot be relocated because it is based on the territory’s assets. This logic led to the territory’s involvement in labeling the Pont d’Arc ornate cave, known as the Chau- vet Cave, as a World Heritage site, and above all to achieve the construction of the world’s largest replica of such a cave. The discovery offer was cer- tainly strengthened but the limited access for visi- tors did not make obvious the immediate economic benefits.
Without forgetting the scientific and educational dimensions, the priority for the 2010s was to in- crease the number of visitors to the area, in partic-
ular by increasing the range of activities linked to volcanic heritage, which represents the majority of the 61 Geosites in the Monts d’Ardèche UNESCO Global Geopark.
To achieve this goal, a strategy was implemented based on a four-year cycle of validation periods for 2015-2018 then 2019-2022 (the 2023-2026 strat-
egy is currently being drafted) (Klee 2019). The third axis of this strategy is entirely devoted to the promotion of geoheritage and the strengthening of the tourist offer. It has been conceived as a mix of:
-Scientific mediation by humans (specialists and guides), delivering of first information by the pre- scriptors (tourist offices, museums, and hotels).
-Digital tools: data online, creating a hiking appli- cation with heritage information (P.O.I.) and cre- ating the first prototypes of virtual and augmented realities while waiting for them to become a reality.
-Paper: publication of brochures, maps and a walk- ing guide.
-Physical support: through the development of sites and paths, installation of interpretation dis- plays, and new museography.
The most visible aspect for the public - but the greatest financial impact - is probably the creation of signage and interpretation equipment for the geosites. During the past eight years, more than 120 totems have been installed at more than 60 sites to welcome visitors and explain the geolog- ical heritage (Fig. 19). They allow all visitors to have an initial presentation and explanation of the site they are about to discover. During this time, 25 geosites and visitior areas were equipped with interpretation tools to really immerse the visitor in the discovery of the site and to give meaning to the visit. In most cases, an original approach was cho- sen through a discovery scenario linking the geo- logical heritage, the local buildings, the flora and fauna and the agricultural activities. The example of the Geosite of the prehistoric cliff of Longetraye
is truly representative of the type of development that has been carried out on the sites (Fig. 9).
Human intervention and discovery panels on the geosites (Fig. 9, 17) are complementary approach- es and address different audiences. Over eight years, more than 260 tourism professionals have been trained in the basics of the geology of the Monts d’Ardèche (Fig. 22). The idea is obviously not to turn them into geologists, but to give them the key elements to better understand geological history and to help them to be better advisers about sites and visitor activities. The other central point of the territorial dynamics is the annual animation program (Fig. 23). Since its launch in 2015, the 93 events organized or supported by Geopark have
welcomed 4611 participants, a demonstration of the public’s undeniable curiosity, if not for geolo- gy, at least for the discovery of landscape heritage in the broadest sense. Finally, in terms of commu- nication, many efforts have been made to make the Geopark concept, on the one hand, and the geolog- ical history and geosites, on the other, as intelligi- ble as possible for the inhabitants and visitors.
The geosite map, published every year in 20,000 copies, is the basic document that makes it easy to locate the places of interest. In addition, the booklet “Everything you need to know about the Monts d’Ardèche Geopark” provides a first level of reading with a quick presentation on the geo- logical history of the region, and an in-depth look
Figure 23. Geological rendez-vous at Issarlès lake. A) 7th of May 2022 (photo P. Bohle). B) 1st of April 2023 (photo E. Defive).
at two major subjects: volcanism and ichnology in the Monts d’Ardèche. The lithoscope, which is more educational, allows the reader to identify the rocks of the region and their use in buildings. The latest edition, the guide of walks and hikes uses these to bring a better understanding of the heritage encountered. Already reissued twice, it also wit- nesses the public’s interest in discovering the area, through geology in the broad sense and volcanism as an essential component.
However, since the COVID-19 pandemic, tourist activity in the Geopark territory has changed. In 2022, Geopark welcomed 19.9 million excursion- ists (ADT/PNRMA/FVT 2022 data), an increase of 59 % compared to 2019, the reference year be- fore the pandemic. There was a peak on 16th of July 2022 with 231,000 excursionists present in the area, whereas the 2019 peak was on 17th of August with 101,000 visitors. Visitor numbers exploded after the pandemic for many reasons that are now well documented (need to get away from it all, to reconnect with nature, etc.). Beyond the visiting activities, the number of overnight stays also rose sharply: + 9 % compared to 2019. On 13th August 2022, 85,175 people who do not live in the area all year round slept in the Monts d’Ardèche, which is much more than the number of permanent residents. The counters set up on many of the territory’s vol- canoes, at the Ray-Pic waterfall, at Mont Signon, at Mont Gerbier de Jonc, also showed an exponential increase in the number of visitors to the territory and to these volcanoes, the main places for excursions and walks. This significant increase in visitor num- bers has had a number of impacts on the geosites: in the volcanic areas, erosion has been greatly acceler- ated in recent years, causing problems for the flora and landscape. The quality of the visitor experience has also been affected, with fewer positive feelings linked to parking difficulties, a feeling of saturation and less immersion in the landscape.
These new data and this new context requires a
change of the objectives of the tourist component of the Geopark label: it is no longer a question of welcoming more but better, and above all of of- fering discoveries and providing knowledge to the inhabitants. The territory’s volcanic heritage re- mains relatively unknown. Above all, knowledge of the “volcanic dimension” of the area needs to be strengthened. All the best-known sites in the area are volcanic ones: Mont Gerbier de Jonc, Mont Mézenc, the Ray-Pic waterfall, the village of Antraigues and the Devil’s Bridge at Thueyts. The priority work of the post-pandemic years is, therefore, the presentation of the volcanic heritage of the Ardèche, which remains difficult to read and understand for most of the inhabitants. A partic- ular focus is therefore placed on educational and pedagogical actions aimed at the inhabitants. The new communication campaign about the “Ardèche Young Volcanoes”, for example, carried out by the Monts d’Ardèche Geopark and three tourist offic- es, is aimed primarily at residents of the region or secondary residents, and in particular families, through the prism of walks and familial games (Fig. 24).
Because of occasional over-visiting suffered by a number of the volcanic sites, the subject of conser- vation, which was not very present in the previous years of the label (except for the fossil sites), is now at the heart of discussion about site facilities and communication. The marketing strategy now aims both to deconcentrate visitor numbers on the major volcanic sites (Gerbier, Mézenc) and to preserve the lesser-known ones (Sara, Cinq Sucs). On the old volcanoes as well as on the more recent ones, the challenge of maintaining visitor numbers on the marked trails is a priority, both for reasons of ero- sion, particularly on the strombolian volcanoes, and for landscape reasons and to preserve the biodiver- sity linked to the volcanic rocks. On Mont Mézenc, a very substantial renaturation and erosion control operation has committed the Geopark to an inten- sive two years’ work (Fig. 25). Glory’s price.
Roxane Buso thanks the Région Auvergne-Rhône- Alpes for funding her doctoral thesis (project Vol- cAURA; Pack Ambition Recherche 2018).
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