The Chaîne des Puys volcanoes of the Auvergne, France

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© Blackwell Publishing Ltd, The Geologists’ Association & The Geological Society of London , Geology Today, Vol. 24, No. 6, November–December 2008

Classic localities explained 1

The Chaîne des Puys volcanoes of the Auvergne, France

David NowellNew Barnet, Herts, UK

The Chaîne des Puys is a series of remarkably fresh volcanic cones in the Auvergne region (Fig. 1), which covers a significant portion of the Massif Central, the large upland area in the southern half of cen-tral France, which at its greatest extent is about 360 km wide. Over a hundred assorted cones, domes and maars are mostly strung out along a north-south axis a few kilometres to the west of Clermont-Ferrand (Fig. 2), the administrative capital of the Auvergne and the Puy-de-Dôme department, which is named after the highest of these volcanoes (Fig. 3).

The 96 Departments into which France was di-vided into after the French Revolution are named after geographical features, with two-thirds of them named after rivers. Thus the Puy-de-Dôme must not be confused with Le Puy-en-Velay, the ‘county town’ of the Haute-Loire which is the neighbouring depart-ment to the southeast, and also famed for being built around its slightly older Miocene volcanoes.

The Massif Central

The Massif Central, in which this later Cenozoic vol-canism occurs, is mainly composed of Precambrian and older Palaeozoic sediments that were metamor-phosed and intruded by extensive granites during the Variscan mountain-building phase that also affected south-west England. There are also isolated Carbon-iferous sedimentary basins and narrow rifts, along with rhyolitic volcanism. This includes the Sillon Houiller, a major tectonic feature that cuts through the Massif Central for 300 km from NNE to SSW, some 20 to 30 km west of the main axis of the Chaîne des Puys. In addition, parts of the Massif Central, including the area around Clermont-Ferrand, are dissected by much younger Oligocene sedimentary troughs, which resulted from east–west extensional movements related to the collision between Africa and Europe, pushing the Alps northwards to the east of this relatively stable region. Nevertheless, the thin-ning of the lithosphere associated with the develop-ment of these structures, mainly on the eastern side of the Massif Central, resulted in the onset of Cenozoic volcanism.

During the Miocene the largest stratovolcano in western Europe, which is over 60 km across, devel-oped in the Cantal, some 80 km to the south, along with more widespread volcanism. During the Quater-nary, however, volcanism was confined to just four districts within France over the last two million years. The Chaîne des Puys is the youngest of these vol-canic fields, and the first eruptions started not long after volcanism had ceased in the Monts Dore just to the south some 200 000 years ago. Along with the Devès area of the Velay district in the Haute-Loire, the Chaîne des Puys remained active until the present glacial cycle that started some 110 000 years ago with the end of the last warm interglacial before mod-ern times. However, only the Chaîne des Puys had numerous and frequent eruptions that culminated in an apparent burst of activity coinciding with the end of the last ice age, and volcanism persisted into the Holocene, which started around 11 600 years ago. The last dated eruption was around 5840 years ago, give or take 120 years, relative to the year 2000. Ash from this unknown vent was found in a peat deposit filling one of the earlier volcanic crater lakes known as maars. The youngest known eruptions were at the Puy de Cliersou (Clierzou) (496·8 5071·2 – this

Fig. 1. Location of the Chaîne des Puys near Clermont-Ferrand within France.

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grid reference, like the others quoted, refers to the blue GPS km grid superimposed on 1 : 25 000 IGN topographic maps) at around 6330 years ago, and from lac Pavin just to the south of the main volcanic field, on the south-eastern flanks of the Monts Dore, at around 6500 years ago.

Chaîne des Puys volcanoes

The core of the Chaîne des Puys volcanic field is a string of cones and domes, some 40 km long and at most 5 km wide, that were erupted on a relatively high plateau parallel to the Limagne bounding fault about 7 km to the east of it (Fig. 4). This fault in a series of downward steps separates this plateau with its relatively ancient bedrock from the much younger Limagne sedimentary basin, in which lies Clermont-Ferrand with a population of 137 000 and a further 80 000 in the ten adjoining suburban communes (town councils). Further eastwards about 10 km

away in this valley, the river Allier flows northwards towards its confluence with the river Loire, which has a catchment area draining nearly a quarter of France. To the west of the main volcanic field the Pe-tite Chaîne des Puys occurs between Pranal and Ban-son (Fig. 2), while a few vents extend eastwards into the Limagne graben, including the eroded remnants of an early eruption beneath the centre of Clermont-Ferrand. Beyond the main chain the volcanic field extends a further 25 km southwards past the flanks of the Monts Dore to La Godivelle, which is some 1200 m up on the moors of the Monts du Cézallier.

The magmas from which the Chaîne des Puys were erupted have a wide variety of mineral compositions. These range from basic magma producing runny ba-salt lava flows with low silica content in the different minerals they contain, to relatively viscous trachytic material derived from acidic magma with a high silica content and quartz (SiO

2), which results in explosive

eruptions, as trapped gas cannot escape easily. This composition will change to reflect the chemistry of the parent magma and the physical processes that happen to it as it rises towards the surface. Thus, if a

Fig. 2. Map showing the distribution of volcanic vents in the Chaîne des Puys, including the Petite Chaîne des Puys, about 10 km to the west, along with the north-south escarpment above Clermont-Ferrand situated in the Limagne graben along with a few scattered vents to the east of the main chain. The different types of vents are as follows: 1, basaltic strombolian cones; 2, basaltic strombolian cones in an initial maar; 3, trachyandesitic systems with tuff-rings; 4. basaltic maars; 5, trachytic protrusions; 6, trachytic domes. The geographic co-ordinates relative to the Paris meridian in grades (gr) define the boundaries of the local BRGM 1 : 50 000 geological maps: 669 – Aigueperse (1986); 692 – Pontgibaud (1989); 693 – Clermont-Ferrand (1973); 716 – Bourg-Lastic (1981); 717 – Veyre-Monton (unpublished).

Fig. 3. View NNE to the Puy de Dôme 1464 m, from the hamlet of Espinasse (493.4 5055.4) just over 13 km away: see outline drawing to identify the other Puys seen from here.

Fig. 4. Block diagram through the Chaîne des Puys looking northwards, with the highest point the Puy de Dôme at 1464 m on a plateau around 900 m high with Clermont-Ferrand at around 400 m in the Limagne graben which drops to below 300 m further east along the river Allier. After Boivin and others 1991.

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mineral phase crystallizes out of the semi-liquid mush from which volcanoes are erupted, some of the other crystals growing in the remaining fluid will show gradational changes around their rims. In other cases fresh magma entering the system, or material gener-ated by it melting either solidified magma (which is still hot) or the surrounding country rocks will be reflected in the mineral petrology of the erupted lava. This is possible, as the temperature of such basaltic la-vas at the surface is about 1100 to 1200 °C which is much hotter than around 900 °C for the more acidic material erupted in the Chaîne des Puys. Thus heat transfer can generate quite considerable amounts of melting, which in other volcanic fields can be several orders of magnitude greater and occasionally result in the formation of massive calderas in which the roof of a shallow magma chamber collapses along radial faults. The resulting eruptions, like the Valles Caldera and earlier Toledo Caldera in New Mexico, which were both at least 20 km wide and erupted twice in the same place during the early Quaternary, can be hundreds of cubic km in volume. This is even bigger than the nearby Monts Dore caldera, which is about 5 km wide and 250 m deep, and which first collapsed around 3.1 million years ago.

In contrast, most individual volcanoes in the Chaîne des Puys were built up over relatively short periods of time, before the near-surface magma sup-ply simply ceased rising or was blocked by a clogged vent and fresh material headed towards the surface in different places. In simple terms this broadly resulted in a series of basic cones and acidic domes, each with a cumulative volume of between 0.05 and 0.5 cubic km, in the case of the Puy de Dôme—although some isolated eruptions were much smaller and others generated considerable lava flows, which can extend 8 km from the vent and be 3 km wide in places. On the eastern flanks of the Monts Dore, the Puy du Tar-taret above the village of Murol blocked a glaciated valley to form Lac Chambon and produced a narrow basaltic lava flow 22 km long down to Neschers, only 4 km short of reaching the river Allier. Since the crys-tals from which these lavas are composed can only be seen in thin section under a microscope, as once at the surface these rocks cooled rapidly, only dark and light types can be easily distinguished in the field.

Lava types

In general, dark lavas are low in silica, ranging from about 43 to 58 per cent in this volcanic field. They form a continuous series from more basic basalts to trachybasalts and hawaiites followed by trachyandes-ites (strictly speaking basaltic trachyandesites) and mugearites, as the total amount of silica (SiO

2) along

with sodium (Na2O) and potassium (K

2O) oxides in-

creases. Hawaiites and mugearites are simply the

names used for these subdivisions when they contain more sodic material than their potassium rich coun-terparts in this series. However the exact composition and names for these subdivisions can vary depend-ing on the different classifications that are still in use. For more detailed studies, a chemical analysis of lava expressed as the percentages of different oxides is needed, in addition to deducing its mineralogical composition and texture from examining it under a microscope in thin section. While gas is an important driving force in bringing such lavas to the surface, these eruptions produce flows or volcanic bombs, lapilli or ashes, depending on the rate at which this happens and the pressure of the gases they contain. This degassing can produce an uninterrupted fire fountain or propel material out of the vent. In this way a cinder cone of ejected scoria (Fig. 5) containing cavities left by the gas builds up to form a strombolian cone, which form about 80 per cent of the volcanoes in the Chaîne des Puys. These cones usually reach up to 200 m in height, though the Puy de Côme is 350 m higher than its surroundings with a basal diameter of 1.5 km and thus a volume of around 0·2 cubic km. As their original shapes are well preserved the slopes of these cones still rest at between 20 and 30 degrees. When exposed the core of such volcanoes is a distinct rusty red (Fig. 6), as the ejecta that fell nearest the vent remained hot enough to oxidize in contact with the air before it was able to cool.

If such rising magma hits enough groundwater this can flash over and boil to produce very explosive phreatomagmatic eruptions that blast the bedrock

Fig. 5. Series of basaltic air fall deposits in a quarry at the base of the Puy de Louchadière (496.0 5078.2), just off the D941: A, looking southwards up to an exposed section about 8 m high on the lower flanks of a strombolian cinder cone; and, B, section about 10 m high on the eastern side of this quarry.

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out of the way to form maars. When such finely fragmented material is ejected from the vent, the wa-ter soon condenses and most of the upward moving column collapses. This forms a density current known as a base surge cloud which behaves like a liquid and can reach speeds of 20 to 50 km/h as it rushes away from the vent. As larger and heavier fragments fall out of the cloud first, graded bedding is formed in addition to the other typical features (Fig. 7) that often develop during repeated explosions as more wa-ter enters the system. Furthermore, denser material tends to be deposited nearest the crater, while lighter ash deposits dominate the distal zone away from the vent. Once volcanic activity has ceased, maars often fill with water to form lakes. Such maars can be ideal locations to preserve later lake sediments that provide an important record of regional changes in vegetation and climate over time, in addition to trapping any later nearby ash fall deposits (tephras), which can also be used to date and correlate such deposits.

In contrast, lighter lavas (which feel rough) range in colour from pearl grey to beige and white, have silica contents above 60 per cent, and in the Chaîne des Puys consist of trachytes and benmoreites. As gases cannot escape so easily from the magma, very high pressures can build up and result in sudden and very violent pyroclastic explosions at any time dur-ing an eruption. If this happens before it reaches the surface this can form a maar in the classic sense of the word. Given the high viscosity of the lava, protru-sions and domes with steep sides can be formed, in addition to generating hot gas and ash clouds, which can roll down the flanks of a vent as pyroclastic nuées ardentes.

Visiting the Chaîne des Puys

Weather permitting, any visit to the Chaîne des Puys should start with the Puy de Dôme, which dominates the whole area and provides an excellent central lo-cation from which to view the rest of the volcanic field. This volcano grew out of the remains of an earlier cinder cone, which was flanked by the Petit Puy de Dôme to the north and the much smaller Puy Lacroix to the south. These basaltic strobolian eruptions were superseded by a much more viscous trachytic lava which grew into a cumulodome, as the sticky lava pushed up to form the first western dome with minor pyroclastic flows down its flanks. This was followed by the explosive collapse of its eastern

Fig. 6. Quarry cutting through the flanks of the Puys de Barme looking northeastwards from the entrance (494.13 5064.57) just off the D 941a. Note the red colour of this basaltic strombolian cinder cone is due to the iron content of the freshly erupted scoriae having been oxidised by circulating air at temperatures over 600 °C in the inner part of the cone.

Fig. 7. Typical features of maar deposits with increasing distance from the volcanic centre: 1, blocks with bomb sag from ballistic projectiles; 2, blocks which do not warp the underlying beds; 3, erosion channels caused by high energy surge clouds; 4, coarse ungraded beds deposited by density currents; 5, surface channels; 6, planar beds with normal graded bedding; 7, scarce cross bedding; 8, antidunes; 9, dunes; 10, planar low energy finely laminated beds; 11, accretionary lapilli (‘ash hailstones’); and, 12, undulations parallel to topography. After Boivin and others, 1991.

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side, which generated a nuée ardente similar to the 1980 eruption of Mount St Helens in the western United States. In the aftermath a second eastern cu-mulodome grew along with its pyroclastic apron and developed a central spine, which later collapsed. The Puy de Dôme eruptions have been dated to around 10 820, 9720 and 9390 years ago, though the two older dates are rather more uncertain as they could be within a thousand years either way. Not soon after these events, when the dome was already covered in pine forest, the Kilian crater exploded around 9080 years ago, near its south-western base just over a km away. The resulting nuée ardente blanketed the sum-mit of the neighbouring Puy de Dôme with a layer of ash and fragments up to 20 cm across, which in

places is over a metre thick, some 400 m above the vent from which they originated.

While you can pay to drive up the Puy de Dôme, at peak times the summit can only be reached via a shuttle bus and apart from the splendid views there is a small museum and restaurant. From this southern side the Monts Dore volcanic field can be seen in the distance (Fig. 8) while nearby in the foreground there are a series of heavily wooded strombolian cones. Walking up to the northern side of the summit near the transmitter tower at the very top there is a view-ing table and from here another series of almost bare cones can be seen. To the north-west round towards the north the relatively nearby Puy de Côme and le Grand Suchet can be looked down upon (Fig. 9),

Fig. 8. View southwards from the Puy de Dôme (1464 m). From left to right: the village of Laschamp (~980 m), the distant Puy de la Vache (1167 m), the Puy de Mercœur (1249 m) and the nearer Puy Laschamp (1255 m), the Puy Montchié (1210 m) forms the wooded foreground, and behind it is the Puys de Barme (1102 m). On the horizon are the Monts Dore caldera and the Puy de Sancy (1885 m), some 30 km away.

Fig. 9. View northwards from the Puy de Dôme (1464 m). From left to right: the Puy de Côme (1253 m) and le Grand Suchet (1231 m), with further away the Puy de Lantegy quarry (~1000 m) and Puy Chopine (1181 m), with behind it the Puy Louchadière (1198 m) in the distance on the skyline. See outline drawing to identify their locations relative to each other.

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with further round just behind le Petit Suchet the fairly insignificant looking Le Cliersou, which is the youngest dated volcano in France. Behind this in the mid-distance are the Puy des Gouttes and Puy Cho-pine which developed on top of each other. Firstly a basaltic cone built up before a series of massive explosions produced an off-centre trachytic maar and then much more viscous lava was extruded to form the Puy Chopine. The Puy des Gouttes is the remains of the former strombolian cone covered in the depos-its of the later trachytic nuée ardente, deposited by the subsequent explosions after quite an interval, as implied by the significant change in the chemistry of the erupted material.

From the Puy de Dôme looking more towards the north-east, the Puy Pariou and Puy des Goules can be seen (Fig. 10). The Puy Pariou has a particularly complex history of increasingly basic eruptions, start-ing with a quite acidic benmoreite tuff ring. A lava lake with a trachyandesite composition then formed, which produced two lava streams that flowed over the edge of the escarpment some 6 km away. Then in a final strombolian phase a new cone developed and a second lava lake emptied, resulting in base surge features which have even been identified as far away as Clermont-Ferrand. This suggests that the Puy Pariou eruptions were fed from a zoned or stratified magma chamber with denser and thus more basic products towards the bottom. Directly behind the Puy des Goules is the Grand Sarcoui, which is traditionally known as the Chaudron as it resembles an overturned cauldron. After a tuff-ring developed in an initial explosive phase, a cumulodome was ex-truded in a series of eruptions.

The Puy de Dôme is also famous for Blaise Pascal in 1648 getting his brother-in-law to measure the air pressure at its top compared with that in Clem-ont-Ferrand at the same time. From this difference in altitude of over 1000 m in a relatively short distance, he was able to show that pressure fell with increas-ing height and deduced that a vacuum really existed. This proved that it is the weight of the atmosphere that keeps a liquid from flowing out of a barometer.

As a result of this key discovery the SI unit of pres-sure is named after Pascal, and given its significance it is a pity the museum does not have a barometer with a live link to another one in the town below. If it is cloudy the Maison du Parc des Vocans (496.1 5060.0) on the D5 nearly a mile northeast of the N89 junction with the D983 is always worth a visit as it has a good bookshop and information about the volcanoes natural park. This regional park stretches for over 120 km and covers the whole of the Chaîne des Puys, Monts Dore and Cantal far to the south.

Other places of interest

While there is plenty to explore in the Chaîne des Puys, the area just to the west of the village of Volvic has a number of interesting localities above the source of the mineral water (Fig. 11). In addition to

Fig. 11. Water flowing from springs in the valley above (floored by trachyandesite lava from the Puy de la Louve), before being piped to the Volvic bottling plant. Viewed through a window in the small Eaux de Volvic building (502.1 5079.7) with on the side a relief of Docteur Pierre Moity, Maire de Volvic 1912–1938, who promoted the capture of the waters.

Fig. 10. View northwards from the Puy de Dôme (1464 m). From left to right: the edge of the Puy de Côme, le Grand Suchet (1231 m), the Puy de L’Aumône (le Petit Suchet) (1198 m), le Traversin. a flat area in the foreground, the Puy Pariou (1209 m) and the Puy des Goules (1146 m) with Grand Sarcoui (Sarcouy) (1147 m) directly behind it. In the distance, the Puy Louchadière (1198 m) on the skyline, the Puy de la Coquille (1152 m) with the Puy de Jumes (1161m) behind it, and the Puy de la Nugère (993 m), with in mid distance the Puy Chaumont (1111 m) in between the last two vents.

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the small building through which the water flows, there is a visitors’ centre and a fountain in the car park (which comes in handy if you need to clean your windscreen). From Volvic take the D986 and turn up the D90 to get to a large abandoned quarry (Fig. 12) opposite the station on the eastern flank of the Puy de la Nugère, which exposes the initial tuff-ring. In addi-tion to bits of country rock and material recycled from earlier eruptions, two fresh magma types are found in the first layers, with blocks and bombs contrasting with black scoriae and cauliflower bombs indicative of ground water infiltration producing phreatomag-matic explosions. This suggests two vents were erupt-ing simultaneously, and the very marked rhythmic bedding that could be seen throughout the entire sequence confirms the phreatomagmatic nature of one of them, even if scoriae products become domi-nant towards the top. On the floor of the old quarry near the entrance there is a rather fine information board (Fig. 13), which explains the evolution of this volcano and the origins of the Volvic mineral water source.

About a kilometre farther up the road and beyond the Maison du Miel there are a number of quarries in later Puy de la Nugère lava flows, just north of the volcano. These were generated when a lava lake

subsequently developed and overflowed eastwards for 4 km towards Volvic at the edge of the escarpment. This followed the route of an earlier and much more extensive basalt flow that spread out for further 4 km into the valley below. One of these quarries has a fine section through the overlying clinkery aa lava flow consisting of trachyandesite, down into the Volvic stone (Fig. 14). While this featureless rock with fairly scattered fractures looks like basalt at first glance, it is rather lighter than might be expected and indeed turns out to have an intermediate mugearite com-position. Since some rocks from this volcano show evidence of mixing between basic hawaiitic and acid-ic trachytic magmas, increasingly vigorous volcanic activity may have produced this homogenised lava. The overlying trachyandesite marks the final phase when a number of small strombolian cones developed within the volcano. When this quarry was visited quite a lot of equipment could be seen lying around (Fig. 15) which showed that traditional methods of working the stone were still being used.

The future

Though volcanic activity has ceased in the Chaîne des Puys, it is more than likely that eruptions will resume at some time in the future. The question why there have been no recent eruptions led to a statistical analysis of their frequency over the last two million years. This showed that during glacial (Milankovich) climate cycles there have been bursts of volcanic ac-tivity. Thus at the end of the last ice age there were 53 dated eruptions in the Chaîne des Puys between 17 400 and 5840 years ago. This is probably linked to massive changes in the extent of ice sheets during climate cycles, which have oscillated roughly every 100 000 years from relatively brief warm intervals to longer increasingly cold periods over the last 800 000 years, with more frequent but less pronounced cli-mate cycles before then. Given the past pattern of

Fig. 12. Rather degraded section approximately 60 m high near Volvic station (499.9 5078.9). This is on the eastern flank of the Puy de la Nugère, and consists of trachyandesitic products of a strombolian eruption, exposed in a roughly 40 ha area surrounded by younger deposits from the Suc de la Louve (Puy de la Louve) which erupted from near the summit of the earlier volcano.

Fig. 13. Close-up of ceramic information board set up on the northern side of the large abandoned quarry near Volvic station (500.1 5079.1).

Fig. 14. Volvic stone quarry in lavas from the Puy de la Nugère (499.1 5079.6), looking southeastwards with a clinkery aa lava flow several metres thick above the main face which has been removed in places by quarrying (q on Fig. 2).

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activity, infrequent events can be expected at present roughly once every 10 000 years. However, the next major burst of volcanic activity would be expected at the end of the next ice age, assuming global warming does not irreversibly override the natural pattern of cyclical variations in our climate.

Suggestions for further reading

Boivin, P., Canus, G., de Goër, A., Gourgaud, A., Ki-effer, G., Mergoil, J. & Vincent, P., [translated by Litto-DeGoër, C. & Murray, J.B.] 1991. Volcanology of the Chaîne des Puys. Parc Naturel Régional des Volcanis d’Auvergue, 63970 Aydat, France.

Middlemost, E., 1997. Magmas, Rocks and Planetary Development—A Survey of Magma/Igneous Rock Sys-tems. Longman, Harlow.

Fig. 15. Old quarrying equipment in basalt quarry about 1 km north of Puy de la Nugère (499.1 5079.6) near the Mason du Miel. Note the wedges for splitting the rock, along with levers, chains and steel hawser.

Nowell, D.A.G., 2006. Le Puy-en-Velay: gateway to the volcanoes of the Haute-Loire. GA Magazine of the Geologists’ Association , v.5(4), pp.16–18

Nowell, D.A.G., Jones, M.C. & Pyle, D.M., 2006. Epi-sodic Quaternary volcanism in France and Ger-many. Journal of Quaternary Science, v.21, pp.645–675

Nowell, D.A.G., 1996. Gravity Modelling of the Valles Caldera. New Mexico. In: Goff, F, Kues, B.S., Rog-ers, M.A., McFadden, L.D. & Gardner, J.N. (eds) Geological Society Guidebook, 47th Field Conference, Jemez Mountains Region, New Mexico, pp.121–128.

Richet, P., 2003. Guide des Volcans de France. Brgmédi-tions/Belin, Paris.

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The Chaîne des Puys volcanoes of the Auvergne, France