Auvergne with OUGS Mainland Europe,19 to 28 July 2003

Arrival

We assembled on Saturday from the various corners of Europe in, inevitably, the bar of the place we were staying. Everyone was introduced and the pace of the week was set. Our accommodation was in a gîte d’étape, like renting out a bunk bed instead of a room or the whole building. It was a big old farmhouse type thing in a very small and quaint village called Laschamp.

Our group was made up of 17 people, who were: Nico (intrepid leader), Elisabeth, Janet, Brenda, Nadine, Mary, Gerry, Stuart, Dave, Jane, Paul, Mike, Brigitte, Frank (otherwise known as Brian), Lynn and myself. I’ll let you put names to faces.

To top

Introduction : A personal view

A character mélange of Jacques Tati, Zebedee, Tigger and a balletomane to boot, comes close to describing the style and teaching methods of another star in the OU geological firmament, one Nicholas Fournier, leader of a most successful recent field trip on his home ground in the Auvergne and Massif Central. Parted from his rub-out pen and drawing board only to keep his fluid levels commensurate with his appetite (at suitable intervals of course), and volubly fluent in three languages but using mainly English, he made a good job of inculcating (Lat. to ram down!) in us the various types of vulcanicity to be found within a reasonable travelling radius of Clermont Ferrand.

Any apprehension I might have felt about Gîte accommodation (and I admit to plenty), was quickly dispelled by the welcoming amorphous group emerging from all parts of the European compass. Ever a survivor and first to bag a bottom bunk, I would now cheerfully accept the sobriquet ‘the oldest Youth Hosteller in the business!’ Apart from the geology interest we all had in common, the trip brought out in us other elements of the OU membership; drinking and eating (in that order), friendship, good conversations and expansion of our individual ‘mind sets’.

The OUGS Mainland European branch has scant mention in run-of-the-mill branch newsletters. Three years young now, it is a small branch of forty-three, with members living as far north and east as Oslo and Moscow, and west and south as Bordeaux and Athens. With the trip leader living in Oxford, based at Milton Keynes and doing in research in Nicaragua, the treasurer in Munich, trip-organiser in France and coordinator in Switzerland, the eventual meeting of the group coming from Belgium, Germany, France, Scotland and East Midlands was a logistic achievement to put us locals to shame! As distances are vast, the AGM is a planned weekend ‘event’, the main opportunity of a ‘get together’; who could resist Brussels or Vienna as a venue?

Running concurrently with our generous consumption of wine and victuals was some good ‘hard rock’ stuff when, to have the various types of volcanic eruptions (‘explosive’, maar, phreatomagmatic, ‘effusive’ ‘etc.) and lava flow consistencies at immediate recall at all times, was a distinct advantage. Nicolas, a.k.a. Nikki, Nico (preferred) but never, under any circumstances in French company or otherwise, Neek, (don’t ask!!) knew his stuff, that being his PhD thesis. So we enjoyed excursions first to the northern part of the Massif to the Puy de Dôme, (1465m) then moving on to Mont Dôre and Puy de Sancy.

The Puy de Dôme is really a double dome, reaching a height of 550m. Consisting of highly viscous, acid, lava (trachyte, locally ‘domite’), it emerged at different times from a magma chamber in the mantle, getting stuck, in a somewhat constipated manner, as it cooled at the top of the conduit. After the first explosive phase, as the lava was too viscous to flow like a basic lava, it accumulated around the exit point to form a dome covering the initial crater. Close to extensive remains of a ‘Temple de Mercure’, a nineteenth century discovery constructed of lava blocks, of high place value even to the Romans who sanctified the site, the needle-like man-made construction on the summit serves as an early-warning system, radio mast, weather station and volcanic research centre. In good visibility the volcanoes, roughly arranged in line, the famous ‘Chaîne des Puys’, can be seen extending over a wide area in a north-south orientation. Nico’s opinion, and as recent research has it, sees this unusual number of volcanoes in such close proximity and of widely varying ages as a result of fissures and faulting, rather than of an underlying hot spot, an earlier opinion now discredited as this widely researched area is always providing new insights.

The whole area of the Massif Central, with much of France, was affected by the Variscan (Hercynean) orogeny towards the end of the Palaeozoic (570-245 Ma). Further faulting and rifting occurred in the Alpine orogeny of the Tertiary (65- 1.64 Ma); its volcanic development spanned millions of years ending only about 8000 years ago. Basically it is a horst, underpinned at great depth by granitic crystalline rocks; Alpine folding, uplift and downfaulting formed the escarpment and Limagne basin to the east, later filled by Tertiary sedimentary material to a depth of 3000 m. To the west the Massif dips more gently into Aquitaine and the Dordogne, the whole vast landscape shaped by later glaciation, river erosion and changes in sea level. The Massif Central, although agriculturally poor, is a ‘bocage’ area famed for cattle (Limousin and Charolais) and cheeses (Roquefort) on the ‘planèzes’ (slopes). The Auvergne produces much of France’s uranium for nuclear power; the Cantal region, further south than our proscribed area, produces tungsten. Around Limoges kaolin prompted the porcelain industry, now in some decline.

Clermont Ferrand itself is ‘Michelin Tyre Country’, a business emanating from the nineteenth century rubber invention of a certain Mr. Mackintosh whose niece married a Frenchman of exceptional entrepreneurial turn of mind. Situated on a butte on the edge of a maar with many caves carved out over the centuries but now mostly used for wine like the one we visited, its thirteenth century Gothic Cathedral is built of extremely strong volcanic blocks quarried mainly from Volvic (of the TV bottled water ad.). This made for a gloomy interior, but enabled the nave to soar to a height of 300 ft. The statue in the cathedral square is of Pope Urban I of the First Crusade ‘fame’ and whose legacy still endures. Volvic supplied almost all of Clermont’s building material until the end of the nineteenth century. Modern buildings have used lighter materials and colours, relieving the rather austere black of the lava. Strong yet with good carving qualities, this is commonly used for lintels where basement blocks or local stone is used as facing material.

We followed our leader, like the Pied Piper of Hamlyn, through the intricacies, consistencies and vagaries of lava flows, domes, maars and pumice ‘nappes’, ‘necks’ and sills, pleased and awed in turn by dammed lakes and surprise caves. This last not always from natural action of huge water/gaseous ‘bubbles’ but a relict of masons quarrying the lava for sarcophagi in Puy Sarcoui. Regarding caves, and deserving special mention here is the intrepid band, (only five opted out), who camped out one night in a cave near the Puys de la Vache and Lassolas. By all accounts they were mightily sustained and rewarded after a three mile uphill hike in the dark, by a fully comprehensive variety of liquid refreshment. Returning downhill at the crack of dawn for early breakfast at the Gîte proved another formidable task! One evening much excitement and comment ensued during a terrific thunderstorm with hailstones like golf balls, the top of at least one member’s car sustained damage. It must have been international news as a friend over here heard all about it.

Our second stay was two nights in a beautifully converted barn, another rural Gîte near Puy de Sancy and Mont Dôre. These volcanoes founded on an older sequence of lavas, evolved over a period from 3 to 0.25 Ma .A major event occurred 3 Ma when 5 km3 of trachyte magma erupted from the Mont Dôre area, pouring out in pyroclastic flows. We saw a spectacular ignimbrite (pumice) deposit at Nappe de Ponce de Rochefort-Montagne reminding me of that in Alaska in the Valley of a Thousand Smokes left by the Katmai eruption of 1912. After driving up to see La Dent du Marais, the backwall of a landslip induced by the Tartaret eruption and soaring behind Lac Chambon, the lake provided a restful morning’s swim. We spent the afternoon in the Chaudefour Valley where several years ago, after prolonged heavy rain, holiday makers were drowned at a camp site downstream by a horrendous flash flood. The Massif du Sancy and Mont Dôre are extensive, covering an area larger than the Puy de Dôme.

The end is in the beginning, and we had a jolly gathering for thanks, gifts and much sampling of local viticulture products before dispersing to spread the OU message across Europe once again, vowing to keep in touch with people, courses and news by e-mail (its life blood, how else?) and to make it to the AGM in Vienna in 2004. This must be the most attractive of all geology AGM locations, just in time for the traditional Viennese New Year Concert and seasonal performances at the Vienna State Opera (but I betray my natural bent)! All power to your elbow, Mainland Europe Branch; special thanks to all the organisers for their hard work, Elizabeth, Annette, Monaco Mike and last but not least, Nico with his knowledge, enthusiasm, dexterity, agility, volubility and supremely physical visual aids!! Life-long learning at its best. Merci mille fois; enchantée de faire votre connaissance!

Mary Holborow

Glossary

pouzzolanes - friable volcanic cinders used in antiquity as natural cement by the Romans, today in roadworks and for gritting roads instead salt which would ultimately pollute the water table, and in water purification (also, see mention of pouzzolane and pozzolana in Kirsty’s account of Day 1).

eleutriation - new word for a layer of dust deposited after ‘dusty stuff’ has been disturbed!

‘eh....way...way’
(with actions appropriate to steering a ‘deux chevaux’) - rising lava trying to exit a conduit before blowing its top like a champagne bottle.

References :

Volcans d’Auvergne - Dominique Decobecq
Parc Naturel Régional des Volcans d’Auvergne.
GA Field Excursion - 1990-Druitt, Gibbons and Horak (Cardiff)
A map Book of France - Tussler and Alden (Macmillan) Acknowledgement: Inaccuracies, other ‘booboos’ etc. are entirely mine!

Day 1 : Regional Setting & Tectonics: what are we doing here?

Elisabeth pointing out sulphur staining around an old fumarole in the crumbly, white trachyte during the walk to the top of the Puy de Dôme. Photograph: Kirsty Crocket
Elisabeth pointing out sulphur staining around an old fumarole in the crumbly, white trachyte during the walk to the top of the Puy de Dôme. Photograph: Kirsty Crocket

Nico used the first day to take us up the Puy de Dôme (see Stuart’s account of day 3 for a guide to pronunciation) and explain the regional tectonic setting as well as to admire the view of the Chaîne des Puys. Most tackled the ascent of the Puy de Dôme on foot while others preferred to arrive in style (as opposed to hot and sweaty but ice-cream definitely well earned). On the way up, Nico pointed out places where we could see what the Puy de Dôme was made of – trachyte.

The Puy is 1480 m high and is 10.8 ka old. The age was calculated by carbon-14 dating of tree remains, among other methods. The Puy de Dôme has also lent its name to the “département” (Administrative region.), the boundaries of which were determined in the olden days according to the distance a man on a horse could travel in one day from a central point.

Once the group had assembled at the top and admired the temple built by the Romans in honour of Mercury, Nico got out his map and proceeded to fill us in on the technical details of what had happened in this part of the world over the last few millennia. The main points were the regional tectonic setting, the maars in the graben and the alignment and different compositions of the Puy volcanoes.

Regional tectonic setting : Volcanism in the Auvergne resulted from E-W extension during the Oligocene which formed the Limagne Valley (Clermont-Ferrand is located in this valley). In Europe as a whole, the extension which has taken place since the Eocene generated the West European Rift, of which the Limagne Valley is a part (the Rhine Valley, the Eifel and Vogelsberg areas, the area east of Cologne in Germany, and the area north of Prague are all associated with the West European Rift). It can be thought of as an aborted rift. The Massif Central (and the Auvergne which is located within the Massif Central) is still tectonically active with many earthquakes every year.

The extensional tectonics thinned the continental crust and decreased the pressure on the underlying mantle. Decompressional melting took place and the magma rose up through fractures generated by the extensional tectonics (path of least resistance). These faults run in a N-S direction and controlled where the magma would reach the surface. This is why the Auvergne volcanoes are aligned in a N-S direction.

The associated thermal anomaly still exists today and is monitored by scientists at the Blaise Pascal University in Clermont Ferrand.

Maars : In the graben, huge thicknesses of sediments accumulated, up to 3 km in places. The water in these sediments meant that the heat from the upwelling magma generated explosive phreatic and eventually phreatomagmatic eruptions. The fragmentary nature and short period of activity of the eruptions in the graben led to rapid erosion. Should this area become active again, the Limagne Valley would be the most dangerous place to be.

The Puy volcanoes : The horsts either side of the graben are crystalline and contain far less water than the valley sediments. Volcanic activity here was less explosive as a result and the deposits were able to build up to larger sizes than in the graben, although not on the scale of stratovolcanoes such as the Mont Dore (more of that in the next edition). A geological map of the Chaîne des Puys will show you that the erupted lavas range in composition from basalt to trachyte (or dômite as it is known locally). This is a result of the rising magma residing for different lengths of time in the magma chamber, where short residence times mean basaltic lavas erupt and longer residence times allow for fractional crystallisation and segregation to take place leading to the eruption of more evolved lavas. The composition affects the shape of the volcano as well. The more evolved the lava, the greater its viscosity and therefore its resistance to flow. The Puy de Dôme is a good example of a well-rounded dome bulging out of a volcano rather than a hollowed out cone shape (see the figure 1).

The timescales of activity involved for each individual volcano range from a few days to a few months both in the graben and on the horsts. Volcanism in the Chaîne des Puys started about 40 ka ago although extension in the area started a lot earlier at about 15 Ma ago.

From the top of the Puy de Dôme, Nico pointed out the various shapes of the volcanoes. Those with flat to domeshaped tops formed from evolved, viscous lavas such as trachyte but others were shaped like horse-shoes. He explained this was because the cones were breached by fluid lava flows (a lot of the volcanoes are no more than giant scoria cones). These shapes are easily identifiable on the geological map from the contours. Once breached the lava would flow out and empty the central part of the volcano.

The horst/graben structure of the Limagne Valley directed lava flows to the east, where shallow slopes induced wide and thin flows and steep slopes generated narrow and deep flows. These are recognisable on the geological map as well. Lavas flows to the west did not spread out. As lavas flowed into the Limagne Valley, they covered and protected older sediments from erosion. Sediments not protected by a covering layer of lava have since been eroded leaving lavacovered sediments standing proud in the valley and forming mesas (similar to inselbergs).

The surface accumulations of scoriaceous material and the fractured rocks allow ground water to flow through. Although it is claimed by certain water companies that the scoria acts to filter and purify ground water, this isn’t strictly true. The ground water in the area is clean in any case and the basaltic rocks through which the water flows do not add any contamination. So high is the porosity of this material that salting the roads in winter is out of the question as this would percolate down and cause pollution. The naming of the scoria “pouzzolanes d’Auvergne” by locales is a misnomer as the original “pozzolana” from Pozzuoli near Naples refers to acidic volcanic ash used by Romans to make cement rather than the strombolian ejecta found in the area.

The Puy de Sancy was visible in the distance from the top of Puy de Dôme. This is a very much older and larger stratovolcano that has erupted periodically over millennia. The magma chamber is still at quite a shallow depth of 5 km although the area is tectonically quiet. Geothermally heated ground water in the area of Puy de Sancy is extracted and used to heat local houses. One of the mysteries of the Auvergne is why no stratovolcanoes developed along the chain instead of the large scoria cones and trachyte domes.

After this introduction to our week from the top of the Puy de Dôme, we visited various volcanoes (Gravesnoires, Puy de la Vache, Puy de Lassolas) that have since been turned into quarries where scoria is extracted for local industrial uses such as construction and road works. Scoria is highly sought after locally for construction because of its properties: low density, resistance to fire, chemical attack and frost shattering, and its high porosity but low permeability mean it is a good thermal insulator. It is too expensive to transport long distances and so is only used in the immediate vicinity. Some locals have voiced concern over the destruction of the Auvergne volcanoes by turning them into quarries. In fact, on our arrival in Laschamp, some were immediately targeted by a local and talked into buying a large car sticker protesting against the quarrying. The cause was no doubt aided by some degree of incomprehension combined with a desire to dump bags and get to the bar.

At the Gravesnoires quarry, Nico explained how the difference in colour of the scoria between red and black is explained by proximity to the central vent and oxidation of the iron in the scoria. The high heat flow over longer periods in the region of the vent speeds up reaction times and allows for oxidation whereas scoria ejected away from the vent cools and does not oxidise rapidly. The colour difference at Gravesnoires was very clear.

We also looked at dykes and tried to determine their direction of flow from observation of features within the flows. Small shear zones and small phenocrysts where visible were alignment to the direction of flow.

After all that excitement under a blazing hot sky, we retired to the terrasse.

By Kirsty Crocket

Day 2 : Effusive Volcanism: when the Earth bleeds (Nico likes dramatic titles)

After a night of spectacular thunderstorms Monday dawned wet and grey, such a change from the hot weekend. However, it was lovely after breakfast to be driven from our gîte at Laschamp in Gerry’s and Mary’s car, whilst listening to Canteloube’s ‘Songs of the Auvergne’; one version sung by Victoria de los Angeles (1973) and the other by Elysium (2001).

We travelled up the D767a and D941 roads and then down the D68. On this last leg, we were following La Vallée de la Tiretaine eastwards to the spa town of Royat, as the lava from the Petit Puy de Dôme had done some tens of thousands of years earlier. The flow had been controlled by the palaeotopography, just as the course of the river is now controlled by the present-day topography. The lava has been dated to between 41,000 and 44,000 years of age. Royat, on the outskirts of Clermont Ferrand, at the very edge of the Limagne basin, is built on this lava. The Petit Puy de Dôme is a basaltic strombolian cinder cone, adjacent to the Puy de Dôme.

The road zigzags back and forth across the lava flow, straying either side onto the granitic basement that underlies the lava of the valley bottom. Our first stop was to inspect this granitic basement, visible in a roadside cutting. We looked for zoning in the euhedral feldspars, to see if the remaining liquid melt composition had been changing while they crystallised. Were their edges being eaten by the melt? What sort of fabric did the granite have? If it were a migmatite, it could have inherited a fabric from the metamorphic rock. Was there a lineation from movement of the cooling granite during emplacement so that crystals were orientated in the direction of movement? These were the questions put to us by our leader, Nico.

The exposures in Royat itself were to be found lining two open-air car parks. In the first of these, there was a basaltic lava flow with columnar joints in which small pyroxene crystals, plagioclase crystals and olivine could be seen, aligned in a glassy (as evidenced by conchoidal fractures), grainless matrix. This clearly cooled very quickly at or near the surface, so it was the product of extrusive volcanism. Basalt flows further than more silicic compositions. Nico told us how he once detected flowing lava on Etna by the sound of breaking glass – just before his boots started to melt!

Columnar basalt in the car park at Royat. Photograph: Kirsty Crocket
Columnar basalt in the car park at Royat. Photograph: Kirsty Crocket

The columns were created as a result of contraction. On cooling to solid matter the lava becomes more organised, so the volume reduces and joints form perpendicular to the cooling surface. Our columns were leaning over into the valley. This was consistent with the way columnar jointing forms in valleys, with the joints at right angles to the sloping sides.

The joint faces showed ridges on a smaller scale, at right angles to the length of the columns. These are thought to mark small steps in the propagation of the fracture and it was suggested that they are related to shearing in the direction of flow. The coarser grains at the top of these steps have not been fully explained. Jointing also occurs at right angles to the sides of the columns in some flows, dividing the columns into stacked tablets, although this had not developed at our site.

Lava flows cool faster at their upper surfaces, but there is less friction with the air than with the ground beneath, so the lava travels more quickly at the top than at the bottom of the flow. This causes shearing along the direction of flow. The columns, too, sometimes lean into the direction of flow. Nico caused much merriment by turning himself into a lava flow to demonstrate these dynamics, with his head and feet as dragged-back cooling surfaces and his tummy thrust forward as the internal, faster-flowing lava! The top of the flow is blockier and more vesicular than the rest. The upper, rubbly, surface at the leading front of the flow gets dragged over, falls and ends up underneath the base.

Nico was also surprised to see slickensides in the joints between the columns and we wondered when it was that the movement had occurred and what had caused it. To discover the direction of movement on slickensides you can run your hands over the surface. More resistance will be felt in the opposite direction to the direction of crystal growth.

Royat is a spa town that must have been very fashionable at one time, judging by the smart old hotels, but it was pretty empty while we were there. During lunch some people sampled the waters from fountain taps in a pavilion.

After lunch we visited a second car park, higher up in the town. Here we found the Grotte des Laveuses (Washerwomen’s Cave), a hollow with a pool beneath another lava flow. The authorities had put up a chain barrier, which prevented close inspection, but many of us suspected that the cave had been a lava tube. However, there were no basaltic stalactites and there was a chilled margin, so Nico explained that it had, in fact, been formed where the lava had trapped a bubble of steam. When the lava flowed in the original valley, there was almost certainly a river flowing then, too. The vaporization of groundwater beneath a lava flow sometimes produces huge bubbles of steam that may remain trapped under the lava flow to produce caves such as this one.

There are two main types of lake in the Chaîne des Puys: barrage lakes and crater lakes. Our next two stops were to two barrage lakes formed where rivers had been dammed either side of a set of combined lava flows that appear to come from the Puy de Lassolas and Puy de la Vache. These were Lac de la Cassière and Lac D’Aydat. Lava flows are full of mineral nutrients but don’t have soil deep enough for agriculture, so are often covered in forests, as was the case with these lavas.

The weather had improved considerably by this time and we walked part of the way around Lac D’Aydat on the lava obstruction before returning to the beach by the lake where some of us had an afternoon swim. Part way round the lake little Germaine earned our admiration by turning into Superwoman and climbing up and over a wall and the road barrier, but a little further on she pulled out the signpost to the lake and turned it round! She feigned innocence, but could it be that she has anarchic tendencies?

After our swim someone noticed a very clear exposure of a lava flow over some granite by the road home. We could see the blocky base that had rolled over from the upper surface and a layer of well-sorted air fall deposits from the same volcano, over the weathered basement granite. However, the rest of the exposure was in somebody’s back garden, so we had to view it at a distance.

While we waited for dinner that evening we had another electric storm with an amazing downpour of hailstones that were at least 4cm in diameter. They bounced wildly off the grass and the patio. Nico, Nadine & Paul ran out into the storm to collect them and bring them back for closer inspection. Luckily most of the cars escaped serious damage but I felt sorry for animals in the fields.

Some of us had previously elected to sleep in a Roman cave that night, but we decided to postpone this, because of the rain. Nico rounded off the evening with a debriefing in the gîte, explaining the formation of lava tunnels by the growth of levées along their flanks (there aren’t any of these in the Auvergne), the theory of mantle plumes, mantle convection and hot spots (no evidence for this in the Auvergne, either) and the formation of maars, or crater lakes. Those in the Auvergne have mostly lost their water. Elisabeth explained about the last roadside flow we’d seen, for the benefit of those who hadn’t stopped for a swim.

Lynn Everson

Day 3 : Roche Merle, Bois de Charmes, Grand Sarcouy

En route (on root) to our first locality, a quarry in the Bois de Charmes (bwaa-d-sharm), Nico took us to a massive block of rubbley lava, the Roche Merle (rosh-merl) which sat isolated on a small hillock in a meadow close to Laschamp (la- chawmp). Measuring roughly 5m in diameter and 4m high with no other outcrops nearby – how did it get there? - too large to be a bomb at 2-3km from the Puy de Dome (pwee-d-dome), could it have rolled there – highly unlikely. After much debate and educated guesswork our illustrious leader pronounced this to be a lava block which had been carried along buried within a flow. This solved the transportation problem. When the flow met an obstacle, our block’s momentum carried it onwards and upwards breaking through the crusty lava flow surface.

More Joan Baez in Elisabeth’s ‘car’ and we pulled up in a lay-by in the Bois de Charmes, in the Foret de Puy de Dome, _ km SE of Le Puy de Dome. Nico sprinted off back down the road – “another leader gone”, we thought, but no, a locality check was needed and why walk when you can run.

The quarry wall, 4m high, presented us with a pale, light coloured, poorly sorted pyroclastic deposit. It contained large and small blocks, both rounded and angular, in a fluffy, unconsolidated matrix. This was unlayered ash with biotite flakes, not orientated. The harder clasts had some feldspars in them. Nico then explained the principles of pyroclastic eruptions.

Initially cold air is drawn into the erupted material as it leaves the vent causing the cloud of lava, air and ash to billow. Because of the cooler nature of the cloud the blocks of lava and ash fall fairly close to the vent and roll down slope. Subsequent eruptions roll over earlier flows encasing the blocks. Speeds of 220km/hr have been recorded and the resultant pressure wave in front of the flow can cause a lot of destruction. Many deaths are caused by this and research carried out on victims has shown that dehydration occurs, shrinking the body which is then encased in ash – a sort of artificial fossil.

Next was a visit north to another part of the Parc des Volcans a sort of National Park where we had lunch (chicken legs – again!!) in the col between the Petite (peteet) and Grand Sarcouy (grawnd sarcoo-i) before climbing the SE side of the Grand version, a 30,000 years old. Puy, bypassing a scoria cone to the south.

Cave in trachyte on Grand Sarcouy. Photograph: Stuart Fairley
Cave in trachyte on Grand Sarcouy. Photograph: Stuart Fairley

A large cave near the summit was an ideal resting spot in the heat (well I do come from Scotland!) The trachyte here was light grey in colour, softish, and had biotite, plagioclase and orthoclase in a crystalline matrix. The eruptions had not been explosive, no pressure build up like in this mornings example.

Interestingly there were a series of hummocks lying to the east. These were the result of mud flows which had run off the side of the dome as the lava flowed out. Smooth clays had been created due to the presence of water in the cone. If the pressure increases due to the influx of new magma these avalanche off producing hummocks where they come to rest.

We carried on to the top which was a marvellous place. Covered in all sorts of interesting vegetation, with a slight depression towards the centre of the vent., there smack bang in the centre was a Scots Pine (Brenda tried to tell me it was a Corsican pine which was probably correct but to complete the dream it had to the Scots variety!).

Last call of the day was to a quarry on the north side of the Sarcouy. Amazing place – weathered ash overlying scoria pumice rubble, with large bombs, lava tubes lying like toppled over tree trunks, pumice scree. Everything was rusty red coloured and not too pleasant to walk on in sandals. Close to the vent, the larger deposits displayed onion skin weathering. Within weeks of deposition chemical weathering had started due to leeching by chemicals in the gases. Air bubbles trapped in the rock had popped off with the heat and pressure to give everything a frothy appearance. Some smaller blocks displayed layered flows, grey in colour and containing no gas bubbles. They had cooled significantly during fallout.

Olivine crystals were seen in some clasts and in the sunlight the reflected in all directions due to their multiple cleavages – quite stunning! An interesting, nae very amusing, large fish–shaped bomb grabbed everyone’s attention. Our mentor explained that large bombs held on to their internal heat for much longer than the smaller ones. After landing the expanding heat continued to produce swelling of the block and some interesting shapes could result.

That evening some of us, the more foolhardy, spent a pleasant evening in caves somewhere near the top of a small cone within site of the Puy de Dome. Songs, light refreshments and some concoction of Nico’s called Ti Poche ? (sore head?)2 all made for an unforgettable party.

Stuart Fairley