Arrival

Group photo taken on the steps of the gîte. Photograph: Kirsty Crocket

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.

Regional Setting & Tectonics: what are we doing here?

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” ¹⁾, 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.

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

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

1) Administrative region.