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.

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.

Cave in trachyte on Grand Sarcouy. Photograph: Stuart Fairley

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.

By Stuart Fairley

2) I think Stuart means "p'tite ponche" (rum punch) but the sore head bit is correct - Kirsty