Day 5: Thursday 24 July 2003

Strato-volcanoes, pumice, calderas and the Two Towers

On Thursday, we moved southwest towards Monte Dore and the Massif du Sancy. We were relocating to new accommodation today so an extended company left Laschamps to travel to our next Gîte in Courbanges. Our first stop en route was Rochefort Montagne. This was a huge deposit of pumice estimated to be 25-30 metres deep which was the airfall deposit of a strato volcano.

Strato-volcanoes build over thousands of years. When they erupt, they initially produce a very high plume – a plinian eruption. On collapse, it is followed by a pyroclastic flow and then by huge volumes of ignimbrites as the magma chamber empties. If the magma chamber empties rapidly the top of the volcano collapses forming a caldera (calderas being much bigger than craters.)

Pumice has a low density. It is a glass and is full of elongated bubbles. The distinctive shape of the bubbles was created by air pressure due to the force of the eruption from the volcano. Vast quantities must have been produced as it can be found 10-15 km from the source and is still very deep. It is believed that, deep as this deposit is, it was laid down within an extremely short time scale of hours, a day at the most.

Rochefort Montagne

We moved on to Orcival. This pretty village was on the pilgrim route. We enjoyed a leisurely lunch and took the time to visit the very picturesque Romanesque church which has a beautiful chapel in the crypt.

Driving on south through the Col de Guery, we stopped and climbed down to a viewpoint to see the Roches de Tuillières and Sanadoire in the distance. The road down to these impressive rocky towers (did JR Tolkien pass this way?) took a little searching out – we almost managed to lose a car or two in our company by swiftly turning into various car parks, completing a quick circuit and doubling back. Enough to confuse anyone not concentrating on our elusive leader, Nico. Succeeding in locating the correct road, we found ourselves at the foot of these huge basalt columns. The loose rubble lying around here is phonolite, a very basic rock, undersaturated and very viscous.

Our final stop of the day was at a quarry of Le Chiex. This apparently deserted quarry in a country location was surprisingly well guarded because within minutes of us scrambling under or around the barrier, security guards arrived. While most of the group stood around at a loss, Nico used his charm to reassure the guards that, despite appearances to the contrary, we were harmless and just liked rocks! We were allowed to proceed. On a previous visit, Nico had seen debris avalanches at this location but much work had been done in the interim and he needed to run up and down the (steep) paths to locate this exposure while most of us followed more slowly behind him, ‘admiring the view’.

The gîte at Courbanges turned out to be most attractive and comfortable. This small hamlet is at an altitude of 1143m with spectacular views from the garden of the surrounding mountains which rise to 1800m and proved to be a most enjoyable final destination for our final 2 days.

By Janet Hiscott