Day 4:Wednesday 24 July 2003

Phreatomagmatism: When magma meet water

Wednesday began with the return of dazed or speechless individuals at breakfast, all had been hit by remote effects of the Clierzou volcano, to be precise, the night spent in a cave on its slope. After this brief return to cave life, there were obvious signs of exhaustion and grumbles of low tolerance for such anachronisms as pneumatic mattresses. Modern brews, however, had been welcome.

Reasons to be dazed and speechless…

After breakfast we left our rural resort of Laschamps and drove East, down to the Limagne graben (see €u(ro)ck November 2003), where is located the city of Clermont Ferrand. In spite of its 1/4 million population the step there is slow on a hot day in the narrow streets of the old centre.

The cathedral

Our first visit was to the cathedral, a prominent landmark on a hill. From the earliest stages of its construction, in 1248, the cathedral still retains its gothic style, although it took another 650 years to complete. The building material is mainly the dark Pierre de Volvic (Volvic lava). With the distant backdrop of the Puy De Dome, its 50-meter high Bayette tower signals the volcanic character of the town. The cathedral had been built on top of an early basilica, of which the only visible memory is the crypt. Although the region had been rocked by religious conflicts from the 12^th century onwards, this cathedral was relatively unscathed as it had been soberly furnished. Today its dark atmosphere contrasts with colourful stained glass windows of the 12^th, 13^th, 15^th and 20^th century. It still contains treasures of roman art, 13^th century frescoes, statues and polychromes pietas, and a very beautiful 13^th century carving of the Virgin in Majesty, painted black in 1830.

Our next visit was to Clermont University’s Department of Earth Sciences and volcanology laboratory: Experimental Magmatology

Nico guided us to his alma mater and introduced us to professor Benjamin De Vries and his assistant Stephanie. Professor De Vries presented the modelling experiments conducted in the department’s experimental magmatology section.

In these small-scale models of volcanic events, a 5 mm layer of laboratory -prepared mixture may represent a 200 meter high sediment layer, while pressures obtained in pascals stand for corresponding rock pressure equivalents of tens of mega pascals. Materials used include silicon putty, sand and plaster, which can be mixed in the desired proportions to obtain viscous, brittle, elastic, or bouncy pastes of variable cohesions as well as to build dykes.

These experiments are very sensitive, and the slightest error in slopes can falsify the effects obtained. With great precision tectonic processes are reproduced and their effects on volcanoes observed. The modelling experiments cover many aspects of volcanic activity. They can focus on the behaviour of different types of magma or ejecta on different slopes, and lateral deformation from sloping strata.

Extension zones are also recreated in small-scale, as well as debris avalanche, caldera formations, intrusions as well as extrusions.

Pyroclastics can be fluidized by introducing gases through the column, or modified by raising the temperature, possibly as high as 600 degrees celsius. Waterbased experiments are also conducted, to obtain a more viscous flow than in gas-supported pyroclastics.

The predictive value of these experiments is high, without any strong contradiction observed so far.

The faculty has a very international involvement, with publications in English and Spanish, and work on site in such active volcanic regions as Indonesia, South America, The Reunion.

Parc and Musee Lecoq

After this visit, the nearby Parc Lecoq, with its cooling green and pond was a perfect site for lunch. The park is named after the 19^th century professor Henri Lecoq. A nearby natural history museum, le Musee Henri-Lecoq, houses his collections in a bourgeois building which had been the professor’s residence.

Some of our group chose to visit this museum. There the displays testify of his eclectic mind, with excellent presentations in mineralogy, petrography, paleontology, zoology and botanics. It was a surprise to see a 19^th century display cabinet housing the 59 native orchid types of the Auvergne. Professor Lecoq’s personal library also remains opened to researchers.

My personal bias took me to the mineralogy collection, which is organised in two sections. Samples of world minerals are excellent, but the local mineralogy was of particular interest. The most striking specimens of fluorite are on display, as the Auvergne is an important source of this mineral. Very sizeable samples include many colours – along with tables describing the causes of colour - as well as various mineral overgrowths, such as barytine. A special display is dedicated to minerals directly associated to volcanism. This includes the constituents of volcanic rocks, minerals linked to the eruptions and those which formed in later stages, such as calcites and aragonites, of hydrothermal origin.

Amethysts, also abundant in Auvergne, holds an important place, in the rough as well as in 19^th century artifacts and carvings.

The wine shop

Although many go to the Auvergne “pour prendre les eaux” and enjoy the healthy effects of thermal waters, our group’s objective to complete the day was Nico’s favourite wine shop. As in most regions in France local wines are produced. To be fair the shop had relatively few bottles of the local goods - Chateaugay, St Pourcain - which are not produced in very large quantity. But down narrow winding steps we reached a very well stocked cellar, with wines from all origins. On closer inspection the vast cellar, whose very constant temperature provides an optimal environment for wine conservation, consists of pale material with a brecciated structured. Dug deep in the ground it offers a fine image of was the the Limagne maars.

The phreatomagmatic events in the Limagne are 156 000 years old. They involved the encounter of magma with a deep watertable within the oligocene sediments in the graben. The hydrostatic pressure opposes the rise of magma but as the water temperature rises water vapour builds up to a point of pressure superior to the mass of the water table. At this point the violence of the eruptions projects to great distances a mix of debris of all sizes. In the Limagne much of this debris originated from the sedimentary substratum, which accounts for the relatively small size of the breccia seen in the wine cellar.

Nico explained how difficult it is to assess risks in such eruptions: Although the rising magma creates seismic activity, it is not so easy to monitor this in sediments. Since the main trigger is the meeting of magma and water, how deep the water is, at what depth the meeting will occur as well as the volume of water and magma involved are the essential controles in the eruption, much of this, however, very difficult to monitor.

The wine of the month was tasted, some of the ware purchased, and time had come to return to leave Clermont Ferrand and return for a last night in Laschamps.

By Brigitte Revol MacDonald