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

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.

By Lynn Everson