The mass of water to a depth of 90 m is frigid

The green tender grass slice on the Red-Brown of the trail. Under a blazing sun, our small troupe continued the rise of le puy of the Ghouls. We just leave the shadows of a beech forest and tackling the trail which exhales the heat accumulated since the morning of this day. "Note the size of the stones and colour", says Jean-Marcel Morel, our guide a volcanologist. "You have an explanation". No answer. Our fully equipped was more reactive at the foot of the volcanic cone... The rise of the 150 m of declivity blunt our thirst to learn! But our doctor of geology and volcanology pleases not the ear, and we press answer. "The colour is iron." "Blocks are bigger here because they have projected less far than the slag", launches just randomly, Martine, a seasoned hiker, seduced by the concept of scientific dimension ride. The answer seems to satisfy those who wish to "teach us to interact with the rocks." And already now, our expert who played under the grassy layer to make us feel the smell of this reddish Earth.

It is better not to lose the thread of these academic explanations. The life sciences where it was the genesis of these volcanoes are, for each of us, a distant past. Not without talent, our guide provides a point of honour to capture our attention. "These are outcrops that allow us to identify the family of the rocks and know how they reached here and at what time," he continued. And issues in answers, small stones in strata, the volcanologist shows us how to dissect the geological history of the site.

Eighty buildings

Major relief of the Hexagon, the chain of the Puys angering the central Massif of its eighty volcanic buildings on approximately 30 kilometres. Grandiose and dramatic landscape this geological set is doubly: by its youth and its morphology. The last eruption dates from 6,700 years, while scientists trace the presence of hominids in the region since at least 400,000 years. Our nomadic ancestors, gatherers and hunters thus attended the birth of the "Lunar" Auvergne, today attracts so many visitors to the risk of weakening many sites (see below).

"To understand the geology, the ideal is to position itself on a high point and see the large structures," explains Jean-Marcel wish. Our Observatory, at the top of the puy of the Ghouls at 1.146 m, fully meets this requirement. This volcano is a cinder cone that offers a crater of about 250 m in diameter and 40 m of depth, forming a circle almost perfect. The edges of the cone leaves appear in places, reddish pozzolans, these very friable volcanic ash on which the vegetation does not cling. Not only the field is not conducive to the plants, but any face is, inside the crater, is subject to the harshness of the action of the prevailing westerly winds. On this slope soft internal vegetation is miteuse, while the other side, shrubs are able to take root. A thick grass furniture the flanks of the vent "formed by a deficit of accumulation of slag".

Dome or Cone

Before descending into the crater, the teacher reminds us the difference between dome and cone. Initially, it is the nature of the magma that makes the difference between eruptions. With its fluidity, its viscosity and its richness in gas, the magma was more or less quickly and falls more or less far. Then occurs, as a result of intensive degassing, a cinder cone while the heart of the volcano settle and will widen to form a crater. If the magma is viscous, he continues to climb after the eruption and its ash plume caused by the accumulation of gas. It forms a dome because there is an accumulation of lava. "In the face of you, the puy de Dome", punctuates Jean-Marcel.

This falls well. The site is too good to not look at the nature of the ground at our feet. In the distance, to the South, stands the crest overlooking the chain of Puys with its 1.465 m the puy de Dôme, young volcanic edifice of 11,000 years... Behind, to the North, the grand and petit Sarcoui play the lookouts. Then, Western, pint, Chaumont and the site of Vulcania, animation of volcanism Park.

Full is, at the foot of our promontory, the basin of the Limagne plain extends to the spine of the Livradois Forez. "It is a corridor between Clermont-Ferrand and Thiers collapse." "This is a link in the Great West-European rift," commented the volcanologist. Because the chain of the Puys is associated with a series of ditches (rifts), which erode the European plate to the North Sea and Baltic Sea to the Mediterranean.

Thus, one of its puys (from latin podium: point top relief) that is the beauty of the regional natural park of the volcanos of Auvergne, the glance embraces miles round, Eruptive forms of the Massif central. The natural park of Livradois-Forez and his beautiful foothills expected him also the hiker in poorly large spaces and unspoilt nature.

The dangers of the Lake

Go to the onslaught of the great Sarcoui. He was born phréatomagmatiques explosions and the effusion of the viscous magma, once cooled, forming the rock called trachyte. "Not too fast." How this is written "School reflexes stand out, while rocks of pyroclastic flows roll beneath our feet. And we are now carefully noted that the trachyandesite is a grey stone, porous and non-frost.

A forest sente leads us to the small Sarcoui, whose slopes, Red-Brown, consist of iron, silica, magnesium-rich minerals. The stones are puffy and cauliflower. Some are bluish-black iridescent. Other indigo or sepia. They are rough and all light. Of real sculptures. Jean-Marcel takes us in digressions on volcanic bombs, spindle, almond, or even in cow dung... And while it evokes the Fiery clouds flowing in ash and gas at more than 200 km/h, the fumaroles escape cracks at the corners of the eruptive vents, we are immersed in the fires of volcanology. None of us surprised is if one of the puys came to wake...

New day. Branch Lake Plavin, about 40 kilometres south of the puy de Dôme. The almost circular Lake fills a large crater of explosion from the meeting of the magma and a water table. He formed what is called one maar, another type of different eruption of those leading to the cinder cone or dome. With its 6,700 years, this eruption is the most recent officially identified in the region. The diameter of the Lake is approximately 800 metres, it is materialized by the parts of the crater where the vegetation is managed to hold on despite of ancient lava flows cut in the explosions. But, at the bottom of the Lake, no life except anaerobic bacteria can be installed without oxygen. The mass of water to a depth of 90 m is frigid. Jean-Marcel says: "Lake Pavin is very monitored to verify that no gas does accumulate as had been the case in Lake Nyos, Cameroon" in 1986, a massive eruption of gas had killed nearly 1,800 people.

Suddenly, the sky is darkened. The atmosphere at the foot of the Lake becomes heavy, conducive to an imaginary of terrifying monsters, volcanic eruptions, viscous magma. The Tower of the Lake however began in the rain. The edges of the trails offer the view the strata of different geological eras. We feel that the land is still in motion. Red-Brown, dark brown tones are all straight from the bowels of the Earth. "Here, one day, it is safe, it will wake up," concludes our guide.