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  The
reindeer hunters who roamed the area during the last ice age
probably saw the final manifestations
of the Chaîne des Puys. There is also no doubt that the
Romans who stopped there later compared
these particular reliefs with active Sicilian and Italian volcanoes.
But from these ancient times we have no trace of scientific
observation and it was not until the 18th
century that the volcanic character of the region
was recognized. At this time, Jean-Etienne
Guettard, by comparing the nature of local volcanic
rocks with those of Italian volcanoes, highlighted this.From
then on, Auvergne established itself as the mecca of French
volcanology. On this study site, the observations of the first
naturalists and then the work of national geologists such as
Dolomieu, Lacroix, as well as those of the renowned local geologist,
Philippe Glangeaud, contributed to the development of volcanology
in France and the influence of this Science well
beyond national borders.
Finally, constant progress in geological
disciplines and technical innovations at the end
of the 20th century - dating, analysis of volcanic minerals
or digital terrain models (DEM) - have made it possible to significantly
deepen knowledge of this region.
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 The gentleness of the reliefs constitutes the main
characteristic of ancient
mountain formations : rounded summits, shallow
gorges, low altitudes. The monotony of the reliefs is rarer
in the Massif Central. It has undergone many transformations
over the course of geological times. In cycles of subsidence,
sedimentary deposits, uplifts, due to movements of the earth's
crust, volcanism very significantly
completed this transformation. This process began
45 million years ago (Menat maar),
then intensified around 35 to 25 million years ago with the
formation of the Limagnes.
Witness the eroded volcanic structures that line them, such
as the necks and lava flows
that make up, for example, the buttes of Nonette, Usson near
Issoire, the Montagne de la Serre in inverted relief, Puy Saint-Romain,
the volcanoes of La Comté, the slopes of Clermont, Montrognon,
etc. Later, Cantal, Aubrac,
Mont-Dore/Sancy, Velay
(Devès and Velay O.), the Chaîne
des Puys, and on the southern limits, Vivarais,
Coirons and Escandorgue
were established on the old massif. The construction of all
these formations took place over the second half of the Tertiary
or Cenozoic Era,
during the Oligocene, Miocene,
Pliocene and Quaternary
Periods.
The Central
Massif, a segment resulting from the Hercynian
Chain, was obviously the site of significant
fractures during its formation during the Primary
Era (or Paleozoic)
and its dismantling during the Secondary
Era (or Mesozoic). The collision of the Eurasian
and African plates in the Tertiary,
generating the Alpine Massif in the East, replayed these fracturings,
impacting the deep structures
of the lithosphere, creating collapse
basins and crustal uplifts. All this dynamic has
favored mantle rises at the origin of volcanism (see "The
formation of the Chain" - Why volcanoes in Auvergne).
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