Remote from ridges of accretion and from
big subduction zones the volcanism of the Central Massif is
of more complex origin.
It constitutes with the tectonic accidents of Limagnes to
which it is associated, a western important geologic constituent
European. The first theories are debated by the scientists
for the benefit of models supported by the last researches
today which connect his origin with the continuous
push of the Africa plate on the Eurasian plate.
In fact, the ascents of Italy at the origin of the genesis
of the Alps upset the crust and the mantle. This confrontation
creates it was 35MY ago an east-west
extension of the earth's crust at the origin of
the rifts of Limagnes, followed by thermic
ascents in the mantle. In this process magmas
form by decompression and rise up then in wills of
the weaknesses of the crust. This volcanic phase develops
there is 25MY ago.
The conjugation of these phenomena so favor the conditions
of forming a continental rift valley
of the North in the South of the Central Massif, which goes
on more in the northeast with the Rhenish ditch and the Eger.
The Auvergne volcanism
in place begins it was 25MY ago, with the phase of Limagnes,
but the most intense period will be situated later by the
construction of Cantal (13 MY in 3 MY), Cézallier (6
MY in 3 MY), the Sancy (3 MY in 200 000 years), more in the
southeast group Devés Velay (13 MY in 1 MY.
The first volcanoes of the Chaîne des Puys appear
150 000 years ago (see
the chronology). The size,
relatively small of the buildings, shows that the activity
was not continuous until our days, but rather occasional.
Indeed the average life cycle of a volcano has been of some
dozens in some hundreds of years at the most.