88
(Cantagrel and Baubron 1983 ), before the beginning of the
Chaîne des Puys activity. Pyroclastic deposits, including
ignimbrites (pumice-and-ash fl ows), nuées ardentes (block-
and- ash fl ows), pumice falls (Plinian deposits) and surges
represent about 20–30 % of the volcanic products (Brousse
1963 ). Volcanic products belong to both silica- undersaturated
(basanites and phonolites, which are poorly represented in
Sancy), and –oversaturated (basalts to trachytes) alkaline
series. About 75 % of the lavas have an intermediate compo-
sition (trachyandesites).
The volcanic activity started about 1.1 Ma ago with a
sequence of pyroclastic eruptions dated between,
1.1 ± 0.01 Ma and 1.01 ± 0.01 Ma. This period of intense
activity seems to be followed by debris avalanche deposits
well exposed in Le Cheix quarries, in the eastern part of the
volcano. These deposits are capped by basaltic lava fl ows
dated at 900 ka. Therefore, early as 1.0 Ma, the Sancy strato-
volcano was already partly edifi ed (Fig. 4.2d ).
The collapse of a small summit caldera (Fig. 4.2e ) accom-
panied the eruption of trachytic pumiceous fl ows (Lavina
1985 ) and air falls, about 0.72 Ma ago (Féraud et al. 1990 ;
Nomade et al. 2012 ). The caldera was later infi lled during
construction of the present-day summit. The period immedi-
ately following caldera collapse was characterized by mafi c
hydromagmatic activity and deposition of lacustrine sedi-
ments. Subsequent activity was predominantly trachyandes-
itic. It involved the emplacement of several domes with
associated pyroclastic fl ows, the lava succession of the
Grande Cascade (0.38 Ma), the basaltic trachyandesite fl ow
of the Plateau de l’Angle (0.36 Ma) and the dykes which
today crop out around the summit of Sancy and in the valley
of Chaudefour. Basaltic lava fl ows were also emitted in distal
position, such as at La Clé du Lac (0.5 Ma).
The global tephrostratigraphy of the Sancy volcano has
been recently revised and updated by Nomade et al. ( 2012 ),
on the basis of 13 new 40Ar/39Ar age determinations and
re-interpretation of the Sancy pyroclastic deposits. Four
cycles, termed C.I to C.IV, of pyroclastic activity have
been determined, the duration of each being about 100 ka.
The volcanic activity started with a major pyroclastic
phase 1.1 Ma ago, i.e 100 ka earlier than previously
thought. Following Cycle I (1.1–1.0 Ma), the main pyro-
clastic cycle II (818–685 ka) encompassed at least 8 large
eruptions, while the most important occurred c. 719 ka
ago. This major event recorded as a 1.4 m-thick layer as far
away as 60 km SE of Sancy (Alleret maar) may be corre-
lated to the formation of the small summit caldera (“C2”
event of Lavina 1985 ; “Rioubes haut” pumice-fl ow deposit
c. 718 ka: Féraud et al. 1990 ). Cycle III (642–537 ka),
including the 0.58 Ma-old Neschers pumice-fl ow deposit
(Lo Bello et al. 1987 ), and Cycle IV (392–280 ka) pro-
duced individual tephra deposits mostly towards the north
and NE of Sancy.
4.5 Conclusions
This brief summary shows that there never existed a single
Monts Dore stratovolcano, but at least two composite, long-
lived, and independent edifi ces separated in space and in
time. The Aiguiller massif, as well as the Massif Adventif,
may also represent distinct eruptive centres, which were
more or less independent and constructed above discrete
magma reservoirs.
At the present time, only 230 ka lapsed since the last erup-
tion of Sancy. This is less than half that of the previous period
of dormancy in the region. It is therefore quite possible that
a new volcano might one day be constructed. Moreover, the
volcanic history of the last 100 ka has been marked by the
construction of scattered scoria cones and maars in the Monts
Dore area. These are probably best regarded as satellites of
the Chaîne des Puys.
The Pavin was built on the south-east fl ank of Sancy stra-
tovolcano, which itself was located SE of the previous Monts
Dore s.s. Thus, the Pavin young volcanic group (including
scoria cones such as Montchal and Montcineyre, and associ-
ated lava fl ows), located at SE of the previous Sancy volcano,
could also be interpreted as witnessing the fi rst activity of a
future stratovolcano.Acknowledgments R-C Maury and S. Nomade are thanked for review-
ing this chapter.References
Baubron JC, Cantagrel JM (1980) Les deux volcans des Monts Dore
(Massif Central français): arguments chronologiques. C R Acad Sc
Paris 290(D):1409–1412
Bayer R, Cuer M (1981) Inversion tri-dimensionnelle des données aéro-
magnétiques sur le massif volcanique du Mont-Dore; implications
structurales et géothermiques. Ann Geophys 37(2):347–365
Besson JC (1978) Les formations volcaniques du versant oriental du
massif des Monts Dore (Massif central français); Feuille 1/25 000
Veyre-Monton No 5–6. – Thèse 3ème cycle, Université de Clermont
Fd II, 167 p
Boudon G, Mossand P (1984) Les facès d’écoulement et de déferlante:
deux composants d’un dépôt cendro-ponceux montdorien (Massif
Central français). Société Géologique de France (Editor), 10è
RAST, Bordeaux, 78 p
Briot D (1988) Génèse d’une série alcaline intraplaque continentale.
Etude géochimique (éléments traces et isotopes Sr-Nd-O) du volcan
des Monts-Dore (Massif Central français). Thèse Doctorat
Université Clermont-Fd: 195 p
Briot D (1990) Magma mixing versus xenocryst assimilation: the gen-
esis of trachyandesites in Sancy volcano, Massif Central, France.
Lithos 25:227–241
Brousse R (1961a) Minéralogie et pétrographie des roches volcaniques
du Massif du Mont-Dore (Auvergne) I: Les minéraux; II:
Pétrographie systématique. Bull Soc Fr Min Crist LXXXIV:131–186
Brousse R (1961b) Minéralogie et pétrographie des roches volcaniques
du massif du Mont-Dore (Auvergne) III. Les deux associations
magmatiques. Bull Soc Franç Miner Crist LXXXIV:245–259A. Gourgaud