Eruption and emplacement dynamics of a thick trachytic lava flow of the Sancy volcano (France)
Résumé
A 70-m-thick, 2200-m-long (51 × 106 m3) trachytic
lava flow unit underlies the Puy de Cliergue (Mt. Dore,
France). Excellent exposure along a 400-m-long and 60- to
85-m-high section allows the flow interior to be accessed on
two sides of a glacial valley that cuts through the unit. We
completed an integrated morphological, structural, textural,
and chemical analysis of the unit to gain insights into eruption
and flow processes during emplacement of this thick silicic
lava flow, so as to elucidate the chamber and flow dynamic
processed that operate during the emplacement of such systems.
The unit is characterized by an inverse chemical stratification,
where there is primitive lava beneath the evolved
lava. The interior is plug dominated with a thin basal shear
zone overlying a thick basal breccia, with ramping affecting
the entire flow thickness. To understand these characteristics,
we propose an eruption model that first involves processes
operating in the magma chamber whereby a primitive melt
is injected into an evolved magma to create a mixed zone at
the chamber base. The eruption triggered by this event first
emplaced a trachytic dome, into which banded lava from the
chamber base was injected. Subsequent endogenous dome
growth led to flow down the shallow slope to the east on
which the highly viscous (1012 Pa s) coulée was emplaced.
The flow likely moved extremely slowly, being emplaced
over a period of 4–10 years in a glacial manner, where a thick
(>60-m) plug slid over a thin (5-m-thick) basal shear zone. Excellent exposure means that the Puy de Cliergue complex
can be viewed as a case type location for understanding and
defining the eruption and emplacement of thick, high-viscosity,
silicic lava flow systems.