Eruption and emplacement dynamics of a thick trachytic lava flow of the Sancy volcano (France)

Abstract

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.

Appendices

Appendix 1: Textural analyses of the different facies

We report here some examples of scanned rock faces, scanned of thin section, and SEM images for each facies.

Appendix 2: Flow dynamics model

We first calculate the critical thickness (h ₀) for the lava of the Puy de Cliergue. This critical thickness corresponds to the point in the flow where shear stress (τ) is greater than the yield strength (τ ₀). Above this point, the fluid will deform, and flow will occur. This point, in terms of h ₀, is given by

$$ {h}_0=\frac{\tau_0}{\rho g \sin \left(\theta \right)} $$

(1)

where τ ₀ here is set to 10⁵ Pa, this being the yield strength found for Santiaguito’s dacite lavas by Harris et al. (2004); ρ is the lava density (2100 kg/m³—the mean of our samples), g is acceleration due to gravity (9.81 m/s²), and θ is the underlying slope (5°). Note: we take maximum values to obtain an upper limit on our calculations. The value of h ₀ will define the thickness of the non-deforming plug in the upper part of the flow.

We next calculate the velocity profile below the non-deforming plug zone using (Moore 1987)

$$ V(z)=\frac{\rho g \sin \left(\theta \right)\times \left({h}^2-{z}^2\right)}{4\eta}\times \left[1-\frac{4{\tau}_0}{3\tau (z)}+\frac{1}{3}\times \frac{{\tau_0}^4}{3\tau {(z)}^4}\ \right] $$

(2)

where h is the flow depth (60 m) and η is the lava viscosity (10⁸ Pa s). At the base of the flow, z = 0, increasing to z = h at the top of the flow. We apply this equation until τ = τ ₀, at this point z = h ₀. Here, deformation will not occur and we will define a plug that rides along on top of the shear zone. Shear stress (τ) is varied as a function of flow depth

$$ \tau (z)=\left(h-z\right)\rho g \sin \left(\theta \right) $$

Thus, τ will decrease from a maximum at the flow base to a point where τ = τ ₀. At this point, deformation no longer occurs. We thus define a basal shear zone across which velocities increase to a maximum at the base of the plug. The resulting velocity profile is given in Fig. 15. Using the velocity profile, we can now calculate strain rate (ε), this being defined by dv/dz. Strain rate (in s⁻¹) is plotted against height (in m) in Fig. 15.

Finally, we calculate the apparent viscosity profile which allows us to vary the viscosity with the strain rate following (Avard and Whittington 2012)

$$ \mathrm{Log}\ \left(\eta \right)\mathrm{app}=-0.738+9.24\times \frac{103}{T(K)}-0.654\times \log \left(\varepsilon \right) $$

(3)

The results are plotted in Fig. 15 where we see that viscosity decreases from a minimum of 10⁴ Pa s under the high shear stress and strain rate conditions at the flow base to 10⁵ Pa s, 4 m higher in the flow (toward the plug base).