Basaltic ignimbrites in monogenetic volcanism: the example of La Garrotxa volcanic field

Abstract

Ignimbrites are pyroclastic density current deposits common in explosive volcanism involving intermediate and silicic magmas and in less abundance in eruptions of basaltic central and shield volcanoes. However, they are not widely described in association with monogenetic volcanism, where typical products include lava flows, scoria and lapilli fall deposits, as well as various kinds of pyroclastic density current deposits and explosion breccias. In La Garrotxa basaltic monogenetic volcanic field, part of the Neogene-Quaternary European rift system located in the northeast of the Iberian Peninsula, we have identified a particular group of pyroclastic density current deposits that show similar textural characteristics to silicic ignimbrites, indicating an overlap in transport and depositional processes. These deposits can be clearly distinguished from other pyroclastic density current deposits generated during phreatomagmatic phases that typically correspond to thinly laminated units with planar-to-cross-bedded stratification. The monogenetic ignimbrite deposits correspond to a few meters to several tens of meters thick units rich in lithic- and lapilli scoria fragments, with an abundant ash matrix, and internally massive structure, emplaced along valleys and gullies, with run-out distances up to 6 km and individual volumes ranging from 10⁶ to 1.5 × 10⁷ m³. The presence of flattened scoria and columnar jointing in some of these deposits suggests relatively high emplacement temperatures, coinciding with available paleomagnetic data that suggests an emplacement temperature around 450–500 °C. In this work, we describe the main characteristics of these pyroclastic deposits that were generated by a number of phreatomagmatic episodes. Comparison with similar deposits from silicic eruptions and previous examples of ignimbrites associated with basaltic volcanism allows us to classify them as ‘basaltic ignimbrites’. The recognition in monogenetic volcanism of such pyroclastic products, which may extend several kilometres from source, has an important consequence for hazard assessment in these volcanic fields, which previously have been considered to present only minor hazards and risks.

Appendix 1

Approximate minimum flow velocities have been calculated using the approaches proposed by Pittari et al. (2007) and Roche (2015), both aimed at calculating the minimum flow velocity required by the parent flow to move lithic clasts captured from an underlaying substrate to a certain distance. Following Pittari et al. (2007), if we assume that the force required to move a static clast of certain dimension and density resting on bedrock is

$$ F1={u}_s m g $$

(1)

and that the force applied to a clast at rest by a pyroclastic flow is

$$ F2=0.5{\rho}_{fl}{\nu}^2{C}_D A $$

(2)

where u _s is the coefficient of static friction, m is the clast mass, g is the acceleration due to gravity, ρ _fl is the bulk density of the pyroclastic flow, ν is the pyroclastic flow velocity, C _D is the coefficient of aerodynamic drag and A is the clast surface impacted by the flow force, and making F1 = F2, we have that

$$ {\nu}^2={u}_s m g/0.5{\rho}_{fl}{C}_D A $$

(3)

Considering a lithic clast size dimensions of 0.8 × 0.5 × 0.3 (volume = 0.12 m³), which corresponds to the maximum lithic clast (Eocene feldspathic sandstone) size we have recorded, and the following input values: u _s  = 0.7; A = 0.15 m²; m = 300 kg (clast density = 2500 kg/m³); g = 10 ms⁻²; ρ _fl  = 1800 kg/m³; C _D  = 0.8, we obtain v = 4.41 m/s (15.87 km/h). If we used the equation proposed by Roche (2015)

$$ {v}^2=\left({\rho}_p-{\rho}_f\right) g C\div \gamma \rho $$

(4)

where ξ is a shape factor equal to 2.3 for an ellipsoid, ρ _p is the clast density, ρ _f is the gas density (∼1 kg/m³), g is the acceleration of gravity, C is the short length of the clast, γ ≈ 0.06 is an empirical factor and ρ is the bulk flow density, and using the same input values that in the previous case, we obtain a minimum flow velocity of 7 m/s, equivalent to 24 km/h.