Topographical controls on small‐volume pyroclastic flows

Abstract

Emplacement of small‐volume (<0·1 km3) pyroclastic flows is significantly influenced by topography. The Arico ignimbrite on Tenerife (Canary Islands) is a characteristic small‐volume pyroclastic flow deposit emplaced on high relief topography. The pyroclastic flow flowed down pre‐existing valleys on the southern slopes of the island. In proximal areas deep (up to 100 m) valleys acted as efficient conduits for the pyroclastic flow, which was mostly channelled; in this particular area the ignimbrite corresponds to a homogeneous, moderately welded deposit, consisting of flattened pumices in an abundant ashy matrix with a relatively low lithic fragment content. In intermediate zones significant changes occur in the steepness of the slope and, although still channelled, here the pyroclastic flow was influenced by hydraulic jumps. In this area, two different units can be clearly distinguished in the ignimbrite: the lower unit is composed of a lithic‐rich ground‐layer deposit that formed at the turbulent, highly concentrated head of the flow; the upper unit consists of a well welded pumice‐rich deposit that occasionally reveals a basal layer formed by shearing with the lower part. This division into two units is maintained as far as distal areas near the present‐day coastline, where the slope is very gentle or null and the ignimbrite is not channelled. The ground layer is not found in distal areas. The ignimbrite here only consists of the upper unit, which is occasionally repeated due to a surging process provoked by the lower flow speed, as the pyroclastic flow spread out of the channelled zone. A theoretical model on how topography controlled the deposition of the Arico ignimbrite is derived by interpreting the observed lithological and sedimentological variations in terms of changes in topography and bedrock morphology. This new model is of general applicability and will help to explain other deposits of similar characteristics.

Characteristics and emplacement mechanisms of the Coranzulí ignimbrites (Central Andes)

“…In the fine-grained crossstratified facies of ignimbrite unit 1 to the north of the caldera some lenticular lithic-rich facies were identified in the stratified succession. As these deposits are very proximal, we believe that they were derived from drag forces that picked up lithics from the substrate, which were then locally deposited by granular jumps (cf., Martí et al, 2019). This hypothesis is supported by the character and composition of the fragments (rounded to subrounded fragments of dacites and metapelites and quartzites from the Ordovician basement).…”

“…The first equation of the model defines the accumulation rate A r from the dense PDC as the rate at which the final deposit builds up from the substrate to the surface (after Doronzo et al, 2016;Giordano and Doronzo, 2017; see also Martí et al, 2019, for application to welded deposits) (Eq. 1) where the deposit thickness h refers to the cumulative or total deposit emplaced by various flow pulses (each one emplacing a depositional unit, i.e., lamina, massive layer), ρ p is the average density of all the particles involved, t is the emplacement timescale, T dep is the average temperature of the flow at emplacement, and T p is the particle (weighted average of juvenile and lithic fragments) temperature.…”

“…8 separates the fields characterised by heat loss (T dep /T p <1) and strain heating (T dep /T p >1). The physical meaning of the relationship between the accumulation rate and the dimensionless temperature is that the faster the accumulation, the more thermally conservative the deposit (cf., Michol et al, 2008;Lavallée et al, 2015;Doronzo et al, 2016;Giordano and Doronzo, 2017;Trolese et al, 2017;Martí et al, 2019). For example, welded ignimbrites will form at relatively high accumulation rates, whereas lava-like lithofacies will occur at extremely high accumulation rates (~1000 kg/m 2 s).…”

“…It is thought that dense PDCs emplacing ignimbrites are thermally conservative flows (McClelland et al, 2004;Lesti et al, 2011;Cas et al, 2011;Sulpizio et al, 2014;Giordano and Doronzo, 2017;Platzman et al, 2020). They can move whilst retaining temperature for several kilometres under the action of sustained pyroclastic fountaining with high mass eruption rates (MERs) during caldera collapse (Martí et al, 2009;Cas et al, 2011;Willcock et al, 2013;Roche et al, 2016;Trolese et al, 2017) or under the action of flow channelization (Doronzo et al, 2016;Martí et al, 2017Martí et al, , 2019Pensa et al, 2019).…”

Provenance of Jurassic siliciclastic deposits, determined by geochemistry and petrology: a case study from a Cimmerian intramontane basin, the Binalud Mountains, Iran

“…DeCelles and Giles 1996;Chiang et al 2004). Martí et al (2019) argued that the high topographic relief of Tenerife (Canary Islands) could be a significant factor for the formation of braided rivers, and this may also apply to the origin of the conglomerates and pebbly sandstones of the Bazeh-Howz Formation.…”