The dynamics of a channel-fed lava flow on Pico Partido volcano, Lanzarote

Abstract

A short length of channel on Pico Partido volcano, Lanzarote, provides us the opportunity to examine the dynamics of lava flowing in a channel that extends over a sudden break in slope. The 1-2-m-wide, 0.5-2-m-deep channel was built during the 1730-1736 eruptions on Lanzarote and exhibits a sinuous, well-formed channel over a steep (11°slope) 100-m-long proximal section. Over-flow units comprising smooth pahoehoe sheet flow, as well as evidence on the inner channel walls for multiple (at least 11) flow levels, attest to unsteady flow in the channel. In addition, superelevation is apparent at each of the six bends along the proximal channel section. Superelevation results from banking of the lava as it moves around the bend thus causing preferential construction of the outer bank. As a result, the channel profile at each bend is asymmetric with an outer bank that is higher than the inner bank. Analysis of superelevation indicates flow velocities of~8 m s -1 . Our analysis of the superelevation features is based on an inertia-gravity balance, which we show is appropriate, even though the down-channel flow is in laminar flow. We use a viscosity-gravity balance model, together with the velocities calculated from superelevation, to obtain viscosities in the range 25-60 Pa s (assuming that the lava behaved as a Newtonian liquid). Estimated volume fluxes are in the range 7-12 m 3 s -1 . An apparent down-flow increase in derived volume flux may have resulted from variable supply or bulking up of the flow due to vesiculation. Where the channel moves over a sharp break in slope and onto slopes of~6°, the channel becomes less well defined and widens considerably. At the break of slope, an elongate ridge extends across the channel. We speculate that this ridge was formed as a result of a reduction in velocity immediately below the break of slope to allow deposition of entrained material or accretion of lava to the channel bed as a result of a change in flow regime or depth.

Construction dynamics of a lava channel

“…The development of wall-accreted sheets to build layers of lava plaster is also a common process in lava tubes (Calvari and Pinkerton 1999). Wall accretions described here also have a similar morphology to those described in Etnean channels by Kilburn and Guest (1993) and on Lanzorote by Woodcock and Harris (2006). Such layering thus appears to be a common feature in a channel or tube subject to variable flow levels, with each layer marking a distinct flow level.…”

“…Accretionary levees (Sparks et al 1976) will also form by smearing of ductile lava at the margins of the channel-contained stream onto the levee tops and walls. This then welds to form solid levees, with textures that usually show elongation of welded clasts in the downflow direction (e.g., Kilburn and Guest 1993;Woodcock and Harris, 2006). Accretion may also form lateral benches, or nested levees, within the main channel (e.g., Lipman and Banks 1987;Linneman and Borgia 1993;Kilburn and Guest 1993).…”

“…Variations in flow level due, for example, to oscillations in the erupted volume flux or lava ponding behind channel blockages to starve lower reaches of supply, as well as down-channel surges following blockage failure, will cause channel flux and flow level to vary. This will thus result in the construction of compound levees, inner-wall coatings and syn-channel structures such as lateral benches (e.g., Lipman and Banks 1987;Naranjo et al 1992;Kilburn and Guest 1993;Bailey et al 2006;Woodcock and Harris 2006). Time varying flow dynamics will result in the construction of a complex channel with levee morphology varying over short temporal and spatial scales (e.g., Lipman and Banks 1987;Kilburn and Guest 1993).…”

Explosive lava–water interactions I: architecture and emplacement chronology of volcanic rootless cone groups in the 1783–1784 Laki lava flow, Iceland

“…5) may reflect a transition from a tube to an open channel along the same lava pathway. The "chute and pool" geometry of most lava tubes (e.g., Guest et al 1984;Calvari and Pinkerton 1999) and, to a lesser extent, lava channels (e.g., Woodcock and Harris 2006;Harris et al 2009) may complicate simple rootless cone alignments by generating a state of stress that favors failure at the base of the flow along off-axis sites. Isolated rootless cones may also form in association with tumuli above pools in an internal pathway system (e.g., "H" in Fig.…”

Lee‐slope katabatic winds on the Philippine island of Mindanao

“…The apparent 'edge' of disturbed water observed on 25 December may have marked a 'hydraulic jump' (Woodcock 2003;Boucher, personal communication), where a new flow regime became established as the speed of flow rapidly decreased at the break of slope. * The relatively low rate of temperature fall in the lowest 5 km of atmosphere at Koror (approximately 5.1 degC km -1 ) on this date supports both the likely existence of strong lee-slope winds and the possible existence of a hydraulic jump when Fr >1 (Barry 1992, p. 135).…”