THE EFFECTS OF UNSTEADY EFFUSION RATES ON LAVA FLOW MORPHOLOGY AND EMPLACEMENT: RESULTS FROM LABORATORY ANALOGUE WAX EXPERIMENTS

Lava flows provide insight into the formation and evolution of volcanoes and large igneous provinces (LIPs). Experiments using polyethylene glycol (PEG) 600 wax established psi (hereafter denoted by Ψ), a dimensionless parameter that relates crust formation (t_s) and lateral advection (t_a) timescales of a viscous gravity current, along with five corresponding flow morphologies (Fink and Griffiths (1990) J. Fluid Mech., 221). More recent studies have used wax experiments with pulsatory source flow rates to address LIPs and flow inflation (e.g., Rader et al. (2017), EPSL, 477). Here we use similar approaches to address the influence of unsteady effusion rates (Q) on three emplacement styles common to lava flows: resurfacing, inflation, and lava tubes. We conducted 120 experiments using a peristaltic pump to inject dyed PEG wax into a chilled bath (~ 0° C) in a tank with a roughened base at a slope of 0°. The experiments were divided into two conditions: Conditions 1 (decreasing Q with time) and 2 (increasing Q with time). For each run 200 ml of wax was injected at an initial Q varying between 1–6 cm³/s across each of the five Ψ regimes. The temperature of the wax was adjusted according to Q to achieve the targeted Ψ value. After 200 ml of wax was injected, there was a pulse – either 10 or 50 s – during which Q either increased to 2Q or decreased to 1/2Q. The run ended after the pulse. We documented flow morphology and measured the flow dimensions. Our preliminary results suggest an interplay between the three emplacement styles, with a primary dependence on Q and local wax rheology. The injection pulse influenced these variables and added to the complexity of the flow. Resurfacing occurs across a range of experimental conditions, but is inhibited by a competent yet flexible crust. Inflation requires a competent, flexible crust and intermediate to low effusion rates. Tube formation required a more coherent crust, intermediate to low effusion rates, and steady flow. Longer pulse lengths favored inflation and tube formation. Resurfacing did little to increase the overall thickness of the flow, whereas inflation increased flow thickness and decreased areal extent. The primary controls on morphology were the crust strength and Q, which along with the pulsatory flow rate, also influenced emplacement style.