The role of ice cavities in lava lobe formation Hannah Reynolds*, Duncan Woodcock, Jennie Gilbert and Steve Lane Lancaster Environment Centre, Lancaster University LA1 4YQ, UK *(hannah.i.reynolds@gmail.com) • The mechanism for the creation of lava lobes remains controversial. • This work investigates the feasibility of lava lobes forming within ice cavities, using an analogue experimental approach. • Observations of cavity morphology, and quantification of cavity growth with time, allow for comparison with field observations. • A model of the melting process was constructed based on experimental observations. Glaciovolcanism produces distinctive large scale volcanic features such as tuyas and sheet-like lava extruded between bedrock and ice. In each case ice surrounding the lava may define the morphology of the edifice and deposit. Rhyolitic lava lobes, which are smaller scale features up to 10 m tall 1 , have also been observed (fig. 1). Tuffen et al 2 proposed that these lobes were emplaced within pre-formed ice cavities, which were themselves generated by volcanic fumaroles (fig. 2). 1. Introduction References: 1. McGarvie, D (2009) Rhyolitic volcano-ice interactions in Iceland. Journal of Volcanology and Geothermal Research 185: 367-389. 2. Tuffen, H; Gilbert, J; McGarvie, D (2001) Products of an effusive subglacial rhyolite eruption: Bláhnúkur, Torfajökull, Iceland. Bulletin of Volcanology 63: 179-190. 3. Tuffen, H; McGarvie, D; Pinkerton, H; Gilbert, J S; Brooker, R A (2008) An explosice-intrusive subglacial rhyolite eruption at Dalakvísl, Torfajökull, Iceland. Bulletin of Volcanology 70: 841-860. Figure 2: Fumarolic melting forms ice cavities (left) and lava intrudes into the cavities (right) to form lava lobes 1,2 . Figure 1: Lava lobes on Hvalvörðugil valley, Öræfajökull, Iceland 1 . Three individual lobes are visible within the red circle. VMSG, Bristol 2013 59_A (S) 2. Methods Figure 4: Schematic cross-sectional diagram of apparatus. The approximate height of the ice block was 15 cm. Diagram not to scale. 3. Cavity growth 4. Field comparisons • Observed natural lava lobes are conical in morphology, whereas the laboratory generated cavities are truncated prolate spheroids. This difference may be explained in two ways: (1) enhanced melting may have taken place at the cavity bases of the natural lava lobes if the run- off had ponded rather than draining, and (2) the natural lava lobes may not represent their original morphology, due to partial collapse or erosion. • The growth rate equates to a period of approximately 7 days to generate a cavity 4 m in height (a typical height for observed lava lobes). This is considered realistic when compared to the ascent time of rhyolitic magma. Figure 6: Three data sets, two taken from video footage, and one from putty moulds. Figure 5: Visualisation of the model for the melting process as observed during analogue experiments. Figure 3: Photograph of a cavity growing within the ice block. • The steam condenses on the cavity ceiling and melting occurs. • The run-off water drains down the walls of the cavity, resulting in further melting and almost equal horizontal and vertical cavity growth (approx. 4:5). • The temperature difference between the run-off and the ice decreases near the base, causing the melting rate to decrease. This results in a lip feature (fig. 5). • The cavity volume was found to grow at a constant rate of 1.8 x 10 -6 m 3 s -1 (fig 6). • Using a scaling factor of 100 gives an estimate for the cavity growth rate of 1.8 x 10 -4 m 3 s -1 . • An analogue experiment was developed to simulate fumarolic activity beneath a glacier using a steam jet and ice block (approx. 12 cm thick) (figs 3, 4). • A suite of experiments of different durations were carried out, ranging from 30 s – 90 s. • All significant heat transfer was constrained using heat and mass measurements of the steam, ice and liquid run-off water. • Video footage of the experiment was used to record time- series measurements of cavity growth. • Putty moulds of the cavity were made following each experiment to reveal further detail of cavity morphology. • Cavity morphology is best described as a truncated prolate spheroid (fig 7). • The run-off water drains from the system once it has reached the base of the cavity. Figure 7: Cavity mould showing truncated spheroid morphology. Mould taken after a 90 s experiment. Ruler used for scale, major divisions are in cm.