Complexity in Lava Flows: Surface Features and Structural Morphology.

Applegarth, Louisa Jane (2008) Complexity in Lava Flows: Surface Features and Structural Morphology. PhD thesis, Lancaster University.

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Abstract

Lava flows are complex phenomena, with the potential to cause severe damage to property and infrastructure. In order to accurately forecast the way in which lava will behave upon eruption, it is necessary to understand the complex processes that occur during emplacement. This thesis employs both analogue modelling, and observations of an active flow on Mt. Etna, to study the surface morphology of lava flows, aiming to relate this to their internal dynamics. Analogue models were used to investigate the interaction between a viscous flow interior and a brittle crust, and the relationship between crustal thickness and flow advance and morphology. Using a high viscosity (10 4 Pa s) silicone gel, and a mixture of sand and plaster of Paris, the behaviour of rheologically stratified, channelised lava flows was simulated. The experiments were conducted on an inclined board with a reservoir constructed at one end. Silicone was released from the reservoir through a sliding gate, where it encountered a seed flow consisting of a silicone sheet topped with a crust of known depth and constrained by levees, in order to represent the influx of fresh lava into a channel. Sequential digital images taken over the course of each experiment allowed marker points on the flow surface to be tracked, and these data were used to construct surface velocity maps. Several experiments were recorded using stereo imagery, allowing changes in the surface relief to be monitored. The observations showed that crustal thickness was a major control on surface morphology and flow behaviour. Thin crust flows were largely controlled by the fluid interior, and the crust rested passively on the surface of the silicone. Thick crusts were less easily deformed in compression at the channel head, but tensile fracturing at the flow front led to a greater degree of interaction between the crust and the flow interior than seen in thin crust flows, which has implications for cooling-limited lava flows. The experiments were successful in reproducing structures seen on lava flows, but represented the emplacement of simple flow units. Compound 'a'a flow fields emplaced during long-lived eruptions often contain many complex features of enigmatic origin, whose significance needs to be evaluated. The 2001 flank eruption of Mount Etna (17th July - 9th August) provided an excellent opportunity to observe the emplacement of a compound lava flow field. Twenty three days of activity at a single vent on the southern flank produced the lower flow field, which reached its maximum length in eight days and thereafter grew by the superimposition and juxtaposition of younger flow units, and break-outs from earlier channels. Several sets of data were collected during the eruption, and these have been used, together with detailed analysis of post-emplacement aerial photographs and field observations, to unravel the temporal and structural evolution of the flow field. The surface morphology of the developing flow was examined, and it was possible to distinguish between features reflecting emplacement processes in individual units; those resulting from channel drainage, inflation, squeezing of lava from an older unit when overridden by a younger unit; and post-emplacement changes including the emergence of lava in a range of rheological states through flow surfaces, flow margins and levee-channel boundaries. This 'squeeze up' lava represents the latest stages of activity, and as such may provide information about the most advanced rheological state in which lava can flow. Although no mature lava tubes were encountered during the field survey, there is clear evidence of preferential thermal pathways within stationary flow units. Had the eruption continued, there is a high probability that a tube system would have developed, as was the case during the 1983, 1991-3 and 2004 eruptions. This would have allowed the flow to extend significantly further, resulting in greater damage to property on this heavily populated side of the volcano. It is concluded that the widespread occurrence of the structures described indicates that the underlying processes responsible for their development are common and repeatable, and therefore amenable to inclusion in future models aiming to accurately forecast flow behaviour.