Modelling volcano degradation through analogue experiments: the impact of volcano slopes and summit craters on erosion patterns

Analogue experiments can enhance our understanding of complex natural volcanic landscapes formed by eruptions, intrusions, remobilisation of volcanic material and erosional processes. Experimental setup in a laboratory offers a controlled setting to investigate the development of rainfall-induced radial drainage basins on scaled volcano cones. It allows to simulate surface runoff, a prominent sediment transport process in volcanic landscapes, primarily influenced by climate, lithology, and topography. By controlling the flowrate within the setup, maintaining a uniform lithology, and using initial axisymmetric cones with the same size, this study aims to record the variations in erosion patterns caused by systematic cone slope and shape changes.

Analogue volcanic cones made up of water-saturated 70 μm silica powder were built upon a drainage layer of coarse sand at the VUB volcanology analogue laboratory. The cones were scaled based on the height/basal width ratios of natural pristine composite volcanoes with a scaling factor of 4.5*10^-6 for the basal width. Initial cones had basal widths of 33 cm with two sets of cones with heights ranging from 4.2 to 6.9 cm. For the highest cones, the lower flank was 21 degrees and the upper slope 30 degrees, with a break-in-slope at 45% of the cone height. Experiments included cones with and without summit craters, the craters were 5 cm wide and 0.5 cm deep. Rainfall-induced erosion was simulated with two atomizer sprinklers, creating a mist of droplets of circa 30 μm. Experiments were run for 3 to 5 hours, simulating erosion taking place over several millions of years at natural volcanoes. We generated a minimum of ten Digital Elevation Models (DEMs) by photogrammetry with sub-millimetre spatial resolution, enabling the estimation of volume loss and erosion rates. The automated algorithms MorVolc and DrainageVolc were used to extract morphometric and drainage parameters (e.g., height/basal width ratio, drainage density, irregularity index) from the DEM of each timestep.

The analogue models' drainage networks and morphological characteristics replicate those found on natural volcanoes. Having a steeper slope for the upper flank of the cone delayed the forming of erosional features on the lower flank, while the top part of the volcano incised deeper than the cones with one slope gradient. The cones without a summit crater develop a radial drainage network from an initial set of narrow gullies to a more stable pattern with fewer valleys that gradually widen. The introduction of a summit crater substantially modifies the resulting erosional patterns: the incision of the crater rim forms two to four dominant watersheds that widen faster than the basins of cones lacking a crater. Migration of drainage divides ceases when equilibrium in the landscape is attained, with cones featuring a summit crater reaching this equilibrium later than those without. Analogue experiments are a valuable tool for studying erosional processes in a controlled manner and give insight into complex volcanic landscapes, thereby improving our understanding of long-term volcanic landscape evolution.