The growth of polygenetic volcanoes in the laboratory: Control of eruptive volume and magma viscosity on edifice development and morphology

Morphometric parameters have been used to describe and classify the shape of volcanic edifices, but the processes that control these parameters are not fully understood. Here we investigate the influence of the changes in eruptive volume and magma viscosity on the edifice morphology of a simple polygenetic volcano with a fixed stable vent using analogue experiments in the laboratory. We ran the experiments by ejecting vegetable oil repeatedly at a static location in a temperature-controlled room with an adequate interval between ejections to allow this material to solidify. The experiments comprised four different scenarios: constant ejection volume and viscosity, linearly decreasing ejection volume with constant viscosity, exponentially decreasing ejection volume with constant viscosity, and increasing viscosity with constant ejection volume. We built digital elevation models from images captured by digital cameras after each ejection using photogrammetry method. We describe the shapes of the edifices using morphometric parameters, such as height, basal width, H/W ratio, volume, slope, circularity, and regularity. The experiments with decreasing ejection volume produced taller edifices with steeper slopes, especially near the summit, compared to the edifice produced with constant ejection volume. A similar finding was also observed for the edifice resulting from the experiment with increasing viscosity. The circularity and regularity indexes were insignificantly influenced by ejection volume and viscosity changes. Instead, these parameters vary with the height fraction of the edifice where the lower part is more circular, but irregular compared to the upper part. Based on the changes of morphometric variables throughout the experiments, we propose three development stages of volcanic edifice growth: basal foundation, flank construction, and vertical build-up. The development stage in which the edifice currently grows can be inferred from the relative changes in the eruptive volume and the magma viscosity. Therefore, the quantitative characterization of a natural volcanic edifice's morphology can be interpreted to explain the processes that influenced it and its stage of growth.