Analysing the topographic form of stratovolcanoes

Abstract

The goal of this study is to explore “ideal” analytic functional forms that best fit the topographic shape of N = 190 isolated stratovolcanoes from around the world. Using a stochastic model for the piling of lava flows to demonstrate one set of physical processes that give rise to a stratovolcano's topography, we find that although the ideal form fits well for many stratovolcanoes, there exist deviations from this shape—particularly where the edifice is more protuberant than expected. We explore sub-surface factors—such as the emplacement of magma beneath a volcano edifice—that may be responsible for these deviations and estimate the relative contribution of dyke intrusions on the height and shape of a volcanic edifice versus the constructional piling of lava. From this comparison, we are able to gain more insight into the role of important subsurface physical factors, such as chamber depth and total intruded volume.We find that low basal ellipticity is a characteristic of nearly all volcanoes that fit the ideal stratovolcano form. By fitting along each quadrant bisected by either a semi-major or semi-minor ellipse axis, we find that the volcanoes chosen in this study fall into four groups: (1) volcanoes that are axisymmetric and fit the ideal form well, (2) volcanoes that are not axisymmetric but may be fit to the ideal form using different fitting parameters in each quadrant, (3) volcanoes that have axisymmetric protuberant deviations in all quadrants, and (4) volcanoes that have non-axisymmetric protuberant deviations that are only present in a couple of quadrants. We show that, in some cases, the non-axisymmetry of volcanoes in category (2) may be understood as the result of the piling of eruptive products along a regional planar trend. For edifices in categories (3) and (4), we model protuberant deviations as the result of a statistical distribution of elastic tensile dislocations representing dykes, which emanate from a magma reservoir and cause uplift at the surface. We support this interpretation of the “excess” topography in a few case studies where independent geologic and geophysical data corroborate our model results.