STRUCTURE-FROM-MOTION PHOTOGRAMMETRY APPLIED TO MORPHOLOGIC STUDIES OF NATURAL AND EXPERIMENTAL BASALTIC LAVA FLOWS

High-resolution digital elevation mapping of meter-scale basaltic lava flows and lobes is achieved through the application of Structure-from-Motion (SfM) photogrammetry, a well-proven method in morphologic studies. Surface topography of lava flows can provide insight into lava rheology, from which composition and emplacement conditions may be inferred. Specifically, we use this method in conjunction with spectral analysis to quantify fold geometry along the crust of ropy pahoehoe lavas. Pahoehoe textures form because the viscoelastic crust of a lava flow is coupled to a viscous core and deforms into semi-regularly spaced folds with wavelengths proportional to the crust-core viscosity contrast and the thickness of the crust (Fink and Fletcher, 1978). At a critical crustal thickness, a surface experiencing sufficient down-slope gravity-driven stresses may be further deformed into folds of larger wavelength. This process, termed multiple-generation folding, has been hypothesized to reflect the relative forcing of strain-rate versus cooling-rate, as well as give insight into composition of the lava (Gregg et al., 1998). Using SfM-derived digital elevation models and Fourier analysis, we test the hypothesis that fold wavelength ratios may be related to lava composition. Results from natural lava examples (Iceland and Reunion Island), and experimental examples from the Syracuse Lava Project, show second-to-first generation wavelength ratios of ~5:1 for basalt and ~3.5:1 for basaltic andesite. These are consistent with a previous report of wavelength ratios across several lava compositions (Gregg et al., 1998). Observations and video analysis from the Syracuse Lava Project, however, show that finite strain patterns (i.e. preserved fold trains) are not always representative of incremental strain history. Further quantitative analyses are needed to understand the full strain history of this complex system and how these complexities may affect interpretations of finite strain.