Multiple-generation folding and non-coaxial strain of lava crusts

Abstract

Viscoelastic strain in lava flows is commonly expressed as gravity-driven buckling of the lava crust. This surface folding process creates the well-known ropy pāhoehoe texture of basaltic lavas and the ogives and surface ridges of more compositionally evolved lava flows. Previous work has shown that surface fold wavelengths are proportional to the viscosity contrast between the lava crust and core, and to the thickness of the crust. Thus, fold analysis can be an important tool for understanding lava flow rheology. We analyze fold wavelength patterns of solidified natural lava flows from the Myvatn lava fields (Iceland), Piton de La Fournaise (La Reunion), and experimental lava flows from the Syracuse University Lava Project. In each case, lava flows exhibited two dominant wavelengths, consistent with multiple generations of coaxial folding. The ratio of the two dominant wavelengths for basalt (Iceland, La Reunion) is ~ 5:1 whereas the wavelength ratio for basaltic andesite (Syracuse) is ~ 3:1, suggesting a compositional control on deformation, as proposed by previous studies. Video analysis of incrementally folded Syracuse lava crusts reveals significant non-coaxial strain, which violates the assumptions of plane strain used in crustal buckling models. These results show that interpreting lava rheology from finite strain requires careful consideration of complex three-dimensional strain fields. Despite these complexities, the correlation between fold wavelength ratios and lava flow composition persists and may provide important insight into flow characterization.