REGULARLY SPACED RIDGES ON SILICIC LAVA FLOWS AND IMPLICATIONS FOR INFERRING THE RHEOLOGICAL PROPERTIES OF PLANETARY LANDFORMS

Topographic ridges on the surfaces of terrestrial lavas and other planetary landforms are commonly used to identify the rheological properties of the host material, using modifications of Biot’s fold theory. The major assumption necessary to make such interpretations is that the topographic ridges and corresponding troughs were caused by layer-parallel compressional stresses that resulted in upright, buckle-style folds of the upper surface. This assumption is accurate for some features such as pahoehoe on basalt lavas which can be observed forming directly. However, for other landforms like silicic lavas, folding of the upper surface has only been inferred. We sought to test the validity of the fold assumption by comparing hundreds of topographic profiles and corresponding power-spectrum densities from six rhyolite and three basalt pahoehoe lavas. We hypothesize that if the ridges on rhyolite lavas formed under compression there should exist multiple wavelengths (i.e. fold generations), and that the dominant ridge wavelength would decrease towards the flow margin. Instead, scale-normalized comparisons of topographic profiles yield differences in the shape of pahoehoe ridges and rhyolite ridges. Pahoehoe ridges are rounded at the peak, and cuspate in the trough, whereas rhyolite peaks and troughs are both cuspate. The power-spectrum densities yielded dominant ridge wavelengths of 33 to 120 meters for rhyolites and 0.20 to 0.08 meters for the pahoehoe. Some rhyolites have similar distributions of ridge wavelengths when compared to basalts on a normalized scale. Variations in frequency response indicates that basalt ridges are more regularly spaced than ridges on the surface of rhyolitic lavas. Results of this study suggest that morphology is more diagnostic than spatial frequencies of ridges for constraining likely emplacement stresses. We propose that regular ridge spacings that form during extension of the upper surface are controlled by crustal thickness, (i.e. depth to brittle-ductile transition), and are not suitable for viscosity estimates. Future work will expand our analysis to other gravity driven flow landforms on Earth and other planetary bodies.