Regularly spaced ridges on rhyolite flows are analyzed through the use of a surface-folding model that was first applied to ropy structures on pahoehoe basalt flows. The requirement that there be a strong folding instability to produce regularly spaced ridges places constraints on three dimensionless parameters related to the properties of the lava and the geometry of the channel: R > 35,S < 0.02, and Ldγ > 28. R is the ratio of surface to interior viscosities, S is a ratio between the stress due to the weight of the lava and the compressive stress due to folding, and Ldγ is a dimensionless form of the ridge spacing. Estimates of strain rates and measurements of ridge spacings and thicknesses of thermal boundary layers of flows allow these three parameters to be calculated independently for a given flow lobe. For the Big Glass Mountain rhyolitic obsidian flow in northern California, R ≅ 104, S < 6.5 × 10−3, and Ldγ > 44. This compatibility between theory and observation supports the folding interpretation for ridges. Furthermore, the model allows calculation of the minimum viscosity of many flows for which such data are otherwise unavailable. The viscosities of a dacite flow in Chile and of a. possible lava flow on Mars are calculated as examples.