Evolved lavas on Mars? Observations from southwest Arsia Mons and Sabancaya volcano, Peru

An extensive (10⁶ km²) lava flow field SW of Arsia Mons contains individual lobes that are 10²-10³ km long, are ∼65 m thick, and display a surface ridging similar to the textures found on terrestrial andesite, dacite, and rhyolite flows, raising the question of whether large-volume evolved lava flows exist on Mars. The regularly spaced ridges on these Martian flows have wavelengths of ∼100 ± 34 and amplitudes of ∼27 ± 11 m. An andesitic lava flow field at Sabancaya volcano, Peru, is a morphologic analog, with flow thicknesses of ∼60-170 m and surface ridges. One intensely studied flow lobe at Sabancaya has a ridge wavelength of ∼67 ± 24 m and amplitude of ∼5 ± 2 m. Despite morphologic similarities, comparing the calculated rheologic and emplacement parameters of the two lava flow fields reveals large differences. Calculated yield strengths for the Martian lavas are 10³ Pa; those for lavas of Sabancaya, Peru, are 10⁵ Pa. Effusion rates of 10³-10⁴ m³ s^-1 calculated for the Martian flows are larger than those calculated for the Sabancaya lavas (1 m³ s^-1). Calculations of emplacement times for the Martian and Sabancaya flows are 1-10² days and 1-10³ days, respectively. Estimates of viscosity are 10⁴-10⁶ Pa s for southwestern Arsia Mons and 10¹⁰-10¹² Pa s for Sabancaya. We therefore conclude that the flows southwest of Arsia Mons are most likely not the same composition as those at Sabancaya volcano, but rather are basaltic or basaltic andesite in composition.