Lava flows on Mars: Analysis of small surface features and comparisons with terrestrial analogs

Abstract

Small‐scale surface features on lava flows provide information on the eruption parameters and the volcanic history of the flows in which they occur. High‐resolution images of four sites on Mars reveal lava flows with a different type or combination of features including pressure ridges, tumuli, festoon ridges, and ring ridges. Pressure ridges, tumuli, and pressure plateaus are spaced irregularly and tend to be oriented parallel to flow. They are formed by inflation of small flow lobes and are indicative of compound flows emplaced by sporadic eruptions with low effusion rates. Festoon ridges are oriented transverse to flow direction, mimic the flow margin, and have a regular spacing. Using a model by Fink and Fletcher (1978) and Fink (1980 a) interior viscosity values of approximately 108 Pa s were calculated for festoon ridges tens to hundreds of meters apart and a few meters high on two basalt flows in Iceland and a flow on Mars. This viscosity is comparable to results from similar studies of more silicic flows and suggests that high viscosities are necessary to form large festoon ridges. By analogy to the Icelandic flows, Martian flows containing festoon ridges are considered to be large, single unit basalt flows emplaced through flood‐style eruptions. High viscosity of the basaltic flows may result from cooling, a high crystal content, or low effusion temperatures. Disruption of the surface of some of these large sheet flows possibly by lava‐water interaction is suggested by the presence of ring ridges and rugged terrain consisting of mesa‐like mounds and channel‐like troughs.