The Spatial Distribution of Lava Flow Surface Features on Earth and Mars

Introduction: Lava flow surface morphology results from processes that occur during emplacement. Features such as tumuli and lava rises are indicators of flow inflation [1]. Tumuli in particular have been identified as possible indicators of lava tube location [2-5], indicating that their distribution on the surface of a lava flow is a function of the internal pathways of lava present during flow emplacement. However, the distribution of tumuli on lava flows has not been examined in a statistically thorough manner, nor have these features been positively identified on other terrestrial planets. In order to more rigorously examine the spatial distribution of tumuli, we examined 3 different flows: two on Earth that display numerous lava rises and tumuli, and another south of Elysium Mons with features morphologically similar to tumuli (Figures 1 and 2). The terrestrial flows include a discrete lobe of the 1969-1974 Mauna Ulu flow at Kilauea, Hawaii. The lobe is located in the distal portion of the flow below Holei Pali, and is characterized by hummocky pahoehoe flows emplaced from tubes [6]. This flow allows complete mapping of surface morphologies because of its well-defined boundaries, wellconstrained emplacement parameters, and known flow thicknesses. In addition, tube locations for this flow were mapped by Holcomb [7] during flow emplacement. We also examine the distribution of tumuli on the distal portion of the hummocky Thrainsskjoldur flow field from maps provided in Rossi and Gudmundsson [8]. MOC image 20-01192 (centered at 1.86°N, 186.11°W) depicts a portion of a lava flow to the southwest of Orcus Patera that may display primary flow surface features. Based on Viking data, the flow extends for over 60 km; we have mapped a 3 x 4.5 km section of the flow imaged at 5.8 m/pixel. The flow is relatively dark with an irregular surface, little apparent mantling, and few impact craters. It appears to superpose a bright unit to the southeast. The surface of the flow has many positive relief features that are at the same scale (<10 – 50 m) as terrestrial inflation features (tumuli, lava rises). The flow is characterized by raised elliptical to circular mounds, some with axial cracks [1,5]. One potential avenue of determining whether they are tumuli is to look at the spatial distribution to see if any patterns similar to those of tumulidominated terrestrial flows can be identified. Since tumuli form by the injection of lava beneath a crust, the distribution of tumuli on a flow should represent the distribution of thermally preferred pathways beneath the flow surface. That distribution of thermally preferred pathways may be a function of the evolution of a basaltic lava flow. As a longer-lived flow evolves, initially broad thermally preferred pathways would evolve to narrower, more well defined tube-like pathways [9-10]. The final flow morphology clearly preserves the growth of the flow over time, with inflation features indicating pathways that were not necessarily contemporaneously active. Here, we test using statistical analysis whether this final flow morphology produces distinct distributions that can be used to readily determine the distribution of thermally preferred pathways beneath the surface of the crust.