EXTENDING INTERFIELD ANALYSIS OF TUMULI ON TERRESTRIAL INFLATED LAVA FLOWS TO MARS

To better understand lava flow emplacement history and dynamics, we focus our study on tumuli—small-scale, positive topographic features (~10 meters in width) found both on terrestrial and Martian inflated lava flows. Inflated lava flows are aptly named for their domed, rigid upper crust that insulates and is lifted by a fluid interior. Locally high magmatic pressures arising from the transport of lava through a network of subsurface lava pathways within these flows can cause a section of the upper crust to tilt upwards and outwards on opposite sides of an axial fracture and produce a tumulus. As tumuli form over lava tubes, we believe that tumuli can be used as records of a flow’s interior structure. We aim to quantitatively investigate hypothesized relationships between tumuli morphometrics (such as tumuli sizes, shapes, and orientations) and larger-scale lava flow emplacement structure (flow directions, placement of flow boundaries, and scale of the flow). Listed are some of our recent results: (1) Tumuli form predominantly over very low slopes (<2^o), where lava tubes can develop roofs, (2) Tumuli long-axis orientations broadly correlate with the general (local) flow direction, (3) Tumuli size (area) does not change while tumuli density in the field increases further from the source vents, due to increasing bifurcations in lava pathways within the distal portions of the flow, and (4) Tumuli elongation (length/width ratio) decreases away from the source vent. Our measurements of >2000 tumuli within six diverse terrestrial fields and subsequent measurements of >2000 tumuli in seven fields within the Elysium Planitia region of Mars all generally comply with the results outlined above. Furthermore, basic tumuli dimensions (length, width, elongation, etc.) are strikingly similar between both planets—suggesting commonality in tumuli formation. Additionally, we have not yet found tumuli on Martian lava flows outside of very young flows within Elysium Planitia (<30 Ma). One possible explanation is that tumuli-forming lava flows only have occurred within a specific period of Martian volcanism. Alternatively, inflation features may be very susceptible to degradation and have eroded beyond recognition on older flows. Our study may thus provide information about regional or global changes in volcanism on Mars.