Surface roughness characterization of the 2014–2015 Holuhraun lava flow-field in Iceland: implications for facies mapping and remote sensing

Surface roughness is a commonly used parameter for the quantitative analysis and characterization of geological terrains on Earth, as well as on other planetary bodies, particularly where detailed optical data may not be available. Here, we statistically investigate if surface roughness can be used to distinguish between different lava facies in remote sensing data by examining the entire 2014–2015 Holuhraun lava flow-field in Iceland. Root-mean-square (RMS) height deviation, Hurst exponents, and breakpoints were calculated to quantify the surface roughness characteristics of eight facies using stereo-derived topographic datasets at three different pixel scales, ranging from 0.05 to 0.5 m/pixel. We show that most of the investigated lava facies (rubbly, spiny, undifferentiated rubbly–spiny, shelly, pāhoehoe, and flat-lying–knobby) are indistinguishable by surface roughness down to the 5 cm baseline, with the exception of topography-building facies like the vent-proximal edifice and the exceptionally blocky channel interior facies. Additionally, we considered baselines corresponding to radar S-band (10 cm), L-band (20 cm), and P-band (90 cm). Our findings imply that when transitional lava types are considered in addition to traditional end-members, topographic roughness data, including RMS height deviation and Hurst exponent values alone, cannot be used to uniquely identify lava facies at these baselines.