DOES SMOOTHING OF LAVA FLOWS FROM SURFACE DEGRADATION CORRELATE WITH EMPLACEMENT AGE?

Lava flows pose a significant hazard to communities surrounding active and dormant volcanoes, and understanding flow ages and trajectories over time is key to identifying the most hazardous areas. Recent research has demonstrated that surface roughness can be used to establish relative ages of certain young and topographically disruptive landforms like landslides (LaHusen et al., 2020), as the surfaces will generally smooth over time as surface irregularities are weathered. Such an approach for relative age dating has not yet been tested on lava flows. To that end, this study utilized LiDAR-derived DEMs to map nearly two dozen postglacial lava flows around the Mount Adams Volcanic Field in south central Washington, eleven of which have been previously dated by K-Ar and Ar-Ar techniques (see dating by Hildreth and Lanphere, 1994; Jicha et al., 2009). Qualitative comparison shows a clear smoothing over time, particularly when comparing youngest and oldest end members of the mapped population. However, due to compositional and morphological differences among lava flows, a variety of characteristics must be assessed to rigorously test the relationship between surface roughness and relative emplacement age. For instance, the roughness in some pahoehoe flows can vary dramatically between the flow interior and associated chilled margins. As such, relative age dating of lava flows based on surface roughness must first consider a variety of complicating variables that influence surface roughness upon initial lava flow emplacement. These variables include flow type (e.g., pahoehoe, a’a), viscosity, bulk composition and crystallinity, position in the flow (chilled margins versus flow interiors), and original slope during emplacement. By documenting these characteristics in flow fields where a large proportion of the lava flows are dated, the impact these factors have on surface roughness and its relationship to emplacement age can be assessed. Similarly, flows or parts of flows that minimize all differences in these characteristics can be compared to test the surface roughness-age correlation. If a calibration between roughness and relative age is robust, calibration to published absolute ages may permit the development of a more extensive relative chronology of lava emplacement for multiple flows in a volcanic field.