Pyroclastic deposit differentiation from LiDAR roughness texture at Mount St. Helens

Topographic data were used to differentiate 1980 Mount St. Helens pumice flow deposit facies based on surface roughness and roughness textures. Roughness elements consist of primary volcanic features and clasts (e.g. pumice-lobes, blocks, and bombs) as well as erosional features (e.g. remnant mounds and channels). The distribution of these features produce roughness textures that depend on the local depositional and erosional history. Roughness textures were derived from a ~2 m spaced LiDAR point cloud, acquired in 2004 by the USGS, The Puget Sound LiDAR consortium, and NASA. Data covering the “Pumice Plain” were processed in ITT- ENVI to produce statistically robust roughness rasters with 5 m resolution. Roughness is the standard deviation of residuals of a best-fit plane calculated for each 5 pixel. Textural statistics were than calculated on the roughness rasters to produce maps of roughness textures. Textures are based on probabilistic statistical measures of the distribution of roughness elements in a 7x7 pixel neighborhood (e.g. Entropy [ENT], Contrast [CON] and Second Moment [SMT]). The scene was then segmented based on the roughness textures. A 14-day field season in Aug 2010 confirmed that 3 textural segments observed in the remote-sensing data represent geologically significant patterns. The segments are: [A] Steep walled channels with 2-4 m blocks produce a texture with high ENT and CON but low SMT. [B] Plateaus with 2 m wide 0.5 m tall 10 m long pumice rich lobes that are quasi-pristine produce a texture with high ENT but low CON and SMT. [C] Gradual slopes with broad parallel channels partially filled with sub meter bombs and blocks produce a texture with high SMT but low CON and ENT. The degree of inundation by lahars is responsible for the majority of the differences among textural segments. Between 1980 and 2004, lahars brought blocks down from the edifice and onto the “pumice plain”. Fast moving and sediment rich lahars incised many of the pumice deposits. Both segments [A] and [C] are significantly altered by lahar erosion and deposition. In-situ pumice from the 1980 deposits can only be found in channel walls and beneath the lahar deposit, in these segments. [A] and [C] only differ in their block populations (blocks in [A] are larger) and steepness of channel walls (walls in [A] are steeper). In segment [B], while some amount of lahar inundation has occurred (e.g. some pumice lobes in [B] have a carapace of lahar-transported blocks), much of the original lobe and levee morphology is preserved. Textural segmentation of roughness rasters is a useful tool for deposit mapping. For whichever planetary body high resolution topographic data are available, roughness texture mapping would be a useful tool for deposit differentiation.