Use of digital aerophotogrammetry to determine rates of lava dome growth, Mount St. Helens, Washington, 2004-2005

Beginning in October 2004, a new lava dome grew on the glacier-covered crater floor of Mount St. Helens, Washington, immediately south of the 1980s lava dome. Seventeen digital elevation models (DEMs) constructed from vertical aerial photographs have provided quantitative estimates of extruded lava volumes and total volume change. To extract volumetric changes and calculate volumetric extrusion rates (magma discharge rates), each DEM surface was compared to preeruption DEM reference surfaces from 1986 and 2003. Early in the 2004-5 eruption, DEMs documented deforming glacier ice and crater floor that formed a prominent “welt” having a volume of 10×10⁶ m³ and a growth rate of 8.9 m³/s before dacite lava first appeared at the surface on October 11, 2004. Afterward, the rate was initially 5.9 m³/s but slowed to 2.5 m³/s by the beginning of January 2005. During 2005, the extrusion rate declined gradually to about 0.7 m³/s. By December 15, 2005, the new dome complex was about 900 m long and 625 m wide and reached 190 m above the 2003 surface. More than 73×10⁶ m³ of dacite lava had extruded onto the crater floor. Successful application of aerophotogrammetry was possible during the critical earliest parts of the eruption because we had baseline data and photogrammetric infrastructure in place before the eruption began. The vertical aerial photographs, including the DEMs and calculations derived from them, were one of the most widely used data sets collected during the 2004-5 eruption, as evidenced in numerous contributions to this volume. These data were used to construct photogeologic maps, deformation vector fields, and profiles of the evolving dome and glacier. Extruded volumes and rates proved to be critical parameters to constrain models and hypotheses of eruption dynamics and thus helped to assess volcano hazards.