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 10×10 6 m 3 and a growth rate of 8.9 m 3 /s before dacite lava first appeared at the surface on October 11, 2004. Afterward, the rate was initially 5.9 m 3 /s but slowed to 2.5 m 3 /s by the beginning of January 2005. During 2005, the extrusion rate declined gradually to about 0.7 m 3 /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 73×10 6 m 3 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. east Crater Glacier west Crater Glacier 1980s lava dome 2004-5 dome A B that address loading effects of the growing dome on surface deformation (Lisowski and others, this volume, chap. 15). Photogrammetry based on vertical aerial photographs has been used previously to monitor, model, map, and measure surface change and deformation at volcanoes (Achilli and others, 1998; Baldi and others, 2000, 2005; Zlotnicki and others, 1990). A recent photogrammetric study of the Mount St. Helens crater (Schilling and others, 2004) tied a block of overlapping vertical aerial photographs to a network of global positioning system (GPS) stations on the volcano's flanks, dome, and crater floor (fig. 2A) resulting in a digital elevation model (DEM) of the volcanic edifice and entire crater configuration in October 2000. The 2000 DEM has served as a baseline for comparison with past DEMs.
