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.